contents contributors vii about the editor ix preface xi acknowledgments xiii 1. introduction 1...

Graphene for Flexible Lighting and Displays

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Woodhead Publishing Series in Electronicand Optical Materials

Graphene for FlexibleLighting and Displays

Edited by

Tae-Woo Lee

Department of Materials Science andEngineering, 1 Gwanak-ro, Gwanak-gu,Seoul, Republic of Korea

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Contents

Contributors viiAbout the editor ixPreface xiAcknowledgments xiii

1. Introduction 1Tae-Woo Lee and Sung-Joo Kwon

References 2

2. Structure and properties of graphene 5Yong Seok Choi, Je Min Yoo and Byung Hee Hong2.1 Structure of graphene 52.2 Synthesis of graphene 82.3 Electronic band structure of graphene 112.4 Optical properties of graphene 142.5 Electrical properties of graphene 162.6 Mechanical properties of graphene 22

References 23Further reading 26

3. Preparation of graphene electrode 27Wencai Ren3.1 Solution casting of graphene oxide 273.2 Transfer methods of CVD grown graphene 33

References 52

4. Graphene doping for electrode application 59Amirhossein Hasani and Soo Young Kim4.1 Chemical doping of graphene 594.2 Metal oxide doping of graphene 654.3 Stability of the doped graphene electrodes 67

Acknowledgments 70References 70

5. Technical issues and integration scheme for graphene electrodeOLED panels 73Jaehyun Moon, Jin-Wook Shin, Hyunsu Cho, Jun-Han Han,Byoung-Hwa Kwon, Jeong-Ik Lee and Nam Sung Cho5.1 Introduction 735.2 Graphene preparation for OLED applications 745.3 Technical issues of OLEDs having graphene film electrodes 745.4 Integration schemes for realizing large area graphene electrode

OLED panels 875.5 Summary and future outlook 94

Acknowledgments 95References 95Further reading 98

6. Graphene-based buffer layers for light-emitting diodes 99Quyet Van Le and Soo Young Kim6.1 Introduction 996.2 Graphene oxide buffer layer 996.3 Graphene-based composite buffer layer 1106.4 Conclusion 113

References 113

7. Graphene-based quantum dot emitters for light-emitting diodes 117Park Minsu, Yoon Hyewon and Jeon Seokwoo7.1 Introduction to graphene quantum dots 1177.2 Synthetic strategies for GQDs 1217.3 Toward highly efficient fluorescence from GQDs 1337.4 Lighting applications of GQDs 1387.5 Summary and outlooks 143

References 145

8. Graphene-based composite emitter 151Hong Hee Kim and Won Kook Choi8.1 Grapheneemetal/metal oxide hybrid composite 1518.2 Graphene-based composite emitter 152

References 170

9. Stretchable graphene electrodes 175Shuyan Qi and Nan Liu9.1 Introduction 1759.2 Preparation of stretchable graphene electrodes 1779.3 Applications of stretchable graphene electrodes 1919.4 Summary and outlook 199

References 201

10. Conclusions and outlook 205References 207

Index 209

vi Contents

Contributors

Hyunsu Cho Flexible Device Research Group, Electronics and TelecommunicationsResearch Institute (ETRI), Daejeon, Republic of Korea

Nam Sung Cho Flexible Device Research Group, Electronics and Telecommunica-tions Research Institute (ETRI), Daejeon, Republic of Korea

Yong Seok Choi Department of Chemistry, College of Natural Sciences, SeoulNational University, Gwanak-Gu, Seoul, Republic of Korea

Won Kook Choi Center for Optoelectronic Materials and Devices, Post-SiSemiconductor Institute, Korea Institute of Science and Technology (KIST), Seoul,Korea

Jun-Han Han Flexible Device Research Group, Electronics and Telecommunica-tions Research Institute (ETRI), Daejeon, Republic of Korea

Amirhossein Hasani School of Chemical Engineering and Materials ScienceChung-Ang University, Seoul, Republic of Korea

Byung Hee Hong Department of Chemistry, College of Natural Sciences, SeoulNational University, Gwanak-Gu, Seoul, Republic of Korea; Program in Nano Scienceand Technology, Graduate School of Convergence Science and Technology, SeoulNational University, Yeongtong-Gu, Suwon-Si, Republic of Korea

Yoon Hyewon Department of Materials Science and Engineering, KAIST Institutefor the NanoCentury, Korea Advanced Institute of Science and Technology (KAIST),Daejeon, Republic of Korea

Hong Hee Kim Center for Optoelectronic Materials and Devices, Post-Si Semicon-ductor Institute, Korea Institute of Science and Technology (KIST), Seoul, Korea

Soo Young Kim Department of Materials Science and Engineering, KoreaUniversity, Seongbuk-gu, Seoul, Republic of Korea

Sung-Joo Kwon Department of Materials Science and Engineering, Pohang Univer-sity of Science and Technology (POSTECH), Pohang, Gyungbuk, Republic of Korea

Byoung-Hwa Kwon Flexible Device Research Group, Electronics and Telecommu-nications Research Institute (ETRI), Daejeon, Republic of Korea

Tae-Woo Lee Department of Materials Science and Engineering, Seoul NationalUniversity, 1 Gwanak-ro Gwanak-gu, Seoul, Republic of Korea

Jeong-Ik Lee Flexible Device Research Group, Electronics and TelecommunicationsResearch Institute (ETRI), Daejeon, Republic of Korea

Nan Liu College of Chemistry, Beijing Normal University, Beijing, P.R., China

Park Minsu Department of Materials Science and Engineering, KAIST Institute forthe NanoCentury, Korea Advanced Institute of Science and Technology (KAIST),Daejeon, Republic of Korea

Jaehyun Moon Flexible Device Research Group, Electronics and Telecommunica-tions Research Institute (ETRI), Daejeon, Republic of Korea

Shuyan Qi College of Chemistry, Beijing Normal University, Beijing, P.R., China

Wencai Ren Shenyang National Laboratory for Materials Science, Institute of MetalResearch, Chinese Academy of Sciences, Shenyang, Liaoning, China

Jeon Seokwoo Department of Materials Science and Engineering, KAIST Institutefor the NanoCentury, Korea Advanced Institute of Science and Technology (KAIST),Daejeon, Republic of Korea

Jin-Wook Shin Flexible Device Research Group, Electronics and Telecommunica-tions Research Institute (ETRI), Daejeon, Republic of Korea

Quyet Van Le Institute of Research and Development, Duy Tan University, DaNang, Vietnam

Je Min Yoo Department of Chemistry, College of Natural Sciences, Seoul NationalUniversity, Gwanak-Gu, Seoul, Republic of Korea

viii Contributors

About the editor

Dr. Tae-Woo Lee is a professor in the Department of Materials Science and Engineer-ing at Seoul National University, Korea. He received his PhD in Chemical Engineeringfrom Korea Advanced Institute of Science and Technology, Korea, in 2002. He joinedBell Laboratories, Lucent Technologies, USA, as a postdoctoral researcher in 2002and then worked at Samsung Advanced Institute of Technology as a member ofresearch staff (2003e08). He was an assistant and associate professor in the depart-ment of materials science and engineering at Pohang University of Science and Tech-nology, Korea until August 2016.

He received a prestigious Korea Young Scientist Award from the President of Korea in2008 and the Scientist of the Month Award from the Ministry of Science, ICT andFuture Planning in 2013. He is author and coauthor of 205 papers in prestigious jour-nals including Science, Nature Photonics, Science Advances, Nature Communica-tions, PNAS, Energy and Environmental Science, Angewandte Chemie, AdvancedMaterials, and ACS Nano. He is also the inventor or coinventor of 375 patented tech-nologies (187 Korean patents and 188 international patents). He currently serves as aneditorial board member on the Journals of FlatChem (Elsevier), EcoMat (Wiley), andSemiconductor Science and Technology (IOP).

His research focuses on organic, organiceinorganic hybrid perovskite and carbonmaterials, and their applications to flexible electronics, printed electronics, displays,solid-state lightings, solar energy conversion devices, and bioinspired neuromorphicdevices. His work in graphene-related fields includes environmentally benign synthe-sis of graphene from solid carbon sources such as inexpensive polymers and carbonwastes, chemical doping of graphene for tuning of its work function and conductivity,preparation of graphene quantum dots, and fabrication of efficient organic and halideperovskite light-emitting diodes using graphene electrodes.

Preface

The demand for lighting and display technologies is driving research to diversify theforms of devices. Future lighting and displays should be bendable, foldable, andstretchable to satisfy consumers’ desire for convenience and efficient use of space.Flexible components of lighting and display devices should be mechanically tolerantof repeated severe flexion. However, conventional light-emitting diodes (LEDs) forlighting and displays are mostly fabricated on a brittle transparent conducting oxideelectrode (e.g., indium tin oxide [ITO]), which has poor tolerance to mechanical strain.Therefore, alternative flexible transparent conducting materials have been evaluated toreplace ITO.

Graphene is a two-dimensional single-atom-thick sheet of carbon atoms in ansp2-bonded hexagonal configuration. Graphene’s unique structure yields excellentelectrical and optical properties as well as mechanical robustness, so it is regardedas a strong candidate for use as a flexible electrode in lighting and displays. However,pristine graphene has several characteristics that limit its practical applications inflexible self-emissive LEDs. Its sheet resistance Rs is too high, and its work function(WF) is too low for graphene to be effective as an anode in LEDs. Chemical dopingof graphene can control Rs and WF, so the charge injection from graphene electrodeto overlying layers in LEDs can be significantly improved. As a result of appropriatechemical doping, especially with organic and polymeric dopants, the luminous prop-erties of organic light-emitting diodes (OLEDs) based on graphene electrodes havebeen increased to be comparable to those of OLEDs that use ITO electrodes. Thedoping can also make graphene very stable against moisture, organic solvents, andacids.

Graphene-based materials can be used as an interfacial layer to improve the chargeinjection in LEDs. Pristine graphene has no bandgap, but chemical functionalization(e.g., oxidation, hydrogenation) can induce one and provide an intermediate step forcharge injection, so the luminous properties of LEDs can be improved.

Graphene quantum dots (QDs) can also themselves be light emitters. The grapheneQDs have advantages (e.g., nontoxicity, high chemical stability, high carrier mobility)over inorganic QDs.

Graphene can also be stretchable, so it can be used in stretchable electronics anddisplays. To date, several structural modifications or composite with other stretchableconducting materials have increased the stretchability of graphene and allowed it to beused in stretchable electronics.

This book will cover the fundamental electrical, optical, and mechanical propertiesof graphene; the preparation of pristine graphene, doped graphene, graphene-derivedinterfacial and graphene QD emitting materials and their composites; and treatments tomodify its electrical properties by adsorbed molecules and deposited films. Then thebook presents the use of flexible or stretchable electrodes with graphene or its com-posites in various LEDs for lighting and displays (e.g., OLEDs, inorganic LEDs,QD-LEDs, and halide perovskite LEDs). It will also describe the use of graphene-derived materials as interfacial buffer layers or as light-emitting layers in LEDs.

This book is written by leading experts who are working on graphene-based mate-rials and optoelectronic devices. It provides in-depth information on use of graphene inlight-emitting devices. The overall goal of this book is to provide comprehensiveinformation about fundamental properties of graphene; on methods to synthesize gra-phene; on techniques to prepare graphene electrodes and composite electrodes; onmethods to dope graphene electrodes; and on applications of graphene-based flexibleelectrodes, interfacial buffer layers, nanoscale emitters, and graphene-based stretch-able electrodes. The ultimate objective is to inspire further research on practicaloptoelectronics applications of graphene.

This book will be of interest to the large community of researchers who are workingon applying graphene in various electronic and optoelectronic devices and may stim-ulate research to develop practical uses of graphene sheets in next-generation displaysand lightings. The book will additionally provide future prospects and suggest furtherdirections for research on graphene-based next-generation displays and lightings.Therefore, this book will be helpful for students, professors, researchers, and engineerswho work on graphene or graphene-derived materials or on graphene-based displaysand lighting technology.

xii Preface

Acknowledgments

The publication of this book is possible only because of the support of many people. Istart by thanking the authors who contributed to this book although they are very busywith their research projects and education.

I would like to say a big thanks to all the members of my research group (PrintedNano-Electronics and Energy Laboratories) and especially the students who performedresearch related to this field: Dr. Tae-Hee Han, Dr. Hong-Kyu Seo, and Dr. Sung-JooKwon.

I sincerely thank to Kayla Dos Santos, an Acquisitions Editor in charge of the fieldof Electronic, Magnetic, and Optical Materials in Elsevier’s Science and TechnologyBooks. She encouraged me to write this book and monitored its progress to ensure thatit was published on time. I also thank Dr. Peter W. Adamson, Editorial Project Man-ager in Elsevier, who handled manuscript and cover design and other related tasks ofbook editing.

Finally, I would like to express my sincere thanks and gratitude to my family mem-bers including my wife, Mun-Hee, and my daughters, Seohyun and Chaehyun, fortheir immense understanding, support, and encouragement that were crucial andinvaluable for successful achievement of research projects and this book.

Introduction 1Tae-Woo Lee 1, Sung-Joo Kwon2

1Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-roGwanak-gu, Seoul, Republic of Korea; 2Department of Materials Science and Engineering,Pohang University of Science and Technology (POSTECH), Pohang, Gyungbuk, Republic ofKorea

Humans have always had need for lighting and have recently developed a reliance oninformation displays (e.g., computer screens) based on lighting. The invention of theincandescent light bulb drove a revolutionary change in human life by freeing usfrom dependence on fire. However, this technology has poor luminous efficiencyand severe heat generation, so it is about to be supplanted by light-emitting diodes(LEDs). Significant advance in LED research has achieved blue LEDs andwhite LEDs with luminous efficiency >100 lm/W [1]. Therefore, LEDs have diverseapplications, including use in displays to visualize information for human interpre-tation. Advances in information technology have increased humans’ need to shareinformation, so emphasis on display technology has increased.

Organic lighteemitting diodes (OLEDs) are also promising next-generation lightsources. OLEDs have advantages such as light weight, easy color tunability, design-able form, and suitability for large-area fabrication, so they also have applications indisplays [2]. Alternative emitters (e.g., quantum dots (QDs), perovskite) are beingevaluated for use in LEDs, and their emissive properties have been continuouslyimproved. The luminous properties of display and lighting technology have beenadvanced remarkably, and the forms of lighting and display devices have been diver-sified in response to the demands of industry. To satisfy future requirements, lightingand displays should be bendable, foldable, and stretchable.

Conventional lighting and displays are mostly fabricated on a transparent electrodethat is formed from a conducting oxide (e.g., indium tin oxide (ITO)), which is brittle.To achieve flexible lighting and displays, these brittle elements must be replaced withflexible components. Therefore, many flexible transparent conductors (e.g., graphene,carbon nanotubes, metal nanowires, and conducting polymers) have been evaluated asmaterials to replace ITO [3e6].

Graphene has remarkable electrical, optical, and mechanical properties and, there-fore, has good potential as a flexible electrode to replace ITO in lighting and displaydevices (Fig. 1.1). However, pristine graphene has high sheet resistance (RS) and lowwork function (WF), so OLEDs with graphene electrode have poor luminous properties[3]. Chemical doping of graphene can modify Rs and the WF, and thereby substantiallyimprove the charge injection from the graphene electrode to overlying layers. By thisapproach, the luminous properties of OLEDs based on graphene electrodes have beenincreased to levels comparable with those of OLEDs that use ITO electrodes [3,7,8].

Graphene for Flexible Lighting and Displays. https://doi.org/10.1016/B978-0-08-102482-9.00001-0Copyright © 2020 Elsevier Ltd. All rights reserved.

https://doi.org/10.1016/B978-0-08-102482-9.00001-0

Further modifications in the properties of graphene are possible. Insertion of agraphene-based interfacial layer can improve the charge injection and increase theluminous properties of LEDs that use graphene. Chemical functionalization (e.g.,oxidation, hydrogenation) can induce a bandgap in the electronic structure of graphene[9,10]; the bandgap can provide an intermediate step for charge injection, so the lumi-nous properties of LEDs can be improved.

Graphene can also be used as an emitting layer. After chemical functionalization bysurface passivation, a graphene QD emitter is less toxic, more chemically stable, andhas higher carrier mobility than conventional inorganic QDs [11].

Ideally, flexible electronics should also be stretchable. Graphene has outstandingmechanical flexibility, but strong in-plane stiffness (340 N/m) and Young’s modulus(0.5 TPa), which impede the use of graphene in stretchable electronics that requiresstretchability above 10% [12]. Mechanical stress cannot be dissipated in the graphenelattice because of the strong bonding between the carbon atoms. For instance, CVDgrown graphene on elastic substrate lose its electrical conductivity under 6% of me-chanical strain [13]. Several structural modifications or combinations with otherstretchable conducting materials have improved the stretchability of graphene andhave been used in stretchable electronics [12,14e16].

In this book, we first introduce the electrical, optical, and mechanical properties ofgraphene and present the preparation of graphene and treatments to modify its electri-cal properties. Then we examine the use of flexible or stretchable electrodes with gra-phene in various lighting and displays (e.g., LEDs, OLEDs, QD-LEDs, perovskiteLEDs). We also review the use of graphene in interfacial materials or emitting mate-rials of LEDs.

References

[1] Y. Narukawa, J. Narita, T. Sakamoto, K. Deguchi, T. Yamada, T. Mukai, Ultra-high ef-ficiency white light emitting diodes, Jpn. J. Appl. Phys. 45 (41) (2006) L1084eL1086.

[2] M.-H. Park, T.-H. Han, Y.-H. Kim, S.-H. Jeong, Y. Lee, H.-K. Seo, H. Cho, T.-W. Lee,Flexible organic light-emitting diodes for solid-state lighting, J. Photonics Energy 5 (1)(2015) 053599.

Flexible lighting Flexible display

Figure 1.1 Schematic drawings of future lighting and displays using graphene electrodes.

2 Graphene for Flexible Lighting and Displays

[3] T.-H. Han, Y. Lee, M.-R. Choi, S.-H. Woo, S.-H. Bae, B.H. Hong, J.-H. Ahn, T.-W. Lee,Extremely efficient flexible organic light-emitting diodes with modified graphene anode,Nat. Photonics 6 (2012) 105e110.

[4] E.C.-W. Ou, L. Hu, G.C.R. Raymond, O.K. Soo, J. Pan, Z. Zheng, Y. Park, D. Hecht,G. Irvin, P. Drzaic, G. Gruner, Surface-modified nanotube anodes for high performanceorganic light-emitting diode, ACS Nano 3 (8) (2009) 2258e2264.

[5] Z. Yu, Q. Zhang, L. Li, Q. Chen, X. Niu, J. Liu, Q. Pei, Highly flexible silver nanowireelectrodes for shape-memory polymer light-emitting diodes, Adv. Mater. 23 (5) (2011)664e668.

[6] M. Cai, Z. Ye, T. Xiao, R. Liu, Y. Chen, R.W. Mayer, R. Biswas, K.-M. Ho, R. Shinar,J. Shinar, Extremely efficient indiumetin-oxide-free green phosphorescent organic light-emitting diodes, Adv. Mater. 24 (31) (2012) 4337e4342.

[7] T.-H. Han, S.-J. Kwon, N. Li, H.-K. Seo, W. Xu, K.S. Kim, T.-W. Lee, Versatile p-typechemical doping to achieve ideal flexible graphene electrodes, Angew. Chem. Int. Ed. 55(21) (2016) 6197e6201.

[8] S.-J. Kwon, T.-H. Han, T.Y. Ko, N. Li, Y. Kim, D.J. Kim, S.-H. Bae, Y. Yang, B.H. Hong,K.S. Kim, S. Ryu, T.-W. Lee, Extremely stable graphene electrodes doped with macro-molecular acid, Nat. Commun. 9 (2018) 2037.

[9] T.-H. Han, S.-J. Kwon, H.-K. Seo, T.-W. Lee, Controlled surface oxidation of multi-layered graphene anode to increase hole injection efficiency in organic electronic de-vices, 2D Mater. 3 (2016) 14003.

[10] J. Son, S. Lee, S.J. Kim, B.C. Park, H.-K. Lee, S. Kim, J.H. Kim, B.H. Hong, J. Hong,Hydrogenated monolayer graphene with reversible and tunable wide band gap and its field-effect transistor, Nat. Commun. 7 (2016) 13261.

[11] Z. Luo, G. Qi, K. Chen, M. Zou, L. Yuwen, X. Zhang, W. Huang, L. Wang, Microwave-assisted preparation of white fluorescent graphene quantum dots as a novel phosphor forenhanced white-light-emitting diodes, Adv. Funct. Mater. 26 (16) (2016) 2739e2744.

[12] N. Liu, A. Chortos, T. Lei, L. Jin, T.R. Kim, W.-G. Bae, C. Zhu, S. Wang, R. Pfattner,X. Chen, R. Sinclair, Z. Bao, Ultratransparent and stretchable graphene electrodes, Sci.Adv. 3 (9) (2017) e1700159.

[13] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, J.-H. Ahn, P. Kim, J.-Y. Choi,B.H. Hong, Large-scale pattern growth of graphene films for stretchable transparentelectrodes, Nature 457 (2009) 706e710.

[14] M.K. Blees, A.W. Barnard, P.A. Rose, S.P. Roberts, K.L. McGill, P.Y. Huang,A.R. Ruyack, J.W. Kevek, B. Kobrin, D.A. Muller, P.L. McEuen, Graphene kirigami,Nature 524 (2015) 204e207.

[15] M. Chen, T. Tao, L. Zhang, W. Gao, C. Li, Highly conductive and stretchable polymercomposites based on graphene/MWCNT network, Chem. Commun. 49 (2013)1612e1614.

[16] M.-S. Lee, K. Lee, S.-Y. Kim, H. Lee, J. Park, K.-H. Choi, H.-K. Kim, D.-G. Kim, D.-Y. Lee, S. Nam, J.-U. Park, High-performance, transparent, and stretchable electrodesusing grapheneemetal nanowire hybrid structures, Nano Lett. 13 (6) (2013) 2814e2821.

Introduction 3

Structure and properties ofgraphene 2Yong Seok Choi 1, Je Min Yoo 1, Byung Hee Hong 1,2

1Department of Chemistry, College of Natural Sciences, Seoul National University,Gwanak-Gu, Seoul, Republic of Korea; 2Program in Nano Science and Technology,Graduate School of Convergence Science and Technology, Seoul National University,Yeongtong-Gu, Suwon-Si, Republic of Korea

2.1 Structure of graphene

2.1.1 Atomic structure of graphene

Graphene, the world’s thinnest two-dimensional material with hexagonal carbon-based honeycomb network, is one of the major sp2-hybridized carbon allotropes alongwith 0D fullerenes (wrapped-up graphene), 1D carbon nanotubes (rolled-up graphenemonolayers), and 3D graphite (stacked-up graphene monolayers). Graphene has beenan important material in various academic areas due to many of its unprecedentedproperties including electron mobility, thermal conductivity, atomic thinness, opticaltransparency, and mechanical strength based on the elongated p conjugation [1e8].To understand the atomic structure of graphene, it is necessary to comprehend theorbital hybridization feature of carbon atoms. Conventionally, four sp3 hybrid orbitalsare formed from one 2s and three 2p orbitals (2px, 2py, and 2pz). The four valence elec-trons on each carbon atom are thus occupied in a single sp3 orbital. In graphene, on theother hand, one 2s and two 2p orbitals participate in the formation of sp2-hybridizedorbitals, leaving 2pz orbital unoccupied. The sp2 orbitals are oriented in xey planewith trigonal planar shape, and the remaining 2pz orbital is perpendicularly positionedto the plane. The sp2 carbon atoms form covalent in-plane s bonds with adjacent car-bon atoms, and p bond is formed by overlapping with two unhybridized 2pz orbitals(Fig. 2.1(b)). Fig. 2.1(c) shows the hexagonal lattice of graphene with distinctivearmchair and zigzag edges. Graphene consists of two carbon atoms per unit cell, result-ing in two nonequivalent carbon atom sublattices (A and B). The real space basisvectors of unit cell are written as follows:

a1¼ a

2

�3;

ffiffiffi3

p �and a2 ¼ a

2

�3; �

ffiffiffi3

p �


https://doi.org/10.1016/B978-0-08-102482-9.00002-2

(a) (b)

2s

1s 1s 1s

2px 2py 2pz 2pz

Ground state

A B

a1

b1

b2

δ1Γ

δ2

δ3

a2

ky

kxK’

K

M

Orbitalhybridisation

sp3 hybrid orbitals sp2 hybrid orbitals

Diamond Graphene

(c) (d)

Figure 2.1 Schematic representation of major carbon allotropes. (a) Graphene can be considered as the basic building block for other carbon allotropesincluding 0D fullerene (wrapped-up), 1D carbon nanotube (rolled-up) and 3D graphite (stacked). (Reproduced with the permission from MacmillanPublishers Ltd: Nat. Mater., Ref. [1]). (b) Ground state atomic orbital of a carbon atom, sp3 hybridized orbitals in diamond and sp2 hybridized orbitalsin graphene. (c) Crystal structure of graphene. 2D hexagonal lattice of graphene in real space with vectors a1 and a2. Two nonequivalent carbon atomsA and B compose the unit cell. (d) The first Brillouin zone with high symmetry points G K, M and the reciprocal lattice with lattice vector b1 and b2 isindicated (Reproduced with the permission from American Physical Society, Ref. [9]).

6Graphene

forFlexible

Lighting

andDisplays

where a ¼ 1.42 Å is the distance from adjacent carbon atoms. The correspondingreciprocal lattice vectors are as follows:

b1¼ 2p3a

�1;

ffiffiffi3

p �and b2 ¼ 2p

3a

�1; �

ffiffiffi3

p �

The structure of graphene can be expressed in reciprocal lattice geometry with thetwo high symmetry points K and K0 within the Brillouin zone, which are known as theDirac points. Their vectors in K-space can be expressed as follows:

K¼ 2p3a

�1;

ffiffiffi3

p �and K0 ¼ 2p

3a

�1; �

ffiffiffi3

p �

This atomic structure of graphene results in a zero bandgap, where the conductionband and the valence band meet at the cone-shaped Dirac point. This enables thoroughinvestigations on the properties of single-layer graphene. In addition, graphene’s infin-ite carbon network structure would ideally give rise to various applications includingflexible display, solar cell, and transparent electrode for its perfect crystallinity andhigh chemical stability and impermeability.

2.1.2 Nanoscale morphology of graphene

Although graphene is only a single atomethick material, it is easily observable withan optical microscope. Fig. 2.2(a) shows chemical vapor deposition (CVD) grapheneon SiO2 wafer with multidots, wrinkles, and residues of polymethyl methacrylate(PMMA) supporting layer. Although multidots, wrinkles, and PMMA residuesmay degrade the electrical properties and induce undesirable doping effects, theseattributes are inevitably accompanied during the transfer process. Atomic forcemicroscopy (AFM) analysis provides more detailed surface information includingthe thickness and morphology. While the thickness of graphene is 0.34 nm in theory,the average thickness of a single-layer graphene ranges from 0.7 to 1.0 nm due to thebonding force between substrate and graphene, as well as the PMMA residues(Fig. 2.2(b)). Transmission electron microscopy (TEM) analysis is employed toexplore not only the thickness but also the grain boundaries, diffraction patterns,and carbon atomic lattice networks of graphene. Fig. 2.2(c) shows TEM imagewith an atom-by-atom analysis of graphene (mid inset). The diffraction patternanalysis indicates that the atomic structures of graphene are highly crystalline (lowerinset). Scanning tunneling microscopy (STM) topography also gives evidence ofgraphene’s high crystallinity and shows the honeycomb structure expected for thefull hexagonal symmetry of graphene (Fig. 2.2(d)). The absence of observabledefects in the STM image with equivalently positioned carbon atoms with thesame intensity indicates the high quality of graphene. Raman spectroscopy is alsoone of the most widely exploited tools in investigating the crystallinity of graphene,which will be discussed in the later section.

Structure and properties of graphene 7

2.2 Synthesis of graphene

As graphite is composed of stacked graphene layers through the van der Waals inter-actions, British scientists were able to obtain graphene flakes by mechanically exfoli-ating graphite with Scotch tape in 2004 for the first time [9]. Although the highestquality monolayer graphene can be produced by mechanical exfoliation (Fig. 2.3),the production scale is restricted to micrometer range only; further applications forflexible display are limited. Among a number of approaches to produce large-scale gra-phene film, the CVD method is considered to be optimal to synthesize large-scalemonolayer graphene with respectable quality and is thus deemed to be the most

(a) (b)

(c) (d)

Residue

Wrinkle

0.1 nm

Figure 2.2 The atomic structure and morphology of graphene. (a) Representative OM and (b)AFM images of monolayer CVD graphene with multi-dots, wrinkles and PMMA residuestransferred on SiO2 substrate with designated arrows. Inset image of (b) represents the heightprofile of graphene (Reproduced with the permission from American Chemical Society, Ref.[10] and IOP, Ref. [11]). (c) HR-TEM image of CVD graphene showing the carbon atomnetwork. The mid inset shows the hexagonal honeycomb lattice of graphene through atom-by-atom analysis. The bottom inset represents the diffraction pattern of graphene indicatingthe high crystallinity of graphene (Reproduced with the permission from American ChemicalSociety, Ref. [2]) (d) HR-STEM image of graphene on an insulating surface (Copyright(2007) National Academy of Sciences, USA, Ref. [12]).


suitable for display applications. The very first attempt to produce CVD graphene wasrather a serendipitous discovery while researchers attempted to synthesize CVD dia-mond by adsorbing carbon gas precursor (i.e., methane) on copper foil, which led tothe formation of very thin yet highly crystalline graphite. Along with the previous find-ings in the 1960s that carbon gas precursors may be adsorbed in a highly crystalline,sp2-hybridized structure onto a hot metal catalyst, this sparked a renewed interest ofresearchers to synthesize large-scale graphene film through the CVD method.

In the early years, some researchers reported the synthesis of micrometer-scalemonocrystalline graphene on top of monocrystalline ruthenium catalyst, which stilldid not overcome the issues with dimension and uniformity [10,11]. Later, other sci-entists suggested the epitaxial growth of graphene on silicon carbide substrate [12].However, in addition to the fact that the price of silicon carbide substrates isextremely costly, the processing conditions are difficult to achieve (high vacuum,high temperature (1300�C)), and selective transfer of graphene from the substrateis nearly impossible, epitaxially grown graphene cannot be processed for practicalapplications. Through further efforts, researchers have demonstrated that a few tran-sition metal catalysts provide the optimal conditions to produce large-scale CVD gra-phene film and play pivotal role in understanding the growth mechanisms based onthe difference in carbon solubility [13e19]. The growth of graphene on transitionmetal catalysts occur by two major mechanisms: (1) segregation and (2) surface-mediated reaction (Fig.2.4). In case of nickel catalysts, which exhibit relativelyhigh carbon solubility, carbon-based gas precursors are decomposed into individualcarbon atoms and are subsequently dissolved and diffused in nickel at high temper-ature. During cooling, dissolved carbon atoms emerge and segregate on the surface.The thickness of graphene is strongly related to the cooling rate; exceedingly fast

Single-layer graphene

Figure 2.3 Representative experimental procedures of mechanical exfoliation (Reproducedwith the permission from Royal Society of Chemistry, Ref. [14]).


cooling provokes the loss of carbon mobility from bulk to result in quenching effect,and slow cooling provides sufficient time for the carbon atoms to diffuse into bulk toprevent surface segregation of them. For copper catalysts, which possess relativelylow carbon solubility, the formation of graphene undergoes surface-mediated reac-tion. Copper’s fully filled 3d-shell gives rise to its low carbon affinity and preventsthe formation of carbide phase. For such reasons, decomposed carbon atoms formseeds and laterally nucleate on the surface until the entire copper surface is coveredwith graphene. It can be thus inferred that copper-mediated CVD growth is prefer-able for the production of large-scale monolayer graphene. It must be noted, howev-er, that as-grown graphene on copper foil is intrinsically polycrystalline; nucleatedseeds inevitably form grain boundaries and defects that impede the electrical proper-ties of graphene. Researchers have endeavored to increase the size of individualgrains to minimize the number of grain boundaries, which could thus enhance theproperties. Based on the fact that the crystallinity of copper foil is one of the decisivefactors to determine the grain size of graphene, some researchers physically extendedcopper foil in the gravitational orientation to increase its crystal grain size. As copperundergoes recrystallization during annealing process, the crystalline domain size ofgraphene increases accordingly, which eventually contribute to the formation

(a) (c)

(b) (d)

Hydrocarbon metalgas

Carbondissolving

Surface Body

Extremely fast cooling Fast/medium cooling Slow cooling

CH4

H

1

215

2

3 4

Segregation, Ni Surface-mediated reaction, Cu

Figure 2.4 Schematic representation of the growth mechanism of graphene by segregation andsurface-mediated reaction on Ni and Cu substrates, respectively. (a, b) Schematic illustration ofgraphene synthesis mechanism on Ni foil. (Reproduced with the permission from AmericanInstitute of Physics, Ref. [15]) (c, d) Schematic illustration of graphene synthesis mechanismon Cu foil (Reproduced with the permission from American Chemical Society, Ref. [16]),(Reproduced with the permission from Elsevier, Ref. [17]).


of fewer grain boundaries and thus superior electrical properties (Figs. 2.5 and 2.6).A recent publication from Professor Hong group demonstrated vertical synthesis ofgraphene with larger grain size by applied tension to copper foil through a verticalCVD system. In addition, the report suggested the continuous mass production of gra-phene film using a roll-to-roll vertical CVD system, which enables the synthesis oflarge-scale, high quality graphene with minimized grain boundaries (Fig. 2.7) [20].

2.3 Electronic band structure of graphene

The energy band structure of graphene is closely related to its p electron system. Theelectrons of graphene behave relativistically from the graphene Dirac equation, and theband structure was first obtained in 1947 by P.R. Wallace [21].

E�ðkÞ z � vFjqj

(a)

(c)

(b)

Vertical Vertical Vertical w/ tension

Jig

Cu foilWeight

Gas out Gas in

Gas in

Gas out

Horizontal - VerticalSwitchable

Figure 2.5 (a,b) Representative images of a vertical CVD system. The gravitactic tension isapplied to Cu foils with additional force applied by weight. (c) Schematic representation ofa horizontal/vertical convertible CVD system (Reproduced with the permission from IOP,Ref. [25]).


(a) (b) (c) Vertical w/ tensionVerticalHorizontal

Grain size (μm2) Grain size (μm2) Grain size (μm2)0 5 10 15 20 25 30 35 40 45 0 5 10 15 20 25 30 35 40 45 0

00

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5

10

15

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30

1

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5 10 15 20 25 30 35 40 45

Cou

nts

Cou

nts

Cou

nts

Figure 2.6 Representative analyses of grain boundaries in CVD graphene by scanningdiffraction mapping in TEM, indicating that the size of grain boundary can be increased withapplied tension to Cu foils (Reproduced with the permission from IOP, Ref. [25]).

(b)(a)

(c)

Figure 2.7 Representative images of vertical roll-to-roll (R2R) CVD system with a tensioncontrol unit. The tension is controlled by the gravitational force and the winding roll. Thisvertical R2T CVD system enables the synthesis of high quality large-scale graphene at a speedof 300 mm per min (Reproduced with the permission from IOP, Ref. [25]).


The charge carriers near the Dirac point are massless, and Dirac Fermions act with aFermi velocity of vF w 1 � 106 m/s. The pluseminus represents p*(unoccupied orconduction) and p (occupied or valence) bands, respectively. The energy band struc-ture of graphene with the nearest neighbor is shown in Fig. 2.8 and exhibits a lineardispersion at the Fermi energy at the high symmetry K and K0 points of the Brillouinzone [22]. The conduction band and the valence band are in contact with the cone-shaped vertices, and each vertex meets at the K-point of the Brillouin zone to formthe Fermi level. Unlike ordinary semimetallic materials, graphene has no bandgap.However, it is still deemed semimetallic as it has zero density of state of electronsat the Fermi level. Because of this unique electronic structure, graphene has a bipolarconduction characteristic that can be easily changed by the type of doping. Whiledoping in semiconductors is achieved by implanting impurities, the electrical proper-ties of graphene can be modulated by inducing chemical doping on the surface. Whendoping with an electron donor, called n-type doping, the electron levels become higherand the conductivity is increased. The upward-shifted Fermi level leads to decreasedwork function (Fig. 2.9) [23]. Conversely, doping with an electron acceptor, calledp-type doping, induces graphene with increased conductivity as well, but thedownward-shifted Fermi level results in increased work function. In summary, chem-ical dopingeinduced changes in the Fermi level of graphene enables the work functioncontrol and subsequent electrical properties. Doping can thus be regarded as one of the

4

4

2

2

24

0

0

0–2

–2 –2–4

–4

EK

kx

ky

Figure 2.8 Representative electronic band structure of graphene. The conduction band andvalence band meet at the Dirac point. Circled inset shows a cone-shaped linear dispersion(Reproduced with the permission from MDPI, Ref. [27]).

E(k)

C-P

DO

S

EF

Intrinsic

Intrinsic

n - doped

n - doped

p - doped

p - doped

EF

E

Figure 2.9 Schematic representation of the Fermi level and Dirac point shifts according to thetype of doping (Reproduced with the permission from American Chemical Society, Ref. [28]).


important techniques for employing graphene in various applications. Studies on gra-phene doping have been vastly carried out, and the most widely employed chemicaldoping method is the use of strong acids such as nitric acid, a strong p-dopant. p-doped graphene has a lower sheet resistance than pristine graphene, and the Dirac pointshifts from 0 V to (þ) direction in graphene field effect transistor (FET) devices [24].

2.4 Optical properties of graphene

2.4.1 Transparency of graphene

Because of the growing worldwide demand for transparent display electrodes, relevantstudies have been vastly carried out. As of today, indium tin oxide (ITO) is the mostwidely employed transparent electrodes for display, touch panel, and solar cell appli-cations. However, the unit price of ITO has been steeply rising owing to the depletionof indium; developing alternative materials is highly demanded. In addition, ITO’sfragility and inflexibility have complicated its uses for broader applications. Graphenehas thus attracted much attention as a promising material for the next-generation trans-parent electrode because of its flexibility and stretchability, along with its relativelyfacile synthesis and patterning processes. If mass production of graphene is achievablepotentially through roll-to-roll method, it is expected to bring technological break-throughs on the next-generation flexible electronics industry. Fig. 2.10(a) shows thatthe UV-vis transmittance spectra on a quartz substrate with increasing number of gra-phene layers. Theoretically, monolayer graphene reduces the light transmittance by2.3% in the visible region and the transmittance decreases as the layer number in-creases. Fig. 2.10(b) shows comparative UV-vis transmittance spectra of graphene

(b)(a) 100

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No. of layers (Tr at 550 nm) = 1 (97.4%)2 (95.1%)3 (92.9%)4 (90.1%)

Graphene only

Graphene/PET~220Ω/sq.

Graphene on PETITO on PET

~180 Ω/sq.ITO/PET

Figure 2.10 Optical characterization of graphene film. (a) UV-vis transmittance spectra ofgraphene film with an increasing number of layers on quartz substrates. The inset shows thetransmittance with and without HNO3 doping (Reproduced with the permission fromAmerican Chemical Society, Ref. [2]). (b) UV-vis transmittance spectra of graphene film andITO on PET substrates. ITO is less transparent in the short visible wavelengths (Reproducedwith the permission from Macmillan Publishers Ltd: Nat. Nanotechnol., Ref. [29]).


and ITO films on polyethylene terephthalate (PET) substrate. While graphene-basedfilm is transparent for all visible ranges, ITO film is relatively opaque in short visiblewavelengths, which is observed to be slightly yellowish with naked eyes.

2.4.2 Raman spectroscopy analysis

Raman spectroscopy is a widely used technique for investigations on the molecularstructures and bonding effects in various areas including physics, chemistry, and ma-terials science. It is known to be the most effective and facile method to analyze thevibrational structure and electronic properties of different materials without damagingthe sample. In particular, due to the unique electron band structure of graphene, thethickness, crystallinity, and doping state can be easily and rapidly analyzed. In addi-tion, the Raman scattering of graphene is amplified by the charge resonance phenom-enon, and the surface remains intact despite its exposure to strong laser due tographene’s high chemical and thermal resistance. Graphene generally exhibits threecharacteristic bandsdG, D, and 2D bandsdwhich can be exploited to identify a num-ber of different properties (Figs. 2.11 and 2.12) [25]. The G band near 1580 cm�1 iscommonly detected in graphitic materials because of the phonon vibration mode cor-responding to the stretching of the carbonecarbon bond. As a result, the carbon atomsin a hexagonal structure oscillate in opposite directions to the adjacent atoms. Becausethe energy level of the G band is determined by the density of surplus charge doped ingraphene, it is possible to quantify the electrons or holes injected during doping.Furthermore, the change of the Fermi level can be estimated by using the electronicdensity state of graphene. Namely, the doping information can be obtained byanalyzing the position and full width half maximum (FWHM) of the G band. In

Conduction band

Valence band

Intervalley Intervalley

IntervalleyK

K’ K’ KK

K

f

f

fi

b

b

ba

a ac

a

cD band

D’ band

2D band

Phonon

Phonon Phonon

PhononDefect

Phonon

Def

Phonon

Phonon

i i

hω 0

hωvib

(a) (b)

(c) (d)

Figure 2.11 Resonant scatterings of the valence and conduction bands. (a) Non-resonantphonon scattering. (b) Second-order Raman process for intravalley scattering of the D’ band ingraphene (c) Second-order Raman process for the D band. (d) Second-order Raman process forthe 2D band. Two phonons have opposite wavevectors to conserve the total momentum in thescattering process (Reprinted with the permission from MDPI, Ref. [32]).


both p- and n-doping, the G band is blue shifted and exhibits narrower FWHM. Inaddition, as the FWHM decreases, the intensity is increased with decreased relative2D/G ratio (Figs. 2.13 and 2.14) [26,27]. The D band near 1380 cm�1 appearswhen there is a bond with sp2 crystal structure. Generally, graphene obtained throughmechanical exfoliation has a high crystallinity and hardly presents any D band. How-ever, if graphene is impaired from chemical reaction or physical treatment, the inten-sity of D band increases; it is thus often used as an indicator of graphene defects. The2D band is located atw2780 cm�1 due to the secondary scattering where two phononsof D band are emitted. This band is determined by the double resonance phenomenoncaused by the electronic structure of graphene, so the thickness and number of layers ofgraphene can be measured through the ratio of the G band and the 2 D band. Fig. 2.12shows increased G/2D ratio by decreased 2D band intensity with an increasing numberof graphene layers [28,29]. Unlike the G band, doping-induced 2D band shifts aredetermined by the type of doping; the position is either red shifted (n-type) or blueshifted (p-type). Because doping process changes the equilibrium lattice parameters,n-doping expands the lattice that softens the 2D mode, while p-doping inducesshrinkage of lattice and phonon stiffening.

2.5 Electrical properties of graphene

2.5.1 Graphene field effect transistor

The FET is a device that controls the source and drain currents by adjusting the flow ofelectrons/holes to the channel by applying a gate voltage to the electrode. In 2004,

(a) (b)

Ram

an in

tens

ity

Raman shift (cm–1)

I(2D

)/I(G

)

G band 2D band532 nm

Graphite

5 layers

4 layers

3 layers

2 layers

1 layer

11200 1600 2000 2400 2800 3200 1 2 FL MLGraphene layers

2

3

4

GrapheneGraphene +LCCs

Figure 2.12 (a) Representative Raman spectra of graphene film with an increasing number oflayers (Reproduced with the permission from, Springer Science+Business Media, Ref. [33]).(b) The peak intensity ratio of 2D/G as a function of number of ayers (Reproduced with thepermission from Elsevier, Ref. [34]).


Novoselov and Geim of the University of Manchester showed that graphene-based de-vices exhibit linearly increasing conductivity by applied gate voltage, confirming thatgraphene has the properties of FETs [9]. In FETs with graphene channel, the positionof the Dirac voltage varies by the doping state of graphene, and the degree of graphenedoping level and state can thus be analyzed. Fig. 2.15 shows a typical graphene FETstructure and IeV curve (currentevoltage curve) with applied gate voltage [30]. Whena gate voltage is applied to a graphene FET fabricated with pristine graphene, the cur-rent flow changes according to the applied gate voltage. If a negative gate voltage isapplied, positive charge is induced in the graphene channel, and the current flowsthrough the holes as major carriers. When the gate voltage is increased to 0 V, theFermi level is located in the zero energy state. Ideally, the current does not flow,and this point is the Dirac voltage of the IeV curve. On the contrary, if a positivevoltage is applied, a negative charge is induced in graphene and the current flowsby electrons as major carriers. On the other hand, the Dirac voltage of doped graphene

0 406–406–574 574 703–703 811 0 406–406–574 574 703–703 811

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0 406–406–574 574 703–703 811Fermi energy (meV)

Electron concentration (×1013 cm–2)

Electron concentration (×1013 cm–2)0 1–1–2 2 3–3 4

Electron concentration (×1013 cm–2)

0 1–1–2 2 3–3 4Electron concentration (×1013 cm–2)

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cm–1

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M(G

) (cm

–1)

/(2D

)/I(G

)

p-doping n-doping

p-doping n-doping

p-doping n-doping

p-doping n-doping

Figure 2.13 Raman spectral analysis of graphene depending on the doping type. The blue linesindicate the theoretical values and the black lines represent the experimental data. (a) Theposition shifts of the G band and (b) the 2D band. (c) The FWHM of the G band depending onthe doping type. (d) 2D/G Raman spectra intensity ratio depending on the doping type(Reproduced with the permission from Macmillan Publishers Ltd: Nat. Mater., Ref. [35]).


(either n- or p-type) is shifted to the negative or positive directions depending on thedoping state. In case of p-doped (hole-induced) graphene FETs, the current flows withmore holes induced by a negative gate voltage. Even when the voltage becomes 0 V,the current continues to flow due to the presence of holes. At a positive voltage, elec-trons begin to appear and combine with the holes and thus provide the lowest currentvalue; this point becomes the Dirac voltage of a p-doped graphene. When a negativevoltage is applied to n-doped graphene, the current flows by holes as the major carriersup to some point, depending on the degree of doping. However, if the gate voltage ex-ceeds a certain value, the holes are equaled out with electrons in n-doped graphene.Likewise, the point with the lowest current value becomes the Dirac voltage of ann-doped graphene FET. Fig. 2.16 shows the Dirac voltages of pristine graphene andn- and p-doped graphene. In brief, the changes in the Dirac voltage position of dopedgraphene can be analyzed by the current flow at different gate voltages. The Dirac volt-ages of n- and p-doped graphene are located in the negative and positive voltage re-gions, respectively. The graphene FETs are thus one of the most important

(a) (b)

(c) (d)

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rb. u

nit)

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eak

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–1)

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io

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2684

2900280027002600160015000

60

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2688

2692

2696

2700

Figure 2.14 Raman spectral analysis of n-doped graphene with different n-dopants having anincreasing number of amine groups. (a) Representative Raman spectra of pristine and n-doped graphene with different n-dopants. (b) The G and 2D band position shifts for pristineand n-doped graphene. (c) 2D/G intensity ratio changes as function of the G band positionwith different n-dopants. (d) The FWHM of the G band with pristine and n-doped graphenewith different n-dopants (Reproduced with the permission from American Chemical Society,Ref. [38]).


techniques for analyzing the electrical properties of graphene because the dopinglevels can be validated from the shifts in Dirac position (Fig. 2.16) [31,32]. The gra-phene FETs can also be used to evaluate the electrical conductivity and resistivity.The electrical conductivity and resistivity of graphene are linearly related to the gatevoltage near the Dirac point. Therefore, the slope of the IeV curve near the Diracvoltage indicates the carrier mobility, which can be calculated by the followingequation:

m¼ 1Ci

dsdVG

where Ci is capacitance of SiO2, s is conductivity of graphene, and VG is applied gatevoltage. Although CVD graphene has relatively high mobility, it cannot reach the

(a) (b)

SiO2

VG(V)–20 0

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60

80

100

20

I D(μ

A)

Right after fabrication (atmosphere)After 10 h vacuum (in vacuum)After annealing (in vacuum)

Source

Drain

Gate

Dielectricp++ Si

Figure 2.15 (a) Schematic image of a graphene FET device. (b) Representative I-V curve from agraphene FET (Reproduced with the permission from Elsevier, Ref. [37]).

(a) (b)20

15

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0–180 –150 –120 –90 –60 –30 0 0

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I sd(μ

A)

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p-dopingdirac point shift

(e2 /h

)σ

Pristine FET3 min15 min8 h (overnight)

μ = 550 cm2V–1s–1

3100 cm2V–1s–1

Figure 2.16 The shifts in the Dirac voltage in (a) n-doped and (b) p-doped grapheneFET devices (Reproduced with the permission from American Chemical Society,Ref. [38, 39]).


theoretical level owing to the presence of defects, grain boundaries, ripples, andcracks. Graphene FET devices may also be fabricated on flexible substrates such aspolydimethylsiloxane (PDMS) and PET films and are more advantageous in terms oftransparency and flexibility. Like the ones on SiO2 substrate, graphene FETs on PEThave a source, drain, and gate electrodes system. While SiO2-based FETs aredielectric, PET-based FETs utilize ion gelebased top gate system. Fig. 2.17 shows arepresentative graphene FET on PET film. Although the device is transparent andflexible, the mobility is much lower than that of SiO2-based FETs because the surfacemorphology of PET is much rougher [33,34].

2.5.2 Sheet resistance

Because graphene is a two-dimensional film, the electrical properties can be evaluatedby measuring the sheet resistance. It may also provide the information on doping asdoping generally induces decreased sheet resistance from increased amount of elec-trons/holes (Fig. 2.18(a),(b)) [35,36]. The linear resistance is measured with twoprobes at a certain distance, but in the case of sheet resistance, a four-point probe isemployed. The four-point probe technique is the most widely used method formeasuring the sheet resistance of semiconductors, especially the resistivity of a metalfilm formed on an insulator, as it is a simple and accurate method that does not requirecomplicated calibration procedures. The most significant feature of a four-point probeis that the probes are linearly aligned with consistent intervals (Fig. 2.18(c)). A con-stant current is applied between probes 1 and 4, and the voltage between probes 2and 3 is measured to calculate the sheet resistance using the ratio between voltageand current through the equation:

r¼ 2psðV = IÞ

PET

Graphenetransfer

Ion gel prepolymerdrop-casting

UV exposure

Mask

Graphene patterning

PEDOT:PSS transfer

(a) (b)

Figure 2.17 Representative fabrication processes and characterization of ion gel-gated flexible,transparent graphene FETs (Reproduced with the permission from IOP, Ref. [40] andAmerican Chemical Society, Ref. [41]).


where V is the voltage, I is the current, r is the resistance, and s is the distance betweenthe probes. The correction factor (CF) can be applied to calculate the sheet resistanceðRs

�based on the resistance measured using a four-point probe. The CF is the value of

which the size, the thickness, and temperature of the sample are reflected. Therefore,the sheet resistance of graphene can be expressed as follows:

Rs¼ 4:532 � rðohm = sq.Þ

where 4.532 is the CF used for graphene-based films. In like manner, the van der Pauwmethod provides an average resistance using a four-point probe placed as a squareform. The sheet resistance can be calculated by the following formula [37]:

Rs¼pRln2

where R is measured resistance. Probe-based sheet resistance measurement can pro-voke damage to sample owing to the direct contact between probe and sample surfaceand is unsuitable for large-scale films as well. Therefore, noncontact sheet resistancemeasurements using eddy currents may be alternatively used for transparent electrode

(a) (b)

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/sqr

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Figure 2.18 (a, b) Representative sheet resistance data with and without doping. (Reproducedwith the permission from IOP, Ref. [42] and Hindawi, Ref. [43]). (c) Schematic representationof a linear 4-point probe. (d) Representative sheet resistance mapping image of a large-scalegraphene film using the eddy currents.


and display applications. This is additionally advantageous for large-scale analysiswithout significant damages. Fig. 2.18(d) shows sheet resistance of large-scale CVDgraphene film (10 � 10 cm) on PET measured using eddy currents.

2.6 Mechanical properties of graphene

As mentioned above, although ITO is widely employed in touch screen, transparentelectrode, and display, the applications in metal-based electrodes in foldable, stretch-able electronic devices are intrinsically unachievable as they break even with a strainof 0.1%. On the other hand, although graphene consists of only one layer of atoms,the sp2 carbon atoms are connected through covalent s-bonds, which are known asthe strongest bond. Therefore, graphene exhibits excellent stretchability and flexi-bility. The intrinsic strength of graphene was first revealed by Hone Group. Theypunched holes in the SiO2 wafer to fabricate suspended graphene structure andanalyzed with AFM. Its theoretical breaking strength is w40 N/m, and the elasticstiffness isw1.0 Tpa [38]. Based on these properties, researchers have demonstrateda number of studies which employed large-scale graphene film as a flexible, bend-able, and stretchable transparent electrode. The mechanical flexibility was investi-gated by measuring the electrical resistance change against mechanicaldeformation. To evaluate the flexibility of graphene, Kim et al. transferred grapheneon PET and measured the resistance change with the bending radius (Fig. 2.19(a),(b))[40]. As shown in Fig. 2.19(a), the resistance of graphene changed only slightly forthe bending radius up to 2.3 mm (tensile strain of 6.5). When the film was bent0.8 mm (tensile strain of 18.7%), the resistance sharply increased initially butrestored the original value as it returned to the original state. In addition, graphenewas transferred to longitudinally prestrained PDMS substrate, and the tensile strainwas measured (Fig. 2.19(b)). The resistance of graphene film of both horizontal andvertical directions remained stable with 11% strain, and 25% stretching induced onlya single-order change. Notably, the results were consistent through repeated bendingcycles. Fig. 2.19(c) indicates the mechanical properties of graphene-based touch-screen devices compared with ITO/PET electrodes, showing the resistance changewhen tensile strain is applied [24]. In case of ITO electrodes, the resistance changeincreases sharply to 2%e3% strain value because ITO cannot endure the strain. Onthe other hand, graphene-based panel is intact up to 6% strain. It can thus beconcluded that the elasticity and flexibility of graphene is far superior to that ofITO or other metal candidates. Bunch et al. fabricates a graphene balloon by expand-ing suspended graphene using the pressure difference between inside and outside ofgraphene membrane (Fig. 2.19(d)) [41]. This demonstrates the strong mechanicalstrength and impermeability of graphene [3,5,41,42]. Based on these properties,researchers have endeavored to exploit graphene as a selective membrane andstretchable encapsulation barrier for organic lighteemitting diode applications.


References

[1] A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (3) (2007) 183.[2] J. Ryu, Y. Kim, D. Won, N. Kim, J.S. Park, E.-K. Lee, D. Cho, S.-P. Cho, S.J. Kim,

G.H. Ryu, Fast synthesis of high-performance graphene films by hydrogen-free rapidthermal chemical vapor deposition, ACS Nano 8 (1) (2014) 950e956.

[3] K. Choi, S. Nam, Y. Lee, M. Lee, J. Jang, S.J. Kim, Y.J. Jeong, H. Kim, S. Bae, J.-B. Yoo,Reduced water vapor transmission rate of graphene gas barrier films for flexible organicfield-effect transistors, ACS Nano 9 (6) (2015) 5818e5824.

[4] J.-H. Ahn, B.H. Hong, Graphene for displays that bend, Nat. Nanotechnol. 9 (10) (2014)737.

[5] D. Shin, J.B. Park, Y.-J. Kim, S.J. Kim, J.H. Kang, B. Lee, S.-P. Cho, B.H. Hong,K.S. Novoselov, Growth dynamics and gas transport mechanism of nanobubbles in gra-phene liquid cells, Nat. Commun. 6 (2015) 6068.

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Figure 2.19 (a) Representative resistance changes with respect to the bending radius and (b)strain. Left inset of (b) shows the stretching cycling test results. (Reproduced with thepermission from Macmillan Publishers Ltd: Nature, Ref. [46]). (c) Representative resistancechanges of graphene-based touch-panel compared with ITO/PET films in bent and flat states(Reproduced with the permission from Macmillan Publishers Ltd: Nat. Nanotechnol., Ref.[29]). (d) Representative images of graphene balloon fabricated by the pressure difference ofthe suspended graphene membrane (Reproduced with the permission from MacmillanPublishers Ltd: Nat. Nanotechnol., Ref. [47]).


[6] U. Sim, J. Moon, J. An, J.H. Kang, S.E. Jerng, J. Moon, S.-P. Cho, B.H. Hong, K.T. Nam,N-doped graphene quantum sheets on silicon nanowire photocathodes for hydrogen pro-duction, Energy Environ. Sci. 8 (4) (2015) 1329e1338.

[7] S. Shin, H.-H. Choi, Y.B. Kim, B.-H. Hong, J.-Y. Lee, Evaluation of body heating pro-tocols with graphene heated clothing in a cold environment, Int. J. Cloth. Sci. Technol. 29(6) (2017) 830e844.

[8] S. Lee, I. Jo, S. Kang, B. Jang, J. Moon, J.B. Park, S. Lee, S. Rho, Y. Kim, B.H. Hong,Smart contact lenses with graphene coating for electromagnetic interference shielding anddehydration protection, ACS Nano 11 (6) (2017) 5318e5324.

[9] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos,I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science306 (5696) (2004) 666e669.

[10] E. Sutter, P. Albrecht, P. Sutter, Graphene growth on polycrystalline Ru thin films, Appl.Phys. Lett. 95 (13) (2009) 133109.

[11] S. Marchini, S. G€unther, J. Wintterlin, Scanning tunneling microscopy of graphene on Ru(0001), Phys. Rev. B 76 (7) (2007) 075429.

[12] C. Riedl, U. Starke, J. Bernhardt, M. Franke, K. Heinz, Structural properties of thegraphene-SiC (0001) interface as a key for the preparation of homogeneous large-terracegraphene surfaces, Phys. Rev. B 76 (24) (2007) 245406.

[13] Q. Yu, J. Lian, S. Siriponglert, H. Li, Y.P. Chen, S.-S. Pei, Graphene segregated on Nisurfaces and transferred to insulators, Appl. Phys. Lett. 93 (11) (2008) 113103.

[14] H. Mehdipour, K. Ostrikov, Kinetics of low-pressure, low-temperature graphene growth:toward single-layer, single-crystalline structure, ACS Nano 6 (11) (2012) 10276e10286.

[15] C.-M. Seah, S.-P. Chai, A.R. Mohamed, Mechanisms of graphene growth by chemicalvapour deposition on transition metals, Carbon 70 (2014) 1e21.

[16] X. Li, W. Cai, L. Colombo, R.S. Ruoff, Evolution of graphene growth on Ni and Cu bycarbon isotope labeling, Nano Lett. 9 (12) (2009) 4268e4272.

[17] W. Cai, Y. Zhu, X. Li, R.D. Piner, R.S. Ruoff, Large area few-layer graphene/graphitefilms as transparent thin conducting electrodes, Appl. Phys. Lett. 95 (12) (2009) 123115.

[18] A. Reina, X. Jia, J. Ho, D. Nezich, H. Son, V. Bulovic, M.S. Dresselhaus, J. Kong, Largearea, few-layer graphene films on arbitrary substrates by chemical vapor deposition, NanoLett. 9 (1) (2008) 30e35.

[19] X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc,Large-area synthesis of high-quality and uniform graphene films on copper foils, Science324 (5932) (2009) 1312e1314.

[20] I. Jo, S. Park, D.J. Kim, J. San Moon, W.B. Park, T.H. Kim, J.H. Kang, W. Lee, Y. Kim,D.N. Lee, Tension-controlled single-crystallization of copper foils for roll-to-roll synthesisof high-quality graphene films, 2D Mater. 5 (2) (2018).

[21] P.R. Wallace, The band theory of graphite, Phys. Rev. 71 (9) (1947) 622.[22] A. Maffucci, G. Miano, Electrical properties of graphene for interconnect applications,

Appl. Sci. 4 (2) (2014) 305e317.[23] G. Jo, M. Choe, S. Lee, W. Park, Y.H. Kahng, T. Lee, The application of graphene as

electrodes in electrical and optical devices, Nanotechnology 23 (11) (2012) 112001.[24] S. Bae, H. Kim, Y. Lee, X. Xu, J.-S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H.R. Kim,

Y.I. Song, Roll-to-roll production of 30-inch graphene films for transparent electrodes,Nat. Nanotechnol. 5 (8) (2010) 574.

[25] A. Merlen, J.G. Buijnsters, C. Pardanaud, A guide to and review of the use of multi-wavelength Raman spectroscopy for characterizing defective aromatic carbon solids: fromgraphene to amorphous carbons, Coatings 7 (10) (2017) 153.


[26] A. Das, S. Pisana, B. Chakraborty, S. Piscanec, S. Saha, U. Waghmare, K. Novoselov,H. Krishnamurthy, A. Geim, A. Ferrari, Monitoring dopants by Raman scattering in anelectrochemically top-gated graphene transistor, Nat. Nanotechnol. 3 (4) (2008) nnano.2008.67.

[27] I. Jo, Y. Kim, J. Moon, S. Park, J. San Moon, W.B. Park, J.S. Lee, B.H. Hong, Stable n-type doping of graphene via high-molecular-weight ethylene amines, Phys. Chem. Chem.Phys. 17 (44) (2015) 29492e29495.

[28] Y. Liu, Z. Liu, W.S. Lew, Q.J. Wang, Temperature dependence of the electrical transportproperties in few-layer graphene interconnects, Nanoscale Res. Lett. 8 (1) (2013) 335.

[29] L. D’Urso, G. Forte, P. Russo, C. Caccamo, G. Compagnini, O. Puglisi, Surface-enhancedRaman scattering study on 1D-2D graphene-based structures, Carbon 49 (10) (2011)3149e3157.

[30] S.K. Jang, J. Jeon, S.M. Jeon, Y.J. Song, S. Lee, Effects of dielectric material properties ongraphene transistor performance, Solid State Electron. 109 (2015) 8e11.

[31] Y. Kim, J. Ryu, M. Park, E.S. Kim, J.M. Yoo, J. Park, J.H. Kang, B.H. Hong, Vapor-phasemolecular doping of graphene for high-performance transparent electrodes, ACS Nano 8(1) (2013) 868e874.

[32] B. Lee, Y. Chen, F. Duerr, D. Mastrogiovanni, E. Garfunkel, E. Andrei, V. Podzorov,Modification of electronic properties of graphene with self-assembled monolayers, NanoLett. 10 (7) (2010) 2427e2432.

[33] S.-K. Lee, B.J. Kim, H. Jang, S.C. Yoon, C. Lee, B.H. Hong, J.A. Rogers, J.H. Cho, J.-H. Ahn, Stretchable graphene transistors with printed dielectrics and gate electrodes, NanoLett. 11 (11) (2011) 4642e4646.

[34] S.-K. Lee, S.H. Kabir, B.K. Sharma, B.J. Kim, J.H. Cho, J.-H. Ahn, Photo-patternable iongel-gated graphene transistors and inverters on plastic, Nanotechnology 25 (1) (2013)014002.

[35] K.K. Kim, A. Reina, Y. Shi, H. Park, L.-J. Li, Y.H. Lee, J. Kong, Enhancing the con-ductivity of transparent graphene films via doping, Nanotechnology 21 (28) (2010)285205.

[36] S.-H. Chan, S.-H. Chen, W.-T. Lin, C.-C. Kuo, Uniformly distributed graphene domaingrows on standing copper via low-pressure chemical vapor deposition, Adv. Mater. Sci.Eng. 2013 (2013).

[37] L. Van der Pauw, A method of measuring specific resistivity and Hall effect of discs ofarbitrary shape, Philips Res. Rep. 13 (1958) 1e9.

[38] C. Lee, X. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsicstrength of monolayer graphene, Science 321 (5887) (2008) 385e388.

[39] X. Wang, X. Li, L. Zhang, Y. Yoon, P.K. Weber, H. Wang, J. Guo, H. Dai, N-doping ofgraphene through electrothermal reactions with ammonia, Science 324 (5928) (2009)768e771.

[40] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, J.-H. Ahn, P. Kim, J.-Y. Choi,B.H. Hong, Large-scale pattern growth of graphene films for stretchable transparentelectrodes, Nature 457 (7230) (2009) 706.

[41] S.P. Koenig, N.G. Boddeti, M.L. Dunn, J.S. Bunch, Ultrastrong adhesion of graphenemembranes, Nat. Nanotechnol. 6 (9) (2011) 543.

[42] L. Wang, L.W. Drahushuk, L. Cantley, S.P. Koenig, X. Liu, J. Pellegrino, M.S. Strano,J.S. Bunch, Molecular valves for controlling gas phase transport made from discreteångstr€om-sized pores in graphene, Nat. Nanotechnol. 10 (9) (2015) 785.


Further reading

[1] A.C. Neto, F. Guinea, N.M. Peres, K.S. Novoselov, A.K. Geim, The electronic properties ofgraphene, Rev. Mod. Phys. 81 (1) (2009) 109.

[2] S.J. Kim, T. Choi, B. Lee, S. Lee, K. Choi, J.B. Park, J.M. Yoo, Y.S. Choi, J. Ryu, P. Kim,Ultraclean patterned transfer of single-layer graphene by recyclable pressure sensitive ad-hesive films, Nano Lett. 15 (5) (2015) 3236e3240.

[3] C.J. Shearer, A.D. Slattery, A.J. Stapleton, J.G. Shapter, C.T. Gibson, Accurate thicknessmeasurement of graphene, Nanotechnology 27 (12) (2016) 125704.

[4] E. Stolyarova, K.T. Rim, S. Ryu, J. Maultzsch, P. Kim, L.E. Brus, T.F. Heinz,M.S. Hybertsen, G.W. Flynn, High-resolution scanning tunneling microscopy imaging ofmesoscopic graphene sheets on an insulating surface, Proc. Natl. Acad. Sci.U.S.A. 104 (22)(2007) 9209e9212.

[5] M. Yi, Z. Shen, A review on mechanical exfoliation for the scalable production of graphene,J. Mater. Chem. 3 (22) (2015) 11700e11715.

[6] W.H. Lee, J.W. Suk, J. Lee, Y. Hao, J. Park, J.W. Yang, H.-W. Ha, S. Murali, H. Chou,D. Akinwande, Simultaneous transfer and doping of CVD-grown graphene by fluoropol-ymer for transparent conductive films on plastic, ACS Nano 6 (2) (2012) 1284e1290.


Preparation of grapheneelectrode 3Wencai RenShenyang National Laboratory for Materials Science, Institute of Metal Research, ChineseAcademy of Sciences, Shenyang, Liaoning, China

3.1 Solution casting of graphene oxide

Among the various synthetic ways, graphene-based transparent conductive films(G-TCFs) can be fabricated by solution casting of graphene oxide (GO) from solutionand subsequent reduction through thermal of chemical ways. The properties of finalG-TCFs are closely related with the properties of the raw GO, the methods for filmformation and reduction.

3.1.1 Properties of GO and GO solution

Because of high aspect ratio, large specific surface area, and strong van der Waalsattraction, graphene sheets tend to stick together. Well-graphitized graphene have asurface that is inert to most commonly used solvents, so the interaction between gra-phene sheets and solvents is hard to balance against the attraction between graphenesheets, which results in poor dispersion or even reaggregation [1e4]. But the proper-ties of GO is much different from that of pristine graphene. GO is usually produced byexfoliation of graphite oxide, which is a highly oxidized graphite produced by interca-lating and oxidizing with a strong acidic oxidant. During oxidation, the graphenelayers of the graphite are decorated with a large amount of oxygen containing groups,e.g., epoxy, hydroxyl and carboxyl, etc. (Fig. 3.1(a)) with a typical carbon/oxygenatomic ratio lower than two. This structure changes the surface of the graphene sheetsfrom hydrophobic to hydrophilic and enables them to be well dispersed in polarsolvents such as water (Fig. 3.1(b)), N, N-dimethylformamide, and 1-methyl-2-pyrrolidinone. As a result, graphite oxide is much easier to exfoliate into mono- orfew-layer sheets in a solvent (mostly water), and these sheets are named as GO. GOsheets are intrinsically insulating due to the disturbed long rangeeconjugated structureduring oxidation, and reduction treatment is necessary before or after film formation torestore the conductivity of the films.

Figs. 3.1(c,d) show the typical optical microscope images of GO sheets before andafter reduction. After reduction, accompanying with the elimination of oxygen-containing groups, part of the insulated sp3 structures can be restored into conductivesp2 structures, which restores the long-range conductivity of reduced GO (rGO).


https://doi.org/10.1016/B978-0-08-102482-9.00003-4

Because of the increase of free electrons for conductivity, the transparency of rGOsheets decreases, which increases the contrast as compared in Fig. 3.1(c,d).

Generally, the stable and homogeneous GO suspensions allow the easy assembly ofultrathin film with thickness as low as several nanometers, and the reducibility of GOenables the fabrication of GO-based TCF and subsequent applications.

3.1.2 Reduction of GO film

As-fabricated GO films are insulating. As a result, the reduction of the GO after filmassembly is unavoidable for the fabrication of GO-based TCFs. Generally, three typesof reduction methods have been used, chemical reduction (CR), high temperatureannealing (HTA), and a combination of both CR and HTA, to realize the reductionof GO, and some typical results are listed in Table 3.1.

For details on the methods and mechanism of GO reduction, the reader can refer toour review on the reduction of GO [16]. Specific to the production of TCFs, hydrazinewas the most frequently used reduction reagent. The reduction treatment can removemost of the oxygen-containing groups attached to the carbon plane and the conjugatedstructure of graphene can be partly restored to make rGO electrically conductive [17].However, reduction by hydrazine alone is not sufficient to achieve maximum reduc-tion; a subsequent annealing can well improve the conductivity of rGO-TCFs

(a)

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Figure 3.1 (a) LerfeKlinowski model of graphene oxide (GO) with the omission of minorgroups (carboxyl, carbonyl, ester, etc.) on the periphery of the carbon plane of the GO, (b) GOsolution, and (c,d) GO and reduced GO (rGO) sheets on a 300 nm SiO2/Si substrate [5].


Table 3.1 Optoelectrical property comparison of rGO-TCFs produced by different reduction methods.

Reduction methods s [S/cm] Rs [kU/sq] T@550 nm [%] sDC/sOP References

CR HI acid w1000 1 85 2.2 [5]

NaBH4 e 4.4 81 0.38 [6]

Hydrazine e 30 80 0.05 [7]

SnCl2/EtOH e 0.82 83 1.55 [8]

Hydrazine e 1.4 � 105 92 <0.001 [9]

HTA 1100�C 600 2.3 85 0.97

1100�C 1786 0.8 82 1.5 [10]

1100�C 5e1000 >80 e [9]

1000�C 1425 70 0.552 [11]

1100�C 550 1.8 70 0.42 [12]

CR þ HTA Hydrazineþ1100�C 550 85 0.64 [13]

Hydrazineþ200�C 100 65 0.022 [14]

Hydrazineþ200�C e 11.3 87 0.29 [15]

Preparation

ofgraphene

electrode29

[14,18]. An rGO-TCF with Rs of 102e103 U/sq at an 80% transparency was obtained

by a combination of hydrazine reduction and annealing at 1100�C [18]. HTA also isfound to be efficient to reduce the electrical conductivity of rGO films, and the reduc-tion effect is significantly affected by the heating temperature [12,18]. For GO filmswith the same thickness, the conductivity is 50, 100, and 550 S/cm when the annealingtemperature is 500, 700, and 1100�C, respectively. The rGO-TCF reduced at 1100�Chas an Rs of 1.8 kU/sq at 70.7% transparency.

Although it is highly effective, HTA is not suitable for the production of TCFs. Oneimportant obstacle is that the most commercial transparent substrate materials, e.g.,glass and polymers, cannot stand a temperature higher than 500 �C. As a result, CRat low temperature is important for rGO-TCFs. Sodium borohydride (NaBH4)[6,19] and hydroiodic acid (HI) [20,21] were reported to be more effective than hydra-zine to reduce GO, especially GO films. An rGO-TCF reduced by NaBH4 has an Rs of4.4 kU/sq at 81% transparency [6], while an rGO-TCF reduced by HI has a lower Rs of1 kU/sq at 85% transparency [5]. Recently, Ning et al. reported another effective CRmethod using SnCl2 as reductant, which results in an rGO film with 820 U/sq sheetresistance and 83% transmittance [8].

3.1.3 Fabrication of GO-based TCF

3.1.3.1 Fabrication methods of solution casting

Research on nanocarbon-based TCFs started in 2004 when Wu et al. [22] and Saranet al. [23] reported using single-walled carbon nanotube films (SWCNTs) to makeTCFs by filtration transfer and dip coating, respectively. After that, hundreds of papershave published on this topic. Till now, more than 10 methods for the TCF fabricationhave proposed, and the basic processes are shown in Fig. 3.2. They demonstrate theinnovation of researchers as well as the inspiration from a highly developed printingindustry on film formation using traditional materials. Hu et al. have done a detailedreview of the methods for the fabrication of CNT-TCFs [24]. These methods arealso suitable for the fabrication of GO-based TCFs because the dispersion of grapheneand GO has very similar characteristics to those of CNTs. After 10 years of researchand development, some methods are still widely considered to be lab-scale only, suchas filtration transferring [22,25e29], spin coating [30e32], and LangmuireBlodgett(LB) coating [33,34], because these methods are mainly limited by a low-coatingefficiency, and the nature of the equipment is intrinsically hard to scale up. Somemethods have potential to be scaled up on industrial scale, like dip coating [23,35],spray coating [22,36e40], blade coating, and rod coating [41,42].

Although lab-scale fabrication methods are less likely to be used to realize theindustrial production of G-TCFs, most fundamental research on TCFs such as perco-lation behavior, temperature- and frequency-dependent transport, and factors thataffect their properties are usually based on films fabricated using these methods.This is because the filtration transferring method leads to uniform and reproduciblefilms, and the network density of G-TCFs can be precisely controlled by varyingthe dispersion concentration. The LB-coating method can be used to preciously


control the film thickness down to a monolayer of graphene sheets. These featuresfacilitate the fabrication of TCFs with precisely controlled microstructures that mayhave desired properties.

3.1.3.2 Fabrication of GO-based hybrid TCFs

To summarize the properties of graphene-based TCFs fabricated by various solutioncasting methods, G-TCFs, no matter whether using directly exfoliated graphene orrGO, mostly have a sDC/sOP value lower than 1, which is far from what is requiredfor applications. As a result, in recent years, the fabrication of GO-based hybridTCFs has been the main trend for this topic.

Silver nanowires (AgNWs) have been utilized as a transparent conductive electrodein organic photovoltaic devices because of their excellent conductivity and transpar-ency [43]. However, the AgNWs deposited on the substrate had a relatively low adhe-sion force and a low resistance to corrosion because they are composed of a metal andare quite fragile because their dimensions, which are several tens of nanometers indiameter and micrometers in length. To overcome this problem, Ahn et al. [44,45]improved electrodes based on AgNWs by combining them with graphene.

GO solution

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Figure 3.2 Depiction of various solution casting methods for graphene oxideebased TCFs.

Preparation of graphene electrode 31

In particular, rGO could prevent chemical reactions and thermal oxidation of AgNWsat high temperature and humidity.

Zhang et al. reported a kind of solution-processible TCF consisting of rGO sheetsand AgNWs. The sandwiched structure of rGO/AgNWs/rGO is readily deposited vialayer-by-layer blade coating on the rigid glass or flexible PET substrate at mild anneal-ing temperature. The rGO nanosheets sandwiching the AgNW networks not onlyinduce close contact of the AgNWs networks as a clipper to improve the conductivitybut also link the discrete AgNWs as a connector to improve the uniformity of the filmconductivity (Fig. 3.3). The transparent rGO/AgNWs/rGO film exhibits sheet resis-tance as low as 14.29 U/,, with transmittance over 90% at 550 nm. Moreover, therGO/AgNWs/rGO composite film shows good ambient stability due to the rGOcoverage and the existing charge interaction between AgNWs and rGO [46,47].

Kim et al. reported the hybrid coating based on the rGO, CNTs, and AgNWs using aspraying method. The overall characteristics of multilayers based on rGO, CNTs, andAgNWs were found to be much better than those of the single-layer AgNW coating.The rGO and CNT layers served to protect the AgNW layer from damage due tobending, contact sliding motions, corrosion, and oxidation due to the variation oftemperature and humidity. Furthermore, the rGO and CNT layers showed a reductionin the haze of 20%e55% compared with the single layer of AgNWs [48].

(a) (b)

(c) (d)

Figure 3.3 SEM images of (a) reduced graphene oxide (rGO) nanosheets, (b) pristineSilver nanowires (AgNWs), (c) rGO/AgNWs, and (d) rGO/AgNW/rGO structures on glasssubstrates [46].


Hu et al. reported a fabrication of poly(3, 4-ethylenedioxythiophene):poly(styrenesulfonate):graphene:ethyl cellulose (PEDOT:PSS:G:EC) hybrid electrodes by roll-to-roll (R2R) process, which allows for the elimination of strong acid treatment. Thehigh-performance flexible printable electrode includes a transmittance of 78% at550 nm and a sheet resistance of 13 U/sq with excellent mechanical stability. Thesefeatures arise from the PSS interacting strongly with the ethoxyl groups from ECpromoting a favorable phase separation between PEDOT and PSS chains, and thehighly uniform and conductive G:EC enable rearrangement of the PEDOT chainswith more expanded conformation surrounded by G:EC via the pep interactionbetween G:EC and PEDOT (Fig. 3.4). The hybrid electrodes are fully functional asuniversal electrodes for outstanding flexible electronic applications [49].

3.1.3.3 Continuous fabrication of GO-based TCFs

The scaling-up of TCF production relies on the design of highly automatic equipmentwith intelligent programming based on some lab-scale fabrication methods. Thecontinuous fabrication of SWCNT-TCFs now has been realized through an R2R print-ing process by slot casting and printing can be performed at a high speed up to 100 m/min on flexible or rigid substrates with several meters wide [50]. Some other methodsthat have been widely used in the printing industry, such as a gravure press [8], reverseroll painting, and fountain/curtain coating, are also proposed to realize the industrialproduction of CNT- and G-TCFs [51]. Recently, fabrication of PEDOT:PSS:G:EChybrid electrodes by R2R allows for the elimination of strong acid treatment. Thehybrid electrodes are fully functional as universal electrodes for outstanding flexibleelectronic applications (Fig. 3.5) [49].

Hu et al. reported the continuous fabrication of hybrid rGO-TCF by the shear forceof slot-die printing during R2R process for fabricating oriented PEDOT:PSS:rGOcomposite. The subsequent HI acid posttreatment can remove insulating content ofconductive polymer as well as reduce the GO to enhance the conductivity of TCFsto 1949 S cm⁻1 with a sheet resistance of 51 U/sq and 82% transmittance. In addition,these FTEs demonstrate remarkable flexural endurance even under extreme bendingsituation. The TCFs possess a low cost of only 2.8 $ for per square meter due to thecarbon materials and R2R technologies [52].

3.2 Transfer methods of CVD grown graphene

Chemical vapor deposition (CVD) on metals has been extensively investigated to pre-pare high-performance flexible transparent electrode because highly conductive andtransparent graphene film can be synthesized in a scalable way [53]. CVD grapheneelectrode has demonstrated its great potential in a wide range of flexible lightingand display devices [54]. For all these applications, CVD-grown graphene must betransferred from metals to flexible substrates. Although high-quality graphene canbe readily grown on metals, the transferred graphene is prone to structural damageand contamination. As a result, the device performances fall far behind the


EC

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Coating radio (web speed/roll speed)

0

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Figure 3.4 Diagram of the interaction between PEDOT chains and G:EC, and the electrical properties and thickness of PEDOT:PSS composite filmsvia roll-to-roll process at different coating ratio [49].

34Graphene

forFlexible

Lighting

andDisplays

expectations as compared with those fabricated by using the exfoliated graphene flakes[55,56]. Therefore, improving the transfer process has currently become the bottleneckto realizing the application of CVD graphene as high-performance flexible transparentelectrode.

The application in flexible lighting and display devices poses several challenges tothe transfer of CVD graphene. It is a fundamental requirement to retain the structural

(I) Cleaning andpretreatment for PET

substrates

(II)Gravure printing:PEDOT:PSS:G:EC

(III) Slot die:ethylene glycol

(IV) Annealing treatmentand encapsulation

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

Figure 3.5 Process and devices of the four steps in the roll-to-roll process for a PEDOT:PSS:G:EC film. The system mainly comprised of unwinding, cleaning, rectification, corona, gravureprinting, slot die coating, thermal annealing, and rolling units [49].


integrity and uniformity of graphene by using a controllable transfer, which is espe-cially difficult for large-area film. For some applications such as organic lighteemission diode (OLED), clean surfaces are needed to avoid current leakage andeven electrical short between electrodes without the residue contamination such aspolymer or metal particles. A continuous and low-cost method is equally importantin terms of scalable production of large-area graphene electrode. Furthermore, somestringent requirements must be satisfied in specific applications. For example, intactgraphene needs to be transferred onto a rough thin-film transistor array substrate ifit is integrated with typical liquid crystal display panel. The above challenges stimulateextensive research efforts in the development of more effective and efficient transfermethods, and significant advances have been made in recent years.

In this section, we introduce the representative progresses in the transfer methods ofCVD graphene in terms of the different interaction between graphene, supportinglayer, and target substrate. Supporting layers are widely used to improve both the struc-tural integrity of graphene and the consistence of transfer, which has a significantimpact on the performances of transferred graphene. For simplicity, typical transfermethods are classified into support-dissolving transfer, adhesion-mediated transfer,and target-supported transfer according to different separation/attachment mechanismsof graphene.

3.2.1 Support-dissolving transfer

Support-dissolving transfer is the most representative method for CVD-grown gra-phene, which allows the transfer of graphene onto either rigid or flexible substratein a versatile and reproducible manner [57]. A typical transfer process involves thedeposition of supporting layer on graphene, separation of graphene from metal, attach-ment of supported graphene onto the target substrate, and removal of supporting layer(Fig. 3.6(a)). Chemical etching is generally used as a relatively mild method to separategraphene by completely dissolving metal substrates with etchant solution. Thin poly-mer films such as poly(methyl-methacrylate) (PMMA) are preferred as the supportinglayer because they are not only readily removed by organic solvents but also flexiblewith sufficient mechanical strength and stable to the etchant solution [58]. The typicalPMMA-supported transfer involves spin coating and curing PMMA film, etchingmetal substrate, water rinsing graphene/PMMA film, attaching graphene/PMMAonto target substrate, baking, dissolving PMMA, and drying. The quality of transferredgraphene is determined by several factors such as the property of PMMA film, inter-facial contact between graphene and target substrate, as well as the posttreatmentprocess. An adequate thickness is crucial for simultaneously realizing a flexible andstrong PMMA film. However, structural damages such as cracks and tears arefrequently observed in the transferred graphene even with the use of PMMA layer.This issue was attributed to the formation of gaps (e.g., under wrinkles or folds)between graphene and target substrate, which will lead to the damage of grapheneon removing the PMMA layer [57]. Redissolution of PMMA coating was reportedto be effective in reducing cracks arising from the unmatched topography ofPMMA by mechanically releasing PMMA/graphene layer to improve its contact


(a) (b)As-grown Gr on Cu

Deposit PMMA and cure

Etch away Cu

Wash PMMA/Gr in DI water

Place PMMA/Gr on substrate Remove PMMA with acetone

Redeposit PMMA and cure

Remove PMMA and acetone

Good transferGraphene

Cracks

Bad transferTransfer

Low surface tension liquid High surface tension liquid

Contact

Lamination

Dry

PMMAremoval

Flat

Pinned TCL

Direction of liquid flow

PMMA/graphene

Organic liquidsubstrate

PMMA/graphene

Pinned TCL

Lamination wrinkles

Graphene folds and cracks

Watersubstrate

Direction of film deformation

Defect-free graphene

Old

New

Quartz

N2 blowing

N2 plasma

Without pretreatmentCu etching

Cu sputter and CVD growth Cu etching Capillary bridges

Delamination

Baking

EVA

EVA

/graphene

Rough substrate

Rough substrate

Rough substrate

Si

Si Si

Cu

Si

PMMA

Si

PMMA PMMA

PMMA

Si

Si

Si

Tight attachmentwithout void

Removal of

(c) (d)

Figure 3.6 (a) Processes for transfer of graphene films with and without the redissolution of poly(methyl-methacrylate) (PMMA) layer [57].(b) Comparison of the PMMA/graphene film transfer process onto low surface energy substrates using liquids of low (heptane) or high (water)surface tension [60]. (c) Transfer of graphene onto a rough surface with the ethyleneevinyl acetate supporting layer [63]. (d) Illustration of the face-to-face method for transferring graphene mediated by capillary bridges [64].

Preparation

ofgraphene

electrode37

with target substrate. However, the presence of interfacial water is responsible for thegaps in most cases, which originates from the remaining water used for rinsing etch-ants [59]. To solve this problem, several methods have been developed. The use ofhydrophilic target substrate together with enhanced baking at 150�C facilitates thespreading and then the evaporation of trapped water, thus yielding nearly crack-freegraphene on silicon wafer [59]. Alternatively, water can be replaced by volatile liquidswith low surface tensions such as heptane, which spread readily over different solidsurfaces (Fig. 3.6(b)) [60]. As a result, high-quality and uniform graphene was trans-ferred onto a variety of target substrates, which showed significantly improved carriermobility. Support-dissolving transfer is preferred in transferring graphene onto holey[61,62] or rough substrates [63] because the mild support-dissolving process can mini-mize the damage of suspended graphene caused by removing the support and the useof thin-supporting layer can improve the contact between graphene and rough surface.Particularly, ethyleneevinyl acetate (EVA) was found to be superior to PMMA intransferring graphene onto rough surface in terms of enabling conformal contact be-tween graphene and large surface steps, which was attributed to its lower contact stiff-ness and elastic modulus (Fig. 3.6(c)) [63].

In general, the typical support-dissolving transfer is complicated and time-consuming from the perspective of transfer efficiency. The quality of graphene attach-ment is also susceptible to the variation of operation skill. A face-to-face technique wasdeveloped to realize the spontaneous transfer of wafer-scale graphene onto target sub-strate, which uses graphene grown on metal film deposited onto plasma-treated targetsubstrate (Fig. 3.6(d)) [64]. In contrast to the independent separating and attachingprocesses, PMMA-supported graphene was spontaneously attached to the underlyingsilicon substrate by the capillary bridges of nascent gas bubbles during etching themetal film. This strategy is promising for batch production of wafer-scale graphenewith improved structural integrity and consistence of large-area graphene. As the targetsubstrate is involved in the CVD growth, however, it is difficult to transfer grapheneonto flexible plastic substrate using this method.

The major problem in the support-dissolving transfer is the formation of contami-nation including the residue of support layer, particles of etching product, and interfa-cial water. In particular, residues of polymer support such as PMMA not only degradethe electrical performances of electronic devices (e.g., FET) by increasing the scat-tering of carriers and the contact resistance to metal electrode but also lower the yieldand stability of lighting and display devices (e.g., OLED) by causing large currentleakage and even electrical short of graphene electrode. This issue becomes evenworse when multilayer graphene was prepared by using the layer-by-layer transfer,in which the surface roughness is multiplied due to the accumulation of residues.Although this issue can be fundamentally solved by using the support-free transfer,it is difficult to scale-up with a typical transferred film of centimeter size [65,66].Therefore, most of research efforts focus on the techniques to reduce residue or alter-native supporting material for clean transfer.

The common methods to reduce the PMMA residue include the composition modi-fication and posttreatments. It is easier to dissolve the cured PMMA layer bydecreasing the concentration of precursor solution or using UV irradiation [67,68].


In the latter case, the intermolecular interaction between PMMA and graphene can beweakened by cleaving the side chain of ester groups in PMMA. Posttreatments typi-cally involve the enhanced solvent rinsing, vacuum annealing, thermal oxidation, orradiolized water etching [55,68e74]. PMMA residues were reduced by usingprolonged immersion in acetone, but they cannot be completely dissolved [71]. Thedissolution can be significantly enhanced by applying both high pressure (w1 MPa)and heating (e.g., 140�C) [70]. Annealing was generally used after solvent rinsingto further reduce the polymer residue, which was reported to enable a twofold increasein carrier mobility [55]. However, such thermal treatment limits the available substrateto materials with high melting points such as silicon wafer or TEM grid. More impor-tantly, even such harsh treatment cannot completely remove polymer residue and athin layer of PMMA was found to remain on the surface of graphene [69]. Therefore,it is desirable to identify new support materials that are not only robust but also easy tobe removed.

Although alternative polymers such as poly(bisphenol A carbonate) and polysty-rene were reported to form less residue than PMMA [75,76], small organic moleculesseems to allow cleaner transfer due to their significantly lower sublimation tempera-tures and higher solubility in solvents. For instance, cyclododecane was found to bea clean support material for graphene because its low sublimating point allowed itto be removed by simple air exposure after transfer [77]. Yet, the transferred graphenesuffered from structural damage with the presence of D peaks in its Raman spectra,which might be related to its relatively low mechanical strength. Similarly, polycyclicaromatic hydrocarbons such as pentacene also allow clean transfer as sublimablesupporting layers [78,79]. A novel solvent intercalation strategy was used to exfoliatepentacene layer from graphene rather than the sublimation treatment that was lesseffective in removing the pentacene (Fig. 3.7(a)) [79]. Wafer-scale transfer wasdemonstrated with this intercalation strategy. The absence of D peaks in Ramanspectra and high carrier mobility together with a nearly zero Dirac point voltage indi-cated a clean and intact transfer. The added advantage of aromatic hydrocarbonsupports is that the residue would not change the band structure and Fermi level of gra-phene because their weak interaction with graphene causes negligible charge transfer[79]. Recently, ultraclean and damage-free transfer of graphene was accomplished byusing rosin as a novel support layer (Fig. 3.7(b)) [80]. This method enabled large-areaintact graphene film with a low surface roughness of 0.66 nm, thus yielding a uniformsheet resistance with significantly reduced deviation of 1% over an area of10 � 10 cm2. The superior effect of rosin-supported transfer was attributed to itsgood solubility, weak interaction with graphene, and adequate support strength. Theuse of rosin-transferred graphene electrode allowed the fabrication of four-inchflexible graphene-based OLED, which further demonstrated the clean and intact trans-fer. Interestingly, even liquid organics such as hexane can be used as a clean support inthe form of an organic/aqueous biphasic configuration [81]. Obviously, this transfermethod is more suitable for small size applications such as conductive coating ofAFM tip or support film of TEM grid because it is difficult to provide strong anduniform support for large area film with liquid layer.


(a) Pentacene ongraphene

Pentacene ongraphene Pentacene

removal(thermal orchemical)

Cu etching

transferCu foil

Pentacene/graphene Graphene

Graphene

Substrate Substrate

40Graphene

forFlexible

Lighting

andDisplays

(b)20 nm

0 nm5

4

3

2

1

00

Distance (μm)

Shee

t res

ista

nce

(Ω p

er s

quar

e)

Width (cm)Length (cm)

Distance (μm)

12 3

4 5

(c)

565

560

555

550

02

46

810

24

68

10

Gr/Cu

Golddeposition

Etchinggold layer

Gold etch maskdeposition

RemovingPMMA

Gold/Gr/CuGold/Gr/substrate

Gr/substrate

S-D/Gr/substrate

PMMAspin coating

PMMA/Gr/CuPMMA/Gr/substrate

Gold pattern/Gr/substrate

20 um

20 um

0 um 10 um 20 um

20 um

20 um

0 um 10 um 20 um

Patterned Gr/substrate

S/D electrodedeposition

Copper etching

and transfer

Grapheneetching

and removalof gold

Copper etching

and transfer

Figure 3.7 (a) Transfer procedure of graphene from Cu foil to an arbitrary substrate using the pentacene-supporting layer [79]. Photographs ofpentacene/graphene and graphene before and after pentacene removal (left) and the transferred graphene on 6-inch SiO2/Si wafer with correspondingoptical microscopy images after chemical removal of pentacene layer (right). (b) Photograph of a 10 � 10 cm2 monolayer graphene film transferredonto PET using rosin with its surface roughness characterized by AFM and sheet resistance map; a four-inch flexible green organic lighteemittingdiode with the rosin-transferred graphene anode [80]. (c) Schematic of the transfer methods and surface topography of Gr layers by PMMA (left) andgold (right)-supporting layers [84].

Preparation

ofgraphene

electrode41

Noble metal film such as gold can also be used as alternative to PMMA layerbecause it is not only readily dissolved by a special solution but also highly stableto water and transfer etchants [82e84]. Typically, thin film of gold (<100 nm) wasdeposited onto the as-grown graphene by using vacuum evaporation technique, whichcan be reinforced by using a second coating such as PMMA layer or adhesive tape. Ingeneral, gold-supported transfer generated less residue and even less defects thanPMMA-supported transfer, but gold residue can still be observed (Fig. 3.7(c)) [84].As a result, thus-prepared FET devices showed smaller shift of Dirac point withimproved carrier mobility and negligible change in hysteresis.

It should be pointed out that the residue or the complete support layer can in fact beused as functional coatings to improve the electrical performance and stability of gra-phene if clean or exposed surface is not required in specific devices. A fluoropolymerCYTOP was used as an alternative support to PMMA layer, and its residue causedstrong p-type doping on graphene after activation with thermal annealing or soakingin solvent, which was attributed to the electrostatic potential arising from the dipolemoment of rearranged fluorine-containing groups [85]. In addition, a PMMA/PBUbilayer support was completely retained on the surface of graphene as a passivationlayer to enhance the stability of FET device by protecting graphene from detrimentalambient species [86]. The Dirac voltage and field-effect mobility were well preservedin air for more than 130 days, whereas these performances degraded dramaticallywithout the passivation layer.

The widely used chemical etching causes the formation of metal oxides particlesunderneath graphene as another common contamination. In addition, the etching pro-cess is not compatible with noble metal substrate and especially undesirable for large-scale production due to the significant metal consumption and severe environmentalpollution. Although the metal contamination can be significantly reduced by posttreat-ment [59], etching-free method is preferred for both clean and large area transfer. Elec-trochemical bubbling is a powerful technique to accomplish etching-free transfer ofgraphene, which enables the nondestructive delamination of graphene from metalsubstrate by using hydrogen bubbles generated by the water electrolysis. Specifically,graphene grown on metal coated with the polymer support is used as the cathode of anelectrolysis cell and the graphene/polymer film is delaminated by the hydrogen bub-bles generated. The separated film can then be transferred onto the target substrateby a typical wet or dry process. The bubbling transfer of graphene grown on Cu foilwas initially performed with the aid of chemical etchant, which was used to facilitatethe exfoliation of graphene [87]. It demonstrated the wet transfer of an inch-size gra-phene film onto silicon wafer and the reuse of Cu foil for three cycles. The carriermobility of graphene increased significantly with the cycles of growth and bubblingtransfer from the reused Cu foil. Such enhancement was attributed to the release ofhigh density of nanoripples on graphene, which originated from the smoothed Cusurface by the multiple electrochemical etching and heating cycles. However, thedelamination rate was relatively low even in the presence of etchant, with a typicaltime of w30 min for a 5 � 5 cm2

film.The completely etching-free bubbling transfer of graphene was first realized by

using graphene grown on Pt (Fig. 3.8(a)) [88,89]. This method is also compatible


(a)

PMMA 1 M NaOH PMMA/graphene

GrapheneGrapheneEdge bead removed andplasma ashed

CAB polymersupport layer

CuPMMA (EL-9)Kapton tapeTarget substrate

Uniform pressure and heating to 140°C

PMMA removal by acetone and annealing

Upward forcefor properadhesion

H2 bubble

Cathode Bubbling Separate

(b) (c)

+ –

–

DI water (90°C)

Water goes between graphene and Cu

Si

Si

–0.4 V

Cu O

1M KCl

PVD Cu

Figure 3.8 (a) Illustration of the etching-free bubbling transfer process of graphene from a Pt substrate [88]. (b) Schematic of “bubble-free” elec-trochemical delamination of chemical vapor deposition graphene from copper film [94]. (c) Illustration of the hot-water delamination transfer processof graphene from Cu foil [96].

Preparation

ofgraphene

electrode43

with other typical metal substrates such as Cu and Ir. Different from the Cu foil, how-ever, the chemically inert Pt was not involved in any chemical reactions and remainedintact after thousands of transfer, thus allowing repeated use for growing graphenewithout limit. In addition, the transferred graphene was free of metal residues. Bothgraphene single crystals and film were successfully transferred using this nondestruc-tive method, which yielded high carrier mobility up tow7100 cm2 V�1 s�1 with a lowcarrier concentration of w2 � 1011 cm�2. This work also demonstrated an improvedtransfer efficiency of delaminating a centimeter-sized graphene within tens of seconds.In addition, the etching-free bubbling was compatible with dry transfer with the aid ofsemirigid frame, which reduced the amount of wrinkles/holes and improved the repro-ducibility [90]. In general, it provides a versatile strategy to transfer graphene grownon both noble and common metal substrates.

From the viewpoint of scalable transfer, one prominent issue with bubbling transferis the relatively low delamination rate, which would decrease with the size ofgraphene. Therefore, it needs to be dramatically improved without causing structuraldamage. One effective method of acceleration is enhancing the bubble generation. Forinstance, the time required to complete delamination was reduced by 10e27 times viasimply increasing the concentration of KOH or NaOH electrolyte, which was attrib-uted to the increased current flow of the cathode at constant voltage [91,92]. Particu-larly, wrinkle was proposed to act as the nucleation sites for hydrogen bubbles at thecathode perimeter [91]. Furthermore, the surface screening effect of nonreactive ionswas considered at high concentration of NaOH, which might facilitate the enrichmentof hydrogen ion at the grapheneemetal interface and thus the subsequent formation ofhydrogen bubbles [92]. However, the probability of structural damage also increasesby simply enhancing the driving force of bubbling delamination.

In addition, the residual bubbles underlying graphene is also problematic for thePMMA-supported wet bubbling transfer, which would cause structural damage ofgraphene in a way similar to the trapped interfacial water. To solve this issue, abubble-free electrochemical delamination method was demonstrated on graphene/Cusample. Different from the bubbling transfer, a lower voltage was applied to avoidthe bubble formation (Fig. 3.8(b)) [93,94]. The delamination of graphene was realizedby the electrolyte intercalation arising from the electrochemical reduction of the under-lying copper oxide layer, which was formed with air exposure or dissolved oxygen inthe electrolyte. The air-oxidized sample required the aid of an immersion technique tocompletely delaminate graphene [93]. The delaminated graphene was then transferredto silicon wafer using the standard wet transfer process. However, this strategy is onlysuitable for metal substrates that can be mildly oxidized and electrochemically reducedwithout degrading the property of graphene. In addition, such delamination is slowerthan the typical bubbling method.

Alternatively, graphene can be intercalated and delaminated from the metalsubstrate by directly immersing in the hot electrolyte or even hot water without electro-chemical reactions. Graphene grown on Pt film was delaminated by immersing in theNaOH solution at 90�C due to the effect of hydroxyl ion on metals [95]. This method isadvantageous over bubbling delamination in uniformly transferring graphene grownon patterned Pt as a bubble-free process. However, it is a slow process that requires


at least 30 min for delaminating a 1 � 1 cm2 sample even with high concentration ofNaOH. Similar graphene delamination can be obtained by simply using hot water im-mersion, which is compatible with both Cu and Pt foil (Fig. 3.8(c)) [96]. This methodis environmentally benign without the use of corrosive alkali, but it suffers from severestructural damage of graphene and even lower delamination efficiency (e.g., 2 h forcentimeter-size graphene). In this regard, the structural continuity and efficiencywere significantly improved by weakening the graphene/Cu with preoxidation treat-ment [97]. As a result, the continuous graphene film was obtained with R2R transferat a high delamination rate of 1 cm/s.

For electronic application such as FET, the formation of interfacial water by thetypical wet process is another important factor that causes the degradation of electricalperformance. This detrimental effect can be largely diminished by replacing water withvolatile liquid such as IPA or using UHV annealing, which was reported to increase thecarrier mobility of graphene by five times [55]. In addition, the wet transfer is notcompatible with the water-sensitive substrates. A fundamental solution to these issuesis the use of dry process to attach graphene. In this case, graphene was directly attachedto target substrate by van der Waals interaction through heat treatment, gas flow, oreven laser, typically with the aid of additional frame or support as rigid sample holders[61,86,98]. The use of dry transfer reduced the damage of suspended graphene trans-ferred onto holey substrate [61]. More importantly, flexible FETs with dry-transferredgraphene demonstrated significantly lowered gate leakage current as compared withthose fabricated by wet transfer [86].

3.2.2 Adhesion-mediated transfer

Adhesion-mediated transfer allows the direct release of graphene from the supportlayer onto target substrate by using the larger adhesion force between graphene/sub-strate than that of graphene/support without dissolving the support material. Typically,this strategy uses support materials with low surface energy such as polydimethylsilox-ane (PDMS) or that with a tunable adhesion such as the thermal release tape (TRT). Inaddition, high flexibility of support layer is crucial for intact transfer, which allowsconformal contact at the support/graphene and graphene/target interfaces. This strat-egy is superior to the support-dissolving transfer in terms of large-area productionbecause PDMS film and TRT are robust and easy to handle, while fragile thin polymerfilms are prone to damage in scalable transfer.

The PDMS transfer involves the attachment of a reusable PDMS film on graphene/metal by pressing, separation of graphene from metal (typically by etching metal), andrelease of graphene onto target substrates with surface energy larger than PDMS bypressing and peeling-off, which is also called “stamping transfer.” Because PDMSis highly elastic, it allows the conformal contact of graphene to various targetsubstrates even in the form of free-standing thick film. Compared with the support-dissolving transfer, it is a much simpler process for both attaching and releasinggraphene without the time-consuming thermal treatment and solvent dissolution. Itsdistinct advantage is to allow the direct fabrication of graphene pattern by simply usinga prepatterned PDMS film. Early studies used PDMS to transfer graphene grown on


metal film deposited on silicon wafer, the flat surface of which facilitated high-qualitytransfer [99]. Recent studies demonstrated the high-quality R2R patterned transfer ofgraphene grown on Cu foil onto both flexible PET and rigid silicon wafer by using acommercial silicone/PET film (Fig. 3.9(a)) [100]. In combination with the vacuumprocess, the use of thin PDMS film also enabled the transfer of graphene onto three-dimensional surfaces, which was used to form biological graphene coatings on dentaland orthopedic implants. However, graphene is vulnerable to cracking on releasedonto the target substrate as a result of deformed PDMS layer under pressure. Therefore,a delicate control of the applied pressure is critical for intact transfer. Alternatively, thisissue together with the limitation of target substrates can be solved by combiningstamping transfer with the support-dissolving transfer [76]. In this case, PDMS isused as a stamp to transfer the polymer-coated graphene, and the attachment ofgraphene onto target substrate is mainly determined by the mechanism of polymer-dissolving strategy.

Similar to the PMMA transfer, however, the use of PDMS also causes the polymercontamination of graphene surface. Because it is difficult to remove the chemically sta-ble PDMS residue by posttreatments such as rinsing or annealing, alternative lowsurfaceeenergy materials are needed for clean transfer. A silicone-based pressure-sensitive adhesive film (PSAF) was developed to realize clean stamping transfer[101]. Compared with PMMA transfer, PSAF-transferred graphene showed less

PET/silicone

Grapheneon Cu foil

Etching of Cu Patterned roller

TargetSubstratePET Nip force

Nip roll pair 1

Nip roll pair 2(heated)

Grapheneon PET

400 mm

F FF

F F

Grapheneon TRT

(a)

(b)

Figure 3.9 (a) Illustration of roll-to-roll (R2R)epatterned transfer with prepatterned silicone/PET film [100]. (b) Process schematic of the transfer of chemical vapor deposition graphenefrom copper onto hBN by using an hBN/PMMA/PVA/PDMS stack and optical image of thetransferred hBN/graphene/hBN [103]. (c) Illustration of R2R transfer by using thermal releasetape with contact pressure control [107].


p-doping with the electron mobility being improved by over three times. PSAF-supported transfer not only demonstrated large-area transfer of continuous graphenefilm onto both 4-inch silicon wafer and B5-size PET film but also patterned transferof graphene in a scalable way.

The PDMS stamping transfer is also compatible with etching-free mechanicaldelamination by enhancing the interaction between PDMS and graphene, which hasbeen realized by applying surface modification on graphene/metal. After coating thesurface of graphene/Cu with a poly(vinyl alcohol) (PVA) carrier layer, its adhesionwith elastomeric PDMS stamp can be tuned by varying the delamination rate [102].A rapid delamination enabled strong adhesion to separate a 4-inch graphene fromCu film, while slow delamination allowed the release of graphene onto silicon waferor poly(ether sulfone) film by using the intrinsic low adhesion of PDMS, which wascompleted by dissolving the PVA layer. Mechanical delamination can also be realizedby simultaneously enhancing surface adhesion and weakening graphene/metal interfa-cial interaction. Graphene domains grown on Cu foil were first mildly oxidized toweaken the graphene/Cu interaction and were then transferred onto a hexagonal boronnitride stack (hBN/PMMA/PVA/PDMS) by direct attachment and delamination(Fig. 3.9(b)) [103]. The supported graphene can also be stamped onto differentsubstrates followed by dissolution of PMMA and PVA layers [104]. Particularly,thus-obtained hBN/graphene/hBN heterostructures were superior to that obtained bythe wet transfer in terms of both structural quality and residual strain, which was attrib-uted to the absence of interfacial contamination. However, it requires further efforts toapply this strategy to transferring large area graphene film. Alternatively, the graphene/Pt interaction was significantly weakened by a carbon monoxide intercalation methodto graphene from Pt foil [105]. The intercalated graphene domains can bemechanically delaminated and transferred onto the target substrate with the typicalPDMS stamping. However, such gas intercalation is limited to the transfer of smallgraphene domains.

TRT is characterized by a tunable adhesion force after thermal treatment, thusallowing the transfer of graphene onto substrates with binding force larger than thatof heated TRT. Its intrinsically high adhesive force was used to form a strong andconformal binding with rough graphene surface, which can then be minimized by ther-mal treatment to release graphene onto target substrates. Compared with the PDMSfilm, TRT is more suitable for R2R transfer of large-area graphene grown on roughmetal foil because the strong adhesion reduces its detachment from graphene duringtransfer. It demonstrated the first R2R transfer of 30-inch graphene film grown onCu foil onto a flexible PET film on the basis of Cu etching [106]. Systematic investi-gation indicated that relatively low pressure of lamination was crucial for mitigatingstructural damage, yielding lower sheet resistances with narrower deviation(Fig. 3.9(c)) [107]. Although R2R technique is preferred for continuous productionof flexible film, it generates more defects in graphene on rigid substrate (e.g., siliconwafer and glass) due to the large stress formed. In this regard, the hot pressing processwas used to minimize damage on graphene by neutralizing the stress [108]. As a result,lower sheet resistance with small deviation was obtained. However, the expensiveTRT for single use significantly increases the cost of transfer.


3.2.3 Target-supported transfer

Instead of using special supporting layer, the target substrates can be directly used tosupport graphene. This method is attractive for clean transfer because it eliminates thesurface contamination of support residue. In addition, the robust target substrates alsoprovide strong support for transferring large-area graphene. The key to high-qualitytransfer is forming a conformal contact between the surfaces of graphene/metal andtarget substrate, which typically depends on the use of van der Waals or adhesive force.

Target-supported transfer was first used to fabricate small-area graphene supportfilm for TEM grid that required a highly clean surface for observation. A small pieceof graphene was transferred onto the surface of TEM grid by directly attaching gra-phene/Cu foil with the surface tension of solvent evaporation (e.g., IPA) followedby Cu etching [109]. Such polymer-free graphene with high-coverage is promisingfor preparing the graphene liquid cell [110]. Large-area transfer onto silicon waferand PET film was demonstrated by using hydrophobic self-assembled monolayermodification to avoid the water intercalation during etching and rinsing(Fig. 3.10(a)) [111]. This process produced clean and smooth graphene with extremelylow roughness of 0.26 nm, thus improving the carrier mobility by over two times. Thismethod is also compatible with the patterned transfer in combination with the lithog-raphy technique. Similar to the support-dissolving transfer, some polymer supports canalso be used as strong dopant. Graphene film was transferred onto the poly(vinylidenefluoride-co-trifluoroethylene) (P(VDFTrFE)) film, a typical ferroelectric fluoropoly-mer, by using spin coating or lamination followed by Cu etching. The polarizedP(VDFTrFE) film was found to dope graphene with an increased carrier density upto w1013 cm�1 [112]. The graphene/P(VDFTrFE)/graphene multilayer film demon-strated its application in transparent flexible acoustic devices and nanogenerators[113].

Target-supported transfer is highly promising for R2R production of flexible film asa simple low-cost process. Graphene grown on Cu foil was directly transferred ontoseveral polymer films by direct thermal lamination followed by Cu etching [114].However, the films suffered from high sheet resistances, presumably due to the dam-age caused by incomplete interfacial contact. In this regard, conformal contact can beobtained by binding graphene with adhesives, the properties of which determine theperformance of graphene to a large extent. A 40-inch flexible film was obtained bydirectly using the commercial PET film coated with EVA adhesive as the target sub-strate, which showed a relatively lower sheet resistance of w2 kU/sq [115]. By usingepoxy resin as the adhesive, Sony Corp. produced a 100-m long flexible graphenetransparent conductive film with sheet resistance ofw500 U/sq without doping, whichis comparable with the value obtained by typical PMMA transfer (Fig. 3.10(b)) [116].The sheet resistance can be lowered with less cracks by incorporating an interfaciallayer between graphene and adhesive to further improve the interfacial contact [117].

Target-supported transfer is also compatible with bubbling delamination, which is apromising method for clean and low-cost transfer of large-area graphene. A flat gra-phene film grown on Pt(111)/sapphire wafer was first attached onto silicon waferand then delaminated by bubbling under a constant pressure of 0.2 MPa [118].


(a) (b)

(d)

(c)

F-SAMtreatment F-SAM

Cu/graphene Coating and bonding

UV irradiation

Gra./CuBonding

Spray etchingPET/epoxy/gra./Cu

PET/epoxy/gra./Cu

PET/epoxy/gra.

PET filmEpoxy resin

Graphene

PET Gravure coating

Water

CuCl2

Transferred graphene

Copperremoved

0

02

–2

5 10 15

Hei

ght (

nm)

Lenght (μm)

(a) (b) (c)

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Target substrate Target substrate

GrapheneCu

CuCu

Monolayer

6.5 cm

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PET

Physical pressure

PDMS

Glass

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Vacuum

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EVA/PET plastic

Flexible transparent conductive plastics

GrapheneNanowire

EVAPET

Figure 3.10 (a) Illustration of transferring graphene to the F-SAM-modified SiO2/Si wafer and the quality/morphology characterization of transferredgraphene [111]. (b) Illustration of roll-to-roll (R2R) transfer by using epoxy/PET target support [116]. (c) Schematic and structure of graphene/metalnanowire hybrid films produced by an R2R bubbling transfer [121]. (d) Illustration of the MET transfer process and graphene transferred ontodifferent transparent substrates [122].

Preparation

ofgraphene

electrode49

Particularly, the use of water intercalation pretreatment significantly facilitated thedelamination by weakening the graphene/Pt interaction, which was performed byexposing the sample to ambient conditions for several days or submerging it inwarm water before bubbling. Polymer-supported flexible graphene film was firstbubbling transferred by using spin-coated polyimide [119]. This method improvedboth the contact of graphene with the target substrate and its electrical conductivityas compared with the typical PMMA-supported bubbling transfer. Alternatively, com-mercial PVC film is directly available as a target-support, which can be simplyattached by thermal lamination [120]. A continuous R2R process was developed tofabricate high-performance flexible graphene/metal nanowire hybrid transparentconductive film by directly bubbling transfer large-area graphene grown on Cu foilonto the nanowires precoated EVA/PET support film (Fig. 3.10(c)) [121]. Graphenefilm was found to effectively reduce the junction resistance between nanowires andimprove the corrosion resistance at ambient condition. A long-cycle life flexible elec-trochromic device was demonstrated by using this hybrid film.

Target-supported mechanical delamination is the most efficient transfer method forCVD graphene, by which graphene can be directly transferred to the target substratesvia a fast mechanical delamination from the metal substrate. Mechanical delaminationis advantageous in terms of the reusable metal substrate together with the absence ofsurface polymer and interfacial water contamination. However, mechanical delamina-tion can only be accomplished by forming a stronger interfacial interaction of gra-phene/substrate than that of graphene/metal. Several strategies have been developedto significantly improve the interaction between graphene and the target substrate,such as the use of linker molecule, adhesive, thermal deformation, and improvingthe adhesion energy [122e126]. One special azide linker molecule was used to modifythe surface of polystyrene substrate, which formed covalent bonds with graphene/Cuunder thermal pressing [123]. Such strong bonding allowed an inch-size graphene tobe directly transferred from the Cu foil to polystyrene by mechanical delamination,with a sheet resistance similar to that obtained by PMMA method. Strong bindingcan also be realized by using adhesive such as epoxy, which is compatible withboth polymer film and silicon wafer [124]. Flexible FET devices based on the gra-phene/epoxy/polyimide film showed stable carrier mobility over bending tests. How-ever, the electrical transport properties of graphene need to be systematically evaluatedto clarify the effect of additional covalent bond and underlying adhesive layer. Inter-estingly, the adhesion energy of graphene to Cu foil can be directly measured with thismethod. Further study revealed that the graphene/epoxy adhesion was rate dependent,which was higher than that of the graphene/copper interface at the higher separationrate [126]. Alternatively, graphene can be directly transferred onto polymer substrateswith thermal pressing as the plastic nature of polymers allows a strong interfacial inter-action. The effects of transfer are highly dependent on the rheological properties ofpolymers [125]. The interfacial adhesion energy can be dramatically enhanced bycombining the mechanical pressing, electrostatic force, and thermal treatment (theMET process, Fig. 3.10(d)) [122]. As a result, graphene films were successfully trans-ferred onto not only polymers (PET and PDMS) but also glasses. For the flexible


polymers, the improved adhesion energy was attributed to the ultraconformal contactarising from the electrostatic force and van der Waals force, which were maximized bythe viscoelastic effect of polymers. In the case of rigid glass, strong CeO covalentbonding might form under high electrostatic force. Thus-transferred graphene/PETfilm demonstrated a high stability over temperature and humidity.

One prominent problem of target-supported transfer is the relatively high surfaceroughness of graphene transferred from rough metal foil onto flexible substrates,which would reduce the transparency and preclude its application in devices requiringsmooth surface such as OLED and solar cells. Moreover, the impact of the underlyingadhesive on the thermal and solvent stability of transferred graphene film needs to beclarified by systematic studies.

3.2.4 Summary

The rapid progress in developing efficient transfer methods has significantly advancedthe research and application of graphene transparent electrode for flexible lighting anddisplaying devices. It should be pointed out that all current transfer methods have theirown advantages and limitations. The support-dissolving transfer such as the typicalPMMA method is widely used in fundamental studies as it allows the transfer ofgraphene onto different substrates as a versatile and reproducible strategy, which iscrucial for the comparison of results with different origins. Most importantly, signifi-cant advances have been made in clean transfer by using novel supporting materials(e.g., pentacene, rosin) and etching-free transfer with alternative delamination tech-niques (e.g., bubbling, water intercalation). However, it is difficult to realize thecontinuous and large-scale transfer with this strategy. Both the adhesion-mediatedtransfer and target-supported transfer are promising for scalable transfer of large-area graphene film. The stamping transfer shows distinct advantage in patterned trans-fer, and the target-supported transfer is free of polymer and etchant contamination incombination with etching-free delamination. However, both cannot be used as versa-tile methods that are suitable for all target substrates, especially for large-area transfer.

In addition, several challenges need to be solved in further investigations topromote the application of CVD graphene. Uniform intact transfer remains to be achallenge for large-area film, especially in the case of etching-free processes. There-fore, the mechanisms of structural damage need to be clarified and highly controllableprocesses are required to reduce the damage. It is much more difficult to simulta-neously realize intact and clean transfer of large-area graphene film. For instance,bubbling transfer holds great promise for low-cost and scalable production of graphenefilm, but clean bubbling transfer of large-area graphene has not been demonstrated. Inaddition, the relatively low delamination rate falls behind the requirement of scalableproduction. Solutions to the above issues might benefit from the development of noveltransfer strategies and supporting layers that enable precise control over the interactionbetween graphene, metal substrate, and target substrates.


References

[1] Y. Hernandez, et al., High-yield production of graphene by liquid-phase exfoliation ofgraphite, Nat. Nanotechnol. 3 (9) (2008) 563e568.

[2] M. Lotya, et al., Liquid phase production of graphene by exfoliation of graphite in sur-factant/water solutions, J. Am. Chem. Soc. 131 (10) (2009) 3611e3620.

[3] P. Blake, et al., Graphene-based liquid crystal device, Nano Lett. 8 (6) (2008)1704e1708.

[4] A.B. Bourlinos, et al., Liquid-phase exfoliation of graphite towards solubilized graphe-nes, Small 5 (16) (2009) 1841e1845.

[5] J. Zhao, et al., Efficient preparation of large-area graphene oxide sheets for transparentconductive films, ACS Nano 4 (9) (2010) 5245e5252.

[6] H.-J. Shin, et al., Efficient reduction of graphite oxide by sodium borohydride and itseffect on electrical conductance, Adv. Funct. Mater. 19 (12) (2009) 1987e1992.

[7] Y.Q. Liu, et al., Stable Nafion-functionalized graphene dispersions for transparent con-ducting films, Nanotechnology 20 (46) (2009) 465605.

[8] J. Ning, et al., A facile reduction method for roll-to-roll production of high performancegraphene-based transparent conductive films, Adv. Mater. 29 (9) (2017), 1605028.

[9] J. Wu, et al., Organic solar cells with solution-processed graphene transparent electrodes,Appl. Phys. Lett. 92 (26) (2008) 263302e263303.

[10] J.B. Wu, et al., Organic light-emitting diodes on solution-processed graphene transparentelectrodes, ACS Nano 4 (1) (2010) 43e48.

[11] Y. Liang, et al., Transparent, highly conductive graphene electrodes from acetylene-assisted thermolysis of graphite oxide sheets and nanographene molecules, Nanotech-nology 20 (43) (2009) 434007.

[12] C. Valles, et al., Solutions of negatively charged graphene sheets and ribbons, J. Am.Chem. Soc. 130 (47) (2008) 15802e15804.

[13] C. Mattevi, et al., Evolution of electrical, chemical, and structural properties of trans-parent and conducting chemically derived graphene thin films, Adv. Funct. Mater. 19(2009) 2577e2583.

[14] G. Eda, G. Fanchini, M. Chhowalla, Large-area ultrathin films of reduced graphene oxideas a transparent and flexible electronic material, Nat. Nanotechnol. 3 (5) (2008) 270e274.

[15] Y. Zhu, et al., Transparent self-assembled films of reduced graphene oxide platelets,Appl. Phys. Lett. 95 (10) (2009), 103104-3.

[16] S. Pei, H.-M. Cheng, The reduction of graphene oxide, Carbon 50 (0) (2011) 3210e3228.[17] S. Stankovich, et al., Synthesis of graphene-based nanosheets via chemical reduction of

exfoliated graphite oxide, Carbon 45 (7) (2007) 1558e1565.[18] H.A. Becerril, et al., Evaluation of solution-processed reduced graphene oxide films as

transparent conductors, ACS Nano 2 (3) (2008) 463e470.[19] W. Gao, et al., New insights into the structure and reduction of graphite oxide, Nat. Chem.

1 (2009) 403e408.[20] S. Pei, et al., Direct reduction of graphene oxide films into highly conductive and flexible

graphene films by hydrohalic acids, Carbon 48 (15) (2010) 4466e4474.[21] K. Moon, et al., Reduced graphene oxide by chemical graphitization, Nat. Commun. 1 (1)

(2010) 73e78.[22] Z. Wu, et al., Transparent, conductive carbon nanotube films, Science 305 (2004)

1273e1276.


[23] N. Saran, et al., Fabrication and characterization of thin films of single-walled carbonnanotube bundles on flexible plastic substrates, J. Am. Chem. Soc. 126 (2004)4462e4463.

[24] L. Hu, D.S. Hecht, G. Gruner, Carbon nanotube thin films: fabrication, properties, andapplications, Chem. Rev. 110 (10) (2010) 5790e5844.

[25] D. Zhang, et al., Transparent, conductive, and flexible carbon nanotube films and theirapplication in organic light-emitting diodes, Nano Lett. 6 (9) (2006) 1880e1886.

[26] Y. Zhou, L. Hu, G. Gr€unera, A method of printing carbon nanotube thin films, Appl.Phys. Lett. 88 (2006) 123109.

[27] M.D. Lima, et al., In-situ synthesis of transparent and conductive carbon nanotube net-works, Phys. Status Solidi RRL 1 (4) (2007) 165e167.

[28] C.-S. Woo, et al., Fabrication of flexible and transparent single-wall carbon nanotube gassensors by vacuum filtration and poly(dimethyl siloxane) mold transfer, Microelectron.Eng. 84 (2007) 1610e1613.

[29] Y. Wang, et al., Optimizing single-walled carbon nanotube films for applications inelectroluminescent devices, Adv. Mater. 20 (23) (2008) 4442e4449.

[30] J.W. Jo, et al., Fabrication of highly conductive and transparent thin films from single-walled carbon nanotubes using a new non-ionic surfactant via spin coating, ACS Nano4 (9) (2010) 5382e5388.

[31] S.L. Hellstrom, H.W. Lee, Z. Bao, Polymer-assisted direct deposition of uniform carbonnanotube bundle networks for high performance transparent electrodes, ACS Nano 3 (6)(2009) 1423e1430.

[32] J.H. Yim, et al., Fabrication of transparent single wall carbon nanotube films with lowsheet resistance, J. Vac. Sci. Technol. B 26 (2008) 851e855.

[33] X.L. Li, et al., Langmuir-Blodgett assembly of densely aligned single-walled carbonnanotubes from bulk materials, J. Am. Chem. Soc. 129 (16) (2007) 4890e4891.

[34] N. Karousis, et al., Zinc phthalocyanine-graphene hybrid material for energy conversion:synthesis, characterization, photophysics, and photoelectrochemical cell preparation,J. Phys. Chem. C 116 (38) (2012) 20564e20573.

[35] M.H.A. Ng, et al., Efficient coating of transparent and conductive carbon nanotube thinfilms on plastic substrates, Nanotechnology 19 (20) (2008) 205703.

[36] M. Kaempgen, G.S. Duesberg, S. Roth, Transparent carbon nanotube coatings, Appl.Surf. Sci. 252 (2) (2005) 425e429.

[37] A. Schindlera, et al., Solution-deposited carbon nanotube layers for flexible display ap-plications, Phys. E 37 (2007) 119e123.

[38] H.Z. Geng, et al., Effect of acid treament on carbon nanotube-based flexible transparentconducting films, J. Am. Chem. Soc. 129 (25) (2007) 7758e7759.

[39] H.-Z. Geng, et al., Doping and de-doping of carbon nanotube transparent conductingfilms by dispersant and chemical treatment, J. Mater. Chem. 18 (2008) 1261e1266.

[40] V.H. Pham, et al., Fast and simple fabrication of a large transparent chemically-convertedgraphene film by spray-coating, Carbon 48 (7) (2010) 1945e1951.

[41] J. Wang, et al., Rod-coating: towards large-area fabrication of uniform reduced grapheneoxide films for flexible touch screens, Adv. Mater. 24 (21) (2012) 2874e2878.

[42] B. Dan, G.C. Irvin, M. Pasquali, Continuous and scalable fabrication of transparentconducting carbon nanotube films, ACS Nano 3 (4) (2009) 835e843.

[43] B.-Y. Wang, et al., Enhanced light scattering and trapping effect of Ag nanowire meshelectrode for high efficient flexible organic solar cell, Small 11 (16) (2015) 1905e1911.


[44] Y. Ahn, Y. Jeong, Y. Lee, Improved thermal oxidation stability of solution-processablesilver nanowire transparent electrode by reduced graphene oxide, ACS Appl. Mater.Interfaces 4 (12) (2012) 6410e6414.

[45] J.T. Han, et al., Dispersant-free conducting pastes for flexible and printed nanocarbonelectrodes, Nat. Commun. 4 (2013) 2491.

[46] X. Hu, et al., Roll-to-Roll fabrication of flexible orientated graphene transparent elec-trodes by shear force and one-step reducing post-treatment, Adv. Mater. Technol. 2 (12)(2017) 1700138.

[47] B. Zhang, et al., Flexible transparent and conductive films of reduced-graphene-oxidewrapped silver nanowires, Mater. Lett. 201 (2017) 50e53.

[48] C.-L. Kim, et al., A highly flexible transparent conductive electrode based on nano-materials, NPG Asia Mater. 9 (2017) e438.

[49] X. Hu, et al., Roll-to-Roll production of graphene hybrid electrodes for high-efficiency,flexible organic photoelectronics, Adv. Mater.Interfaces 2 (17) (2015), 1500445.

[50] Solution-processed Transparent Electrodes, in: S.H. David, R.B. Kaner (Eds.), MRSBulletin 36 (2011) 749e755.

[51] D.S. Hecht, L.B. Hu, G. Irvin, Emerging transparent electrodes based on thin films ofcarbon nanotubes, graphene, and metallic nanostructures, Adv. Mater. 23 (13) (2011)1482e1513.

[52] X. Hu, et al., Roll-to-Roll fabrication of flexible orientated graphene transparent elec-trodes by shear force and one-step reducing post-treatment, Adv. Mater. Technol. 2 (12)(2017) 1700138.

[53] W.C. Ren, H.M. Cheng, The global growth of graphene, Nat. Nanotechnol. 9 (10) (2014)726e730.

[54] A.C. Ferrari, et al., Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, Nanoscale 7 (11) (2015) 4598e4810.

[55] J. Chan, Reducing extrinsic performance limiting factors in graphene grown by chemicalvapor deposition, ACS Nano (2012) 3224e3229.

[56] N. Petrone, et al., Chemical vapor deposition-derived graphene with electrical perfor-mance of exfoliated graphene, Nano Lett. 12 (6) (2012) 2751e2756.

[57] X.S. Li, et al., Transfer of large-area graphene films for high-performance transparentconductive electrodes, Nano Lett. 9 (12) (2009) 4359e4363.

[58] A. Reina, et al., Transferring and identification of single- and few-layer graphene onarbitrary substrates, J. Phys. Chem. C 112 (46) (2008) 17741e17744.

[59] X.L. Liang, et al., Toward clean and crackless transfer of graphene, ACS Nano 5 (11)(2011) 9144e9153.

[60] H.H. Kim, et al., Wetting-Assisted crack- and wrinkle-free transfer of wafer-scale gra-phene onto arbitrary substrates over a wide range of surface energies, Adv. Funct. Mater.26 (13) (2016) 2070e2077.

[61] J.W. Suk, et al., Transfer of CVD-grown monolayer graphene onto arbitrary substrates,ACS Nano 5 (9) (2011) 6916e6924.

[62] C.-K. Lee, et al., Monatomic chemical-vapor-deposited graphene membranes bridge ahalf-millimeter-scale gap, ACS Nano 8 (5) (2014) 2336e2344.

[63] J.Y. Hong, et al., A rational strategy for graphene transfer on substrates with roughfeatures, Adv. Mater. 28 (12) (2016) 2382e2392.

[64] L. Gao, et al., Face-to-face transfer of wafer-scale graphene films, Nature 505 (7482)(2014) 190e194.

[65] Y.J. Ren, et al., An improved method for transferring graphene grown by chemical vapordeposition, Nano 7 (1) (2012), 1150001.


[66] W.-H. Lin, et al., A direct and polymer-free method for transferring graphene grown bychemical vapor deposition to any substrate, ACS Nano 8 (2) (2014) 1784e1791.

[67] J.W. Suk, et al., Enhancement of the electrical properties of graphene grown by chemicalvapor deposition via controlling the effects of polymer residue, Nano Lett. 13 (4) (2013)1462e1467.

[68] H.J. Jeong, et al., Improved transfer of chemical-vapor-deposited graphene throughmodification of intermolecular interactions and solubility of poly(methylmethacrylate)layers, Carbon 66 (2014) 612e618.

[69] Y.C. Lin, et al., Graphene annealing: how clean can it Be? Nano Lett. 12 (1) (2011)121414e121419.

[70] Z.Y. Chen, et al., High pressure-assisted transfer of ultraclean chemical vapor depositedgraphene, Appl. Phys. Lett. 108 (13) (2016) 132106.

[71] H. Park, et al., Graphene as transparent conducting electrodes in organic photovoltaics:studies in graphene morphology, hole transporting layers, and counter electrodes, NanoLett. 12 (1) (2012) 133e140.

[72] C. Gong, et al., Rapid selective etching of PMMA residues from transferred graphene bycarbon dioxide, J. Phys. Chem. C 117 (44) (2013) 23000e23008.

[73] J. Lee, et al., Clean transfer of graphene and its effect on contact resistance, Appl. Phys.Lett. 103 (10) (2013) 103104.

[74] A.E. Islam, et al., Atomic level cleaning of poly-methyl-methacrylate residues from thegraphene surface using radiolized water at high temperatures, Appl. Phys. Lett. 111 (10)(2017) 103101.

[75] Y.C. Lin, et al., Clean transfer of graphene for isolation and suspension, ACS Nano 5 (3)(2011) 2362e2368.

[76] J. Song, et al., A general method for transferring graphene onto soft surfaces, Nat.Nanotechnol. 8 (5) (2013) 356e362.

[77] A. Capasso, et al., Cyclododecane as support material for clean and facile transfer oflarge-area few-layer graphene, Appl. Phys. Lett. 105 (11) (2014) 113101.

[78] M.G. Chen, et al., Sublimation-assisted graphene transfer technique based on smallpolyaromatic hydrocarbons, Nanotechnology 28 (25) (2017) 255701.

[79] H.H. Kim, et al., Clean transfer of wafer-scale graphene via liquid phase removal ofpolycyclic aromatic hydrocarbons, ACS Nano 9 (5) (2015) 4726e4733.

[80] Z.K. Zhang, et al., Rosin-enabled ultraclean and damage-free transfer of graphene forlarge-area flexible organic light-emitting diodes, Nat. Commun. 8 (2017) 14560.

[81] G.H. Zhang, et al., Versatile polymer-free graphene transfer method and applications,ACS Appl. Mater. Interfaces 8 (12) (2016) 8008e8016.

[82] C.-L. Hsu, et al., Layer-by-Layer graphene TCNQ stacked films as conducting anodes fororganic solar cells, ACS Nano 6 (6) (2012) 5031e5039.

[83] M.G. Lemaitre, et al., Improved transfer of graphene for gated Schottky-junction, vertical,organic, field-effect transistors, ACS Nano 6 (10) (2012) 9095e9102.

[84] M. Jang, et al., Improved performance and stability of field-effect transistors with poly-meric residue-free graphene channel transferred by gold layer, Phys. Chem. Chem. Phys.16 (9) (2014) 4098e4105.

[85] W.H. Lee, et al., Simultaneous transfer and doping of CVD-grown graphene by fluo-ropolymer for transparent conductive films on plastic, ACS Nano 6 (2) (2012)1284e1290.

[86] H.H. Kim, et al., Water-free transfer method for CVD-grown graphene and its applicationto flexible air-stable graphene transistors, Adv. Mater. 26 (20) (2014) 3213e3217.


[87] Y. Wang, et al., Electrochemical delamination of CVD-grown graphene film: toward therecyclable use of copper catalyst, ACS Nano 5 (12) (2011) 9927e9933.

[88] L. Gao, et al., Repeated growth and bubbling transfer of graphene with millimetre-sizesingle-crystal grains using platinum, Nat. Commun. 3 (2012) 699.

[89] S. Koh, et al., Epitaxial growth and electrochemical transfer of graphene on Ir(111)/alpha-Al2O3(0001) substrates, Appl. Phys. Lett. 109 (2) (2016) 043033.

[90] C.s.J.L. de la Rosa, et al., Frame assisted H2O electrolysis induced H2 bubbling transfer oflarge area graphene grown by chemical vapor deposition on Cu, Appl. Phys. Lett. 102 (2)(2013) 022101.

[91] G. Fisichella, et al., Microscopic mechanisms of graphene electrolytic delamination frommetal substrates, Appl. Phys. Lett. 104 (23) (2014) 233105.

[92] L.H. Liu, et al., A mechanism for highly efficient electrochemical bubbling delaminationof CVD-grown graphene from metal substrates, Adv. Mater. Interfaces 3 (8) (2016)1500492.

[93] C.T. Cherian, et al., Bubble-free’ electrochemical delamination of CVD graphene films,Small 11 (2) (2014) 189e194.

[94] F. Pizzocchero, et al., Non-destructive electrochemical graphene transfer from reusablethin-film catalysts, Carbon 85 (2015) 397e405.

[95] J.K. Choi, et al., Growth of wrinkle-free graphene on texture-controlled platinum filmsand thermal-assisted transfer of large-scale patterned graphene, ACS Nano 9 (1) (2015)679e686.

[96] P. Gupta, et al., A facile process for soak-and-peel delamination of CVD graphene fromsubstrates using water, Sci. Rep. 4 (2014) 3882.

[97] B.N. Chandrashekar, et al., Roll-to-Roll green transfer of CVD graphene onto plastic for atransparent and flexible triboelectric nanogenerator, Adv. Mater. 27 (35) (2015)5210e5216.

[98] E.C.P. Smits, et al., Laser induced forward transfer of graphene, Appl. Phys. Lett. 111(17) (2017) 173101.

[99] Y. Lee, et al., Wafer-scale synthesis and transfer of graphene films, Nano Lett. 10 (2)(2010) 490e493.

[100] T. Choi, et al., Roll-to-roll continuous patterning and transfer of graphene via dispersiveadhesion, Nanoscale 7 (2015) 7138e7142.

[101] S.J. Kim, et al., Ultraclean patterned transfer of single-layer graphene by recyclablepressure sensitive adhesive films, Nano Lett. 15 (5) (2015) 3236e3240.

[102] S.Y. Yang, et al., Metal-etching-free direct delamination and transfer of single-layergraphene with a high degree of freedom, Small 11 (2) (2015) 175e181.

[103] L. Banszerus, et al., Ultrahigh-mobility graphene devices from chemical vapor depositionon reusable copper, Sci. Adv. 1 (6) (2015) e1500222.

[104] L. Banszerus, et al., Identifying suitable substrates for high-quality graphene-based het-erostructures, 2D Mater. 4 (2) (2017), 025030 (8 pp.).

[105] D. Ma, et al., Clean transfer of graphene on Pt foils mediated by a carbon monoxideintercalation process, Nano Res. 6 (9) (2013) 671e678.

[106] S. Bae, et al., Roll-to-roll production of 30-inch graphene films for transparent electrodes,Nat. Nanotechnol. 5 (8) (2010) 574e578.

[107] B. Jang, et al., Damage mitigation in roll-to-roll transfer of CVD-graphene to flexiblesubstrates, 2D Mater. 4 (2) (2017) 024002.

[108] J. Kang, et al., Efficient transfer of large-area graphene films onto rigid substrates by hotpressing, ACS Nano 6 (6) (2012) 5360e5365.


[109] W. Regan, et al., A direct transfer of layer-area graphene, Appl. Phys. Lett. 96 (11) (2010)113102.

[110] J.C. Zhang, et al., Clean transfer of large graphene single crystals for high-intactnesssuspended membranes and liquid cells, Adv. Mater. 29 (26) (2017), 1700639.

[111] B. Wang, et al., Support-free transfer of ultrasmooth graphene films facilitated by self-assembled monolayers for electronic devices and patterns, ACS Nano 10 (1) (2016)1404e1410.

[112] G.X. Ni, et al., Graphene-ferroelectric hybrid structure for flexible transparent electrodes,ACS Nano 6 (5) (2012) 3935e3942.

[113] S.H. Bae, et al., Graphene-P(VDF-TrFE) multilayer film for flexible applications, ACSNano 7 (4) (2013) 3130e3138.

[114] L.G.P. Martins, et al., Direct transfer of graphene onto flexible substrates, Proc. Natl.Acad. Sci. U.S.A. 110 (44) (2013) 17762e17767.

[115] G.H. Han, et al., Poly(Ethylene Co-vinyl acetate)-assisted one-step transfer of ultra-largegraphene, Nano 6 (1) (2011) 59e65.

[116] T. Kobayashi, et al., Production of a 100-m-long high-quality graphene transparentconductive film by roll-to-roll chemical vapor deposition and transfer process, Appl.Phys. Lett. 102 (2) (2013) 023112.

[117] C.Y. Cai, et al., Crackless transfer of large-area graphene films for superior-performancetransparent electrodes, Carbon 98 (2016) 457e462.

[118] K. Verguts, et al., Controlling water intercalation is key to a direct graphene transfer, ACSAppl. Mater. Interfaces 9 (42) (2017) 37484e37492.

[119] X.H. Wang, et al., Direct delamination of graphene for high-performance plastic elec-tronics, Small 10 (4) (2014) 694e698.

[120] D.L. Mafra, T. Ming, J. Kong, Facile graphene transfer directly to target substrates with areusable metal catalyst, Nanoscale 7 (36) (2015) 14807e14812.

[121] B. Deng, et al., Roll-to-Roll encapsulation of metal nanowires between graphene andplastic substrate for high-performance flexible transparent electrodes, Nano Lett. 15 (6)(2015) 4206e4213.

[122] W. Jung, et al., Ultraconformal contact transfer of monolayer graphene on metal tovarious substrates, Adv. Mater. 26 (37) (2014) 6394e6400.

[123] E.H. Lock, et al., High-quality uniform dry transfer of graphene to polymers, Nano Lett.12 (1) (2012) 102e107.

[124] T. Yoon, et al., Direct measurement of adhesion energy of monolayer graphene as-grownon copper and its application to renewable transfer process, Nano Lett. 12 (3) (2012)1448e1452.

[125] G.J.M. Fechine, et al., Direct dry transfer of chemical vapor deposition graphene topolymeric substrates, Carbon 83 (2015) 224e231.

[126] S.R. Na, et al., Selective mechanical transfer of graphene from seed copper foil using rateeffects, ACS Nano 9 (2) (2015) 1325e1335.


Graphene doping for electrodeapplication 4Amirhossein Hasani 1, Soo Young Kim 2

1School of Chemical Engineering and Materials Science Chung-Ang University, Seoul,Republic of Korea; 2Department of Materials Science and Engineering, Korea University,Seongbuk-gu, Seoul, Republic of Korea

Doping is a facile approach used to improve the properties of nanostructures. Toenhance electronic and mechanical properties, electron acceptors and donors can beadded to nanostructures [1]. Recently, researchers have introduced various methodsto enhance the properties of graphene such as substitution, chemical doping, and metaloxide doping [2e5]. In the substitution method, nitrogen and boron can be replaced bythe carbon atoms in graphene, achieving a tunable bandgap in its optoelectronic struc-ture. For example, Angoli et al. proposed a method to dope boron in graphene usingchemical vapor deposition (CVD), and the resulting material can be used as a catalystfor the hydrogen evolution reaction [6]. Fig. 4.1 shows a schematic illustration of theCVD deposition of B-G on Cu foils. [7]. However, this method suffers from high costsand the need for complicated equipment.

4.1 Chemical doping of graphene

An important concern for enhancing electronic device performance is the alignment ofenergy bands between semiconductors and electrodes. However, modifying the workfunction (WF) of conducting nanostructures is difficult. Therefore, it is preferable toutilize methods involving surface treatment to adjust the WF. Chemical doping is afacile method that can improve the electronic properties of nanostructures, especiallygraphene and carbon nanotubes (CNTs) [7e10]. The Fermi level of the graphene sheetcan be modulated by chemical dopants, such as metal chlorides [11]. For example, Shiet al. modulated the WF of graphene by using gold trichloride (AuCl3) and CVD; amodified graphene electrode was used in photovoltaic devices with a power conver-sion efficiency of 0.08%, more than 40 times larger than that of the corresponding elec-trode without chemical doping [11]. Fig. 4.2 shows a schematic diagram of thegraphene transferring and device fabrication process. In this process, the gold cationscan accept electrons from the p orbitals of the graphene lattice by immersing the gra-phene in AuCl3 solution. As can be seen in Fig. 4.3, the Au3þ ion has negative Gibbsfree energy, suggesting an increased WF of graphene due to its depletion near the


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CO

Monolayer boron-doped graphene

Figure 4.1 Schematic illustration of chemical vapor deposition of B-G on Cu foils usingphenylboronic acid.Reproduced with permission H. Wang, Y. Zhou, D. Wu, L. Liao, S. Zhao, H. Peng, Z. Liu,Synthesis of boron-doped graphene monolayers using the sole solid feedstock by chemical vapordeposition, Small 9 (2013) 1316e20., Copyright 2010 Wiley-VCH.

CVD-G filmNi

SiO2/Si wafer

NiSiO2/Si wafer

NiSiO2/Si wafer

Spin-coating PMMAannealing at 120°C

PMMA/CVD-G film

PMMA/CVD-G film

Immerge into Ni etchant

Ni etchant PMMA/CVD-G film

Transfer to DI water

Transfer to AuCl3 for doping

PMMA/CVD-G film

DI water

5 mM AuCl3 water solution

AM 1.5Picked up by N-Si waferto fabricate devices

PMMA/CVD-G

N-Si

GlassAu/Ti E

lectro

de

Figure 4.2 Schematic diagram of the transfer and device fabrication process. The graphenefilms were synthesized using an APCVD method on a SiO2/Si wafer (300 nm thermal oxide)with a 300 nm Ni film deposited in advance. The PMMA/CVD-G film was finally transferredto an n-type Si substrate for device fabrication. CVD, chemical vapor deposition; PMMA,methyl methacrylate.Reproduced with permission H.J. Shin, W.M. Choi, D. Choi, G.H. Han, S.M. Yoon, H.K. Park,S.W. Kim, Y.W. Jin, S.Y. Lee, J.M. Kim, Control of electronic structure of graphene by variousdopants and their effects on a nanogenerator, J. Am. Chem. Soc. 132 (2010) 15603e09.,Copyright 2008 American Chemical Society.


Dirac point [12]. The related chemical reactions are described by the followingequations:

Grapheneþ 3AuCl3/Grapheneþ AuCl2� þ AuðIÞ þ AuCl4

� (4.1)

3AuCl2� /Au0 þ 2AuCl4

� þ 2Cl� (4.2)

AuCl4� þGraphene/Grapheneþ þ Au0 þ 4Cl� (4.3)

Choi et al. in 2018 improved the resistance of metal contacts with graphene bydoping the AuCl3 into graphene by CVD growth and solution processing [13]. Thedoped graphene exhibited a low contact resistivity of w897 Umm, lower than thatof the graphene without doping (w1774 Umm) [13]. In addition, the resistivity ofthe doped graphene contact increased 13% after 60 days, better than that of the non-doped graphene (50% increase after 30 days). Therefore, the stability of the contactincreased on doping. Kwon et al. in 2012 investigated the effect of metal chlorides(AuCl3, IrCl3, MoCl3, OsCl3, PdCl2, and RhCl3) on the WF of the few-layer graphene.They achieved a highWF of graphene of 4.7e5.1 eV compared with the 4.2 eV of gra-phene without doping [14]. The increasing WF energy was attributed to electron trans-fer from graphene to the metal ion, as shown in Fig. 4.4. Metal chlorides acceptcharges from other nanomaterials due to their positive reduction potentials. Reductionfrom Me3þ to Me0 is involuntary, and the depletion of electrons near the Dirac pointincreases the WF of graphene. Moreover, the negative Gibbs free energy of the metalatoms plays a vital role in graphene doping.

Doping by BV 0 Doping by Au 3+

EvacEvac

EiEF

EF

e–

e–e–

ΔG = +108.1 kJ/mol

ΔG = –145.7 kJ/mol

BV

Ei

Evac

EF

AuCl3

Ei

Reducingagent

Figure 4.3 Modulation of electronic structure by differences of reduction potentials.Reproduced with permission H.J. Shin, W.M. Choi, D. Choi, G.H. Han, S.M. Yoon, H.K. Park,S.W. Kim, Y.W. Jin, S.Y. Lee, J.M. Kim, Control of electronic structure of graphene by variousdopants and their effects on a nanogenerator, J. Am. Chem. Soc. 132 (2010) 15603e09.,Copyright 2008 American Chemical Society.

Graphene doping for electrode application 61

Recently, researchers have developed methods to dope graphene using surfaceexposed to acidic gas vapor [15]. In a transferable-process graphene synthesis, poly(-methyl methacrylate) (PMMA) remains on the surface, but it is an unwanted dopant ingraphene. As a result, the Fermi level of graphene may not be located at the Dirac pointdue to transfer to another substrate. Therefore, it is desirable to eliminate the PMMAon the graphene surface, resulting in charge transfer from graphene to the other mate-rials on the surface. Kasry et al. in 2010 prepared large area stacked graphene dopedwith nitric acid (HNO3), which is a p-type dopant for carbon-based materials [16].Following Eq. (4.4), an electron can be transferred from graphene to nitric acid:

6HNO3þ 25C/C25 þ NO3�$4HNO3 þ NO2 þ H2O (4.4)

Bare graphene exhibits high Rs due to its low carrier density. The reactiondescribed by Eq. (4.4) indicates a shift in Fermi level associated with increasing con-ductivity and carrier concentration of the graphene sheets (Fig. 4.5). This phenomenonwas first observed in single-wall carbon nanotubes (SWCNTs), where the conductivityof the SWCNTs was enhanced by surface treatment [17]. A considerable number ofstudies have been published regarding the enhancement of electrical conductivity ofcarbon-based materials. D’Arsié et al. in 2016 reported that the Fermi level of gra-phene can be modified by surface treatment using nitric acid. They demonstratedthat the optimized HNO3-doping conditions yielded Rs as low as 180 U/sq andapproximately 95% transmittance at 600 nm. In addition, different concentrations ofnitric acid over a range of temperatures were tested, and it was found that graphenereadily adsorbs NO3 molecules under optimized conditions. Moreover, anionic dop-ants can be used to increase the conductivity of carbon-based materials which canbe attributed to a larger number of charge carriers [18]. Wassei et al. in 2011 used thi-onyl chloride (SOCl2) as a dopant in graphene and carbon-based materials. The Rs ofthe prepared grapheneeCNT thin films was reduced from 425 to 103 U/sq. The doping

P dopant

e–

EF

Ei

Me0

Me3+

Figure 4.4 Schematic of the mechanism of graphene doping by metal chloride.Reproduced with permission K.C. Kwon, K.S. Choi, S.Y. Kim, Increased work function in few-layer graphene sheets via metal chloride doping, Adv. Funct. Mater. 22 (2012) 4724e4731.,Copyright 2010 Wiley-VCH.


likely occurred through bonding of sulfur to the graphitic backbone which is tinyCeCl interaction. For graphene, the chlorine anion in the vapor phase absorbs ontothe surface and interacts with the edge of the delocalized pi system (Fig. 4.6) [19].

Additionally, organic molecules can be used to modify the surface of graphene andact as electron acceptors and donors, improving its electronic properties. Lu et al. in2008 investigated tetracyanoethylene (TCNE) for tuning the electronic structure ofgraphene by density functional theory. Theoretically, they found that p-type dopedgraphene can be achieved by charge transfer between graphene and organic molecules.Organic molecules can control the bandgap and carrier density near the Dirac point.Fig. 4.7 shows the optimized absorption structure of TCNE on the graphene surface.TCNE shifts the Fermi level closer to the Dirac point by controlling the hole concen-tration in graphene. As a result, the obtained bandgap was as large as 0.23 eV due toTCNE coverage on the graphene surface [20]. Kwon et al. in 2014 investigated dopedgraphene electrodes for GaN-based LEDs. The graphene was prepared by CVD oncopper foil and subsequently transferred to a GaN substrate by PMMA. The metalchlorides were then dissolved in coordinating solvents such as nitromethane, acetoni-trile, or methanol for AuCl3, IrCl3, and RhCl3. The D-G was fabricated by spin coatingthe metal chlorides on P-G at 3000 rpm for 60 s after a residual time of 60 s [21]. Theresulting sheet resistance decreased from 220 to 105e140 U/sq and the WF increasedfrom 4.2 to 4.9e5.1 eV after graphene was doped with metal chlorides. The enhancedelectrical properties of the LEDs were attributed to decreasing sheet resistance andincreasing WF value.

Another study was performed to investigate the effect of bis(trifluoromethanesul-fonyl)amide [((CF3SO2)2NH)] (TFSA), poly(ethylene imine) (PEI), and diazoniumsalts on graphene surfaces [22e25]. TFSA is stable in air due to its hydrophobic prop-erties which can also help stabilize graphene. Similarly, TFSA molecules can absorbelectrons from the graphene surface, resulting in p-type graphene. Furthermore, use ofdiazonium and PEI as dopants improves conductivity.

(a) (b)

Graphene layer Nitric acidInterlayer doped Last-layer doped Undoped

high Rs

Dopedlow Rs

EF

EF

Figure 4.5 (a) Schematic illustrating two different doping methods. In the interlayer-dopedcase, the sample is exposed to nitric acid after each layer is stacked, whereas in the lastlayeredoped case, the film is exposed to nitric acid only after the final layer is stacked. (b)Illustration of the graphene band structure, showing the change in the Fermi level due tochemical p-type doping.Reproduced with permission A. Kasry, M.A. Kuroda, G.J. Martyna, G.S. Tulevski, A.A. Bol,Chemical doping of large-area stacked graphene films for use as transparent, conductingelectrodes, ACS Nano 4 (2010) 3839e44., Copyright 2010 American Chemical Society.


(a)

(b)

OH

H

H

O

O

O

O

O O

R

O+

SSCl Cl

Cl–

Cl

Cl

ClO

O–SO2, –HCl

SOCl2

SCl

R

RR

Figure 4.6 (a) Proposed reaction mechanism where thionyl chloride undergoes a nucleophilicinteraction with carboxylic acid, releasing chloride which can functionalize groups present onthe chemically converted graphene and oxidized carbon nanotubes. R represents polyaromatichydrocarbon species; (b) visualization of chloride ion (green circles) bonding to the carbons atthe edge of a graphitic base.Reproduced with permission J.K. Wassei, K.C. Cha, V.C. Tung, Y. Yang, R.B. Kaner, Theeffects of thionyl chloride on the properties of graphene and grapheneecarbon nanotubecomposites, J. Mater. Chem. 21 (2011) 3391e96., Copyright 2011 The Royal Society ofChemistry.

Figure 4.7 Schematic top view of the optimized adsorption structure of tetracyanoethylene(TCNE) at site 2 (Fig. 4.1) at high coverage (1 TCNE molecule per supercell containing 24carbon atoms per graphene layer). The black sticks represent the honeycomb structure ofgraphene, the small orange balls are the carbon atoms of TCNE, and the gray balls are nitrogenatoms of TCNE.Reproduced with permission (lu et al. 2008), Copyright 2008 American Chemical Society.


4.2 Metal oxide doping of graphene

Metal oxides are promising candidates for improving the performance of graphene-based electrodes. Recently, transition metal oxides have attracted an enormous amountof attention due to their excellent properties. Meyer et al. used the interface structure ofgraphene with thermally evaporated molybdenum trioxide (MoO3) layers to improveelectronic properties of organic lighteemitting diodes [26]. The combination of gra-phene with MoO3 results in a surface charge transfer, leading to the stable p-typedoping of graphene with a downshift of 0.25 eV from the Fermi level and reducedsheet resistance below 50 U/sq. Fig. 4.8 illustrates the band energy level formationof MoO3/graphene and its electronic structure. Electronic states due to oxygenvacancies in the MoO3 crystal structure were observed, indicating a highly n-dopedsemiconductor. Therefore, MoO3 improves the band level alignment and allows forthe manipulation of energy level toward the Fermi level [26].

Another study by Fan et al. in 2014 investigated the effect of aluminum oxide(Al2O3) in highly conductive few-layer graphene [27]. The conductivity of the filmwas enhanced to 2.1 � 103 S/m using an optimum amount of Al2O3 in few-layergraphene. The oxygen concentration, in the form of oxides, plays a vital role ingraphene doping. A synergistic effect between the graphene surface and metal oxidematrix was observed [27]. The zero-gap two band model (STB) can be used to describethe thermos-power results. Theoretically, the Seebeck coefficient is described by thefollowing equations [28].

a¼ðshahþ shaeÞ=ðshþaeÞ (4.5)

EVAC

EVAC

EF

MoO3

MoO3

Graphene

p-dopedgraphene

Gapstates

VB

CB

WF

= 4.

7 eV

1.9

eV

IE =

9.4

eV

WF

= 6.

6 eV

EA

= 6.

4 eV

Figure 4.8 Schematic of the graphene/MoO3 interface and energy level alignment diagram ofthe monolayer graphene/MoO3 interface.Reproduced with permission J. Meyer, P.R. Kidambi, B.C. Bayer, C. Weijtens, A. Kuhn, A.Centeno, A. Pesquera, A. Zurutuza, J. Robertson, S. Hofmann, Metal oxide induced chargetransfer doping and band alignment of graphene electrodes for efficient organic light emittingdiodes, Sci. Rep. 4 2014 5380., Copyright 2014 Nature Publishing Group.


where s is the electrical conductivity, and the subscripts e and h are related to theelectrons and holes, respectively. The partial conductivity in terms of carrier con-centration and carrier mobility ratios can be expressed as follows:

a¼ nhne; b ¼ me

mh(4.6)

Therefore, the Seebeck coefficient can be rewritten as follows:

a¼ðaah þ baeÞ=ðaþ bÞ (4.7)

In addition, the energy dependence and Seebeck coefficient at room temperature(300K) can be expressed using the following equations:

ah¼�k

e

�½2F1ðdhÞ =F0ðdhÞ� dh� (4.8)

ae¼��k

e

�½2F1ðdeÞ =F0ðdeÞ� de� (4.9)

FjðdÞ¼ZN

0

hxj = eðx�dÞ þ 1

idx (4.10)

where k is the Boltzmann constant, and j is the order of the Fermi integral.

dh ¼ D

kT; de ¼ �D

kT; (4.11)

are the Fermi-level energies based on the STB model of graphitized materials, and Drepresents the Fermi-level depression [29]. Therefore, the equation can be written as afunction of carrier concentration and Fermi-level depression:

a¼ln

�1þ exp

�D

kT

��

ln

�1þ exp

��D

kT

�� (4.12)

The above equations prove that carrier mobility is not strongly related to the tem-perature. The STB model is a useful calculation to describe graphite-based materials asa semimetal with plane transport. The role of oxides in doping is to raise the surfacestate higher than valence band, accepting electrons from graphene to balance thedifferent WF at the metal oxide/graphene interface [27].


Hong et al. (2016) reported the application of Mn2O3 as a graphene dopant toincrease conductivity, where the resistivity was approximately 10�2 U � m at a pres-sure of w43 GPa [29].

4.3 Stability of the doped graphene electrodes

Stability of the doped graphene electrodes under ambient conditions restricts theirpractical applications. Despite the excellent performance of doped graphene elec-trodes, they degrade under condition with high humidity, high pressure, and high/low temperature. The stability of doped graphene is a major concern for its real-world application under a variety of atmospheric condition. Therefore, the stabilityof doped graphene electrodes must be improved for use in different environmentalconditions. Choi et al. in 2018 investigated the stability of AuCl3-doped grapheneused as an electrode in contact resistance of graphene-based devices. Fig. 4.9 showsthe sheet resistance of the doped graphene electrode as a function of time [13].As can be seen in Fig. 4.9(a), the sheet resistance of AuCl3-doped graphene wasimproved by 15% over 5 days and by 50% after 1 month in air. In addition, the contactresistivity of AuCl3-doped graphene was quite stable (13% increased after 2 months).The high stability of AuCl3-doped graphene can be ascribed to the decoration of thegraphene surface with metals that protect the material from environmental factorssuch as high/low temperature, diffusion of water molecules, and high pressure [13].

Additionally, D’Arsié et al. in 2016 investigated the stability of p-type graphene-doped using nitric acid (HNO3). They defined the stability as a fraction of the recov-ered Rs under different environmental conditions [18]. The graphene doped withHNO3 was not stable due solely to its absorbance of NO3, but it was stable whenthe oxygen bonding is enforced. The key to increasing the stability of doped graphene

0

15%increase

50%increase

13%increase

100

200

300

400

500

600

0

200

400

600

800

1200

1000

1400

As doped As doped5 days 30 days 60 days

(a) (b)

ρ c(Ω

. μm

)

Rsh

(Ω/

)

Figure 4.9 (a) Sheet resistance of AuCl3 as-doped graphene after 5 and 30 days. (b) Contactresistivity of graphene metal measured immediately and after 60 days.Reproduced with permission D.C. Choi, M. Kim, Y.J. Song, S. Hussain, W.S. Song, K.S. An, J.Jung, Selective AuCl3 doping of graphene for reducing contact resistance of graphene devices,Appl. Surf. Sci. 427 (2018) 48e54., Copyright 2018 Elsevier.


is to dope at high temperatures and concentrations via absorption of oxygen on thegraphene surface.

Moreover, Piazza et al. in 2015 studied the p-type doping of graphene and its sta-bility during thermal annealing. In their study, p-type graphene was obtained by ther-mal annealing in the presence of oxygen. However, graphene can be destroyed at highannealing temperatures, resulting in unstable doped graphene electrodes. To preventthe graphene damage at high temperatures, oxygen should be strictly controlled duringthe treatment process. Fig. 4.10(a) shows the distribution of stable formation as config-uration of energy. From Fig. 4.10(a), stable states were populated from 140 to 250�Cand were also stable under vacuum conditions up to 300�C (Fig. 4.10(b)). As a result,an activation energy of 56 meV was obtained for the doping process with a stable con-dition as adjusted bonding energy larger than 49 meV [30]. Another study wasperformed by Kwon et al. in 2013 regarding the impact of Au anion complexes ondoping. Au(OH)3, Au2S, AuBr3, and AuCl3 were used as graphene dopants, resultingin a reduction of Rs from 950 U/sq to 820, 600, 530, and 300 U/sq and a simultaneousincrease in WF from 4.3 to 4.6, 4.8, 5.0, and 4.9 eV, respectively [32]. In addition,thermal annealing was used to investigate the effect of anions with Au cations ongraphene doping. Fig. 4.11 illustrates the Dirac point state corresponding to eachsample and treatment condition. Different ionic conformations can be formeddepending on the coordination state of the solvent. The mechanism consists of twomajor steps: the combination of Au cations with a carbon center and withdrawing elec-trons from the graphene sheet via Au cations. These steps lead to the formation of astrong bond between the reactive carbon center and anions associated with increasingthe overall stability. Therefore, nonchloride Au complex dopants play vital roles in the

T = 140°C

T > 140°C

T > 300°C

T = 250°C

Configuration energy Configuration coordinate

Con

figur

atio

n po

pula

tion

Con

figur

atio

n en

ergy

Activation Ea=56meV

O2 trapping/stability

(a) (b)

Figure 4.10 (a) Schematic distribution of stable doping state populations as a function ofenergy. The temperatures reported in the scheme represent the onset and completion processtemperature determined experimentally. (b) Schematic representation of the doping stateenergy as a function of a generalized configuration coordinate where the barrier of activationand trap were determined theoretically.Reproduced with permission A. Piazza, F. Giannazzo, G. Buscarino, G. Fisichella, A.L. Magna,F. Roccaforte, M. Cannas, F. Gelardi, S. Agnello, Graphene p-type doping and stability bythermal treatments in molecular oxygen controlled atmosphere, J. Phys. Chem. C 119(2015)22718e23., Copyright 2015 American Chemical Society.


long-term stability of graphene-doped materials [31]. In another work, Kwon et al.increased the thermal stability of doped graphene electrode through the formation ofgraphene overlayers. The Rs of the metal chlorideedoped graphene electrode withoutoverlayers increased from 460 to 24,473 U/sq. after thermal annealing at 400�C. Theorigin of this enhanced stability can be described by the following equations [32]:

2grapheneþ 2MeCl3/2grapheneþ þMeCl2� þMeCl4

�ðdominantÞ (4.13)

3MeCl2�/ 2Me0YþMeCl4

� þ 2Cl� (4.14)

MeCl4� þ 3graphene/3grapheneþ þMe0 þ 4Cl� (4.15)

Grapheneþ þCl�/Graphene� Cl (4.16)

Grapheneþ þMeCl4�/Graphene�MeCl4 (4.17)

Pristine graphene

Pristine graphene

Au complex dopingAu3+, AuX, Au0

AnnealingAuX, Au0 aggregation

X2 sublimation orevaporation

Doped state Annealed state

Figure 4.11 The proposed mechanism of annealing-induced degradation of an Au complexedoped graphene. Yellow spheres, orange spheres, and green spheres represent Au0, Au3þ, andX� anions, respectively.Ref. Effect of anions in Au complexes on doping and degradation of graphene. Reproduced withpermission (Kwon et al. 2011), Copyright 2013 The Royal Society of Chemistry.


The composition of graphene chlorine and graphene metal chlorides can beadjusted by annealing process. The chlorine vapor was generated from the unstablepart of MeCl4

�. The amount of electronegativity in the combined chlorineecarbonatoms is higher than carbon atoms in the graphene nanosheet. As a result, temporarydipole electrons are formed by the difference of the electronegativity which trans-formed the graphene to p-type (Eqs. (4.16) and (4.17)). Chlorine gas is also producedby MeCl2

� during the annealing process. Therefore, the improved stability originatedfrom the annealing process and application of an increased number of overlayers ingraphene.

Acknowledgments

This research was supported in part by Creative Materials Discovery Program through the Na-tional Research Foundation of Korea (NRF) funded by Ministry of Science and ICT(NRF-2017M3D1A1039379) and in part by a National Research Foundation of Korea (NRF) grantfunded by the Korean government (MSIP) (No. 2017K1A3A1A67014432).

References

[1] T. Wehling, K. Novoselov, S. Morozov, E. Vdovin, M. Katsnelson, A. Geim,A. Lichtenstein, Molecular doping of graphene, Nano Lett. 8 (2008) 173e177.

[2] X. Wang, X. Li, L. Zhang, Y. Yoon, P.K. Weber, H. Wang, J. Guo, H. Dai, N-doping ofgraphene through electrothermal reactions with ammonia, Science 324 (2009) 768e771.

[3] H. Liu, Y. Liu, D. Zhu, Chemical doping of graphene, J. Mater. Chem. 21 (2011)3335e3345.

[4] K.C. Kwon, K.S. Choi, C. Kim, S.Y. Kim, Effect of transition-metal chlorides on grapheneproperties, Phys. Status Solidi (A) 211 (2014) 1794e1800.

[5] Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier, H. Dai, Co3O4 nanocrystals ongraphene as a synergistic catalyst for oxygen reduction reaction, Nat. Mater. 10 (2011) 780.

[6] S. Agnoli, M. Favaro, Doping graphene with boron: a review of synthesis methods,physicochemical characterization, and emerging applications, J. Mater. Chem. 4 (2016)5002e5025.

[7] H. Wang, Y. Zhou, D. Wu, L. Liao, S. Zhao, H. Peng, Z. Liu, Synthesis of boron-dopedgraphene monolayers using the sole solid feedstock by chemical vapor deposition, Small 9(2013) 1316e1320.

[8] X. Miao, S. Tongay, M.K. Petterson, K. Berke, A.G. Rinzler, B.R. Appleton, A.F. Hebard,High efficiency graphene solar cells by chemical doping, Nano Lett. 12 (2012)2745e2750.

[9] K.C. Kwon, S. Kim, C. Kim, J.-L. Lee, S.Y. Kim, Fluoropolymer-assisted grapheneelectrode for organic light-emitting diodes, Org. Electron. 15 (2014) 3154e3161.

[10] K.C. Kwon, K.S. Choi, B.J. Kim, J.-L. Lee, S.Y. Kim, Work-function decrease of gra-phene sheet using alkali metal carbonates, J. Phys. Chem. C 116 (2012) 26586e26591.

[11] Y. Shi, K.K. Kim, A. Reina, M. Hofmann, L.-J. Li, J. Kong, Work function engineering ofgraphene electrode via chemical doping, ACS Nano 4 (2010) 2689e2694.


[12] H.-J. Shin, W.M. Choi, D. Choi, G.H. Han, S.-M. Yoon, H.-K. Park, S.-W. Kim, Y.W. Jin,S.Y. Lee, J.M. Kim, Control of electronic structure of graphene by various dopants andtheir effects on a nanogenerator, J. Am. Chem. Soc. 132 (2010) 15603e15609.

[13] D.-C. Choi, M. Kim, Y.J. Song, S. Hussain, W.-S. Song, K.-S. An, J. Jung, SelectiveAuCl3 doping of graphene for reducing contact resistance of graphene devices, Appl. Surf.Sci. 427 (2018) 48e54.

[14] K.C. Kwon, K.S. Choi, S.Y. Kim, Increased work function in few-layer graphene sheetsvia metal chloride doping, Adv. Funct. Mater. 22 (2012) 4724e4731.

[15] C.W. Jang, J.H. Kim, J.M. Kim, D.H. Shin, S. Kim, S.-H. Choi, Rapid-thermal-annealingsurface treatment for restoring the intrinsic properties of graphene field-effect transistors,Nanotechnology 24 (2013) 405301.

[16] A. Kasry, M.A. Kuroda, G.J. Martyna, G.S. Tulevski, A.A. Bol, Chemical doping of large-area stacked graphene films for use as transparent, conducting electrodes, ACS Nano 4(2010) 3839e3844.

[17] Q.B. Zheng, M.M. Gudarzi, S.J. Wang, Y. Geng, Z. Li, J.-K. Kim, Improved electrical andoptical characteristics of transparent graphene thin films produced by acid and dopingtreatments, Carbon 49 (2011) 2905e2916.

[18] L. D’Arsié, S. Esconjauregui, R.S. Weatherup, X. Wu, W.E. Arter, H. Sugime, C. Cepek,J. Robertson, Stable, efficient p-type doping of graphene by nitric acid, RSC Adv. 6 (2016)113185e113192.

[19] J.K. Wassei, K.C. Cha, V.C. Tung, Y. Yang, R.B. Kaner, The effects of thionyl chloride onthe properties of graphene and grapheneecarbon nanotube composites, J. Mater. Chem. 21(2011) 3391e3396.

[20] Y. Lu, W. Chen, Y. Feng, P. He, Tuning the electronic structure of graphene by an organicmolecule, J. Phys. Chem. B 113 (2008) 2e5.

[21] K.C. Kwon, B.J. Kim, C. Kim, J.-L. Lee, S.Y. Kim, Comparison of metal chloride-dopedgraphene electrode fabrication processes for GaN-based light emitting diodes, RSC Adv. 4(2014) 51215e51219.

[22] D.B. Farmer, R. Golizadeh-Mojarad, V. Perebeinos, Y.-M. Lin, G.S. Tulevski, J.C. Tsang,P. Avouris, Chemical doping and electron� hole conduction asymmetry in graphene de-vices, Nano Lett. 9 (2008) 388e392.

[23] A. Gupta, G. Chen, P. Joshi, S. Tadigadapa, P. Eklund, Raman scattering from high-frequency phonons in supported n-graphene layer films, Nano Lett. 6 (2006) 2667e2673.

[24] A. Das, S. Pisana, B. Chakraborty, S. Piscanec, S. Saha, U. Waghmare, K. Novoselov,H. Krishnamurthy, A. Geim, A. Ferrari, Monitoring dopants by Raman scattering in anelectrochemically top-gated graphene transistor, Nat. Nanotechnol. 3 (2008) nnano.2008.2067.

[25] A. Kuruvila, P.R. Kidambi, J. Kling, J.B. Wagner, J. Robertson, S. Hofmann, J. Meyer,Organic light emitting diodes with environmentally and thermally stable doped grapheneelectrodes, J. Mater. Chem. C 2 (2014) 6940e6945.

[26] J. Meyer, P.R. Kidambi, B.C. Bayer, C. Weijtens, A. Kuhn, A. Centeno, A. Pesquera,A. Zurutuza, J. Robertson, S. Hofmann, Metal oxide induced charge transfer doping andband alignment of graphene electrodes for efficient organic light emitting diodes, Sci. Rep.4 (2014) 5380.

[27] Y. Fan, L. Kang, W. Zhou, W. Jiang, L. Wang, A. Kawasaki, Control of doping by matrixin few-layer graphene/metal oxide composites with highly enhanced electrical conduc-tivity, Carbon 81 (2015) 83e90.

[28] C.A. Klein, STB model and transport properties of pyrolytic graphites, J. Appl. Phys. 35(1964) 2947e2957.


[29] Y. Hong, M. Wu, G. Chen, Z. Dai, Y. Zhang, G. Chen, X. Dong, 3D printed microfluidicdevice with microporous Mn2O3-modified screen printed electrode for real-time determi-nation of heavy metal ions, ACS Appl. Mater. Interfaces 8 (2016) 32940e32947.

[30] A. Piazza, F. Giannazzo, G. Buscarino, G. Fisichella, A.L. Magna, F. Roccaforte,M. Cannas, F. Gelardi, S. Agnello, Graphene p-type doping and stability by thermaltreatments in molecular oxygen controlled atmosphere, J. Phys. Chem. C 119 (2015)22718e22723.

[31] K.C. Kwon, B.J. Kim, J.-L. Lee, S.Y. Kim, Effect of anions in Au complexes on dopingand degradation of graphene, J. Mater. Chem. C 1 (2011) 2463e2469.

[32] K.C. Kwon, S.Y. Kim, Extended thermal stability in metal-chloride doped graphene usinggraphene overlayers, Chem. Eng. J. 244 (2014) 355e363.


Technical issues and integrationscheme for graphene electrodeOLED panels

5Jaehyun Moon, Jin-Wook Shin, Hyunsu Cho, Jun-Han Han, Byoung-Hwa Kwon,Jeong-Ik Lee, Nam Sung ChoFlexible Device Research Group, Electronics and Telecommunications Research Institute(ETRI), Daejeon, Republic of Korea

5.1 Introduction

Thanks to its versatile chemical bonding capacity, carbon can form many allotropes.Among known eight allotropes, graphene is the only two-dimensional sheet andmay be considered as a large assembly of aromatic molecule. Graphene is atwo-dimensional film, in which carbon atoms are arranged in a hexagonal array [1].Since its isolation in 2004, graphene research has experienced the hype stage [2e4].Graphene has been extensively investigated from atomistic level to device level. Asan active device component, its applications have been probed virtually in all devicearea. Graphene was proposed as a novel material that will push the boundaries of exist-ing technologies to the extreme. Various platforms for transistors, sensors, energycomponents, shielding layer, structural components, and biomedical applicationshave been suggested and reported [5]. In this chapter, we aim to explore the technicalissues relevant to organic lighteemitting diodes (OLEDs) [6]. To be specific, weaddress processing issues relevant to the pattering of graphene films and their integra-tion into large area OLED panels as transparent electrodes [7]. Graphene can offeroptically transparency, electrically conductivity, chemical stability, and mechanicallyflexibility. Thus, from the perspective of optoelectronic applications, of which OLEDbelongs to, graphene emerges as a fitting choice for replacing the dominantly usedtransparent electrode material indium tin oxide (ITO) [8]. However, the reportedmajority of outstanding graphene properties are based on single-domain graphenewhich has not undergone device-level processing. In real applications, single-domain graphene is rarely used, and the characteristics of a graphene containingunit device cannot fully represent the overall performances of a panel-level devicearray. In fact, not the graphene properties but the processing and integration stronglymatter to bring graphene electrode OLED panel into reality.

In this chapter, we first explore important technical features of graphene anodeOLEDs. Here, we contrast the difference between ITO anode OLEDs. Next, weexamine the technical hurdles which impede the realizations of pixelated grapheneanode OLED on large area. We suggest and demonstrate a systematic procedure which


https://doi.org/10.1016/B978-0-08-102482-9.00005-8

allows the implementation of display compatible processes [7]. Finally, we discuss thepossibility of applying graphene for flexible OLED panels. For a longtime, graphenehas been contemplated as an alternative electrode material to its brittle ITO counterpartto realize flexible OLEDs.

5.2 Graphene preparation for OLED applications

Because we aim to use graphene in OLEDs as a transparent electrode, the choice ofgraphene preparation is important. Graphene can be prepared by numerous methods[9]. Technologies related to graphene growth and transfer are quite extensive andcan be easily amounted to an independent chapter. Reader interested in this fieldmay consult a published article [10]. To use graphene or multilayer graphene inOLEDs, several requirements must be met. First, it must be possible to have grapheneover a large area. Second, as a transparent electrode, graphene must bear acceptablelevels of electrical and optical properties. Third, graphene must be patternable intogeometrically accurate shapes and size. So far, the patterning issue of graphene hasnot been a major scientific interest. However, establishing a reliable patterning processis of very high importance in realizing commercial-level products. We will return tothis topic in the forthcoming sections.

For OLED application, chemical vapor deposition (CVD) is the preferred graphenegrowth method [11,12]. In the CVD method, large area graphene films are obtainedmainly by thermally decomposing carbon-containing gas species (e.g., CH4). Carbonatoms absorb on a catalytic metallic surface on which graphene nucleate and growisothermally in a polydomain fashion. The choice of catalytic metal is Cu. Comparedwith Ni, Cu gives monolayeric graphene with excellent surface flatness and unifor-mity. Moreover, layer-by-layer doping technique can be applied to lower the sheetresistance of the graphene film. Thanks to the advancement in CVD growth and isola-tion/transfer process, graphene and multilayer graphene film can be formed on the sub-strate of choice on large area. Graphene film grown by CVD method offers acceptablesheet resistance (w50 U/Sq.), mechanical compliance, and transmittance (w80%)[13]. CVD-grown graphene can be transferred on plastic substrates. This makesCVD graphene as a unique choice for the use in flexible graphene OLEDs [14].

5.3 Technical issues of OLEDs having graphene filmelectrodes

5.3.1 Actual examples. Graphene versus ITO OLEDs

In this section, we investigate the characteristics of graphene anode OLEDs. At firstglance, replacing ITO anode to graphene might look trivial. However, becausegraphene and ITO are different in many aspects, one needs to pay close attention tothe differences to make most of the graphene anode. In particular, optical and interfa-cial considerations are of high importance [15e18].


To grasp the difference, we present actual OLED characteristics measured fromreal devices [6]. Fig. 5.1 summarizes the device performance of bottom emissionetype phosphorescent OLEDs. As the emissive layer, we used a green dopant oftris(2-phenylpyridinato-C2,N) iridium(III) (Ir(ppy)3) and a host of 2,6-bis(3-(carba-zol-9-yl)phenyl)pyridine (DCzPPy). The transparent electrodes were 70 nm ITO andfour-layered graphene film. Our graphene film is grown by a CVD method on a Cufoil. The isolated monolayer graphene was p-doped using benzimidazole (C7H6N2)-containing solution [19]. Four-layered graphene film was obtained by layer-by-layertransfer. The direct transmittance (DT) and sheet resistance of our graphene film were83% (at 550 nm) and 65 U/Sq., respectively. For convenience, we refer an OLEDwith ITO anode as ITO OLED and an OLED with graphene film anode as grapheneOLED. Both the current density (J) and luminance (L) levels of ITO OLED wereobserved to be higher than those of the graphene OLED (Fig.5.1(a)). Accordingly,the ITO OLED showed higher external quantum efficiency (EQE, %) level than

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)d()c(

Figure 5.1 (a) The JVL characteristics of ITO anode and graphene film anode OLEDs. (b) TheEQEs of ITO anode and graphene film anode OLEDs. (c) The EL spectra of ITO anode OLEDas a function of viewing angle. (d) The EL spectra of graphene film anode OLED as a functionof viewing angle. EL, electroluminescence; EQE, external quantum efficiency; ITO, indium tinoxide; OLED, organic lighteemitting diode.Reproduced by permission from Elsevier, J. Moon, J.W. Shin, H. Cho, J.H. Han, N.S. Cho, T.J.Lim, S.K. Park, H.K. Choi, S.Y. Choi, J.H. Kim, M.J. Maeng, J. Seo, Y. Park, J.I. Lee, Technicalissues in graphene anode organic light emitting diodes, Diam. Relat. Mater. 57 (2015) 68e73.

Technical issues and integration scheme for graphene electrode OLED panels 75

that of graphene OLED (Fig.5.1(b)). The EQE of graphene OLED is approximately90% of ITO OLED at luminance (L) level of 1000 cd/m2. It is important to notice thatfor a given organic stack structure, the EQE of graphene OLED is always lower thanITO OLED. The results of Fig.5.1(a),(b) can be attributed to various factors. The holeinjection from graphene surface into the adjacent hole transport layer (HTL) is notnecessarily effective as the case of ITO. The hole injection is closely related to theenergy alignment of the work function of the anode and the highest occupied molec-ular orbital (HOMO) level of the adjacent organics. This part calls for interfacialmodification to enhance hole injection and charge transport characteristics. Theoptical properties of graphene film can be a factor. Generated light passes throughthe transparent anode. Thus, the extinction coefficient (k), which is related to absorp-tion, of the anode is expected to have effect on the OLED efficiency. We will returnto the absorption issue later. Fig. 5.1 (c),(d) shows the angular electroluminescence(EL) spectra of ITO and graphene OLEDs. In the case of ITO OLED, shoulder devel-opment in the EL spectrum is apparent as the viewing angle changes. In addition, themain peak shifts slightly toward lower wavelength as the viewing angle increases.However, in the case of graphene OLED, all EL spectral lines almost superimpose,indicating negligible angular dependency. The presence of angular EL spectra depen-dency is a demerit of light source quality [20,21]. From practical viewpoint, theabsence of angular dependency is highly preferred because the perceived light orcolor will be uniform. The results strongly indicate that ITO and graphene OLEDsare different in electrical performance and EL spectral characteristics, which haveto be considered in designing graphene OLEDs.

5.3.2 Optical issues

In this part, we explore the aspect of internal optics relevant to graphene OLED[22,23]. In a simplified picture, conventional bottom emissiveetype OLEDs can bestructurally described as a vertical stack of a metallic cathode, organic layers, and atransparent anode. Typical organic thickness does not exceed 1 mm. At such thickness,internal interference or microcavity strongly matters. This situation is schematicallydescribed in Fig. 5.2(a). Also an actual scanning electron microscope (SEM) imageof a graphene OLED is shown (Fig. 5.2(b)). The light generated in the emissiveorganic layer travels downward (DEO) and upward (UEO). The optical componentUEO reflects at the highly reflective metallic cathode, resulting in an optical componentUER. Because of the difference in the refractive indices of organics and transparentanode, there exists a weak mirror surface at the HTL/anode interface. The DEO contrib-utes to the appearance of DER. As one may easily imagine, all optical componentsinterfere, and the internal optics is fairly complicated to describe.

Reflection takes places at the surface where the optical contrast is large. Thus,among many factors governing the internal optics, the reflectance of the electrodesis of special interest. The reflectance of Al, which is typical cathode material, exceeds93% in the visible range. Thus, the reflectance of interest is that of anode/HTL inter-face. If the reflectance at the interface is low, it is not easy to form a cavity structure.


The reflectance of anode/HTL can be calculated using a transfer matrix method. Inturn, the reflectance is related to the cavity enhancement factor (Gcav(l)) as thefollowing [24]:

GcavðlÞf Tt ð1þffiffiffiffiffiRr

p Þ2

ð1� ffiffiffiffiffiffiffiffiffiffiRtRr

p Þ2 þ 4ffiffiffiffiffiffiffiffiffiffiRtRr

psin2

�D4

2

�

where Rt and Rr are the reflectance of the transparent electrode and the reflectiveelectrode, respectively, and Tt is the transmittance of the transparent electrode. Df isphase change taking place on reflection. Constructive interference takes place whenthe Df equals 2mp, where m is an integer. The calculated profile of Gcav(l) can beapplied to deduce the emission augmentation or reduction at a specific wavelength. Inaddition, for given wavelength, the influence of reflectance can be estimated. Fig. 5.3shows the calculated reflectance of ITO/organics and graphene/organics interfaces.Also the calculated Gcav(l)s are also shown. In the calculation course, a constructiveinterference condition at a wavelength of 520 nm is assumed. While the reflectance ofITO/organics is showing variation, the reflectance of graphene OLED is hardlyvarying. Also the reflectance value is fairly low. Accordingly, the Gcav(l) at thewavelength of 520 nm, there exists noticeable difference between two devices. In theITO case, the Gcav(l) is approximately 1.3, while the corresponding value of graphenecase does not exceed one.

HTLDEo

Cathode

Anode

ETL UEo

UER

Glass

DER

EL*

*:Emissive layer

(a) (b)

Aluminum

Organic layers

Graphene500 nm

Figure 5.2 (a) Schematics of simulation cell and optical components. (b) An SEM image of agraphene organic lighteemitting diode.(a) Reprinted by permission from Elsevier, J. Moon, J.W. Shin, H. Cho, J.H. Han, N.S. Cho, T.J.Lim, S.K. Park, H.K. Choi, S.Y. Choi, J.H. Kim, M.J. Maeng, J. Seo, Y. Park, J.I. Lee, Technicalissues in graphene anode organic light emitting diodes, Diam. Relat. Mater. 57 (2015) 68e73.(b) Reproduced by permission from American Institute of Physics, J. Hwang, H.K. Choi, J.Moon, T. Kim, J.W. Shin, C.W. J. Joo, J.H. Han, D.H. Cho, J.W. Huh, J.I. Lee, H.Y. Chu,Multilayered graphene anode for blue phosphorescent organic light emitting diodes, Appl. Phys.Lett. 100 (2012) 133304e133307.


To verify the results of Fig. 5.3, we performed optical simulations on devices,which are based on multiple interference theory and dipole oscillation theory(Fig. 5.4). The light source or the dipole was positioned at the emissive layer(EML) side of the electron transport layer (ETL)/EML interface. The theoretical back-ground of the simulations can be found elsewhere [24]. The optical constants of gra-phene were obtained from published literature. In the simulation courses, we havevaried the thickness of the HTL, while fixed the ETL thickness as 60 nm. The thick-nesses of metallic Al cathode and EML were 100 and 20 nm, respectively. In the ITOcase, the efficiency (EQE, %) shows evident sinusoidal oscillatory behavior as theHTL thickness changes. In contrast, in the graphene case, the oscillatory behavior islow and the efficiency changes slightly in narrow widow. The results of Fig. 5.4strongly indicate that the graphene/organics interface cannot form a strong mirror,which is indispensable to induce microcavity effect in OLEDs. In other words, the pos-sibility of enhancing the emission or efficiency is limited when graphene is used as atransparent electrode.

Fig. 5.5(a) shows the normalized angular emission profile of ITO and grapheneOLEDs. Experimental angular emission profiles obtained under different organicthicknesses are useful to gain insights on the microcavity and anode/HTL reflectanceon device level. The OLED structure relevant to Fig. 5.5 can be found elsewhere [18].The HTL thickness was varied to alter the microcavity condition. The internal interfer-ence or microcavity is a function of the length, which corresponds to the total organicthickness in a given angle. If the length in a specific angle matches the constructivecondition of a wavelength, the emission will be enhanced at the corresponding angle.First, we explore the ITO OLEDs. The OLEDs with HTL thicknesses of 70 and

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Figure 5.3 The simulated reflectance from organic layer and cavity enhancement factor ofindium tin oxide (ITO) and graphene electrode.Reproduced by permission from IEEE, H. Cho, J.W. Shin, N.S. Cho, J. Moon, J.H. Han, Y.D.Kwon, S. Cho and J.I. Lee, Optical effects of graphene electrodes on organic light-emittingdiodes, IEEE J. Sel. Top. Quantum Electron., 22 (2016) 7230237.


175 nm exhibited higher luminance in the normal incidence direction (q ¼ 0�) distri-butions. In contrast, OLEDs with HTL thicknesses of 105 and 140 nm exhibited higherluminance in the high angle direction (q > 40o). The variations in the luminancedistribution as a function of HTL thickness clearly show the presence of a microcavityeffect in the ITO OLEDs. The result also strongly indicates the intrinsic presence ofmircocavity in ITO OLEDs. In contrast, the angular luminance distributions ofgraphene OLEDs turned out to be not a strong function of the HTL thickness. Theluminance level varies within 10% in the angular range of 0�<q < 50o, giving distri-butions of Lambertian-like. Technically speaking, to achieve emission enhancement ina specific direction or emission angle, the cavity length must be an integer multiple ofthe wavelength of the emission material in the specific direction. The results of Fig. 5.5are in accordance with the calculated reflectance and Gcav(l). To induce microcavityeffect, mirror surfaces must be present. Because of the low reflectance of graphene/organic interface, it is fairly difficult to have microcavity effect and luminanceenhancement in a specific angle [24]. Fig. 5.5(b),(c) shows the 1931 Commissioninternationale del’éclairage (CIE) color coordinates of OLEDs with ITO or grapheneanodes as a variation of viewing angles. As can be readily noticed, the CIE x,ycoordinates of graphene OLED are hardly changing over the angular range0�<q < 70o. To be specific, the differences in coordinates do not exceed 0.01. Incontrast, the CIE x,y coordinates of ITO OLED tend to fluctuate. Briefly, the reason

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ITO anode (70 nm)

Graphene anode (2 nm)Graphene anode (1 nm)

Figure 5.4 Simulated EQEs of ITO anode and graphene film anode OLEDs as a function ofHTL thickness. Inset is the actual OLED structure used in simulations. EQE, external quantumefficiency; HTL, hole transport layer; ITO, indium tin oxide; OLED, organic lighteemittingdiode.Reproduced by permission from Elsevier, J. Moon, J.W. Shin, H. Cho, J.H. Han, N.S. Cho, T.J.Lim, S.K. Park, H.K. Choi, S.Y. Choi, J.H. Kim, M.J. Maeng, J. Seo, Y. Park, J.I. Lee, Technicalissues in graphene anode organic light emitting diodes, Diam. Relat. Mater. 57 (2015) 68e73.


of the fluctuation in the coordinates is due to the mismatch between the cavity lengthand the wavelength of the emitter. Details of the relevant optics can be found elsewhere[18]. The microcavity effect in OLEDs has pros and cons. Microcavity approach offersa facile method to tune the emission direction and efficiency optically. However, thepresence of microcavity degrades the angular EL spectral uniformity over a broademission angle. Also it distorts the original EL spectra. In the case of white OLEDs,in which at least two colors are mixed to obtain white light, augmentation or reductionof one color component can lead to failure in achieving acceptable white color temper-ature. The advantageous effects of microcavity are difficult to achieve in graphene

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Figure 5.5 (a) The normalized luminous distributions of OLEDs with indium tin oxide (leftportion) and graphene (right portion) anode. (b) The CIE x coordinates as variation of theviewing angle. (c) The CIE y coordinates as variation of the viewing angle. CIE, Commissioninternationale del’éclairage; ITO, indium tin oxide; OLED, organic lighteemitting diode.Reproduced by permission from IEEE, H. Cho, J.W. Shin, N.S. Cho, J. Moon, J.H. Han, Y.D.Kwon, S. Cho and J.I. Lee, Optical effects of graphene electrodes on organic light-emittingdiodes, IEEE J. Sel. Top. Quantum Electron., 22 (2016) 7230237.


OLED. However, the disadvantages of microcavity can be easily overcome by the useof graphene as a transparent electrode. In other words, graphene is useful to preservethe spectral quality of the OLED light.

5.3.3 Electrical issues

In this part, we explore the electrical issue. In OLEDs, holes and electrons recombinein the emission layer to produce photons. Thus, enhancing the current of electrons andholes toward the emission zone is of prime importance to ensure high-performanceOLEDs. To have an effective OLED, the electrical transport must not be hamperedby the presence of barriers at interfaces. Followings are important. First, chargemust be easily transported toward the organics from the electrodes. Second, to havehigh rate of exciton formation, it is desirable to spatially confine hole and electronsin the emissive layer. Because we are exploring graphene as a transparent electrode,we focus on the graphene/HTL interface and the energy alignment associated withthe thin-film stack. To obtain quantitative information, in situ surface analyses arevery useful [25]. The reported work function (f) of graphene is w4.6 eV [26]. Thisvalue is close to that of ITO (w4.8 eV). To explore the electrical characteristic, it isuseful to fabricate hole-only devices (HODs). HODs are a simplified nonradiativedevice which has a stack structure of anode/HTL/cathode. Fig 5.6(a) compares thecurrent density (J) and applied voltage (V) characteristics of the HODs. Our HODshave a stack structure of anode (graphene film or ITO)/HTL (TAPC, 100 nm)/cathode(Al, 100 nm). TAPC refers to 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane and iswidely used as HTL due to its high hole mobility (w10�4 cm2/V). To facilitate thecurrent, a hole injection layer (HIL) is frequently inserted between the anode andHTL. We inserted an HIL (10 nm) of 1,4,5,8,9,11-hexaazatriphenylene hexacarboni-trile (Hat-CN). The Hat-CN is a strong n-type material of lowest unoccupied molecularorbital (LUMO) level located around 5.5 eV (relative to the vacuum level). At a givenvoltage, the J of a graphene HOD is always lower than the J of ITO HOD. In addition,the difference in J between graphene and ITO HODs increases as the applied voltageincreases. The use of Hat-CN as the HIL in graphene HOD improves the J level signif-icantly. From OLED perspective, the results of Fig 5.6(a) strongly imply that the useHat-CN HIL can significantly contribute in enhancing the efficiencies of graphene filmanode OLEDs [16,28]. To elucidate the reason of improvement in J level by inserting aHat-CN HIL layer, we performed in situ ultraviolet photoelectron spectroscopy (UPS)surface analyses and constructed energy-level diagrams. Fig. 5.6 (b),(c) shows theenergy-level diagrams for graphene film/TAPC and graphene film/Hat-CN/TAPCinterfaces, respectively. The work functions (fs w 5.1 eV) of MLG turned out tobe not sensitive to the adjacent organic layer. The hole injection barriers (fhs) wereestimated using the energy difference between the Fermi level (Ef) and the onset ofHOMO peak. Details of the UPS methods in studying organic interfaces can be foundelsewhere [27]. The fhs of graphene film/TAPC and graphene film/Hat-CN/TAPCwere 0.65 and 3.03 eV, respectively. While a fh of 0.65 eV will not preclude holeinjection toward TAPC, fh of 3.03 eV will effectively block hole injection at theMLG/Hat-CN interface. Thus, enhancement in the J cannot be due to the lowered


energy barrier between graphene film and Hat-CN. Hat-CN is a high mobility strongn-type organic semiconductor, which can extract electrons from adjacent organics.Thus, the role of Hat-CN is not an energy barrier modifier but a charge generation layer(CGL) [27]. Under applied voltage, the electrons in the HOMO level of TAPC areextracted toward the Hat-CN LUMO level, which effectively contributes in holegeneration in the TAPC layer. In this picture, the energy barrier between graphenefilm and Hat-CN is irrelevant to the charge transport, but the energy difference betweenthe LUMO level of Hat-CN and the HOMO level of TAPC is. Fig. 5.6(d) illustrates theHat-CN as a CGL under applied voltage. Because UPS measurements cannot locate

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Figure 5.6 (a) The JV characteristics of indium tin oxide anode and graphene film anode hole-only devices. (b) The energy-level diagram of graphene/TAPC interface. (c) The energy-leveldiagram of graphene/Hat-CN/TAPC interface. (d) The charge transport taking place betweenthe LUMO of Hat-CN and HOMO of TAPC.Reprinted by permission from Elsevier, J.H. Kim, J. Seo, D.G. Kwon, J.A. Hong, J. Hwang,H.K. Choi, J. Moon, J.I. Lee, D.Y. Jung, S.-Y. Choi, Y. Park, Carrier injection efficiencies andenergy level alignments of multilayer graphene anodes for organic light-emitting diodes withdifferent hole injection layers, Carbon 79 (2014) 623e630.


the energy level of unoccupied energy levels, the LUMO level of Hat-CN cannot bepositioned exactly but only roughly suggested. However, because the clear enhance-ment in J has been observed by using a Hat-CN HIL, the LUMO level of Hat-CNand HOMO level of TAPC must be very close, which implies the possibility of Fermipinning of Hat-CN LUMO level. In this work, we have introduced Hat-CN to modifythe graphene film/HTL interface. Certainly, other HIL candidates exist and their rolesin relation to the J-V characteristics are not necessary same as we presented here[29,30]. As we have explored in the previous section, optical methods for improvinggraphene OLED is fairly limited. In other words, because of the insensitivity of cavityin graphene OLED, strategies for improving the electrical transport emerge as animportant topic. The significance may be summarized as following. Even the f valuesare similar, the J-V characteristics can be very different. Thus, one needs to be cautiousabout the organic layer directly adjacent to the graphene surface. In this course, thein situ UPS method is particularly useful because it is possible to obtain realisticquantitative energyelevel diagrams.

5.3.4 Absorption and light extraction issues

In the previous section, we gave a technical description on the limit of inducing micro-cavity in graphene OLEDs. In addition to this aspect, optical loss due to absorption is apart which needs attention [31,32]. The BeereLambert law dictates light intensity(I) attenuation as Iw exp(-2pk/lO)IO. Here, lO and IO refer to wavelength and theincident light intensity, respectively. Thus, light passing through a medium with biggerextinction coefficient (k) is expected to experience larger decay in I. The reported kvalue of graphene is 1.3, while the k of ITO is 0.05. The optical absorption is approx-imately 2.3% per one graphene layer. Thus, the light outcoupling is expected to reduceproportionally to the number of graphene layers [33].

The DT of the graphene films on the glass layer is shown in Fig.5.7(a). As the num-ber of graphene layer increases, the DT gradually decreases. The DTs of monolayerand four-layer graphene film were approximately 88.8% and 78.9% at 550 nm wave-length, respectively. Because the glass substrate used in the experiment has a DT valuethat is higher than 91% throughout the visible range, the transmittance is primarilyaffected by the graphene on the glass. To evaluate the influence of graphene’s absorp-tion on the OLED device level, graphene OLEDs with the variation of the graphenelayer as one, two, and four were fabricated. Fig. 5.7(b) presents their performancessuch as the angular distributions of L, the efficiencies and the EL spectra in normaldirections. In accordance to the DT measurement, the OLED with fewer graphenelayer numbers exhibits a higher L in all directions (Fig.5.7(b)). This result is inter-preted as the influence of absorption. Decreasing the number of graphene layersfrom four to one thus improves the EQE of 8.22% and LE of 7.96%, respectively(Fig. 5.7(c)). The EL main peak remained at a same position for all the OLED devices(Fig. 5.7(d)). The EL spectra bear a small shoulder at around 560 nm, which is a minortrace of microcavity effect. From the viewpoint of emission spectrum quality, theangular invariable feature of EL spectra is highly desirable. The reflectance of one,two, and four graphene layers on a glass substrate were obtained as 0.42%, 0.85%,


and 2.38% at 550 nm, respectively. Such low reflectance cannot induce noticeablemicrocavity effect in the device. Efficiency enhancement by microcavity methodand reducing the absorption are not quite effective [6,18]. Thus, there is a technicalneed to boost the efficiency of graphene anode OLED to that of OLEDs with typicaltransparent anodes.

In the context of limited light outcoupling, it is useful to introduce a light extractionlayer into graphene OLEDs. Light extraction enables the outcoupling of opticallyconfined light within the OLED device [34e38]. As an example of light extractionstructure, we use a random scattering layer (RSL). The RSL is positioned beneaththe graphene in the OLED (Fig. 5.8(a)). RSL can significantly induce internalscattering and thereby improve the light extraction efficiency by altering the light pathsand facilitating the outcoupling of the generated light from the device. Specifics of theRSL can be found elsewhere [39]. Briefly, the RSL consists of randomly distributed ofnanopillars and a high refractive index planarization layer. Nanopillars have height ofw500 nm and diameters of 300e500 nm, with a random distribution as shown in

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E (%

)

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. (lm

/w)

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Graphene-OLED1 layer2 layer4 layer

EL

inte

nsity

(A.U

.)Lu

min

ance

(cd/

m2 )

(a)

(c)

(b)

(d)

Figure 5.7 The effects of graphene layer numbers. (a) Direct transmittances. (b) Luminancedistribution of graphene anode OLEDs. (c) External quantum efficiencies. (d) EL spectra. EL,electroluminescence; EQE, external quantum efficiency; OLED, organic lighteemitting diode.Reproduced by permission from OSA, J.W. Shin, H. Cho, J. Lee, J. Moon, J.H. Han, K. Kim, S.Cho, J.I. Lee, B.H. Kwon, D.H. Cho, K.M. Lee, M. Suemitsu, N.S. Cho, Overcoming theefficiency limit of organic light-emitting diodes using ultra-thin and transparent grapheneelectrodes, Opt. Express 26 (2018) 617e626.


Fig. 5.8(b). Surface planarization is essential when using a nanostructure in an OLEDbecause the protruding morphology of the nanopillars can easily degrade the opera-tional stability of the OLED (Fig. 5.8(c)). High refractive index of planarization layeris desired to optically have light into the scattering components, which are thenanopillars.

To verify the light extraction capacity, we have fabricated OLEDs with and withoutRSL. We contrast OLEDs with transparent electrodes of conventional oxide (thick-ness: 100 nm) (IZO OLED) and single layer graphene (SLG OLED). To minimizethe effect of absorption, we intentionally use single layer graphene. Fig. 5.9(a) showsthe angular luminance (L) profiles of each OLED. The angular L profiles of planardevices, which have no RSL embedded, were dependent on the type of transparentanode, reflecting the microcavity effect. The microcavity effect gives angular distribu-tion of the L. In the case of planar SLG OLED, the microcavity is weaker than the IZOOLED due to the lower reflectance. Unlike the OLED with the IZO electrode, theOLED with the SLG electrode exhibited Lambertian-like emission profile. TheOLEDs with RSL show similar L distribution. This is due to the microcavity diminish-ing effect due to the RSL. Fig. 5.9(b) summarizes the EQE and the LE of the devices.The planar SLG- and IZO OLEDs show similar EQE and LE values. Introduction of ascattering layer minimizes the microcavity effect, which on the other hand alters theoptical traveling path and thereby contributes to the elimination of internal reflectionloss [40]. Thus, the use of the scattering layer increased the EQE and the LE values byalmost the same amount for both types of the devices. The scattering layer made theOLEDs less dependent on the cavity length and, hence, the organic thickness. Theseexperimental results demonstrate the possibility of achieving graphene OLEDs withefficiencies similar to those of conventional OLEDs with oxide anodes. In thiswork, we have suggested RSL as a structure to achieve comparable efficiencies.Certainly, there are many candidates which have to be explored to make grapheneOLED efficient [14,41].

Nanopillars

Height : 550nm

RS

L

Cathode

Organic layer

Planarization layer

Substrate

Graphene

Scattering structures

1000 nm 500 nm

Planarization layer

Nanostructure

(a) (b) (c)

Figure 5.8 (a) The cross-sectional schematics of graphene anode OLED equipped with an RSL.(b) The SEM image of nanopillars. (c) The SEM image of planarized nanopillars. OLED,organic lighteemitting diode; RSL, random scattering layer.Reproduced by permission from OSA, J.W. Shin, H. Cho, J. Lee, J. Moon, J.H. Han, K. Kim, S.Cho, J.I. Lee, B.H. Kwon, D.H. Cho, K.M. Lee, M. Suemitsu, N.S. Cho, Overcoming theefficiency limit of organic light-emitting diodes using ultra-thin and transparent grapheneelectrodes, Opt. Express 26 (2018) 617e626.


1600140012001000

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20

10

0

EQ

E (%

)

L.E

(lm

/W)

Lum

inan

ce (c

d/m

2 )

(a) (b)

Figure 5.9 (a) The effect of RLS on the luminance distributions of various OLEDs. (b) The external quantum efficiencies and luminous efficacies.EQE, external quantum efficiency; OLED, organic lighteemitting diode; RSL, random scattering layer.Reproduced by permission from OSA, J.W. Shin, H. Cho, J. Lee, J. Moon, J.H. Han, K. Kim, S. Cho, J.I. Lee, B.H. Kwon, D.H. Cho, K.M. Lee, M.Suemitsu, N.S. Cho, Overcoming the efficiency limit of organic light-emitting diodes using ultra-thin and transparent graphene electrodes, Opt. Express26 (2018) 617e626.

86Graphene

forFlexible

Lighting

andDisplays

5.4 Integration schemes for realizing large areagraphene electrode OLED panels

Broadly speaking, graphene is a suitable transparent electrode choice for OLED appli-cations. The values of sheet resistance and transmittance are acceptable. While thereexists some limitation in light extraction, wide angle spectral stability is a huge meritas a light source. However, graphene devices bearing products are virtually absent inthe commercial market. This not due to the properties of graphene itself but rather thanprocessing issues related to graphene. As a component, graphene has to be integratedinto devices and modules. To overcome the laboratory scale or proof-of-concept level,processing obstacles relevant to the graphene integration must be resolved [6,7].

In this part, processing hurdles relevant to the integration of graphene transparentelectrodes into OLED panels are addressed. OLED itself bears importance and poten-tial in active matrix (AM) display application, and commercial AM-OLED mobile andTV are already commercially available [42,43]. To expand the realm of grapheneOLEDs, it is important to achieve patterning methods which yield graphene pixelsthat are dimensionally correct and defect free [44,45]. We believe that this task is ofvery high priority and importance to witness graphene films as actual componentsin commercial OLED products. Without establishing a stable and reliable grapheneprocessing, graphene will remain in the realm of proof-of-concept level.

5.4.1 Actual examples of patterning hurdles

To figure out processing hurdles, we provide actual examples relevant to patterningand surface quality. Because graphene is very thin and carbonous, several patterningmethods can be readily proposed. In this regard, we examine patterning methods usingshadow mask with oxygen plasma and laser ablation. Fig. 5.10 shows the results ofapplying these patterning methods. In the shadow mask approach, the exposed regionof graphene films is etched off by oxygen plasma, while shadowed region remains,forming patterns. The edge region of oxygen plasmaepatterned graphene film bearsnoticeable irregularity (Fig. 5.10(a)). In addition, because plasma can leak underneaththe shadow mask, structural damage on the graphene film takes place (Fig. 5.10(b)).The Raman profile of patterned MLG clearly shows the presence of the D peak(w1350 cm�1). The D peak appears when the honeycomb bonding of graphene isbroken to result in structural damage. The presence of D peak signifies the deteriora-tion of graphene. The damaged surface will certainly limit the hole injection toward theHTL and result in lower OLED performance. The use of laser ablation in patterningdoes not yield an acceptable standard. Structural observation strongly points theproblem as the intense heat accompanied to laser ablation. First, the dimension isnot accurate. The dotted lines in Fig. 5.10(c) represent the edges, on which graphenefilms were supposed to terminate. However, significant narrowing of the MLG film canbe observed. Second, in the laser exposed regions, graphene films wrap on to formbundles (Fig. 5.10(d)). Such bundles can turn into particles, which are well knownto hamper device fabrication and operations.


To obtain well-defined patterns, photolithography methods can be suggested.Because of its predominant use in the display industry, this method bears significance.Actual examples after the photolithographic patterning process are shown in Fig. 5.11.Graphene films were observed to be peeled off (Fig. 5.11(a)). The surface of thegraphene film was also observed to be torn and crumbled (Fig. 5.11(b)). As a result,the emission image of fabricated OLED using such defective graphene failed to reachan acceptable standard (Fig. 5.11(c)). The emission uniformity is low, and many defec-tive regions are readily observable. Presumably, due to the nature of the transferprocess, graphene film will not necessarily lie globally flat on the surface. The unsuit-ability of direct application of photolithography seems to have originated from the vander Waalselike weak adhesion between the graphene films and the substrate.

Graphene film

Graphene filmGraphene bundle

10 μm

20 μm 2 μm

1000 1500 2000 2500 3000Raman shift (1/cm)

D

G2D

Patterned graphene film

Pristine graphene film

Cou

nts

(Arb

. uni

t)

(a)

(c) (d)

(b)

Figure 5.10 (a) An SEM image showing the periphery of oxygen plasmaepatterned graphenefilm. (b) The Raman spectra of pristine and plasma-patterned graphene film. (c) An SEM imageof laser patterned MLG film. (d) Periphery of IR laser-patterned MLG film.Reprinted by permission from Elsevier, J. Moon, J.W. Shin, H. Cho, J.H. Han, N.S. Cho, T.J.Lim, S.K. Park, H.K. Choi, S.Y. Choi, J.H. Kim, M.J. Maeng, J. Seo, Y. Park, J.I. Lee, Technicalissues in graphene anode organic light emitting diodes, Diam. Relat. Mater. 57 (2015) 68e73.


5.4.2 Overcoming the weak adhesion hurdle

As depicted in the actual examples of Fig. 5.11, the weak adhesion of graphene to itsglass substrate is not strong enough to have photolithographic patterning applicable.To improve the adhesion in a practical way, the “liquid bridging” concept is useful[46]. Liquid bridging provides a connection between two solid surfaces via liquidmolecules and forms an attractive force between interfaces [47,48]. This bridge, whosestability is dependent on the separation distance between the interfaces, is stable belowthe critical separation distance. In addition, the attractive force becomes stronger as thedistance decreases. In conjunction to photolithography, we use water as the medium forbridging. Water is chosen because of its wide usage in photolithography. The overallscheme is illustrated in Fig. 5.12. Because of the nature of the graphene transfer process,there exist numerous pores which weaken the adhesion (Fig. 5.12(a)) [49,50]. Thesepores can act as the defect seeds that may allow solution permeation during photolithog-raphy processes, resulting in unwanted peeling, tearing, and deterioration of graphenefilms. To improve the poor adhesion of graphene, we reduced the density of these poresby utilizing the liquid bridging. By allowing water to fill the air pores that exist betweengraphene and the substrate, liquid bridge can be formed (Fig. 5.12(b)). The water insidethe pores is in physical contact with every part of the internal surface of the graphenefilm. Owing to the mechanical compliance of graphene, on removal of the water, thegraphene film can be stretched to eliminate the air pores and achieve close physicalcontact between the substrate and the graphene film with stronger adhesion(Fig. 5.12(b),(c)). Water is removed by drying in a vacuum oven.

Fig. 5.13 summarizes the results of applying liquid bridging treatment to graphenefilms. The effective adhesion energy value of the graphene with pristine condition andthe liquid bridge treatment was improved from 0.9 � 0.14 Jm�2 to 1.71 � 0.21 Jm�2

(Fig. 5.13(a)). To extract quantitative adhesion information, a double cantilever beamfracture mechanics was used [51,52]. The improvement in adhesion can be interpretedas an increase in van der Waals force which is due to the very close contact between the

Graphene film Graphene film

5 μm 5 μm

(a) (b) (c)

Figure 5.11 Graphene damages from the photolithography pattering process. (a) Peeling andfolding of graphene film. (b) Crumbled graphene film. (c) Influence of graphene damage on theemission of organic lighteemitting diode.Reproduced by permission from IOP, J.W. Shin, J.H. Han, H. Cho, J. Moon, B.H. Kwon, S.Cho, T. Yoon, T.S. Kim, M. Suemitsu, J.I. Lee, N.S. Cho, Display process compatible accurategraphene patterning for OLED application, 2D Mater. 5 (2017) 014003.


Initial state

H2O removal

Dl-H2O dipping and liq. bridge formingGraphene film

Graphene layer

Air pores

Supporting layer

Graphene filmInterface (graphene/liquid)

H2O liq.bridge

(a) (b)

(c)

Figure 5.12 The concept of improvement in adhesion between graphene/substrate by liquidbridging. (a) Initial state. Air pores between the graphene film and the substrate. (b) Formationof liquid bridge by water permeation. (c) Removal the water by drying in vacuum.Reproduced by permission from IOP, J.W. Shin, J.H. Han, H. Cho, J. Moon, B.H. Kwon, S.Cho, T. Yoon, T.S. Kim, M. Suemitsu, J.I. Lee, N.S. Cho, Display process compatible accurategraphene patterning for OLED application, 2D Mater. 5 (2017) 014003.

Pristine Liquid bridge treatment

Histogram : adhesion energyAverage roughness

μm

Adh

esio

n en

ergy

(J m

2 ) Average roughness (nm)

2.4

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Untreated

Treated

(a) (b)

Figure 5.13 (a) The effect of liquid bridging on effective adhesion energy and surfaceroughness. (b) The AFM images of untreated and liquid bridgingetreated graphene films.Reproduced by permission from IOP, J.W. Shin, J.H. Han, H. Cho, J. Moon, B.H. Kwon, S.Cho, T. Yoon, T.S. Kim, M. Suemitsu, J.I. Lee, N.S. Cho, Display process compatible accurategraphene patterning for OLED application, 2D Mater. 5 (2017) 014003.


graphene film and supporting layer. The average surface roughness (Ra) was reducedfrom 2.33 to 0.52 nm (Fig. 5.13(b)). The result signifies the effectiveness of ourapproach to reduce the adhesion and surface roughness.

5.4.3 OLED panels with pixelated graphene electrodes

In this section, we demonstrate the technical possibility of accurate patterning ofgraphene films and the applications of pixelated graphene as transparent electrodesfor fully operational OLED panels. Fig. 5.14 illustrates the processing scheme. Inan effort to effectively investigate the technical issues associated with pixelatedgraphene electrodes, we adopted a simplified bottom emissionetype OLED structure.In this context, instead of the thin-film transistor array, addressing metal lines wereinstalled to control the on/off states of OLED pixels. We have formed a passivationlayer, metal addressing lines, and VIA/metal contact. We refer this structure as integra-tion panel. On the passivation layer, graphene films were transferred and liquidbridging treated and patterned. Then, OLED was formed using a thermal evaporationmethod. Here, we show the applicability of photolithography process, which yieldsclean and geometrically accurate pixel arrays.

Before moving to array fabrication, we examined the applicability of liquidbridging treatment compatibility to photolithography on unit device scale(2 mm � 2 mm). Fig. 5.15(a) shows two juxtaposed images of working OLEDs. Thereis no noticeable difference in the emission images between the nonpatterned grapheneand patterned graphene OLEDs. To pattern graphene films, we have combined liquidbridging and photolithography. The emission geometry is correct. No blemish spot canbe observed. The IVL characteristics and the EL spectra of both devices are almostidentical (Fig. 5.15(b)). Moreover, the Raman spectra reveal no occurrence ofgraphene surface deterioration after the photolithography process (Fig. 5.15(c)). Thetechnical significance may be summarized as the following. Thanks to the liquidbridging effect, graphene film remains physically stable on the support during thephotolithographic patterning process. Finally, it was able to achieve dimensionallycorrect patterns, clean surface, and high OLED performances.

Metal VIA Graphene OLED

Passivation

GlassAddressingmetal line

(1) Integration panel (5) OLED panel fabrication(4) Graphene patterning (2) Graphene transfer(3) Liq. bridging treatment

Passivation

Figure 5.14 Illustrated the processing scheme graphene-pixel electrode organic lighteemittingdiode (OLED) with addressing metal line.


We extended our graphene patterning approach on array scale (Fig. 5.16). Pixelswere designed to have a size of 100 mm � 200 mm. By comparing the layout(Fig. 5.16(a)) and the actual patterning (Fig. 5.16(b)), it can be readily noticed thatthe patterning is successful without undesired features. SEM images show patternswith precise geometry (Fig. 5.16(c)). The interface between the graphene and thepassivation layer is sharp (Fig. 5.16(d)). Also the absence of particle is noticeable.

Fig. 5.17 shows actual images of the integration panel with graphene pixels and anOLED panel. We have integrated a graphene pixel array of which active area is80 � 60 mm2. The basic architecture of individual unit is identical to that of Fig5.16(a). The backplane is composed of 33,000 graphene electrode pixels, connectedby each column and weaved into oddeeven pairs (Fig. 5.17(a)). This arraycorresponds to a resolution of 151.8 pixel/inch. The OLED was formed by a thermalevaporation method. We have used phosphorescent red (l ¼ 620 nm) and phosphores-cent yellow green (l ¼ 560 nm) as our emissive layer materials. The fabricated OLEDpanels were glass encapsulated and connected to a driving board. Our OLED is fullyoperational without noticeable defect (Fig. 5.17(b)). So far graphene has struggled toovercome the hurdle imposed by various processing difficulties. Many graphene appli-cations are not quite successful in going beyond the stage of “proof-of-concept.”

10–1

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. uni

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<Nonpatterned graphene-OLED> <patterned graphene-OLED>(a)

(b) (c)

Figure 5.15 (a) Actual emission images of graphene organic lighteemitting diode (OLED) withnonpatterned graphene film and patterned graphene film. (b) OLED characteristics and (c)Raman spectra of graphene film before and after photolithography patterning.Reproduced by permission from IOP, J.W. Shin, J.H. Han, H. Cho, J. Moon, B.H. Kwon, S.Cho, T. Yoon, T.S. Kim, M. Suemitsu, J.I. Lee, N.S. Cho, Display process compatible accurategraphene patterning for OLED application, 2D Mater. 5 (2017) 014003.


100 μmPAD

Gra

phen

e

VIA 5 μm

Graphene

PassivationLayer

(a) (b)

(d)(c)

Figure 5.16 (a) Layout of the organic lighteemitting diode panel. (b) Optical image ofpixelated graphene on integration panel. (c) SEM image of graphene pixels on integrationpanel. (d) SEM image of the periphery of patterned graphene. Comparison to Fig. 10 isrecommended.Partly reproduced by permission from IOP, J.W. Shin, J.H. Han, H. Cho, J. Moon, B.H. Kwon,S. Cho, T. Yoon, T.S. Kim, M. Suemitsu, J.I. Lee, N.S. Cho, Display process compatibleaccurate graphene patterning for OLED application, 2D Mater. 5 (2017) 014003.

10 mm

(a) (b)

Figure 5.17 (a) Fabricated integration panel on a glass substrate of 100 mm � 100 mm. (b)Addressable two-color organic lighteemitting diode module with pixelated graphene films astransparent electrodes.Partly reproduced by permission from IOP, J.W. Shin, J.H. Han, H. Cho, J. Moon, B.H. Kwon,S. Cho, T. Yoon, T.S. Kim, M. Suemitsu, J.I. Lee, N.S. Cho, Display process compatibleaccurate graphene patterning for OLED application, 2D Mater. 5 (2017) 014003.

The fine operation shown in Fig. 5.17 is an actual demonstration of graphene applica-tion on practically meaningful level. Because all processes implemented are compat-ible to the existing display fabrication, our approach can be readily applied tocommercial level.

Graphene film bears mechanical compliance and can be a very useful alternative tothe brittle ITO. In this context, graphene has been suggested as a material for flexibletransparent electrode. Fig. 5.18 shows a prototype of flexible OLED with patternedgraphene transparent electrodes. We have implemented our patterning process on apolyimide (PI) support, which is formed from a PI varnish on a glass substrate. PIis a preferred choice due to its high temperature (350�C) resistance and chemicalstability [53]. All components were patterned and formed on the PI. Later, the wholeassembly was detached from the glass substrate using a laser lift-off method [54,55].We are positive that the result of Fig. 5.18 can be extended to far larger area and bringonce dreamt display format into an existing reality [56].

5.5 Summary and future outlook

The overall objectivity of this chapter was to recall technical issues relevant to the useof graphene as a transparent electrode for OLED applications. The concept of graphenetransparent electrode might look trivial. However, its integration into a device andforming a graphene pixel array are not simple at all. Substantial science and technol-ogy are required to build a working graphene OLED panel. In the OLED panel case,we have extensively explored and emphasized the importance of substantiating a cleanand reliable graphene patterning process. Currently, virtually all properties of graphenehave been explored and understood on atomistic level. In addition, high-quality largearea graphene film is available. However, so far, graphene has not been successful inestablishing a solid technology platform on system level. From the viewpoint of gra-phene as a component in OLED, we see the problem as a matter of reliable integration

Figure 5.18 Operational flexible organic lighteemitting diode module with pixelated graphenefilms. The flexible substrate was detached from the support using a laser lift-off method.


and processing compatibility. In addition, to have graphene-containing devices flour-ishing, it is very important to find applications in which the uniqueness of graphene isindispensable. Otherwise, existing materials will quickly supersede graphene. As afuture outlook, we view graphene as an indispensable material for realizing flexibleelectronics, in which electrical conductivity, optical transparency, and mechanicalflexibility are required concurrently.

Acknowledgments

This work was supported by Ministry of Trade, Industry and Energy/Korea Evaluation Instituteof Industrial Technology (MOTIE/KEIT), research program “Development of basic and appliedtechnologies for OLEDs with Graphene.”

References

[1] A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (2007) 183e191.[2] S. Bae, H. Kim, Y. Lee, X. Xu, J.-S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H.R. Kim,

Y.I. Song, Y.-J. Kim, K.S. Kim, B. €Ozyilmaz, J.-H. Ahn, B.H. Hong, S. Iijima, Roll-to-rollproduction of 30-inch graphene films for transparent electrodes, Nat. Nanotechnol. 5(2010) 574e578.

[3] F. Bonaccorso, Z. Sun, T. Hasan, A. Ferrari, Graphene photonics and optoelectronics, Nat.Photonics 4 (2010) 611e622.

[4] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos,I.V. Grigorieva, A.A. Firsov, Electric field in atomically thin carbon films, Science 306(2004) 666e669.

[5] A.C. Ferrari, F. Bonaccorso, V. Fal’ko, K.S. Novoselov, S. Roche, P. Bøggild, S. Borini,F.H.L. Koppens, V. Palermo, N. Pugno, J.A. Garrido, R. Sordan, A. Bianco, L. Ballerini,M. Prato, E. Lidorikis, J. Kivioja, C. Marinelli, T. Ryh€anen, A. Morpurgo, J.N. Coleman,V. Nicolosi, L. Colombo, A. Fert, M. Garcia-Hernandez, A. Bachtold, G.F. Schneider,F. Guinea, C. Dekker, M. Barbone, Z. Sun, C. Galiotis, A.N. Grigorenko, G. Konstantatos,A. Kis, M. Katsnelson, L. Vandersypen, A. Loiseau, V. Morandi, D. Neumaier, E. Treossi,V. Pellegrini, M. Polini, A. Tredicucci, G.M. Williams, B.H. Hong, J.-H. Ahn, J.M. Kim,H. Zirath, B.J.V. Wees, H.V.D. Zant, L. Occhipinti, A.D. Matteo, I.A. Kinloch, T. Seyller,E. Quesnel, X. Feng, K. Teo, N. Rupesinghe, P. Hakonen, S.R.T. Neil, Q. Tannock,T. L€ofwander, J. Kinaret, Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, Nanoscale 7 (2015) 4598e4810.

[6] J. Moon, J.-W. Shin, H. Cho, J.-H. Han, N.S. Cho, T.J. Lim, S.K. Park, H.K. Choi, S.-Y. Choi, J.-H. Kim, M.-J. Maeng, J. Seo, Y. Park, J.-I. Lee, Technical issues in grapheneanode organic light emitting diodes, Diam. Relat. Mater. 57 (2015) 68e73.

[7] J.-W. Shin, J.-H. Han, H. Cho, J. Moon, B.-H. Kwon, S. Cho, T. Yoon, T.-S. Kim,M. Suemitsu, J.-I. Lee, N.S. Cho, Display process compatible accurate graphene patterningfor OLED application, 2D Mater. 5 (2017) 014003.

[8] D.S. Ginley, C. Bright, Transparent conducting oxides, MRS Bull. 25 (2000) 15e21.[9] X. Huang, Z. Yin, S. Wu, X. Qi, Q. He, Q. Zhang, Q. Yan, F. Boey, H. Zhang, Graphene-

based materials: synthesis, characterization, properties, and applications, Small 7 (2011)1876e1902.


[10] H. Wang, T. Maiyalagan, X. Wang, Review on recent progress in nitrogen-doped gra-phene: synthesis, characterization, and its potential applications, ACS Catal. 2 (2012)781e794.

[11] J. Hwang, H.K. Choi, J. Moon, T. Kim, J.-W. Shin, C.W.J. Joo, J.-H. Han, D.-H. Cho,J.W. Huh, J.-I. Lee, H.Y. Chu, Multilayered graphene anode for blue phosphorescentorganic light emitting diodes, Appl. Phys. Lett. 100 (2012) 133304e133307.

[12] J. Moon, J. Hwang, H.K. Choi, T.Y. Kim, S.-Y. Choi, C.W. Joo, J.-H. Han, J.-W. Shin,B.J. Lee, D.-H. Cho, J.W. Huh, S.K. Park, N.S. Cho, H.Y. Chu, J.-I. Lee, Large areaorganic light emitting diodes with multilayered graphene anodes, Proc. SPIE (2012) 8476.U1eU5.

[13] Y. Zhang, L. Zhang, C. Zhou, Review of chemical vapor deposition of graphene andrelated applications, Acc. Chem. Res. 46 (2013), 2329e233.

[14] T.-H. Han, Y. Lee, M.-R. Choi, S.-H. Woo, S.-H. Bae, B.H. Hong, J.-H. Ahn, T.-W. Lee,Extremely efficient flexible organic light-emitting diodes with modified graphene anode,Nat. Photonics 6 (2012) 105e110.

[15] C.E. Small, S.-W. Tsang, J. Kido, S.K. So, F. So, Origin of enhanced hole injection ininverted organic devices with electron accepting interlayer, Adv. Funct. Mater. 22 (2012)3261e3266.

[16] Y.-K. Kim, J.W. Kim, Y. Park, Energy level alignment at a charge generation interfacebetween 4, 40 -bis(N -phenyl-1-naphthylamino)biphenyl and 1,4,5,8,9,11-hexaazatriphenylene- hexacarbonitrile, 2009, Appl. Phys. Lett. 94 (2009) 063305.

[17] S.-Y. Kim, J.-J. Kim, Outcoupling efficiency of organic light emitting diodes employinggraphene as the anode, Org. Electron. 13 (2012) 1081e1085.

[18] H. Cho, J.-W. Shin, N.S. Cho, J. Moon, J.-H. Han, Y.-D. Kwon, S. Cho, J.-I. Lee, Opticaleffects of graphene electrodes on organic light-emitting diodes, IEEE J. Sel. Top. QuantumElectron. 22 (2016) 7230237.

[19] S.J. Kim, J. Ryu, S. Son, J.M. Yoo, J.B. Park, D. Won, E.-K. Lee, S.-P. Cho, S. Bae,S. Cho, B.H. Hong, Simultaneous etching and doping by Cu-stabilizing agent for high-performance graphene-based transparent electrodes, Chem. Mater. 26 (2014) 2332e2336.

[20] W. Gaynor, S. Hofmann, M.G. Christoforo, C. Sachse, S. Mehra, A. Salleo,M.D. Mcgehee, M.C. Gather, B. L€ussem, L.M. Meskamp, P. Peumans, K. Leo, Color inthe corners: ITO-free white OLEDs with angular color stability, Adv. Mater. 25 (2013)4006e4013.

[21] C.W. Joo, K. Lee, J. Lee, H. Cho, J.-W. Shin, N.S. Cho, J. Moon, Optical and structuralapproaches for improved luminance distribution and enhanced efficiency of organic lightemitting diodes, J. Lumin. 187 (2017) 433e440.

[22] C.-L. Lin, T.-Y. Cho, C.-H. Chang, C.-C. Wu, Enhancing light outcoupling of organiclight-emitting devices by locating emitters around the second antinode of the reflectivemetal electrode, Appl. Phys. Lett. 88 (2006) 081114.

[23] V. Bulovi�c, V. Khalfin, G. Gu, P. Burrows, D. Garbuzov, Weak microcavity effects inorganic light-emitting devices, Phys. Rev. B 58 (1998) 3730e3740.

[24] H. Cho, J.-M. Choi, S. Yoo, Highly transparent organic light-emitting diodes with ametallic top electrode: the dual role of a Cs2CO3 layer, Opt. Express 19 (2011)1113e1121.

[25] H. Ishii, K. Sugiyama, E. Ito, K. Seki, Energy level alignment and interfacial electronicstructures at organic/metal and organic/organic interfaces, Adv. Mater. 11 (1999)605e625.

[26] S.M. Song, J.K. Park, O.J. Sul, B.J. Cho, Determination of work function of grapheneunder a metal electrode and its role in contact resistance, Nano Lett. 12 (2012) 3887e3892.


[27] J.-H. Kim, J. Seo, D.-G. Kwon, J.-A. Hong, J. Hwang, H.K. Choi, J. Moon, J.-I. Lee,D.Y. Jung, S.-Y. Choi, Y. Park, Carrier injection efficiencies and energy level alignmentsof multilayer graphene anodes for organic light-emitting diodes with different hole in-jection layers, Carbon 79 (2014) 623e630.

[28] L. Zhang, F.-S. Zu, Y.-L. Deng, F. Igbari, Z.-K. Wang, L.-S. Liao, Origin of enhanced holeinjection in organic light-emitting diodes with an electron-acceptor doping layer: p-typedoping or interfacial diffusion? ACS Appl. Mater. Interfaces 7 (2015) 11965e11971.

[29] N. Koch, S. Duhm, J.P. Rabe, A. Vollmer, R.L. Johnson, Optimized hole injection withstrong electron acceptors at organic-metal interfaces, Phys. Rev. Lett. 95 (2005) 237601.

[30] K. Walzer, B. M€annig, M. Pfeiffer, K. Leo, Highly efficient organic devices based onelectrically doped transport layers, Chem. Rev. 107 (2007) 1233e1271.

[31] R.R. Nair, P. Blake, A.N. Grigorenko, K.S. Novoselov, T.J. Booth, T. Stauber,N.M.R. Peres, A.K. Geim, Fine structure constant defines visual transparency of graphene,Science 320 (2008) 1308.

[32] Y. Cai, J. Zhu, Q.H. Liu, Tunable enhanced optical absorption of graphene using plas-monic perfect absorbers, Appl. Phys. Lett. 106 (2015) 043105.

[33] J.-W. Shin, H. Cho, J. Lee, J. Moon, J.-H. Han, K. Kim, S. Cho, J.-I. Lee, B.-H. Kwon, D.-H. Cho, K.M. Lee, M. Suemitsu, N.S. Cho, Overcoming the efficiency limit of organiclight-emitting diodes using ultra-thin and transparent graphene electrodes, Opt. Express 26(2018) 617e626.

[34] T.-W. Koh, J.-M. Choi, S. Lee, S. Yoo, Optical outcoupling enhancement in organic light-emitting diodes: highly conductive polymer as a low-index layer on microstructured ITOelectrodes, Adv. Mater. 22 (2010) 1849e1853.

[35] R. Meerheim, M. Furno, S. Hofmann, B. L€ussem, K. Leo, Quantification of energy lossmechanisms in organic light-emitting diodes, Appl. Phys. Lett. 97 (2010) 253305.

[36] J.-W. Shin, D.-H. Cho, J. Moon, C.W. Joo, S.K. Park, J. Lee, J.-H. Han, N.S. Cho,J. Hwang, J.W. Huh, H.Y. Chu, J.-I. Lee, Random nano-structures as light extractionfunctionals for organic light-emitting diode applications, Org. Electron. 15 (2014)196e202.

[37] J. Moon, E. Kim, S.K. Park, K. Lee, J.-W. Shin, D.-H. Cho, J. Lee, C.W. Joo, N.S. Chom,J.-H. Han, B.-G. Yum, S. Yoo, J.-I. Lee, Organic wrinkles for energy efficient organic lightemitting diodes, Org. Electron. 26 (2015) 273e278.

[38] K. Lee, J.-W. Shin, J.-H. Park, J. Lee, C.W. Joo, J.-I. Lee, D.-H. Cho, J.T. Lim, M.-C. Oh,B.-K. Ju, J. Moon, A light scattering layer for internal light extraction of organic light-emitting diodes based on silver nanowires, ACS Appl. Mater. Interfaces 8 (2016)17409e17415.

[39] J.-W. Shin, J. Moon, D.-H. Cho, C.W. Joo, S.K. Park, J. Lee, J.-H. Han, N.S. Cho, H. Cho,J.-I. Lee, White organic light emitting diodes with a random scattering layer for an internallight extraction, ECS J. Solid State Sci.Technol. 5 (2016) R3126eR3130.

[40] J.-W. Kim, J.-H. Jang, M.-C. Oh, J.-W. Shin, D.-H. Cho, J. Moon, J.I. Lee, FDTD analysisof the light extraction efficiency of OLEDs with a random scattering layer, Opt. Express 22(2014) 498e507.

[41] J. Lee, T.-H. Han, M.-H. Park, D.Y. Jung, J. Seo, H.-K. Seo, H. Cho, E. Kim, J. Chung, S.-Y. Choi, T.-S. Kim, T.-W. Lee, S. Yoo, Synergetic electrode architecture for efficientgraphene-based flexible organic light-emitting diodes, Nat. Commun. 7 (2016) 11791.

[42] J.K. Jeong, J.H. Jeong, J.H. Choi, J.S. Im, S.H. Kim, H.W. Yang, K.N. Kang, K.S. Kim,T.K. Ahn, H.-J. Chung, M. Kim, B.S. Gu, J.-S. Park, Y.-G. Mo, H.D. Kim, H.K. Chung,12.1-Inch WXGA AMOLED display driven by indium-Gallium-Zinc oxide TFTs array,SID Symp. Dig. Tech. Pap. 39 (2008) 1e4.


[43] D.-U. Jin, J.-S. Lee, T.-W. Kim, S.-G. An, D. Straykhilev, Y.-S. Pyo, H.-S. Kim, D.-B. Lee, Y.-G. Mo, H.-D. Kim, H.-K. Chung, World-largest (6.5”) flexible full color topemission AMOLED display on plastic film and its bending properties, SID Symp. Dig.Tech. Pap. 40 (2009) 983e985.

[44] J.-Y. Hong, J. Jang, Micropatterning of graphene sheets: recent advances in techniques andapplications, J. Mater. Chem. 22 (2012) 8179e8191.

[45] Y. Xue, L. Zhu, H. Chen, J. Qu, L. Dai, Multiscale patterning of graphene oxide andreduced graphene oxide for flexible supercapacitors, Carbon 92 (2015) 305e310.

[46] J.T. Lim, H. Lee, H. Cho, B.-H. Kwon, N.S. Cho, B.K. Lee, J. Park, J. Kim, J.-H. Han, J.-H. Yang, B.-G. Yu, C.-S. Hwang, S.C. Lim, J.-I. Lee, Flexion bonding transfer ofmultilayered graphene as a top electrode in transparent organic light-emitting diodes, Sci.Rep. 5 (2015) 17748.

[47] P. Petkov, B. Radoev, Statics and dynamics of capillary bridges, Colloids Surf., A 460(2014) 18e27.

[48] B.N. Persson, Capillary adhesion between elastic solids with randomly rough surfaces,J. Phys. Condens. Matter 20 (2008) 315007.

[49] L. Gao, G.-X. Ni, Y. Liu, B. Liu, A.H.C. Neto, K.P. Loh, Face-to-face transfer of wafer-scale graphene films, Nature 505 (2014) 190e194.

[50] Y. Chen, X.-L. Gong, J.-G. Gai, Progress and challenges in transfer of large-area graphenefilms, Adv. Sci. 3 (2016) 1500343.

[51] T. Yoon, W.C. Shin, T.Y. Kim, J.H. Mun, T.-S. Kim, B. Cho, Direct measurement ofadhesion energy of monolayer graphene as-grown on copper and its application torenewable transfer process, Nano Lett. 12 (2012) 1448e1452.

[52] D. Sen, K.S. Novoselov, P.M. Reis, M.J. Buehler, Tearing graphene sheets from adhesivesubstrates produces tapered nanoribbons, Small 6 (2010) 1108e1116.

[53] H. Lim, W.-J. Cho, C.-S. Ha, S. Ando, Y.-K. Kim, C.-H. Park, K. Lee, Flexible organicelectroluminescent devices based on fluorine-containing colorless polyimide substrates,Adv. Mater. 14 (2002) 1275e1279.

[54] R. Delmdahl, R. P€atzel, J. Brune, Large-area laser-lift-off processing in microelectronics,Phys. Procedia 41 (2013) 241e248.

[55] F. Li, E.C.P. Smits, L.V. Leuken, G.D. Haas, T.H. Ellis, J.L.V.D. Steen, A. Tripathi,K. Myny, M. Ameys, S. Schols, P. Heremans, G. Gelinck, Integration of flexibleAMOLED displays using oxide semiconductor TFT backplanes, SID Symp. Dig. Tech.Pap. 45 (2014) 431e434.

[56] J.-H. Ahn, B.H. Hong, Graphene for displays that bend, Nat. Nanotechnol. 9 (2014)737e738.

Further reading

[1] N. Li, S. Oida, G.S. Tulevski, S.-J. Han, J.B. Hannon, D.K. Sadana, T.-C. Chen, Efficientand bright organic light-emitting diodes on single-layer graphene electrodes, Nat. Commun.4 (2013) 2294.


Graphene-based buffer layers forlight-emitting diodes 6Quyet Van Le 1, Soo Young Kim 2

1Institute of Research and Development, Duy Tan University, Da Nang, Vietnam;2Department of Materials Science and Engineering, Korea University, Seongbuk-gu, Seoul,Republic of Korea

6.1 Introduction

Buffer layers play a crucial role in light emitting diodes (LEDs), promoting charge in-jection by reducing the energy barrier between the electrodes and the active layers. Thefrequently used electrode, indium tin oxide (ITO), exhibits a work function (WF) of4.1e4.7 eV; this is much lower than the ionization energy of organic materials,reducing the possible device performance [1]. To overcome this challenge, a thin layerof poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (WFw5.2 eV) is sandwiched between the ITO and emissive layers. However, the use ofPEDOT:PSS as a hole injection layer (HIL) severely reduces the stability of LEDdevices as a result of its highly acidic and hygroscopic nature. Recently, a numberof inorganic materials have been effectively employed to replace PEDOT:PSS; exam-ples are MoO3 [2], V2O5 [3], WO3 [4], WS2 [5e8], MoS2 [5,9,10], TaS2 [5,11], andgraphene oxide (GO) [12e14]. Among these, GO appears to be the most promising asa result of its tunable WF, high stability, high charge-carrier mobility, low cost, andsolution processability.

This chapter reports on current progress in the application of GO as an HIL inLEDs. First, the structure, electrical, and optical properties are described. Next, theuse of GO as the buffer layer in various LEDs, including organic LEDs (OLEDs),polymer LEDs (PLEDs), and quantum dot LEDs (QLEDs) is reviewed. The effectof the structure and thickness of the GO on the performance of the LEDs as well asthe HIL mechanism of GO are clarified. In addition, the combination of GO with othermaterials to form composite HILs is discussed.

6.2 Graphene oxide buffer layer

6.2.1 Structures, properties, and synthesis

GO is generally derived from graphite through an oxidation and exfoliation process inan organic solvent or water [15,16]. GO contains various functional groups such ashydroxyl, epoxy, carboxy, carbonyl, phenol, lactone, and quinine [17]. However,the precise structure of GO is still under debate because of its complexity [15]. Severalstructural models of GO have been proposed, as shown in Fig. 6.1aee [18]. Hofmann

Graphene for Flexible Lighting and Displays. https://doi.org/10.1016/B978-0-08-102482-9.00006-XCopyright © 2020 Elsevier Ltd. All rights reserved.

https://doi.org/10.1016/B978-0-08-102482-9.00006-X

Figure 6.1 Previous structure models of GO: (a) Hofmann, (b) Ruess, (c) Scholz-Boehm, (d)Nakajima-Matsuo, (e) Lerf-Klinowski. (f) Aberration-corrected TEM image of a single sheetof suspended GO. The scale bar is 2 nm. Expanded view (A) shows (left to right) a 1 nm2

enlarged oxidized region of the material; a proposed atomic structure for this region (carbonatoms in gray and oxygen atoms in red); the average of simulated TEM images (see SupportingInformation) of the proposed structure and of another structure where the position of oxidativefunctionalities has been changed. Expanded view (B) focuses on the white spot in the graphiticregion. This spot moved within the graphitic region but was stationary for three frames (6 s) ata hydroxyl position (left portion of expanded view (B)) and for seven frames (14 s) at a (1,2)epoxy position (right portion of expanded view (B)). The ball-and-stick figures below themicroscopy images represent the proposed atomic structure for such functionalities. Thesimulated TEM image for the suggested structure (see Supporting Information) agrees wellwith the TEM data. Expanded view (C) shows a 1 nm2 graphitic portion using an exit-planewave reconstruction of a focal series of GO images and the atomic structure in this region.(e) Reproduced with permission T. Szab�o, O. Berkesi, P. Forg�o, K. Josepovits, Y. Sanakis, D.Petridis, I. Dék�any, Evolution of surface functional groups in a series of progressively oxidizedgraphite oxides. Chem. Mater. 18 (2006) 2740e2749 Copyright 2006 American ChemicalSociety. (f) Reproduced with permission K. Erickson, R. Erni, Z. Lee, N. Alem, W. Gannett,A. Zettl, Determination of the local chemical structure of graphene oxide and reduced grapheneoxide. Adv. Mater. 22 (2010) 4467e4472, Copyright 2010 Wiley-VCH.


and Holst’s structure is composed of epoxy functional groups that are widely distrib-uted on a basal plane of graphite having an sp2 hybridized configuration, whereasRuess’s consists of hydroxyl groups and an sp3 hybrid system. Later, Scholz andBoehm proposed a model with a corrugated backbone containing regular quinoidalspecies instead of epoxide and ether groups [15]. Nakajima and Matsuo presented amodel with a stage 2 graphite intercalation based on poly(carbon monofluoride),(CF2)n [19]. More recently, based on NMR studies, Lerf and Klinowski introduceda widely used model that describes GO as a random distribution of flat aromatic re-gions (unoxidized benzene rings) and wrinkled regions of alicyclic 6-membered rings[20]. The atomic-scale features of GO were recorded for the first time by Ericksonet al., using high-resolution transmission electron microscopy (HRTEM), as shownin Fig. 6.1f [21]. It can be observed from the high contrast HRTEM image that GOcontains three different types of region: disordered regions, holes, and graphitic re-gions. It was proposed by the authors that the holes which appeared on the GO sheetsoriginated from oxidation and exfoliation, releasing CO and CO2, whereas thegraphitic region results from the incomplete oxidation of the basal plane. It was alsosuggested that the disordered regions of the basal plane originate in the high-densityoxygen-containing functional groups.

GO exhibits superior thermal, mechanical, electrical, and optical properties as aresult of its unique 2D structure, which is embedded with various functional groups.The electrical properties of GO significantly depend on its structure and level of oxi-dization. Specifically, GO is an insulator with a large energy bandgap and high Rs (upto 12 U/sq) arising from a large degree of sp3 hybridization [22e24]. However, thesheet resistance of GO can be decreased through a thermal or chemical reduction treat-ment [25,26]. As a result, GO can be tuned from an insulator to a semiconductor or agraphene-like semimetal [22,27e32]. A typical demonstration of the tunable-resistance behavior of GO is shown in Fig. 6.2aed.

The optical properties of GO have also been intensively studied [33e35]. It wasdiscovered that GO is fluorescent over a wide range of wavelengths, from the ultravi-olet through the visible and into the near-infrared region, as a result of heterogeneousatomic and electronic structures [36]. Differing from other semiconductors, where thefluorescence results from band-edge transitions, in GO it originates from recombina-tion of electronehole pairs in localized electronic states in which sp2 domains areisolated within the carboneoxygen sp3 matrix [37]. The relation between fluorescence,conductivity, and absorbance and GO reduction, which was reported by Eda et al., isshown in Fig. 6.2eeg [37]; the level of GO reduction was controlled through the expo-sure time to hydrazine vapor. The IeV characteristics of GO-based devices for variousreduction times are shown in Fig. 6.2e. The low bias current observed in 2 minereduced GO is drastically increased with 5 min reduction, indicating a significantchange in conductivity. The insulating properties of 2 minereduced GO are furtherconfirmed in Fig. 6.2f. From Fig. 6.2g, it can be seen that the absorption and PL are

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markedly increased after around 20 s of exposure as a result of the increased number oflocalized sp2 sites. It should be noted that the energetic coupling between these sitesremains small in the first stage of reduction. However, in the later stages (after5 min of exposure), the PL of GO is abruptly reduced by the increased interconnectiv-ity between sp2 sites; this facilitates the hopping of excitons to nonradiative combina-tion centers, resulting in PL quenching.

Graphite powder is the most common starting material for synthesizing GO. Theconversion of graphite powder to GO can be carried out by various techniques,such as Brodie’s [38], Hofmann’s [39], and Hummers’s methods [40]. Briefly, thegraphite power is chemically reacted with a strong acid such as HNO3, H2SO4, orHCl, with subsequent intercalation employing an alkali metal compound such asKClO3, KMnO4, or NaNO3. Fig. 6.3 illustrates the oxidation and intercalationprocesses used to obtain GO from graphite, based on the Hummers’s method [41].

6.2.2 GO buffer layer in optoelectronics

The use of GO as a buffer layer in optoelectronic devices was presented by Li et al. in2010 [42]. Typically, a thin layer of GO is sandwiched between ITO and an activelayer consisting of a blend of poly(3-hexylthiophene) and the fullerene derivative,phenyl-C61-butyric acid methyl ester, in an organic solar cell. The use of GO as aninterlayer drastically increases the power conversion efficiency of the solar cell from1.8% to 3.5%, which is then comparable to that of a PEDOT:PSS-based device(3.6%), and indicates a role for GO as a buffer layer. In 2011, Zhong et al. successfullyincorporated GO as a HIL in OLEDs [43]. Specifically, the GO-based OLEDs showeda current efficiency of 23 cd A�1 and a power efficiency of 14 lm W�1, which aresignificantly higher than those of PEDOT:PSS-based devices (15 cd A�1 and 11 lmW�1, respectively). The device structure and device characteristic of the GO-basedOLED are shown in Fig. 6.4bef. To avoid the formation of dark spots and pixelshrinkage over time caused by absorbed water on the GO, as well as by restacking

resistance (red solid curve; left-axis) and temperature profile (dotted blue; right-axis), asfunctions of time. The resistance at point A (temperature 172�C, point 1 in the first heatingcycle) is equal to that at point B (125�C) in the second heating cycle. A similar behavior wasobserved in each subsequent cycle, with the three sample resistances (for the same temperature,roughly 125�C) forming a decreasing sequence. The resistance sequence is indicated by thethree dotted horizontal arrows; temperatures are indicated by boxes on the heating curve.(e) IeV and (f) transfer characteristics of individual GO sheet devices at different stages ofreduction (duration of exposure to hydrazine is noted in the legend). (g) Summary plot for aGO thin film, showing the maximum PL intensity (circles; left axis), absorbance at 550 nm(triangles; right axis), and current at 1 V (squares; right axis), as functions of reduction time.(d) Reproduced with permission I. Jung, D.A. Dikin, R.D. Piner, R.S. Ruoff, Tunable electricalconductivity of individual graphene oxide sheets reduced at “low” temperatures, Nano Lett. 8(2008) 4283e4287, Copyright 2008 American Chemical Society. (g) Reproduced withpermission G. Eda, Y.Y. Lin, C. Mattevi, H. Yamaguchi, H.A. Chen, I.S. Chen, C.W. Chen, M.Chhowalla, Blue photoluminescence from chemically derived graphene oxide, Adv. Mater.22 (2010) 505e509, Copyright 2010 Wiley-VCH.

=


of the GO sheets during the preparation process, a surface grafting with 4-(octoxy)ben-zenediazonium tetrafluoroborate was applied. The phenylated GO (P-GO) wasachieved by simply treating GO with 4-(octoxy)benzenediazonium tetrafluoroborate)for 10e60 min. It should be noted that the functionalization of GO depends signifi-cantly on the reaction time. The solubility of P-GO in dimethylformamide increasesfrom 0.6 to 1.0 and 1.8 mg mL�1 as the reaction time is increased from 10 to 30and 60 min, respectively. A single layer of P-GO can be easily produced by spincoating (Fig. 6.4a). This research also shows that the two-layer P-GOebased deviceexhibits better performance than the one-layer device. The mechanism by which aGO HIL improves the device was later discovered by Lee et al. in 2012 [44]. The struc-ture and energy band alignment of a device employing poly(phenylvinylene): superyellow (SY), (Merck Co., Mw ¼ 1,950,000 g mol�1) as an emissive layer (thickness150 nm) is displayed in Fig. 6.5a. It is reported that GO containing epoxy andepoxy-hydroxyl groups, which disturb the sp2 conjugation in the hexagonal plane,is an insulator with a large optical bandgap of 3.6 eV [42]. To clarify the role ofGO, reduced GO (rGO) and rGO-based devices were also fabricated. The removalof the functional groups in rGO effectively restores the sp2 conjugation and therebyreduces the optical bandgap to 1.15 eV [45]. The device performance is optimizedwith a GO thickness of 4.3 nm, showing a maximum luminance (Lmax) and an externalquantum efficiency (EQE) of 39,000 cd m�2 and 6.7%, respectively, which are supe-rior to those of a PEDOT:PSS-based device (Lmax ¼ 33,800; EQE ¼ 3.5%)(Table 6.1). In contrast, the performance of rGO-based PLEDs is drastically lower,with an Lmax of 8300 cd m�2 and EQE of 1.8%. The turn-on voltage is found to beindependent of the HIL used. To explain these phenomena, the WFs of PEDOT:PSS, GO, and rGO were measured by ultraviolet photoelectron spectroscopy; the

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Figure 6.3 Illustration of GO preparation, based on a newly improved Hummers’s method.Reproduced with permission H. Yu, B. Zhang, C. Bulin, R. Li, R. Xing, High-efficient synthesisof graphene oxide based on improved Hummers method, Sci. Rep. 6 (2016) 36143, Copyright2016 Nature Publishing Group.


WF of GO (4.89 eV) is lower than that of PEDOT:PSS (4.95 eV). This suggests thatthe electron-blocking behavior of GO is more efficient than that of PEDOT:PSS andrGO; this was confirmed using PLEDs with the structure shown in Fig. 6.5a. It isseen in Fig. 6.5b and c that the PLEDs with the device structure ITO/SY/SPR-001/LiF/Al (SPR-001 denotes a red-emitting polymer) display red emission and thosewith ITO/SPR-001/GO/SY/LiF/Al exhibit green emission. In contrast, the device

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Figure 6.4 (a) AFM images of phenylated GO (P-GO) (sample B) films spin coated onto freshlycleaved mica. (b) Structure of the OLED device. (c) The CeVeL characteristics of the OLEDwith the P-GO layer as a hole-injecting buffer layer. (d) Power efficiency. (e) Current effi-ciency. (f) Luminance.Reproduced with permission Z. Zhong, Y. Dai, D. Ma, Z.Y. Wang, Facile synthesis of organo-soluble surface-grafted all-single-layer graphene oxide as hole-injecting buffer material inorganic light-emitting diodes. J. Mater. Chem. 21 (2011) 6040e6045, Copyright 2011 TheRoyal Society of Chemistry.


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ITOITO/PEDOT:PSSITO/GO(i)(2.0nm)ITO/GO(ii)(2.6nm)ITO/GO(iii)(4.3nm)ITO/GO(iv)(5.2nm)

100

10–1

10–2

10–3

PL

inte

nsity

(a.u

.)

–1 0 1 2 3 4 5 6 7 8Time (ns)

0.5

0.4

0.3

0.2

0.1

0.0Quartz GO PEDOT:PSS rGO

Type of substrate

Exc

iton

lifet

ime

(ns)

IRFQuartz/SY/(10nm)

Quartz/GO/SY(10nm)Quartz/GO/SY(10nm)

(a)

(b)

(d)

(f)

(c)

(e)

(g)

Figure 6.5 (a) Device schematics of PLEDs with a GO layer; chemical structure of the GO;schematic energy diagrams of the flat band conditions of PLEDs with a hole-transport layer(HTL) (rGO, GO, and PEDOT:PSS). (b,c) Electron-blocking properties of GO. Schematicenergy diagrams of the devices used to evaluate the electron-blocking behavior of GO: (b)


ITO/SPR-001/(GO or rGO)/super yellow (SY)/LiF/Al. (c) Electroluminescence (EL) spectraof diverse device configurations. (d) Photoluminescence (PL) spectra of SY films on quartz,PEDOT:PSS/quartz, rGO/quartz, and GO/quartz substrates. (e) Transmittance of HTLs(PEDOT:PSS, GO) on ITO, measured using a UV-Vis spectrometer. (f) Time-resolved PLsignal of SY, PEDOT:PSS/SY, rGO/SY, and GO/SY films, measured by time-correlated singlephoton counting (TCSPC). (g) Exciton lifetime of SY, PEDOT:PSS/SY, rGO/SY, and GO/SYfilms.Reproduced with permission B.R. Lee, J.-W. Kim, D. Kang, D.W. Lee, S.-J. Ko, H.J. Lee, C.-L.Lee, J.Y. Kim, H.S. Shin, M.H. Song, Highly efficient polymer light-emitting diodes usinggraphene oxide as a hole transport layer, ACS Nano 6 (2012) 2984e2991, Copyright 2008American Chemical Society.

Table 6.1 Device performance of PLEDs with PEDOT:PSS, GO, and rGO as HILs [44].

Deviceconfiguration

Maximumluminance(cd mL2)(at voltage)

Maximumluminousefficiency (cdAL1) (atvoltage)

Maximumpowerefficiency(lm WL1)(at voltage

MaximumEQE (%)(at voltage)

Turnonvoltage(V)

ITO/SY/LiF/Al

700 1.4 (8.4 V) 0.6 (6.6 V) 0.6 (8.4 V) 2.8

ITO/PEDOT:PSS/SY/LiF/Al

33,800(12.6 V)

8.7 (9.6 V) 3.9 (5.2 V) 3.5 (9.2 V) 1.8

ITO/GO[2.0 nm]/SY/LiF/Al

31,400(12.4 V)

8.8 (9.4 V) 4.2 (5.0 V) 3.3 (8.2 V) 1.8


35,100(12.0 V)

14.3 (8.6 V) 6.6 (5.4 V) 5.0 (8.4 V) 1.8


39,000(10.8 V)

19.1 (6.8 V) 11.0 (4.4V) 6.7 (6.8 V) 1.8


28,500(11.2 V)

13.9 (7.4 V) 8.6 (4.0 V) 5.0 (7.4 V) 1.8

ITO/rGO[4.3 nm]/SY/LiF/Al

8300(13.0 V)

19.1 (6.8 V) 11.0 (4.4V) 6.7 (6.8 V) 1.8

=


with the structure ITO/SPR-001/rGO/SY/LiF/Al shows red emission as a result ofinsufficient electron blocking. These results suggest that electron transport fromSPR-001 to SY, or SY to SPR-001, is efficiently blocked by GO. The enhancementof emission from GO-based PLEDs is further verified by PL measurement, shownin Fig. 6.5deg. Briefly, the PL intensity and exciton lifetime for an SY layer are higheron GO than on PEDOT:PSS or on rGO. Thus, the use of GO improves the hole injec-tion and electron blocking, increasing the electronehole recombination rate in theemissive layer and improving the device performance.

Since the performance of GO as a buffer layer in OLEDs was initially demon-strated, many efforts have been made toward further improvement. For example,Yang et al. found that GO used as a HIL in OLEDs showed better performance afterlight reduction under water vapor at 200�C, compared to pristine GO [46]. Yang et al.also compared the performance of rGO fabricated from a LangmuireBlodgett (rGOLB) film and by spin coating. The device performance for various HILs is summarizedin Table 6.2. The rGO-based OLED exhibits a maximum luminance around 2 timeshigher than the GO-based version and is increased a further 1.5 times by using anrGO LB film. The LangmuireBlodgett film has a better arrangement of the nano-sheets, which increases the surface coverage on the ITO. These data from Yanget al. suggest the important roles of the oxidation level and nanosheet arrangementof the GO film for high device performance. Lee et al. functionalized GO with(3-glycidyl oxypropyl)-trimethoxysilane (GPTMS) and triethoxymethylsilane(MTES), increasing the WF from 4.8 eV to 4.9 and 5.0 eV, respectively. Themaximum luminance efficiencies of PGTMS-GO and MTES-GO were measured tobe 13.91 and 12.77 cd A�1, respectively, which are higher than that for the PEDOT:PSS-based device (12.34 cd A�1) [13].

The role of GO in QLEDs has also been demonstrated. For instance, Park et al. usedPVK/GO/V2O5-x as an efficient HIL in a QLED with the structure PVK/GO/V2O5-x/

Table 6.2 Performance of OLEDs with the structure ITO/HIL/TPD/Alq3/Al, using variousHILs [46].

HIL

Driving voltage(V) for100 mA cmL2

Maximumluminance (cd mL2,12 V)

Luminanceefficiency (cd AL1,12 V)

PEDOT:PSSspin-coatingfilm

11.7 4435 2.9

GO spin-coatingfilm

12.1 2253 1.9

rGO spin-coating film

11.4 4107 3.0

rGO LB film 9.6 6232 3.8


Type A: w/ GO layer Type B: w/ GO layer

(–)Al cathode

ZnO NPQDPVKGOV2O5

ITO

Glassanode

Glass

(+) (+)

(–)

1.2

1.0

0.8

0.6

0.4

0.2

500 550 600 650 700 750Wavelength (nm)

Rel

ativ

e E

L in

tnsi

ty

Rel

ativ

e E

L in

tnsi

ty

0.025

0.020

0.015

0.010

0.005

0.000500 550 600 650 700 750

Wavelength (nm)

(i) (iii) (iv)=0.41 eVΔ –0.15 eV

5.12 eV

0.2 ev

4.97 eV 4.63 eV

1.15 ev0.65 ev

–0.34 eVEvac

EFermi

φ=4.71 eV

Δh=0.16 ev

Sub-gaostates

E =2.8 eV=0.49 eV

Δh1

Δh2=0.50 eV Δh3=0.75 eV

ITO V O GO PVK1.36 eV

QDs(core/shell)NPs

ZnO Al

Type A

e–

h+

Type B (i) (v)

–1.23 eV

3.89 eV

0.16 eV

Δh0=0.16 eV

Δh3, =1.74 eV

ITO V O PVK QDs(core/shell)

ZnONPs

Al

PVK:Lying-down orientation(π–π stacking)GO

Cross-linking(V-O-VC)

Monomeric(and polymeric)

V O

O

O

O OO

O OO

O

OOO OO OO

OOO

O

V VV

OHOH OH

OHOHHydrogenbonding

PVK: Tilted or stand-up orientated

OxygenVanadium

Hydrogenbonding

PVK

V 3d and PVK C 2p wave function hybridization

V O

(a)

(b)

(d)

(f)

(e)

(g)

(c)

O

Figure 6.6 QD-LEDs with and without a GO interlayer. (a) Schematic illustrations depictingstructures of QD-LEDs with and without the GO interlayer. EL spectra of QD-LEDs (b) withGO interlayer and (c) without GO interlayer at the same applied bias voltage of 6.0 V. Insets in(b) and (c) are photographs of red emission from types A and B QD-LEDs, respectively, alsotaken under the same dark room and camera conditions. Electronic energy level alignments of(d) type A and (e) type B QD-LEDs derived from UPS spectra. The insertion of the GO layer


(CdSe/CdZnS core/shell QDs)/(ZnO nanoparticles)/Al, as shown in Fig. 6.6a [47].(PVK designates poly(vinylcarbazole), used as a hole-transport layer (HTL)). TheQLEDs with a GO-based HTL yielded maximum luminance, luminous efficiency,and EQE several times (up to 10 times) higher than those without GO. The EL spectraof QLEDs with and without GO is shown in Fig. 6.6b and c. The improvement in de-vices employing GO results from the reduction of the hole barrier between the QDsand the HIL from 1.74 to 0.75 eV, as seen in the downward shift of the PVK energylevels (Fig. 6.6dee).

6.3 Graphene-based composite buffer layer

Apart from the stand-alone use of GO as a HIL, the combination of GO with other ma-terials such as 2-dimensional transition metal dichalcogenides (2D-TMDs) andPEDOT:PSS has also been actively investigated. The use of a MoS2 nanosheet(NS)/GO composite as a HIL was demonstrated for the first time by Park et al. [12].Typically, MoS2 NSs are synthesized using the lithium intercalation method; theresulting NSs were mixed with GO in the ratios 10:0, 8:2, 6:4, 4:6, and 2:8. The fabri-cation scheme, device structure, and device performance are shown in Fig. 6.7aee.The highest power efficiency among (MoS2 NS)/GO-based OLEDs, 3.77 lm W�1,is found with the (MoS2 NS):GO ratio of 6:4. However, stand-alone GO as a HILshowed the best device performance, with a power efficiency of 4.94 lm W�1, as aresult of the high surface coverage and the GO WF. To improve the device perfor-mance, the uniformity and WF of (MoS2 NS)/GO will need to be improved. Other2D-TMDs such as WS2, TaS2, and TiS2 may be investigated as 2D-TMDs/GOHILs. For comparison, the use of PEDOT:PSS composite as a HIL significantlyimproves the device performance by increasing the maximum luminance from 647to 725 cd m�2 while decreasing the turn-on voltage from 5.35 to 3.65 V [48] [refer-ence]. The improvements in PEDOT:PSS-based devices originate in the increasedconductivity of the PEDOT:PSS layer on GO incorporation. However, excess GOresults in decreased conductivity, transmittance, andWF leading to poor device perfor-mance (Fig. 6.8aec). Therefore, the mixing ratio between PEDOT:PSS and GO is animportant factor in obtaining high-efficiency OLEDs; the performance dependence isshown in Fig. 6.8def and Table 6.3. Similarly, Diker et al. improved the luminous

downshifts the electronic energy levels of poly(vinylcarbazole) (PVK), resulting in a reducedhole injection barrier at the PVK/QD interface. The boxed panels are schematics depicting theheterointerfacial chemical structures of (f) PVK/GO/V2O5ex and (g) PVK/V2O5ex for types Aand B QD-LEDs, respectively.Reproduced with permission M. Park, T.P. Nguyen, K.S. Choi, J. Park, A. Ozturk, S.Y. Kim,MoS2-Nanosheet/Graphene-Oxide composite hole injection layer in organic light-emittingdiodes, Electron. Mater. Lett. 13 (2017) 344e350, Copyright 2010 Wiley-VCH.

=


250 25000

20000

15000

10000

5000

0

200

150

100

50

0 2 4 6 8 10 12 14 16

0

Voltage (V)

MoS2-GO 10–0 MoS2-GO 10–0MoS2-GO 8–2 MoS2-GO 8–2 MoS2-GO 6–4 MoS2-GO 4–4 MoS2-GO 4–6 MoS2-GO 4–6MoS2-GO 2–8 MoS2-GO 2–8MoS2-GO 0–10 MoS2-GO 0–10

Cur

rent

den

sity

(mA

/cm

2 )

4 6 8 10 12 14 16Voltage (V)

12

10

8

6

4

2

0.1 1 10 100

5.0

4.5

3.5

3.02.52.0

4.0

0.1 1 10 100

BuLi

Intercalation

MoS bulk Li MoS

Exfoilation

Hummers method

GraphiteMo S Li C O H

GO

MoS nanosheet

LiF/Al

Alq

Alq

BCP

: C545T

NPB

ITO glass

MoS -GO composite

Lum

inan

ce (c

d/m

2 )

Lum

inan

ce e

ffici

ency

(cd/

A)

Pow

er e

ffici

ency

(lm

/W)

Current density (mA/cm2) Current density (mA/cm2)

(c)

(e)

(d)

(a) (b)

Figure 6.7 (a) Current densityevoltage, (b) luminanceevoltage, (c) luminanceefficiencyecurrent density, and (d) power efficiencyecurrent density characteristics of organiclighteemitting diode devices with MoS2-graphene oxide composite as hole injection layer. (e)Synthesis scheme of materials and structures for organic lighteemitting diode devices.Reproduced with permission M. Park, T.P. Nguyen, K.S. Choi, J. Park, A. Ozturk, S.Y. Kim,MoS2-Nanosheet/Graphene-Oxide composite hole injection layer in organic light-emittingdiodes, Electron. Mater. Lett. 13 (2017) 344e350, Copyright 2017 Springer.


800

92

90

88

86

84

82

80350 400 450 550 600 650 700500

Wavelength (nm)

Tran

smitt

ance

(%)

0.00 wt%0.02 wt%0.04 wt%0.06 wt%0.08 wt%

90

89

88

87

86

91

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.0Concentration (wt%)

Tran

smitt

ance

(%)

700

600

500

400

300

100

–100

0

0 1

200

2 3 4 5 6 7Voltage (V)

0.00 wt%0.02 wt%0.04 wt%0.06 wt%

700

800

600

500

400

300

100

–100

0

200

0.00 wt%0.02 wt%0.04 wt%0.06 wt%

0 2 4 6 8 10 12Voltage (V)

800

750

700

650

600

550

4500.02 0.04 0.06 0.080

Concentration (wt%)

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0500 550 600 650 700

Wavelength (nm)

0.00 wt%0.02 wt%0.04 wt%0.06 wt%

Cur

rent

den

sity

(mA

/cm

)Lu

min

ance

(cd/

m )

Con

duct

ivity

(S/c

m)

EL

inte

nsity

(a.u

.)

(a) (d)

(e)

(f)

(b)

(c)

Figure 6.8 (a) Transmission spectra and (b) transmittance at 550 nm of PEDOT:PSS films, fordifferent doping concentrations of GO (0e0.08 wt%), (c) the conductivity of PEDOT:PSSmixed with 6 wt% DMSO and 0.00e0.08 wt% GO. The effect of GO concentration on the (d)JeV curves, (e) luminanceeV curves, and (f) EL spectra of PLEDs, using PEDOT:PSS/GOcomposite as the HTL.Reproduced with permission H.S. Dehsari, E.K. Shalamzari, J.N. Gavgani, F.A. Taromi, S.Ghanbary, Efficient preparation of ultralarge graphene oxide using a PEDOT:PSS/GOcomposite layer as hole transport layer in polymer-based optoelectronic devices, RSC Adv. 4(2014) 55067e55076, Copyright 2014 The Royal Society of Chemistry.

Table 6.3 Summary of properties for PLEDs with PEDOT:PSS and PEDOT:PSS/GOcomposite HTLs [48].

GO(wt%)

Thresholdvoltage (V)

Turn-onvoltage (V)

Maximumluminance (cdmL2) lEL (nm)

ELintensity(a.u.)

0 5.45 5.35 647.1 552.9 0.736

0.02 4.10 3.85 676.8 555.3 0.734

0.04 3.70 3.65 725.6 558.9 0.735

0.06 3.80 3.75 691.1 559.5 0.738


efficiency from 0.071 to 0.156 mA cm�2, and the power efficiency from 0.043 to0.08 lmW�1, by optimizing the ratio between GO and PEDOT:PSS [49]. These resultsconfirm the importance of GO composites as HILs in light-emitting devices.

6.4 Conclusion

This chapter provides an overview of the current applications of GO as the HIL inlight-emitting devices. GO possesses a 2D structure with various functional groupssuch as hydroxyl, epoxy, carboxy, carbonyl, phenol, lactone, and quinine, resultingin unique optical and electrical properties that can be modified by controlling thosegroups. The use of GO and GO-composites as HILs significantly improves the perfor-mance of LED devices. However, the uniformity of GO films and the reliability ofGO-based devices remain a big challenge for commercialization; these issues willrequire more attention and investigation in future.

References

[1] D.J. Milliron, I.G. Hill, C. Shen, A. Kahn, J. Schwartz, Surface oxidation activates indiumtin oxide for hole injection, J. Appl. Phys. 87 (2000) 572e576.

[2] J. Li, Q. Guo, H. Jin, K. Wang, D. Xu, G. Xu, X. Xu, Low-temperature solution-processedmoox as hole injection layer for efficient quantum dot light-emitting diodes, RSC Adv. 7(2017) 27464e27472.

[3] H. Zhang, S. Wang, X. Sun, S. Chen, Solution-processed vanadium oxide as an efficienthole injection layer for quantum-dot light-emitting diodes, J. Mater. Chem. C 5 (2017)817e823.

[4] Y.H. Kim, S. Kwon, J.H. Lee, S.M. Park, Y.M. Lee, J.W. Kim, Hole injection enhance-ment by a Wo3 interlayer in inverted organic light-emitting diodes and their interfacialelectronic structures, J. Phys. Chem. C 115 (2011) 6599e6604.

[5] C. Kim, P. Nguyen Thang, V. Le Quyet, J.M. Jeon, W. Jang Ho, Y. Kim Soo, Perfor-mances of liquid-exfoliated transition metal dichalcogenides as hole injection layers inorganic light-emitting diodes, Adv. Funct. Mater. 25 (2015) 4512e4519.

[6] T.P. Nguyen, Q. Van Le, K.S. Choi, J.H. Oh, Y.G. Kim, S.M. Lee, S.T. Chang, Y.-H. Cho,S. Choi, T.-Y. Kim, S.Y. Kim, MoS2 nanosheets exfoliated by sonication and theirapplication in organic photovoltaic cells, Sci. Adv. Mater. 7 (2015) 700e705.

[7] Q.V. Le, C.M. Kim, T.P. Nguyen, M. Park, T.-Y. Kim, S.M. Han, S.Y. Kim, (NH4)2WS4precursor as a hole-injection layer in organic optoelectronic devices, Chem. Eng. J. 284(2016) 285e293.

[8] V. Le Quyet, P. Nguyen Thang, Y. Kim Soo, Uv/ozone-treated WS2 hole-extraction layerin organic photovoltaic cells, Phys. Status Solidi (RRL) e Rapid Res. Lett. 8 (2014)390e394.

[9] Q.V. Le, T.P. Nguyen, H.W. Jang, S.Y. Kim, The use of uv/ozone-treated MoS2 nano-sheets for extended air stability in organic photovoltaic cells, Phys. Chem. Chem. Phys. 16(2014) 13123e13128.


[10] G.J. Choi, Q.V. Le, K.S. Choi, K.C. Kwon, H.W. Jang, J.S. Gwag, S.Y. Kim, Polarizedlight-emitting diodes based on patterned Mos2 nanosheet hole transport layer, Adv. Mater.29 (2017) 1702598.

[11] Q.V. Le, T.P. Nguyen, K.S. Choi, Y.-H. Cho, Y.J. Hong, S.Y. Kim, Dual use of tantalumdisulfides as hole and electron extraction layers in organic photovoltaic cells, Phys. Chem.Chem. Phys. 16 (2014) 25468e25472.

[12] M. Park, T.P. Nguyen, K.S. Choi, J. Park, A. Ozturk, S.Y. Kim, MoS2-Nanosheet/Graphene-Oxide composite hole injection layer in organic light-emitting diodes, Electron.Mater. Lett. 13 (2017) 344e350.

[13] C.Y. Lee, Q.V. Le, C. Kim, S.Y. Kim, Use of silane-functionalized graphene oxide inorganic photovoltaic cells and organic light-emitting diodes, Phys. Chem. Chem. Phys. 17(2015) 9369e9374.

[14] K. Soon Choi, Y. Park, K.-C. Kwon, J. Kim, C. Keun Kim, S.Y. Kim, K. Hong, J.-L. Lee,Reduced graphite oxide-indium tin oxide hybrid materials for use as a transparent elec-trode, J. Electrochem. Soc. 158 (2011) J231eJ235.

[15] D.R. Dreyer, S. Park, C.W. Bielawski, R.S. Ruoff, The chemistry of graphene oxide,Chem. Soc. Rev. 39 (2010) 228e240.

[16] R. Ruoff, Calling all chemists, Nat. Nanotechnol. 3 (2008) 10.[17] D. Chen, H. Feng, J. Li, Graphene oxide: preparation, functionalization, and electro-

chemical applications, Chem. Rev. 112 (2012) 6027e6053.[18] T. Szab�o, O. Berkesi, P. Forg�o, K. Josepovits, Y. Sanakis, D. Petridis, I. Dék�any, Evolution

of surface functional groups in a series of progressively oxidized graphite oxides, Chem.Mater. 18 (2006) 2740e2749.

[19] T. Nakajima, A. Mabuchi, R. Hagiwara, A new structure model of graphite oxide, Carbon26 (1988) 357e361.

[20] A. Lerf, H. He, M. Forster, J. Klinowski, Structure of graphite oxide revisited, J. Phys.Chem. B 102 (1998) 4477e4482.

[21] K. Erickson, R. Erni, Z. Lee, N. Alem, W. Gannett, A. Zettl, Determination of the localchemical structure of graphene oxide and reduced graphene oxide, Adv. Mater. 22 (2010)4467e4472.

[22] C. Mattevi, G. Eda, S. Agnoli, S. Miller, K.A. Mkhoyan, O. Celik, D. Mastrogiovanni,G. Granozzi, E. Garfunkel, M. Chhowalla, Evolution of electrical, chemical, and structuralproperties of transparent and conducting chemically derived graphene thin films, Adv.Funct. Mater. 19 (2009) 2577e2583.

[23] D.W. Boukhvalov, M.I. Katsnelson, Modeling of graphite oxide, J. Am. Chem. Soc. 130(2008) 10697e10701.

[24] H.A. Becerril, J. Mao, Z. Liu, R.M. Stoltenberg, Z. Bao, Y. Chen, Evaluation of solution-processed reduced graphene oxide films as transparent conductors, ACS Nano 2 (2008)463e470.

[25] G. Eda, G. Fanchini, M. Chhowalla, Large-area ultrathin films of reduced graphene oxideas a transparent and flexible electronic material, Nat. Nanotechnol. 3 (2008) 270.

[26] S. Wang, P.J. Chia, L.L. Chua, L.H. Zhao, R.Q. Png, S. Sivaramakrishnan, M. Zhou,G.S. Goh Roland, H. Friend Richard, T.S. Wee Andrew, K.H. Ho Peter, Band-liketransport in surface-functionalized highly solution-processable graphene nanosheets, Adv.Mater. 20 (2008) 3440e3446.

[27] G. Eda, C. Mattevi, H. Yamaguchi, H. Kim, M. Chhowalla, Insulator to semimetal tran-sition in graphene oxide, J. Phys. Chem. C 113 (2009) 15768e15771.


[28] C. G�omez-Navarro, R.T. Weitz, A.M. Bittner, M. Scolari, A. Mews, M. Burghard,K. Kern, Electronic transport properties of individual chemically reduced graphene oxidesheets, Nano Lett. 7 (2007) 3499e3503.

[29] I. Jung, D.A. Dikin, R.D. Piner, R.S. Ruoff, Tunable electrical conductivity of individualgraphene oxide sheets reduced at “low” temperatures, Nano Lett. 8 (2008) 4283e4287.

[30] G. Lee, K.S. Kim, K. Cho, Theoretical study of the electron transport in graphene withvacancy and residual oxygen defects after high-temperature reduction, J. Phys. Chem. C115 (2011) 9719e9725.

[31] D. Luo, G. Zhang, J. Liu, X. Sun, Evaluation criteria for reduced graphene oxide, J. Phys.Chem. C 115 (2011) 11327e11335.

[32] Z. Wei, D. Wang, S. Kim, S.-Y. Kim, Y. Hu, M.K. Yakes, A.R. Laracuente, Z. Dai,S.R. Marder, C. Berger, W.P. King, W.A. de Heer, P.E. Sheehan, E. Riedo, Nanoscaletunable reduction of graphene oxide for graphene electronics, Science 328 (2010)1373e1376.

[33] X. Sun, Z. Liu, K. Welsher, J.T. Robinson, A. Goodwin, S. Zaric, H. Dai, Nano-grapheneoxide for cellular imaging and drug delivery, Nano Res. 1 (2008) 203e212.

[34] Z. Liu, J.T. Robinson, X. Sun, H. Dai, Pegylated nanographene oxide for delivery of water-insoluble cancer drugs, J. Am. Chem. Soc. 130 (2008) 10876e10877.

[35] Z. Luo, P.M. Vora, E.J. Mele, A.T.C. Johnson, J.M. Kikkawa, Photoluminescence andband gap modulation in graphene oxide, Appl. Phys. Lett. 94 (2009) 111909.

[36] K.P. Loh, Q. Bao, G. Eda, M. Chhowalla, Graphene oxide as a chemically tunable platformfor optical applications, Nat. Chem. 2 (2010) 1015.

[37] G. Eda, Y.Y. Lin, C. Mattevi, H. Yamaguchi, H.A. Chen, I.S. Chen, C.W. Chen,M. Chhowalla, Blue photoluminescence from chemically derived graphene oxide, Adv.Mater. 22 (2010) 505e509.

[38] B.C. Brodie, Xiii. On the atomic weight of graphite, Philos. Trans. R. Soc. Lond. 149(1859) 249e259.

[39] U. Hofmann, E. K€onig, U.€U. Graphitoxyd, Z. Anorg. Allg. Chem. 234 (1937) 311e336.[40] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80

(1958), 1339-1339.[41] H. Yu, B. Zhang, C. Bulin, R. Li, R. Xing, High-efficient synthesis of graphene oxide

based on improved Hummers method, Sci. Rep. 6 (2016) 36143.[42] S.-S. Li, K.-H. Tu, C.-C. Lin, C.-W. Chen, M. Chhowalla, Solution-processable graphene

oxide as an efficient hole transport layer in polymer solar cells, ACS Nano 4 (2010)3169e3174.

[43] Z. Zhong, Y. Dai, D. Ma, Z.Y. Wang, Facile synthesis of organo-soluble surface-graftedall-single-layer graphene oxide as hole-injecting buffer material in organic light-emittingdiodes, J. Mater. Chem. 21 (2011) 6040e6045.

[44] B.R. Lee, J.-w. Kim, D. Kang, D.W. Lee, S.-J. Ko, H.J. Lee, C.-L. Lee, J.Y. Kim,H.S. Shin, M.H. Song, Highly efficient polymer light-emitting diodes using grapheneoxide as a hole transport layer, ACS Nano 6 (2012) 2984e2991.

[45] J. Yang, M. Heo, H.J. Lee, S.-M. Park, J.Y. Kim, H.S. Shin, Reduced graphene oxide(rGO)-Wrapped fullerene (C60) wires, ACS Nano 5 (2011) 8365e8371.

[46] Y. Yang, X. Yang, W. Yang, S. Li, J. Xu, Y. Jiang, Ordered and ultrathin reduced grapheneoxide lb films as hole injection layers for organic light-emitting diode, Nanoscale Res. Lett.9 (2014) 537.


[47] R. Park Young, S. Choi Kyoung, C. Kim Jong, S. Seo Young, Y. Kim Soo, Y.J. Kim,K. Choi Won, Y. Jeong Hu, S. Yang Woo, J. Hong Young, Graphene oxide insertedpoly(N-Vinylcarbazole)/Vanadium oxide hole transport heterojunctions for high-effi-ciency quantum-dot light-emitting diodes, Adv. Mater. Interfaces 4 (2017) 1700476.

[48] H.S. Dehsari, E.K. Shalamzari, J.N. Gavgani, F.A. Taromi, S. Ghanbary, Efficient prep-aration of ultralarge graphene oxide using a PEDOT:PSS/GO composite layer as holetransport layer in polymer-based optoelectronic devices, RSC Adv. 4 (2014)55067e55076.

[49] H. Diker, G.B. Durmaz, H. Bozkurt, F. Yesil, C. Varlikli, Controlling the distribution ofoxygen functionalities on go and utilization of PEDOT:PSS-GO composite as hole in-jection layer of a solution processed blue oled, Curr. Appl. Phys. 17 (2017) 565e572.


Graphene-based quantum dotemitters for light-emitting diodes 7Park Minsu, Yoon Hyewon, Jeon SeokwooDepartment of Materials Science and Engineering, KAIST Institute for the NanoCentury,Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Republic of Korea

7.1 Introduction to graphene quantum dots

Quantum dot is a zero-dimensional material that is introduced from the quantumconfinement effect when it is sized in nanometer scale and has various electricaland optical properties depending on the size of the particle. The electrons confinedin small areas of nanoscale which are smaller than exciton Bohr radius are quantizedand limited in free motion, and electrons in quantum dot are confined in every threedirection by the quantum confinement effect which causes a finite number of electron,hole, and exciton states, resulting in various characteristics on the size of the particle.In other words, the full confinement in every three direction results in the completequantization or discretization of the energy states of confined charge carriers in quan-tum dot [1]. Therefore, the fewer energy levels are quantified as the energy level of thecarrier decreases as the particle size decreases, resulting in a wider and more discre-tized bandgap [2e4].

In the case of metals having >1022 cm�3 of carrier density, the quantum confine-ment effect is generally expressed when they have less than few nanometer size equalto dozens of metal atom clusters [5]. In contrast, semiconductor quantum dot, consist-ing of covalent bondings, generally form carriers through doping methods which actlike free electrons in metals. In the case of semiconducting materials, they generallyhave a carrier concentration of <1017 cm�3, and the quantum confinement effect ismanifested with a size of few nanometers to hundreds of nanometers from 103 to106 atomic clusters [6e8]. Accordingly, the particle size of which the size effect isrevealed by the quantum confinement effect depends on the intrinsic properties ofcomposing atoms of the materials [4,9,10].

Graphene is a two-dimensional and single-atom thick sheet composed of sp2 hy-bridized carbons, defined as a zero-bandgap semimetallic material due to its specialelectronic structure called “Dirac cone” where its conduction band and valence bandmeet at a point [11,12]. In graphene, each carbon atom is covalently bonded with threeadjacent carbon atoms resulting in hexagonal arrays. Because of the bonding nature ofgraphene, each carbon atom shares one p-electron with adjacent three carbon atoms,resulting in p-electrons two dimensionally delocalized out of the graphene plane. Thedelocalized p-electrons allow freely moving behavior similar to free electrons inmetals. Therefore, graphene has a carrier concentration of w1012 cm�2 (the unit is


https://doi.org/10.1016/B978-0-08-102482-9.00007-1

different from sphere-shaped quantum dots due to its dimensionality) from the exis-tence of p-electrons although it is composed of covalent bondings [13]. Accordingly,graphene takes the intermediate characteristics of metals and semiconductors and mayappear the quantum confinement effect when it has smaller lateral size than severalnanometers to tens of nanometers. It is very natural that graphene with semimetalliccharacteristics without a bandgap may have the chance to have a bandgap by the quan-tization of carrier states when its size is less than tens of nanometers and exhibits lumi-nescent properties [14].

To exploit graphene as a luminescent material for optoelectronic applications, manyresearchers have deliberated on bandgap opening of graphene by cutting graphene forobtaining small-sized graphene to generate discrete energy levels such as conjugatedhydrocarbons from organic fluorescent molecules (i.e., naphthalene, anthracene, coro-nene, etc.) or semiconductor quantum dot with smaller lateral size than its Bohr radius[15]. As a result, nano-sized graphene dots having luminescent properties have beenextensively obtained in various ways by cutting or scissoring graphitic materials.Based on the quantum confinement effect, size effect was considered as the lumines-cence origin (bandgap origin) of graphene quantum dots (GQDs) in the initial study[16,17]. However, many studies have reported that optical properties with regard tothe size effect are inconsistent with the luminescence properties of two-dimensionalGQDs possessing p-electrons which may have similar characteristics to free electrons[18]. Based on the quantum confinement effect on the particle size, it could be ex-pected that GQDs should exhibit similar optical properties to other conjugated organicfluorescent molecules such as pyrene, coronene, and ovalene when the size of GQDsbecomes very small because graphene is composed of sp2 carbon hexagons as like ben-zene molecules (Fig. 7.1) [19,20]. Accordingly, the size of graphene should be under

10501000

950900850800750700650600550500450400350300250200

0.5 1.0 1.5 2.0 2.5 3.0Size of GQDs (nm)

Near IR

Red

OrangeYellowGreen

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UV

Em

issi

on w

avel

engt

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m)

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G4: cir-coronene

G7: hexa-peri-hexabenzocoronene

492.3

572.4

450.5399.5

235.2

G5 G6

G8

Figure 7.1 Calculated emission wavelength (nm) using TDDFT method in vacuum as afunction of the diameter of GQDs [20].Reprinted with permission from M.A. Sk, et al., Revealing the tunable photoluminescenceproperties of graphene quantum dots, J. Mater. Chem. C 2 (34) (2014) 6954e6960. Copyright2014 The Royal Society of Chemistry.


2 nm to exhibit visible luminescence based on a theoretical calculation [20,21].However, numerous results have reported much larger size of GQDs or even grapheneoxide (GO) which exhibits visible light.

In addition, the more interesting point is that GQDs (lateral size >2 nm) fabricatedby top-down methods from graphene or GO to GQDs exhibit visible luminescence,whereas GQDs fabricated by bottom-up methods barely show luminescent properties[22]. Furthermore, photoluminescence (PL) is not observed in physically oxidizedgraphene or GQDs by random oxidation using oxygen plasma treatment, whereaschemically oxidized graphene or GQDs by acidic oxidation or solvent process whichis generally applied to synthesis of GO exhibits PL [23,24]. It could be an evidence thatthe existence of defects or oxygen functional groups arising from chemical oxidationprocess accompanied with graphene cutting procedure is essential for GQDs to exhibitluminescence. In this regard, we could consider GQDs as very smallesized GOs.

In amorphous carbon materials which contain a mixture of sp2 and sp3 hybridizedbondings as like carbon quantum dots or carbon nanodots, it is known that the finitesize of very small sp2 clusters, surrounded by sp3 bondings in the hybridized mixture,causes confinement of p-electrons of the sp2 bondings, resulting in localized electronicstates. The localized electronic states enable the sp2 clusters to have localizedelectronehole pairs and lead to exhibit PL [25]. Therefore, the carbon materials con-taining the mixed phase of sp2 and sp3 become dependent to the characteristics of theconfined sp2 clusters [26]. Similarly, because GO is also composed of two dimension-ally mixed structure of sp2 and sp3 bondings whose sp2 carbons have been hybridizedto sp3 bondings or covalently bonded (functionalized) with epoxy or hydroxyl groups,GO exhibits PL properties by its two dimensionally confined sp2 clusters [27]. In fact,a number of previous studies have reported luminescent GO whose emission is rangingfrom visible to near infrared [28,29]. Eda et al. observed blue emission from reducedgraphene oxide (rGO) and also attributed the emission to the small sp2 clusters isolatedfrom surrounding sp3 carbons or oxygen-functionalized carbons introduced by theoxidation and reduction process for the fabrication of the rGO. Moreover, the PLorigin of graphene or GQDs containing oxygen content was also found to be the evo-lution of small sp2 clusters by observation of the changes in blue PL intensity under thedegree of reduction [30]. Based on the previous studies regarding the PL origin of GO,Chien at al. studied changes in PL characteristics of GO suspension with gradualreduction to explain the PL origin of oxygen containing graphitic materials. As shownin Fig. 7.2, it was confirmed that GO exhibiting reddish emission near 600 nm grad-ually showed blue-shifted emission by undergoing reduction process. Accordingly,they explained the PL origin of the oxidized graphitic materials in two types:(1) blue emission (<450 nm) from isolated small sp2 clusters (we interpret the isolatedsmall sp2 clusters as subdomains) confined by defects or oxygen functional groupsexisting in graphene basal planes and (2) red-shifted emission by excessive defectsor excessive functional groups, which form new energy states (extrinsic states)between the localized pep* states from the sp2 clusters, contributing to the emissionat longer wavelength [31].

In fact, the DFT calculation results in Fig. 7.3 show that a sp2 carbon hexagon hascompletely different bandgap energy when the hexagon exists under two different

Graphene-based quantum dot emitters for light-emitting diodes 119

conditions: (1) existing independently (benzene) and (2) existing within graphene ma-trix as a confined sp2 carbon hexagon by surrounding oxygen functional groups.Furthermore, the bandgap energies for clusters of carbon hexagons vary in betweenindependently isolated structures as like fluorescent organic molecules and confinedstructures within graphene matrix (Fig. 7.4). It is more likely that construction of smallsp2 clusters confined within graphene matrix than independently isolated structures aslike organic fluorescent molecules (i.e., benzene, naphthalene, anthracene, pyrene, cor-onene, etc.) as GQDs have numerous chances to have sp3 bondings or oxygen func-tionalization through their synthetic procedure. Therefore, subdomains surroundedby oxygen functional groups, generated during chemical synthetic process, play arole as isolated (independent) quantum dots which exhibit the quantum confinementsize effect. Then the GQDs become having a discrete bandgap and are able to exhibitintrinsic emission as GO does [32]. Additionally, excessive functional groups or defec-tive sites induce extrinsic emission (defect state emission) by providing additionalstates between the intrinsic bandgap as seen in Fig. 7.2. Plus, the bandgap of GQDsby subdomains can be tuned by electron donating or withdrawing functional groupsattached at the edges or defective sites of GQDs [33]. In this regard, oxygen functionalgroups in GQDs could play two roles: (1) confinement of small sp2 clusters for forma-tion of subdomains and (2) inducing extrinsic emission when excessive. BecauseGQDs have different PL origin from sphere-shaped quantum dots, GQDs generallydo not follow the quantum confinement size effect with their lateral particle diametersbut follow the size effect with the internal sp2 subdomain size as similar to the PLorigin of GO.

Original GO

PL

/ a.u

.

PL

/ a.u

.

400 500 600 700 400 500 600 700λ λ/ nm

π*

π π

π*

Exc

itatio

n

Exc

itatio

n

/ nm

Reduced GO

Reduction

(a) (b)

Figure 7.2 Proposed PL emission mechanisms of (a) the predominant extrinsic emission in GOfrom disorder-induced localized states. (b) The predominant intrinsic emission in rGO fromconfined cluster states [31].Reprinted with permission from C.T. Chien., et al., Tunable photoluminescence from grapheneoxide. Angew. Chem. Int. Ed. 51 (27) (2012) 6662e6666. Copyright 2012 Wiley-VCH VerlagGmbH and Co. KGaA, Weinheim.


7.2 Synthetic strategies for GQDs

First of all, it is necessary for GQDs to be clearly distinguished from the “carbon quan-tum dots” or “carbon nanodots,” namely “carbon dots (CDs).” The GQDs that we aretrying to address is represented as a planar structure of mono- or few layers (w2 nm)with tens of nanometers in size. On the other hand, the CDs are mainly synthesizedfrom the single molecule carbon precursors such as glucose, citric acid by bottom-up process, creating the complex sp2-sp3 carbon clusters that results in quasi-spherical shape [34]. As a result of such structural differences, the PL mechanism ofCDs is usually governed by surface functional groups or defect states [35], in contraryto GQDs. Still, the differences on PL mechanism between the GQDs and CDs arecontroversial. For this matter, some well-organized review papers that cover suchtopics will be helpful for understanding [34e36]. Meanwhile, the terminology for vari-ously called “GQDs” should be clearly subdivided depending on the synthetic route.Especially, as we stated above, the oxidation degree (typically within the range of4.72e48.68 at. % [24,37e40]) plays a key role on determining intrinsic bandgap ofGQDs, which implies that the synthetic strategies must be focused on discovering

PBE PBE

HSE HSE

5.10 eV3.50 eV

4.35 eV7.22 eV

DO

S (a

rb. u

nit)

DO

S (a

rb. u

nit)

–8 –4 0 4 –5 0 5

Energy (eV) Energy (eV)

2 x 2 unit cell(a) (b)

Figure 7.3 (a) The atomic structure of benzene ring which is terminated with hydrogen is inupside. Total density of states of benzene ring with PBE and HSE06 functional is in downside.(b) The atomic structure of graphene confined with hydrogen which made six carbon ringsconfined by hydrogen is in upside. Their bandgap is 3.50 eV with PBE functional and 4.35 eVfor HSE06 results [24].Reprinted with permission from H. Yoon, et al., Intrinsic photoluminescence emission fromsubdomained graphene quantum dots. Adv. Mater. 28 (26) (2016) 5255e5261. Copyright 2016Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim.


n = 1

n = 7

n = 2

n = 10

n = 4

n = 19

Ban

dgap

(eV

)

6

4

2

00 5 10 15 20

Number of hexagons

Figure 7.4 Schematic illustrations of calculated atomic structures and DFT calculation results of local bandgaps of subdomains with increasing numberof confined sp2 carbon hexagons [24].Reprinted with permission from H. Yoon, et al., Intrinsic photoluminescence emission from subdomained graphene quantum dots. Adv. Mater. 28 (26)(2016) 5255e5261. Copyright 2016 Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim.

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the methods in a way that allow the control of oxidation degree. However, because themajority of preparation methods on GQDs have performed by scissoring of GO sheetsor oxidized graphite (which are closer to “graphene oxide quantum dots (GOQDs)”rather than “GQDs”), it has been in trouble with controlling oxidation precisely.Even the reduced GOQDs (rGOQDs) through chemical or thermal reduction seemto have different PL properties from that of GQDs with low oxidation [24,41,42]due to the effect of residual oxygen containing functional groups [33,43]. Judgingfrom those results, it would be desirable for controlling the oxidation through the syn-thetic approaches without acidic oxidizing process. Although various syntheticmethods are well explained by some review papers [35,44,45], using a graphite inter-calation compound (GIC) will be one of the preferred approach to control the oxidationfinely [17,42,53]. With our proposed standard for controlling oxidation in mind, itwould be very helpful to understand the methodological approaches.

In Section 7.2, we will classify the fabrication strategies for GQDs reported so farinto mainly top-down and bottom-up approaches and deal with their representativeresults. Various fabrication methods on two-dimensional (2D) planar structuredGQDs with a thickness of w3 layers (w2 nm thick) have been reported, which aremainly composed of top-down and bottom-up approaches. We primarily introducetop-down methods containing hydro/solvothermal cutting [33,38,40,43,46e53],acid-assisted cutting [16,17,33,39,41,54e60], cutting from GICs [24,61,62], micro-wave treatment [63e66], electrochemistry [67e73], and so on. A majority of GQDsynthesis has been done by “cutting” from the bulk graphite or from the GO producedby well-established Hummers’ method. In this approach, strong acids such as H2SO4and HNO3 are used to preoxidize and exfoliate graphite. Underlying oxygen contain-ing functional groups in GO sheets such as epoxy groups act as cleavage sites to bereadily attacked by either way of chemical or physical treatments, breaking downthe GO sheets into smaller pieces. First principle calculations that Li et al. reportedfor oxidative breakup of graphene indicate the well-controlled oxidation of graphenesheets (GSs) could lead to more smooth edges [74]. Therefore, bulk sp2 carbonecontaining materials such as graphite, GO, carbon fibers, and carbon nanotubes(CNTs) could be within the range of starting materials for top-down approach.

7.2.1 Hydrothermal/solvothermal cutting

The GO sheets could be split into GQDs in aqueous solution via hydrothermal reaction[38,46e50,75,76]. In 2010, Pan et al. [46] proposed a facile method for hydrothermalsynthesis of GQDs from micrometer-sized GO sheets as starting materials(Fig. 7.5(a)). Firstly, the GSs that obtained from thermal deoxidization of GO sheetsat 200e300�C for 2 h were oxidized in concentrated H2SO4 and HNO3 for15e20 h under mild ultrasonication. Then the hydrothermal deoxidization of theoxidized GSs at 200�C for 10 h under weak alkaline conditions (pH ¼ 8) could pro-duce GQDs with average size of 9.6 nm and the height of 1e2 nm (one to threegraphene layers). They explained that poorly ordered GQDs were obtained due tothe low temperature of thermal deoxidization of GO sheets with weak alkaline condi-tion. Later, they improved the hydrothermal method for well-crystallized GQDs with


Hydrothermaldeoxidization

O

O

O

OO

OO

O

Nucleophilic interctionwith ammonia

NH2

NH2NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

Edge destruction

(a)

(b)

Figure 7.5 (a) Mechanism for the hydrothermal cutting of oxidized GSs into GQDs: a mixedepoxy chain composed of epoxy and carbonyl pair groups (left) is converted into a completecut (right) under the hydrothermal treatment [46]. (b) Schematic illustration of the proposedformation mechanism for amine functionalized GQDs (af-GQDs) through the amino-mediatedbond scission by nucleophilic substitution reaction of oxygen-functional groups on oxidizedgraphene sheets (OGSs) with ammonia, under a low-temperature amino-hydrothermal con-dition [48].(a) Reprinted with permission from D. Pan, et al., Hydrothermal route for cutting graphenesheets into blue-luminescent graphene quantum dots, Adv. Mater. 22 (6) (2010) 734e738.Copyright 2010 Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim. (b) Reprinted withpermission from Tetsuka, H., et al., Optically tunable amino-functionalized graphene quantumdots, Adv. Mater. 24 (39) (2012) 5333e5338. Copyright 2010 Wiley-VCH Verlag GmbH andCo. KGaA, Weinheim.


average size of 3 nm, and the height of 1.5e1.9 nm from the high-temperature rGOsheets (600�C) with a fine chemical cutting route under strongly alkaline hydrothermalconditions (pH > 12) [47]. In 2012, Tetsuka et al. [48] extracted amino-functionalizedGQDs (af-GQDs) via amino-mediated bond scission under a low-temperature amino-hydrothermal condition as shown in Fig. 7.5(b). The OGSs were subjected to mildamino-hydrothermal treatment at 70e150�C for 5 h in ammonia solution, followedby thermal annealing at 100�C. During the treatment, ammonia reacted with epoxygroups of OGSs to form primary amines and alcohols by nucleophilic substitution,enabling self-limited extraction of sp2 carbon domains by ring opening of the epoxidewith direct bonding of primary amines to graphene edge. The af-GQDs that edge-terminated with primary amine show the average size of 2.5 nm with the height of1.13 nm. Their fluorescence could be tuned from violet to yellow through the degreeof amine functionalization that can be controlled by changing the initial concentrationof ammonia and the temperature of the amino-hydrothermal treatment. Later, theyexpanded their synthetic approaches to functionalization of various nitrogen-containing functional groups for tuning HOMO/LUMO levels with viable optoelec-tronic applications [49].

Methods on solvothermal cutting of GO are also available using various solventselsewhere [40,43,51e53]. Zhu et al. [40] reported a one-step solvothermal route forthe synthesis of GQDs from GO that dispersed in dimethyl formamide (DMF). TheGO/DMF solutions were under ultrasonication for 30 min, followed by treated solvo-thermal process at 200�C for 5 h. The average size of prepared GQDs was 5.3 nm withthe height of 1.2 nm. They explained that the DMF acted as a solvent as well as a weakreduction agent during the reaction. In 2011, Shen et al. [54] proposed a method forpolyethylene glycol (PEG)epassivated GQDs from rGO (Fig. 7.6(a)). They firstlyfurther oxidized GO by 2.6 M of HNO3 with 70�C reflux for 24 h, cutting into smallGO sheets. After tuning the pH to 8 with Na2CO3, PEG1500N was mixed and heated at120�C for 24 h. The PEG-passivated GO was then reduced by hydrazine hydrate at100�C for 24 h, and finally strong blue-emitting GQDs were obtained after dialyzingfor 7e10 days. The size of the prepared PEG-passivated GQDs was within the range of5e19 nm with an average diameter of 13.3 nm and a PL-quantum yields (PL-QYs) of7.4% using rhodamine B as a reference. Peng et al. [17] fabricated GQDs with differentsize distribution in scalable amounts from pitch-based carbon fibers using the acidicoxidation and etching (Fig. 7.6(b)). Three types of GQDs with blue, green, and yellowemission were produced from the mixture of carbon fibers and concentrated strongacids (H2SO4 and HNO3) with sonication for 24 h at different temperature (80�C,100�C, and 120�C). The produced GQDs were in the size range of 1e4 nm,4e8 nm, and 7e11 nm, respectively, with a thickness of 0.4e2 nm correspondingto one to three graphene layers. They explained that it is easier to control the size asthe carbon fibers already have small sp2 domain structure.

In 2013, Ye et al. [55] reported a facile approach to synthesize tunable GQDs fromvarious types of coal. Fig. 7.6(c) shows the representative scheme for the synthesis ofGQDs from bituminous coal. They said the oxidative displacement of crystalline car-bon within the coal structure is easier than that of pure sp2 carbon structure, resulting innanometer-sized GQDs with amorphous carbon addends on the edges. The coal-


PEG1500N

(a)

(b)

(c)

(d)

N2H4 reduction

CF

Bituminous coal b-GQDs

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Sonication,heating in acids

PEG-diamine, KOH + water

Reductionby N2H4

Reductionby N2H4

1. H2SO4, HNO3

2. 100 ºC

Oxidized GOs1st cutting

2nd cutting 2nd cutting

Attachment of –NHR groups

GOs

GQDs-NHR GQDs

Oxidationcutting

Figure 7.6 (a) Representation of the preparation for GQDs by hydrazine hydrate reduction of GO with surface passivation by PEG [54]. (b)Representation scheme of oxidation cutting of CF into GQDs [17]. (c) Schematic illustration of the synthesis of GQDs from bituminous coal.Oxygenated sites are shown in red [55]. (d) Schematics of GQDs and GQDs-NHR from GOs by N2H4 reduction or attachment of PEG diamine [33].(a) Reprinted with permission from J. Shen, et al., Facile preparation and upconversion luminescence of graphene quantum dots, Chem. Commun. 47(9) (2011) 2580e2582. Copyright 2011 The Royal Society of Chemistry. (b) Reprinted with permission from J. Peng, et al., Graphene quantum dotsderived from carbon fibers, Nano Lett. 12 (2) (2012) 844e849. Copyright 2012 American Chemical Society. (c) Reprinted with permission from Ye,R., et al., Coal as an abundant source of graphene quantum dots. Nat. Commun. 4 (2013) 2943. Copyright 2013 Macmillan Publishers Limited.(d) Reprinted with permission from S.H. Jin, et al., Tuning the photoluminescence of graphene quantum dots through the charge transfer effect offunctional groups. ACS Nano 7 (2) (2013) 1239e1245. Copyright 2012 American Chemical Society.

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derived GQDs were obtained by sonicating the coal in concentrated H2SO4 and HNO3,followed by heat treatment at 100�C or 120�C for 24 h. The size of GQDs from bitu-minous coal was 2.96 � 0.96 nm in diameter, and the height was 1.5e3 nm suggestingthat there are two to four layers of GO-like structures. GQDs were also synthesizedfrom coke and anthracite coals, with their sizes of 5.8 � 1.7 nm, 29 � 11 nm, respec-tively. The three types of GQDs from anthracite, bituminous, and coke showed yellow,blue, and green emission with the maxima of 460 nm, 480 nm, and 530 nm, respec-tively. Meanwhile, Jin et al. [33] prepared amine-functionalized GQDs by two-stepcutting of oxidized GOs as shown in Fig. 7.6(d). Firstly, the GOs were oxidativelycut by H2SO4/HNO3 mixture, and they were treated with chemical reduction byN2H4 for further cutting. To fabricate the amine groupefunctionalized GQDs, theoxidized GOs were reacted with diamine-terminated PEG diamine by the ring-opening reaction of the epoxy groups on the GOs with amine groups under alkalineconditions. The average size of GQDs showed 2.45 � 0.63 nm in diameter with theheight 1.07 � 0.59 nm.

7.2.2 Graphite intercalation compounds

In 2012, Lin et al. [61] reported an effective approach for the preparation of water-soluble GQDs based on exfoliating and disintegrating MWCNTs and graphite flakesusing the high reactivity of potassiumegraphite intercalation compounds (K-GICs)(Fig. 7.7(a)). After a few years, Song et al. [24,62] introduced a preparation methodto obtain low-oxidized and mass-producible GQDs via potassiumetartrate cointerca-lated GICs (Fig. 7.7(b)). Potassiumesodium tartrate salt was used as an intercalantrather than using only potassium metal. Both methods resulted in efficient blue-emitting GQDs (410e420 nm) attributed to the intrinsic emission from small sp2 clus-ters. In addition, they both provided high production yield (w23% for K-GICs,w60%for potassiumetartrate GICs) and introduced low concentration of oxygen functionalgroups by avoiding harsh oxidation, indicating the resultant GQDs from two methodsare not close to GOQDs.

7.2.3 Microwave-assisted cutting

Microwave heating is advantageous for rapid and homogenous heating of reactants,which is a strong tool for the reduction of GO [77,78], as well as cutting the GO[63e66]. In 2012, Li et al. [64] proposed a facile microwave-assisted preparation ofstabilizer-free GQDs through the cleaving GO nanosheets under acidic conditions.By using a microwave heating, the cleaving and reduction processes of GO weresimultaneously accomplished without additional reducing agents. The lateral size ofprepared GQDs was 2e7 nm with a height of 0.5e2 nm. In other groups, Chenet al. [63] reported a one-step microwave-assisted preparation of GQDs as shown inFig. 7.8. A mixture of GO aqueous solution with H2SO4/HNO3 was heated undermicrowave irradiation at 200�C for 5 min. The resultant was neutralized by sodium


carbonate, and the supernatant was collected by centrifugation, followed by dialysis.The obtained GQDs show an average diameter of 3 nm with a height of less than0.7 nm, which is indicative of a single-layered GQD.

7.2.4 Electrochemical cutting

Utilizing electrochemical reaction is also applicable for the preparation of GQDs fromsp2 carbon sources such as graphite, graphene, and MWCNTs [67e73]. Under thehigh redox potential, the oxidative cleavage reaction can occur by hydroxyl and oxy-gen radicals created from oxidized CeC bonds or oxidized water as shown in Fig. 7.9[67]. In 2011, Li et al. [69] firstly reported electrochemical synthesis of functionalGQDs from 0.1 M phosphate buffer solution (PBS, pH 6.86) using a filtration-formed graphene film as a working electrode on the CV scan within � 3.0 V at ascan rate of 0.5 V/s. The produced GQDs have 3e5 nm in size with a height of

Layeredmaterials

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K

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250°Cfor 24 hr

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K+

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QDs

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In water

Exfoliation

Graphite

Oxygen Carbon Potassium Hydrogen Sodium

Graphite intercalation compounds High-quality GQD

(a)

(b)

Figure 7.7 (a) The scheme of the formation of GQDs based on the high reactivity ofpotassiumegraphite intercalation compounds (K-GICs) [61]. (b) Schematic diagram showingthe overall processes of the GQDs based on GICs using potassiumesodium tartrate salt [62].(a) Reprinted with permission from L. Lin, S. Zhang, Creating high yield water solubleluminescent graphene quantum dots via exfoliating and disintegrating carbon nanotubes andgraphite flakes, Chem. Commun. 48 (82) (2012) 10177e10179. Copyright 2012 The RoyalSociety of Chemistry. (b) Reprinted with permission from S.H. Song, et al., Highly efficientlight-emitting diode of graphene quantum dots fabricated from graphite intercalationcompounds, Adv. Opt. Mater. 2 (11) (2014) 1016e1023. Copyright 2014 Wiley-VCH VerlagGmbH and Co. KGaA, Weinheim.


OH

OH

OH

OH

OHHO

OHOHOH

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Room temperature

HNO3 + H2SO4—3 h

NaBH4— 2 h

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O

O

Figure 7.8 Schematic representation of the microwave-assisted preparation route for green-emitting GQDs and blue-emitting GQDs [63].Reprinted with permission from L.L. Li, et al., A facile microwave avenue toelectrochemiluminescent two-color graphene quantum dots, Adv. Funct. Mater. 22 (14) 20122971e2979. Copyright 2012 Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim.

Electrochemistry

BF4–

.OH

O.

Figure 7.9 Illustration of the exfoliation process showing the attack of the graphite edgeplanes by hydroxyl and oxygen radicals, which facilitate the intercalation of BF4

- anion.The dissolution of hydroxylated carbon nanoparticles gives rise to the fluorescent carbonnanoparticles. Oxidative cleavage of the expanded graphite produces graphenenanoribbons [67].Reprinted with permission from J. Lu, et al., One-pot synthesis of fluorescent carbonnanoribbons, nanoparticles, and graphene by the exfoliation of graphite in ionic liquids, ACSNano 3 (8) (2009) 2367e2375. Copyright 2009 American Chemical Society.


1e2 nm. Later, they used N-containing tetrabutylammonium perchlorate in acetoni-trile instead of PBS as the electrolyte and fabricated nitrogen-doped GQDs(N-GQDs) [68]. The N-GQDs have diameters of 2e5 nm with a typical height of1e2.5 nm, implying the one to five graphene layers. Zhang et al. [70] performedthe electrolysis of the graphite rod as an anode with a current density in the rangeof 80e200 mA/cm2 in 0.1 M NaOH aqueous solution. The produced GQDs exhibitthe size of 5e10 nm with a height less than 0.5 nm. Ananthanarayanan et al. [72] pro-posed a facile method to electrochemical synthesis of GQDs from 3D graphene grownby chemical vapor deposition (CVD). They used a 3D graphene electrode with1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6) in acetonitrile (10%,v/v) as the electrolyte. A constant voltage or a cyclic voltage waveform was appliedto 3D graphene with a platinum wire as the counter electrode and an Ag/AgCl elec-trode as the reference. The lateral size of resulted GQDs showedw6 nm with a heightof 1.25 nm.

7.2.5 Other chemical treatment

In 2012, Zhou et al. [79] suggested that the cutting of GO can occur with Fentonreagent (Fe2þ/Fe3þ/H2O2) under a UV irradiation as displayed in Fig. 7.10(a). Fromthe reaction, the GQDs with periphery carboxylic groups could be generated withmass scale production. They explained that the hydroxyl radicals ($OH) from thedissociated H2O2 under the photoassisted catalysis of Fe3þ/Fe2þ in water can actsas strong oxidizing agent. Under the UV irradiation, the hydroxyl radicals and/orperoxide radicals would attack the carbon atoms connected with the hydroxyl andepoxide groups of GO, breaking CeC/C]C bonds. Simultaneously, the newlyformed oxygen-functional groups such as quinone groups or radicals act as furtherphoto-Fenton reaction sites. The GQDs from the photo-Fenton reaction of GO havean average lateral size of 40 nm with a thickness of w1.2 nm. Although numerousmethods have been known for synthesizing GQDs, they have only focused on flaketype graphene reduced from GO, making difficult to separate the size effect ofGQDs from the measured optical properties. Lee et al. [23] have contributed inaddressing this issue, which is selective etching of graphene. The uniform-sizedGQDs were obtained by selective etching using self-assembled block copolymers(BCPs) as an etch mask on graphene films grown by CVD (Fig. 7.10(b)). Theas-prepared GQDs are composed of mono- or bilayer graphene with diameters of 10and 20 nm, corresponding on the size of BCP nanospheres. Fine size tuning with ox-ygen contentedependent PL measurement in this research provides better understand-ing of the effects of size as well as the functionalization on GQDs.

In the bottom-up methods for the preparation of GQDs [21,22,66,80e83], there arenot much reports compared to the top-down methods due to the difficulty in formingthin and planar structure with uniform sp2 carbon domains, which generate poorPL-QYs. Li’s group has reported a stepwise solution chemistry that is based on oxida-tive condensation reactions, for the bottom-up synthesis of large and stable colloidalGQDs with uniform size and shape [22,80]. Their GQDs are composed of fusedgraphene moieties containing 168, 132, and 170 conjugated carbon atoms,


respectively, via oxidation of polyphenylene dendritic precursors (Fig. 7.11). Theresultant colloidal GQDs were stabilized by multiple 20,40,60-trialkyl phenyl groupscovalently attached to the edges of the graphene moieties. They explained that thecrowdedness on the edges of the graphene cores twists the substituted phenyl groupsfrom the plane of the core, positioning alkyl chains along with them in all three dimen-sions. This leads to reduced face-to-face interaction between the graphene layers, thusincreasing the solubility of GQDs.

In 2011, Lu et al. [21] reported geometrically well-defined GQDs that can be ob-tained from ruthenium (Ru) surface using C60 molecules as a precursor. The stronginteraction between the C60 and Ru induced the formation of surface vacancies inthe Ru single crystal and a subsequent embedding of C60 molecules in the surface.The fragmentation of the embedded C60 molecules then produces carbon clusters, coa-lescing to form geometrically well-defined GQDs on thermally activated diffusion(Fig. 7.12(aec)). Meanwhile, Liu et al. [81] proposed a bottom-up fabrication ofGQDs with uniform morphology by using unsubstituted hexa-peri-hexabenzocoro-nene (HBC) as the carbon source. Along with the other large polycyclic aromatic

Brush formation Block copolymer

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Figure 7.10 (a) Schematic representation of the photo-Fenton reaction of the GO sheets toproduce GQDs [79]. (b) Schematic illustration of the fabrication of GQDs including the spincoating of BCP, formation of silica dots, and etching process by O2 plasma [23].(a) Reprinted with permission from X. Zhou, et al., Photo-Fenton reaction of graphene oxide: anew strategy to prepare graphene quantum dots for DNA cleavage, ACS Nano 6 (8) (2012)6592e6599. Copyright 2012 American Chemical Society. (b) Reprinted with permission from J.Lee, et al., Uniform graphene quantum dots patterned from self-assembled silica nanodots, NanoLett. 12 (12) (2012) 6078e6083. Copyright 2012 American Chemical Society.


hydrocarbons, the HBC is considered as nanoscaled fragments of graphene. Theyexplained that the composition, degree of graphitization, and physical and chemicalproperties of these materials can be easily tuned by the pyrolysis conditions and pre-cursors with different aromatic frameworks (Fig. 7.12(d)). The mono-dispersed disk-like GQDs with a large size (w60 nm) and a thickness of 2e3 nm were preparedthrough the processes of carbonization, oxidization, surface functionalization witholigomeric poly(ethylene glycol) diamine (PEG1500N), and reduction with hydrazine.

NH2 NH2

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Figure 7.11 (a) Procedure of bottom-up synthesis of large colloidal GQDs with uniform andtunable size through stepwise solution chemistry with 3-iodo-4-bromoaniline as startingmolecule. (b) The colloidal GQDs have graphene moieties of 168, 132, and 170 conjugatedcarbon atoms for 1, 2, and 3, respectively [22].Reprinted with permission from X. Yan, X. Cui, L.S. Li, Synthesis of large, stable colloidalgraphene quantum dots with tunable size, J. Am. Chem. Soc. 132 (17) (2010) 5944e5945.Copyright 2010 American Chemical Society.


7.3 Toward highly efficient fluorescence from GQDs

7.3.1 Importance of controlling subdomain uniformly

The efficient fluorescence of GQDs is the most important factor in optical applicationsincluding light-emitting diodes. We now discuss about the realization of efficient fluo-rescence in GQDs. In Section 7.1, it was mentioned that the luminescence mode ofGQDs can be divided into two categories, one by the subdomain (intrinsic emission)which is composed of few sp2 carbon hexagons confined by oxygen functional groupsand the other by defect states (extrinsic emission) including functional groups, defects,edges, heteroatoms, etc. [84]. The extrinsic emission is from additional energy states(but not from intrinsic bandgap), formed by defects or functional groups existing inGQDs, smaller than the intrinsic bandgap, whereas the intrinsic emission is lumines-cence from a discrete bandgap enabled by formation of subdomains within grapheneplanes. Therefore, the extrinsic emission exhibits not only stronger dependency onexcitation wavelength than intrinsic emission but also less luminescence efficiencythan the intrinsic emission as those additional energy states take possibility to act asluminescence centers as well as carrier trap sites or quenching sites. To get themaximum PL-QYs and narrow bandwidth, facilitating intrinsic luminescence is thekey to achieve high-performance electroluminescence (EL) devices [24,62].

HBC 1

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(D ≈ 60 nm)

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Self-assembly

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2. Functionalizationand reduction

725 K825 K

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Figure 7.12 (a) Mechanisms using C60. The majority of C60 molecules adsorb on the terrace,and these decompose to produce carbon clusters with restricted mobility. (b) Temperature-dependent growth of GQDs with different equilibrium shape from the aggregation of thesurface diffused carbon clusters. (c) Corresponding STM images for the well-dispersedtriangular and hexagonal equilibriumeshaped GQDs produced from C60-derived carbonclusters [21]. (d) Processing diagram for the preparation of photoluminescent GQDs by usingHBC 1 as carbon source [81].(c) Reprinted with permission from J. Lu, et al., Transforming C 60 molecules into graphenequantum dots, Nat. Nanotechnol. 6 (4) (2011) 247. Copyright 2011 Macmillan PublisherLimited. (d) Reprinted with permission from R. Liu, et al., Bottom-up fabrication ofphotoluminescent graphene quantum dots with uniform morphology, J. Am. Chem. Soc. 133(39) (2011) 15221e15223. Copyright 2011 American Chemical Society.


We have discussed about various types of GQDs and their fluorescence propertiesaffected by their various synthetic methods. As mentioned above, GQDs can be clas-sified as GOQDs (by relatively uncontrolled), rGOQDs (by reduction after relativelyuncontrolled oxidation), and GQDs (by controlled oxidation). Generally, it can beobserved that the GQDs from nonoxidized graphite exhibit better fluorescence effi-ciency than GOQDs and rGOQDs from GO [24,33,62]. Liu et al. analyzed andreported the differences in PL properties between GQDs fabricated via solvent exfo-liation without acidic oxidation and GOQDs fabricated via acidic oxidation.As shown in Fig. 7.13, GQDs made without acidic oxidation exhibit short wavelengthmainly in blue color region (around 400 nm), and relatively narrow and strong PL peakthan green-emissive GOQDs (around 500 nm) made under strong acidic oxidationincluding nitric acid and sulfuric acid is also observed in the GQDs [41]. The mostwidely used synthetic methods for GOQDs and rGOQDs are based on hydrothermalor solvothermal process by chemically cutting GO which contains various oxygenfunctional groups including carboxyl, carbonyl, and epoxy groups under strong acidiccondition. During the chemical cutting process, the graphene layer is undergoneextremely strong attack from the strong acids at least 1e2 times, resulting in attach-ment of various oxygen functional groups to the graphene layer [33,47,85]. Therefore,relatively random-sized clusters are more likely to be distributed although the confinedsp2 clusters (subdomains) are created, and excessive oxygen functional groups whichcontribute to the extrinsic emission and carrier trap sites can also be found easily. Fromthis reason, relatively low possibilities to have subdomains and uncontrolled-sized sp2

subdomains caused by uncontrolled oxidation of the strong acidic oxidation make theemission property of GOQDs become extrinsic-dominant emission which exhibitbroad and excitation-dependent emission [47,76,86,87].

However, it is difficult to suppress the extrinsic emission and promote efficientintrinsic emission by simply removing the excessive oxygen functional groups orreducing oxygen concentration in the GOQDs. As mentioned in Section 7.1, physi-cally oxidized GQDs or even graphene and GO by random attack of oxygen atoms us-ing oxygen plasma treatment barely exhibit PL characteristics because the physicaloxidation it is difficult to form the subdomains [23,24]. The more interesting phenom-enon is that GOQDs undergone only oxidation process and rGOQDs undergone bothoxidation and reduction processes to produce different luminescent properties even ifthey have the same oxygen concentration. Regarding the phenomenon, Jang et al. con-ducted gradual oxidation and reduction process of GOQDs and rGOQDs to studyabout the reversibility of oxidation of GQDs. As shown in Fig. 7.14, the structuralmodels of GOQDs and rGOQDs are totally different, indicating the oxidation andreduction processes occur irreversibly. This finding implies that not the oxygen con-centration but the oxidation methods and formation nature of defects including oxygenfunctional groups in GQDs play key roles in both defect-related extrinsic emission andthe sp2 subdomain formation [24,42]. By combining and summarizing the resultsrelated to the PL origin of GQDs, the most important strategy for the efficient lumines-cence is promoting the intrinsic emission and suppressing the extrinsic emission.


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Figure 7.13 Synthetic scheme (a) and PL spectra (b) of GQDs and GOQDs using chemical exfoliation of graphite nanoparticles (GNPs) [41].Reprinted with permission from Liu, F., et al., Facile synthetic method for pristine graphene quantum dots and graphene oxide quantum dots: origin ofblue and green luminescence. Adv. Mater. 25 (27) (2013) 3657e3662. Copyright 2013 Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim.

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The strategy can be realized by a controlled oxidation process with efficient attachmentof oxygen functional groups for confinement of uniform sized sp2 subdomains and pre-vention of excessive defect states [32].

7.3.2 Maximizing intrinsic emission by controlled oxidation

As mentioned earlier, a great and precise control of the subdomain that contributes tointrinsic emission could be a key factor in the implementation of efficient lumines-cence with high color purity. Because subdomains are clusters of small sp2 hexagonswhich are isolated by defects including oxygen functional groups, a precise controlover the oxidation process that comes with the process of making graphene intoGQDs can be thought of as the key to great control of subdomain formation. A possibleroute to form subdomains with circular or elliptical structures can be chemical oxida-tion of graphene through GIC method. As mentioned in Section 7.2, Lin et al. intro-duced blue-emitting GQDs by exfoliating potassium intercalated GICs in EtOH/H2O solution, and after that, Song et al. also introduced highly efficient blue-emitting GQDs with low oxidation degree compared with GOQDs and rGOQDs[61,62]. Both of two synthetic methods have something in common with their forma-tion mechanism of GQDs. The intercalated potassium ions, which are alkali metal ions,between graphite layers, occur a mild oxidation reaction by meeting the solvents suchas water and ethanol, resulting in GQDs [62,88]. Afterward, Yoon et al. revealed theorigin of the efficient emission from the GIC-based GQDs even with the low oxygenconcentration [24].

The alkali metal ions such as potassium ions are arranged with a particular stoichi-ometry on the graphene plane as seen in Fig. 7.15 when the metal ions are intercalatedbetween the graphite interlayer [89e91]. After exfoliating the alkali metal intercalated

Low oxidation Full oxidation Reduction

Figure 7.14 Structural models of GOQDs and rGOQDs for chemical oxidation and reduction.(White and yellow regions represent sp2 bonding and sp3 bonding formed by oxygen-functional groups, respectively, and the regions v describe vacancies.) [42].(a) Reprinted with permission from V. Gupta, et al., Luminscent graphene quantum dots fororganic photovoltaic devices, J. Am. Chem. Soc. 133 (26) (2011) 9960e9963. Copyright 2011American Chemical Society. (b) Reprinted with permission from W. Kwon, et al.,Electroluminescence from graphene quantum dots prepared by amidative cutting of tatteredgraphite, Nano Lett. 14 (3) (2014) 1306e1311. Copyright 2014 American Chemical Society. (c)Reprinted with permission from S.H. Song, et al., Highly efficient light-emitting diode ofgraphene quantum dots fabricated from graphite intercalation compounds, Adv. Opt. Mater. 2(11) (2014) 1016e1023. Copyright 2014Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim.


GICs in a solvent such as water, the graphene layer adjacent to the alkali metals can beeasily tattered or oxidized by the active reaction of the alkali metal and the solvent.Once an oxygen is attached onto the graphene plane, the other oxygen is preferentiallyattached to the adjacent locations from the firstly attached oxygen to compensate theunstable energy states of the firstly attached oxygen in general oxidation reaction ofgraphene, unless extremely high temperature or strong acids are applied [46]. As aresult of the preferential attachment, a configuration of a confined sp2 carbon hexagonsby the attached oxygens with circular or elliptical shapes become energetically favor-able [24]. Therefore, in the case of alkali metal intercalated GICs which take a partic-ularly arranged alkali metal layer between the graphite layers, it is likely to havesubdomains by the sequential attachment of oxygens (or oxygen functional groups)as seen in the red circles in Fig. 7.15 for the energetic compensation of the attachedoxygens during the reaction between the alkali metals and the solvent. Furthermore,if other molecules (organic molecules) such as tartrate are cointercalated with alkalimetals at the same time, relatively uniformly created subdomains could be promotedby interfering some of alkali metal locations (Fig. 7.15(c)) [92,93]. From this expec-tation, even if not many studies have been reported of GQDs made with GICs and

Graphite intercalationcompounds

The position ofintercalated potassiumacts as an initiation pointof oxidation

K ionKC8 with tartrate

B C AA

γβ

δ

(a) (b)

(c)

Figure 7.15 Various structures of alkali metal intercalation compounds. (a) Plane view of MC8

(M: alkali metal) [89]. (b) Plain view of KC24 [90]. (c) Plane view of KC8 with tartrate [24].(a) Reprinted with permission from Z. Wang, S.M. Selbach, T. Grande, Van der Waals densityfunctional study of the energetics of alkali metal intercalation in graphite, RSC Adv.4 (8) (2014)4069e4079. Copyright 2014 The Royal Society of Chemistry. (b) Reprinted with permissionfrom J. Purewal, Hydrogen Adsorption by Alkali Metal Graphite Intercalation Compounds. Vol.(Chapter 2). 2010 Ph. D. Thesis, California Institute of Technology. (c) Reprinted withpermission from H. Yoon, et al., Intrinsic photoluminescence emission from subdomainedgraphene quantum dots, Adv. Mater. 28 (26) (2016) 5255e5261. Copyright 2016 Wiley-VCHVerlag GmbH and Co. KGaA, Weinheim.


they are limited to blue emission so far, it would be likely to be expected that the size ofsubdomains could be freely and uniformly controlled to tune the emission color or toenhance the color purity if a reliable method is developed to manipulate the position ofalkali metals between the graphite layers. The proof of this concept can be verifiedfrom scanning tunneling microscopy study or realization of longer wavelength emis-sion by cointercalation of longer chain with alkali metals.

7.4 Lighting applications of GQDs

Before of the excellent luminescent properties of the GQDs mentioned above, manyefforts have been made to apply GQDs in various types of light-emitting devices(LEDs) similar to the existing fluorescent materials such as phosphors or semicon-ductor quantum dots. Especially, the GQDs are low cost, eco-friendly carbon nanoma-terials that do not need expensive toxic heavy metals or rare earth metals. With highstability and unique PL properties, GQDs have demonstrated its potential for LEDssuch as color converter in white lighteemitting diodes (WLEDs) and active materialsin electroluminescent devices.

7.4.1 Down-converting white lighteemitting diodes

To make a down-converting type WLEDs, it is crucial to form a solid-state GQD-based composite film with a stable PL property. However, it has been considered asa challenging work to prepare a solid-state film using only GQDs due to the seriousaggregation caused fluorescence quenching (ACQ) [94,95]. Several studies havebeen reported to form a composite film with various polymer matrices to preventACQ and to improve optical properties. Because of the excellent UV stability andbroad spectral shape of GQDs, the white light emission with high color rendering in-dex and enhanced luminous efficiency could be realized.

In 2012, Luk et al. [96] reported a GQDeagar composite as a color-converting ma-terial in blue LEDs for white light emission (Fig. 7.16(a)). They prepared GQDs bymicrowave-assisted pyrolysis from glucose, and WLEDs were fabricated by coatinga GQDeagar composite onto a blue-emitting LED. The emission wavelength ofblue LED at 410 nm served as an excitation source for GQDeagar composite, whichresults in the PL at 524 nm. The white light emission was achieved by combining thesetwo emission peaks, with over w61% light-conversion efficiency and over 100-hourcolor stability. After 2 years, P. Roy et al. [98] prepared a white lighteconverting capusing a plant leafederived green-emitting GQDs. The green-emitting GQDs werefabricated from raw plant leaf extracts such as neem and fenugreek via hydrothermalmethod. The white lighteconverting cap was fabricated by coating onto a lab madePET cap using the PMMA solution containing red-emitting chlorophyll (CPY),green-emitting GQDs, and blue-emitting quinine sulfate. It was mounted on a near-UV LED and emitted white light by an excitation of 380 nm from the LED. Tetsukaet al. [97] made a flexible composite film using cellulose nanofiber wrapped-amino


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Figure 7.16 (a) Fluorescence images of (a-i) the GQD solutions and (a-ii) GQDeagar composites. The photograph of the as-fabricated WLED (a-iii)without and (a-iv) with biased current. (a-v) EL spectra of the WLED under various forward currents [96]. (b) Emission images from the afGQDs@CNFeclay hybrids excited using a UV lamp (365 nm). Each film has contents following afGQDs with different emission colors: I, blue-emittingafGQDs; II, green-emitting afGQDs; III, yellow-emitting afGQDs; IV, orange-emitting afGQDs. (b-i) Luminescence spectra and luminous efficiencychange (inset) of GWLED with yellow and orange-emitting afGQDs under various forward currents. (b-ii) Luminescence spectra and luminousefficiency change (inset) of GWLED with green- and orange-emitting afGQDs under various forward currents. (b-iii) CIE coordinates of the blueLED chip (circle) and GWLEDs [97].(a) Reprinted with permission from C. Luk, et al., An efficient and stable fluorescent graphene quantum doteagar composite as a converting material inwhite light emitting diodes, J. Mater. Chem. 22 (42) (2012) 22378e22381. Copyright 2012 The Royal Society of Chemistry. (b) (Reprinted withpermission from H. Tetsuka, A. Nagoya, R. Asahi, Highly luminescent flexible amino-functionalized graphene quantum dots@ cellulosenanofibereclay hybrids for white-light emitting diodes, J. Mater. Chem. C. 3 (15) (2015) 3536e3541. Copyright 2015 The Royal Society ofChemistry.

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functionalized GQDs (CNF@afGQDs) which exhibited colorful PL (Fig. 7.16(b)).The blue-, green-, yellow-, and orange-emitting afGQDs were prepared from oxidizedgraphene nanosheets with amino-hydrothermal treatments. The multicolor-emittingafGQDs@CNFeclay composite films were produced by incorporating CNF@afGQDs into the flexible clay platelets, and afGQD-based WLEDs (GWLEDs) werefabricated by coating the composite films onto the blue LED. Finally, the GWLEDsshowed pure white light emission with a CIE coordinate of (0.33, 0.37) and luminousefficiencies over 31 lm/W.

7.4.2 Electroluminescent devices

In addition to the color converterebased WLEDs, the efforts have been made to applyGQDs as active materials for electroluminescent devices. It started out by introducingGQDs into the active layer of organic lighteemitting diodes (OLEDs). In 2011, Guptaet al. [99] fabricated active layer using the mixture of MEH-PPV with methylene blue(MB)efunctionalized GQDs (1%). In comparison to the pristine MEH-PPV device,the turn-on voltage of MEH-PPV/MB-GQDs device was decreased from 6 to 4 V asshown in Fig. 7.17(a). They explained that the MB-GQDs dispersed in MEH-PPV pro-vide more charge transport pathways, resulting in the increase of carrier density, andlowering turn-on voltage. At higher than 3% of mixing ratio, charge trapping andshortening effect occur due to agglomeration. After a few years, Kwon et al. [100] pre-pared size-controlled GQDs via amidative cutting of tattered graphite. The size of theGQDs could be tuned by amine concentration, resulting in bandgap tuning with PLfrom blue to brown. Finally, they fabricated OLED active layer with GQDs as dopants,and 4,40-bis(carbazol-9-yl)biphenyl (CBP) as a host material as shown in Fig. 7.17(b).The white EL was achieved by energy transfer from the host to the GQDs withmaximum luminance of 10 cd/m2 and external quantum efficiency (EQE) of 0.1%.Immediately after that, Song et al. [62] reported highly efficient blue GQD-LEDs usingGQDs from GIC method (Fig. 7.17(c)). The device structure of GQD-LEDs is con-sisted of ITO anode, a 30 nm poly(ethylenedioxythiophene):polystyrene sulfonate(PEDOT:PSS) hole injection layer, a 20 nm poly(vinyl carbazole) with 3 wt.%-dopedGQDs active layer, a 20 nm 2,20,200-(1,3,5-benzinetriyl)-tris(1-phynyl-1-H-benzimid-azole) (TPBi) electron transport layer, a 1 nm LiF and 100 nm Al cathode. With theadvantages of additional carrier transport/injection pathways from doped GQDs, thechance of radiative recombination increased. The device with PVK:GQDs (3 wt.%)showed decreased turn-on voltage (8 V) with enhanced luminance (1000 cd/m2) andcurrent efficiency (0.65 cd/A), compared to that of reference device without GQDsshowed 13 V, 10 cd/m2, and 0.49 cd/A, respectively.

Meanwhile, Kim et al. [101] revealed the origin of EL mechanism from GQDsincorporated polymer lighteemitting diodes (PLEDs) (Fig. 7.18(a)). They used a sim-ple device structure with ITO anode, a 40 nm PEDOT:PSS hole injection layer, a80 nm GOQDs doped (0.4 wt.%) PVK active layer, and a 100 nm LiF/Al cathode.The EL emission from GOQD-doped PVK layer showed white color, compared tothat of PVK layer without GOQDs showed violet color. Although the performancesof GOQD-embedded PLEDs were poor (luminance w1 cd/m2), they focused on


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Figure 7.17 (a) Measured current density of MEH-PPV with MB-GQDs (a-i) 0%, (a-ii) 0.5%,(a-iii) 1%, and (a-iv) 3% as a function of the applied voltage (V). The inset plots the elec-troluminescence spectrum of the MEH-PPV (red) and MEH-PPV/MB-GQDs (1%) (black).The inset also shows the band diagram of the MEH-PPV/MB-GQDs and the recordedbrightness of MEH-PPV LED and MEH-PPV/MB-GQDs (1%) LED [99]. (b-i) Physical andelectronic structures of OLEDs employing the GQDs. GraHIL and TPBI (1,3,5-tri(phenyl-2-benzimidazolyl)-benzene) are hole and electron transporting layers, respectively. (b-ii)Normalized EL intensity of a set of OLEDs at a fixed bias (13 V). (b-iii) Photo of white lightemission from an OLED employing the 10 nm GQDs [100]. (c) Device characteristics ofGQD-LEDs. (c-i) Schematic illustration of the GQD-LEDs structure and the correspondingband diagram. (c-ii) Left: luminanceevoltage (L-V) characteristic curves for the fabricatedGQD-LEDs. The highest luminance, exceeding 1000 cd m�2, was measured for a device with3.0 wt% GQDs at 16 V. Right: electroluminescent image of GQD-LEDs, consisting of fiveemitting areas [62].(a) Reprinted with permission from V. Gupta, et al., Luminscent graphene quantum dots fororganic photovoltaic devices, J. Am. Chem. Soc. 133 (26) (2011) 9960e9963. Copyright 2011American Chemical Society. (b) Reprinted with permission from W. Kwon, et al.,Electroluminescence from graphene quantum dots prepared by amidative cutting of tatteredgraphite. Nano Lett. 14 (3) (2014) 1306e1311. Copyright 2014 American Chemical Society. (c)Reprinted with permission from S.H. Song, et al., Highly efficient light-emitting diode ofgraphene quantum dots fabricated from graphite intercalation compounds, Adv. Opt. Mater. 2(11) (2014) 1016e1023. Copyright 2014Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim.


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Figure 7.18 (a-i) Schematic energy diagram of GOQDs embedded PLEDs. (a-ii) digital camera images of PLEDs using pristine PVK and GOQD-blended PVK as emissive layers, respectively (applied bias was 11V). (a-iii) Calculated MO and electric transition. The MO and possible electronictransition associated [101]. (b) GQD-LEDs demonstration. (b-i) Structure of GQD-LEDs. (b-ii) Energy levels in GQD-LEDs. (b-iii to b-vi) Elec-troluminescence spectra (top) and photographs (bottom) of host-only (b-iii), 1- (b-iv), 2- (b-v), and 3-LEDs (b-vi) with PL and EL [102].(a) Reprinted with permission from J.K. Kim, et al., Origin of white electroluminescence in graphene quantum dots embedded host/guest polymer lightemitting diodes, Sci. Rep. 5 (2015) srep11032. Copyright 2015 Macmillan Publishers Limited. (b) Reprinted with permission from W. Kwon, et al.,High color-purity green, orange, and red light-emitting diodes based on chemically functionalized graphene quantum dots. Sci. Rep. 6 (2016) 24205.Copyright 2016 Macmillan Publishers Limited.

142Graphene

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investigating its origin. The results of steady-state PL, time-correlated single photoncounting (TCSPC), and density functional theory (DFT) calculations revealed thatwhite EL is attributed to the hybridized GOQD-PVK complex emission with energytransfer and the interaction between the individual GOQDs and PVK emissions. Later,Kwon et al. [102] reported multicolor-emitting GQD-LEDs with improved efficiencyand color purity as shown in Fig. 7.18(b). They prepared functionalized GQDs withaniline derivatives that generate new extrinsic energy states for green, orange, andred emissions. Results of transient absorption and time-resolved PL spectroscopiesshow that underlying carrier dynamics of extrinsic electronic transitions is related tothe chemical properties of aniline derivatives. The chemically functionalized GQD-based LEDs are fabricated using the structure with an ITO as anode, a SOHIL as ahole injection layer, an emissive layer with GQDs as dopants and TCTA:TPBi as acohost material, a TPBi electron transport layer, and a LiF/Al as a cathode. Becauseof the aniline functionalizationederived extrinsic energy states, the GQD-LEDs dis-played green, orange, and red EL that has narrow linewidths (FWHM < 80 nm).The best performances from green GQD-LEDs with maximum luminance of390 cd/m2, EQE ¼ 1.28%, and current efficiency of 3.47 cd/A are achieved.

Luo et al. [103] reported another recent research for GQD-LEDs using microwave-assisted prepared white fluorescent GQDs (WGQDs) in active layer of OLEDs as dop-ants with CBP host material (Fig. 7.19(a)). The device is consisted of an ITO anode,PEDOT:PSS as a hole injection layer, a GQD-doped (10 wt.%) CBP host as an activelayer, a TPBi as an electron transport layer, and a LiF/Al cathode. The white EL emis-sion was obtained from the WGQDs with a luminance of 200 cd/m2, and EQE of 0.2%.Kim et al. [104] proposed highly efficient GQD-LEDs based on octadecylamine(ODA) functionalized GQDs (Fig. 7.19(b)). The device structure is composed ofITO anode, PEDOT:PSS hole injection layer, a ODA-GQDs doped PVK active layer,a TPBi electron transport layer, and a LiF/Al cathode. The EL emission was positionedat 516 nm with ultrahigh current efficiency of 6.51 cd/A and EQE of 2.67%. This ef-ficiency enhancement of GQD-LEDs was originated from the efficient energy transferbetween the PVK and the ODA-GQDs and the increase of the number of holes injectedinto the ODA-GQDs.

7.5 Summary and outlooks

Emerging new class of fluorescent GQDs with unique PL characteristics, much prog-ress has made in the midst of enormous interest on exploring low cost, simple syntheticroutes, and practical applicability for LEDs. Still, there are many further issues to besolved in utilizing thin and small GQDs for optoelectronics. Although a variety of syn-thetic routes is available, the difficulty in fine control of oxidation is a roadblock. Inthis regard, constantly exploring the nonoxidizing ways for the preparation of high-quality GQDs is of importance. The high PL-QY for GQDs is also critical requirementfor optoelectronic applications. The PL-QYs can be enhanced by introducing matrixmaterials, as well as surface functionalization or passivation. As we discussed above,


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Figure 7.19 (a) Characteristics of WLED based on WGQDs. (a-i) Current densityevoltage (JeV) and (a-ii) schematic energy-level diagram of thedevice (the inset in panel (a-i) is the schematic structure of WLED device). (a-iii) Electroluminescent spectra of the WGQD-based WLED withapplied voltages from 5 to 14 V. (a-iv) Photograph of white electroluminescence of the device operated at 14 V [103]. (b-ii) Schematic energydiagram. (b-iii and iv) Luminanceevoltage and current densityeluminance curves for LEDs with PVK, PVK:ODA NPs, PVK:GQDs, and PVK:ODA-GQDs [104].(a) Reprinted with permission from Z. Luo, et al., Microwave-assisted preparation of white fluorescent graphene quantum dots as a novel phosphor forenhanced white-light-emitting diodes, Adv. Funct. Mater. 26 (16) (2016) 2739e2744. Copyright 2016 Wiley-VCH Verlag GmbH and Co. KGaA,Weinheim. (b-i) Device structure of PVK:ODA-GQDs-LEDs. (b) Reprinted with permission from D.H. Kim, Kim. T.W. Ultrahigh current efficiency oflight-emitting devices based on octadecylamine-graphene quantum dots, Nano Energy 32 (2017) 441e447. Copyright 2017 Elsevier Ltd.

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the implementation of GQDs is spanning to various LEDs such as color convertingfilms, active layers for both of carrier injection type and field-induced EL. In otherwords, the device architecture should be precisely designed depending on the opera-tion types. The device factors including active layer composition, band alignment,and layer thickness become all important in determining the performances of GQD-LEDs. With those studies on fundamentals and applications, we are expecting theresearches on GQD-LEDs to be highly promising for the next-generation flexibledisplays.

References

[1] A. Tartakovskii, Quantum Dots: Optics, Electron Transport and Future Applications,Cambridge University Press, 2012.

[2] A.L. Efros, A.L. Efros, Interband absorption of light in a semiconductor sphere, Sov.Phys. Semicond. Ussr 16 (7) (1982) 772e775.

[3] R. Rossetti, et al., Excited electronic states and optical spectra of ZnS and CdS crystallitesin the |15 to 50 Å size range: evolution from molecular to bulk semiconductingproperties, J. Chem. Phys. 82 (1) (1985) 552e559.

[4] H. Drexler, et al., Spectroscopy of quantum levels in charge-tunable InGaAs quantumdots, Phys. Rev. Lett. 73 (16) (1994) 2252.

[5] R. Haglund Jr., et al., Nonlinear optical properties of metal-quantum-dot compositessynthesized by ion implantation, Nucl. Instrum. Methods Phys. Res. Sect. B BeamInteract. Mater. Atoms 91 (1e4) (1994) 493e504.

[6] J.L. Boone, G. Cantwell, M.D. Shaw, Free electron density measurements by IR ab-sorption in CdS, J. Appl. Phys. 58 (6) (1985) 2296e2301.

[7] A.P. Alivisatos, Semiconductor clusters, nanocrystals, and quantum dots, Science 271(5251) (1996) 933e937.

[8] J.M. Luther, et al., Localized surface plasmon resonances arising from free carriers indoped quantum dots, Nat. Mater. 10 (5) (2011) 361.

[9] D. Gammon, et al., Homogeneous linewidths in the optical spectrum of a single galliumarsenide quantum dot, Science 273 (5271) (1996) 87e90.

[10] O.E. Semonin, J.M. Luther, M.C. Beard, Quantum dots for next-generation photovoltaics,Mater. Today 15 (11) (2012) 508e515.

[11] A.C. Neto, et al., The electronic properties of graphene, Rev. Mod. Phys. 81 (1) (2009)109.

[12] K.S. Novoselov, et al., A roadmap for graphene, Nature 490 (7419) (2012) 192.[13] J.-N. Fuchs, M.O. Goerbig, Introduction to the Physical Properties of Graphene, Lecture

notes, 2008.[14] J. Shen, et al., Graphene quantum dots: emergent nanolights for bioimaging, sensors,

catalysis and photovoltaic devices, Chem. Commun. 48 (31) (2012) 3686e3699.[15] J. Melville, M. Kapelewski, Optical Properties of Quantum Dots, UC Berkley College of

Chemistry, 2015. https://www.ocf.berkeley.edu/wjmlvll/lab-reports/quantumDots/quantumDots.pdf.


https://www.ocf.berkeley.edu/%7Ejmlvll/lab-reports/quantumDots/quantumDots.pdf



[16] Y. Dong, et al., One-step and high yield simultaneous preparation of single-and multi-layer graphene quantum dots from CX-72 carbon black, J. Mater. Chem. 22 (18)(2012) 8764e8766.

[17] J. Peng, et al., Graphene quantum dots derived from carbon fibers, Nano Lett. 12 (2)(2012) 844e849.

[18] Z. Zhang, et al., Graphene quantum dots: an emerging material for energy-related ap-plications and beyond, Energy Environ. Sci. 5 (10) (2012) 8869e8890.

[19] S. M€uller, K. M€ullen, Expanding benzene to giant graphenes: towards molecular devices,Phil. Trans. R. Soc. Lond. A 365 (1855) (2007) 1453e1472.

[20] M.A. Sk, et al., Revealing the tunable photoluminescence properties of graphene quantumdots, J. Mater. Chem. C 2 (34) (2014) 6954e6960.

[21] J. Lu, et al., Transforming C 60 molecules into graphene quantum dots, Nat. Nanotechnol.6 (4) (2011) 247.

[22] X. Yan, X. Cui, L.-S. Li, Synthesis of large, stable colloidal graphene quantum dots withtunable size, J. Am. Chem. Soc. 132 (17) (2010) 5944e5945.

[23] J. Lee, et al., Uniform graphene quantum dots patterned from self-assembled silicananodots, Nano Lett. 12 (12) (2012) 6078e6083.

[24] H. Yoon, et al., Intrinsic photoluminescence emission from subdomained graphenequantum dots, Adv. Mater. 28 (26) (2016) 5255e5261.

[25] K.P. Loh, et al., Graphene oxide as a chemically tunable platform for optical applications,Nat. Chem. 2 (12) (2010) 1015.

[26] T. Heitz, et al., Radiative and nonradiative recombination in polymerlike a� C: H films,Phys. Rev. B 60 (8) (1999) 6045.

[27] W. Cai, et al., Synthesis and solid-state NMR structural characterization of 13C-labeledgraphite oxide, Science 321 (5897) (2008) 1815e1817.

[28] X. Sun, et al., Nano-graphene oxide for cellular imaging and drug delivery, Nano Res. 1(3) (2008) 203e212.

[29] Z. Luo, et al., Photoluminescence and band gap modulation in graphene oxide, Appl.Phys. Lett. 94 (11) (2009) 111909.

[30] G. Eda, et al., Blue photoluminescence from chemically derived graphene oxide, Adv.Mater. 22 (4) (2010) 505e509.

[31] C.T. Chien, et al., Tunable photoluminescence from graphene oxide, Angew. Chem. Int.Ed. 51 (27) (2012) 6662e6666.

[32] T.-F. Yeh, et al., Elucidating quantum confinement in graphene oxide dots based onexcitation-wavelength-independent photoluminescence, J. Phys. Chem. Lett. 7 (11)(2016) 2087e2092.

[33] S.H. Jin, et al., Tuning the photoluminescence of graphene quantum dots through thecharge transfer effect of functional groups, ACS Nano 7 (2) (2013) 1239e1245.

[34] X. Li, et al., Carbon and graphene quantum dots for optoelectronic and energy devices: areview, Adv. Funct. Mater. 25 (31) (2015) 4929e4947.

[35] S. Zhu, et al., The photoluminescence mechanism in carbon dots (graphene quantum dots,carbon nanodots, and polymer dots): current state and future perspective, Nano Res. 8 (2)(2015) 355e381.

[36] X.T. Zheng, et al., Glowing graphene quantum dots and carbon dots: properties, syn-theses, and biological applications, Small 11 (14) (2015) 1620e1636.

[37] C.K. Chua, et al., Synthesis of strongly fluorescent graphene quantum dots by cage-opening buckminsterfullerene, ACS Nano 9 (3) (2015) 2548e2555.


[38] F. Yang, et al., Influence of pH on the fluorescence properties of graphene quantum dotsusing ozonation pre-oxide hydrothermal synthesis, J. Mater. Chem. 22 (48) (2012)25471e25479.

[39] F. Jiang, et al., Eco-friendly synthesis of size-controllable amine-functionalized graphenequantum dots with antimycoplasma properties, Nanoscale 5 (3) (2013) 1137e1142.

[40] S. Zhu, et al., Strongly green-photoluminescent graphene quantum dots for bioimagingapplications, Chem. Commun. 47 (24) (2011) 6858e6860.

[41] F. Liu, et al., Facile synthetic method for pristine graphene quantum dots and grapheneoxide quantum dots: origin of blue and green luminescence, Adv. Mater. 25 (27) (2013)3657e3662.

[42] M.H. Jang, et al., Is the chain of oxidation and reduction process reversible in luminescentgraphene quantum dots? Small 11 (31) (2015) 3773e3781.

[43] Y. Feng, et al., Enhancement in the fluorescence of graphene quantum dots by hydrazinehydrate reduction, Carbon 66 (2014) 334e339.

[44] L. Li, et al., Focusing on luminescent graphene quantum dots: current status and futureperspectives, Nanoscale 5 (10) (2013) 4015e4039.

[45] M. Bacon, S.J. Bradley, T. Nann, Graphene quantum dots, Part. Part. Syst. Charact. 31 (4)(2014) 415e428.

[46] D. Pan, et al., Hydrothermal route for cutting graphene sheets into blue-luminescentgraphene quantum dots, Adv. Mater. 22 (6) (2010) 734e738.

[47] D. Pan, et al., Cutting sp 2 clusters in graphene sheets into colloidal graphene quantumdots with strong green fluorescence, J. Mater. Chem. 22 (8) (2012) 3314e3318.

[48] H. Tetsuka, et al., Optically tunable amino-functionalized graphene quantum dots, Adv.Mater. 24 (39) (2012) 5333e5338.

[49] H. Tetsuka, et al., Molecularly designed, nitrogen-functionalized graphene quantum dotsfor optoelectronic devices, Adv. Mater. 28 (23) (2016) 4632e4638.

[50] S. Kim, et al., Anomalous behaviors of visible luminescence from graphene quantumdots: interplay between size and shape, ACS Nano 6 (9) (2012) 8203e8208.

[51] S. Zhu, et al., Graphene quantum dots with controllable surface oxidation, tunablefluorescence and up-conversion emission, RSC Adv. 2 (7) (2012) 2717e2720.

[52] K. Lingam, et al., Evidence for edge-state photoluminescence in graphene quantum dots,Adv. Funct. Mater. 23 (40) (2013) 5062e5065.

[53] L. Wang, et al., Common origin of green luminescence in carbon nanodots and graphenequantum dots, ACS Nano 8 (3) (2014) 2541e2547.

[54] J. Shen, et al., Facile preparation and upconversion luminescence of graphene quantumdots, Chem. Commun. 47 (9) (2011) 2580e2582.

[55] R. Ye, et al., Coal as an abundant source of graphene quantum dots, Nat. Commun. 4(2013) 2943.

[56] L. Wang, et al., Unraveling bright molecule-like state and dark intrinsic state in green-fluorescence graphene quantum dots via ultrafast spectroscopy, Adv. Opt. Mater. 1 (3)(2013) 264e271.

[57] Y. Sun, et al., Large scale preparation of graphene quantum dots from graphite withtunable fluorescence properties, Phys. Chem. Chem. Phys. 15 (24) (2013) 9907e9913.

[58] P. Luo, et al., Aryl-modified graphene quantum dots with enhanced photoluminescenceand improved pH tolerance, Nanoscale 5 (16) (2013) 7361e7367.

[59] R. Sekiya, et al., White-light-emitting edge-functionalized graphene quantum dots,Angew. Chem. Int. Ed. 53 (22) (2014) 5619e5623.


[60] M. Park, et al., Combination of a sample pretreatment microfluidic device with a pho-toluminescent graphene oxide quantum dot sensor for trace lead detection, Anal. Chem.87 (21) (2015) 10969e10975.

[61] L. Lin, S. Zhang, Creating high yield water soluble luminescent graphene quantum dotsvia exfoliating and disintegrating carbon nanotubes and graphite flakes, Chem. Commun.48 (82) (2012) 10177e10179.

[62] S.H. Song, et al., Highly efficient light-emitting diode of graphene quantum dots fabri-cated from graphite intercalation compounds, Adv. Opt. Mater. 2 (11) (2014)1016e1023.

[63] L.L. Li, et al., A facile microwave avenue to electrochemiluminescent two-color graphenequantum dots, Adv. Funct. Mater. 22 (14) (2012) 2971e2979.

[64] S. Chen, et al., Unusual emission transformation of graphene quantum dots induced byself-assembled aggregation, Chem. Commun. 48 (61) (2012) 7637e7639.

[65] H. Sun, et al., Improvement of photoluminescence of graphene quantum dots with abiocompatible photochemical reduction pathway and its bioimaging application, ACSAppl. Mater. Interfaces 5 (3) (2013) 1174e1179.

[66] L. Tang, et al., Deep ultraviolet photoluminescence of water-soluble self-passivatedgraphene quantum dots, ACS Nano 6 (6) (2012) 5102e5110.

[67] J. Lu, et al., One-pot synthesis of fluorescent carbon nanoribbons, nanoparticles, andgraphene by the exfoliation of graphite in ionic liquids, ACS Nano 3 (8) (2009)2367e2375.

[68] Y. Li, et al., Nitrogen-doped graphene quantum dots with oxygen-rich functional groups,J. Am. Chem. Soc. 134 (1) (2011) 15e18.

[69] Y. Li, et al., An electrochemical avenue to green-luminescent graphene quantum dots aspotential electron-acceptors for photovoltaics, Adv. Mater. 23 (6) (2011) 776e780.

[70] M. Zhang, et al., Facile synthesis of water-soluble, highly fluorescent graphene quantumdots as a robust biological label for stem cells, J. Mater. Chem. 22 (15) (2012)7461e7467.

[71] D.B. Shinde, V.K. Pillai, Electrochemical preparation of luminescent graphene quantumdots from multiwalled carbon nanotubes, Chem. A Eur. J. 18 (39) (2012) 12522e12528.

[72] A. Ananthanarayanan, et al., Facile synthesis of graphene quantum dots from 3D gra-phene and their application for Fe3þ sensing, Adv. Funct. Mater. 24 (20) (2014)3021e3026.

[73] X. Tan, et al., Electrochemical synthesis of small-sized red fluorescent graphene quantumdots as a bioimaging platform, Chem. Commun. 51 (13) (2015) 2544e2546.

[74] Z. Li, et al., How graphene is cut upon oxidation? J. Am. Chem. Soc. 131 (18) (2009)6320e6321.

[75] G.S. Kumar, et al., Amino-functionalized graphene quantum dots: origin of tunableheterogeneous photoluminescence, Nanoscale 6 (6) (2014) 3384e3391.

[76] F. Liu, et al., Tuning photoluminescence of reduced graphene oxide quantum dots fromblue to purple, J. Appl. Phys. 115 (16) (2014) 164307.

[77] W. Chen, L. Yan, P.R. Bangal, Preparation of graphene by the rapid and mild thermalreduction of graphene oxide induced by microwaves, Carbon 48 (4) (2010) 1146e1152.

[78] Y. Zhu, et al., Carbon-based supercapacitors produced by activation of graphene, Science332 (6037) (2011) 1537e1541.

[79] X. Zhou, et al., Photo-Fenton reaction of graphene oxide: a new strategy to preparegraphene quantum dots for DNA cleavage, ACS Nano 6 (8) (2012) 6592e6599.


[80] X. Yan, et al., Large, solution-processable graphene quantum dots as light absorbers forphotovoltaics, Nano Lett. 10 (5) (2010) 1869e1873.

[81] R. Liu, et al., Bottom-up fabrication of photoluminescent graphene quantum dots withuniform morphology, J. Am. Chem. Soc. 133 (39) (2011) 15221e15223.

[82] K. Habiba, et al., Luminescent graphene quantum dots fabricated by pulsed laser syn-thesis, Carbon 64 (2013) 341e350.

[83] J. Kim, J.S. Suh, Size-controllable and low-cost fabrication of graphene quantum dotsusing thermal plasma jet, ACS Nano 8 (5) (2014) 4190e4196.

[84] L. Wang, et al., Direct observation of quantum-confined graphene-like states and novelhybrid states in graphene oxide by transient spectroscopy, Adv. Mater. 25 (45) (2013)6539e6545.

[85] S. Park, et al., Aqueous suspension and characterization of chemically modified graphenesheets, Chem. Mater. 20 (21) (2008) 6592e6594.

[86] J. Shen, et al., One-pot hydrothermal synthesis of graphene quantum dots surface-passivated by polyethylene glycol and their photoelectric conversion under near-infrared light, New J. Chem. 36 (1) (2012) 97e101.

[87] Z. Gan, H. Xu, Y. Hao, Mechanism for excitation-dependent photoluminescence fromgraphene quantum dots and other graphene oxide derivates: consensus, debates andchallenges, Nanoscale 8 (15) (2016) 7794e7807.

[88] R. Schl€ogl, H. Boehm, The reaction of potassium-graphite intercalation compounds withwater, Carbon 22 (4e5) (1984) 351e358.

[89] Z. Wang, S.M. Selbach, T. Grande, Van der Waals density functional study of the en-ergetics of alkali metal intercalation in graphite, RSC Adv. 4 (8) (2014) 4069e4079.

[90] J. Purewal, Hydrogen Adsorption by Alkali Metal Graphite Intercalation Compounds.Vol. (Chapter 2), Ph. D. Thesis, 2010. California Institute of Technology.

[91] G. Yoon, et al., Factors affecting the exfoliation of graphite intercalation compounds forgraphene synthesis, Chem. Mater. 27 (6) (2015) 2067e2073.

[92] T. Maluangnont, et al., Preparation of a homologous series of graphite alkylamineintercalation compounds including an unusual parallel bilayer intercalate arrangement,Chem. Mater. 23 (5) (2011) 1091e1095.

[93] J. Xu, et al., Recent progress in graphite intercalation compounds for rechargeable metal(Li, Na, K, Al)-Ion batteries, Adv. Sci. 4 (10) (2017).

[94] Z. Gan, H. Xu, Y. Fu, Photon reabsorption and nonradiative energy-transfer-inducedquenching of blue photoluminescence from aggregated graphene quantum dots, J. Phys.Chem. C 120 (51) (2016) 29432e29438.

[95] S. Hassanzadeh, K.H. Adolfsson, M. Hakkarainen, Controlling the cooperative self-assembly of graphene oxide quantum dots in aqueous solutions, RSC Adv. 5 (71)(2015) 57425e57432.

[96] C. Luk, et al., An efficient and stable fluorescent graphene quantum doteagar compositeas a converting material in white light emitting diodes, J. Mater. Chem. 22 (42) (2012)22378e22381.

[97] H. Tetsuka, A. Nagoya, R. Asahi, Highly luminescent flexible amino-functionalizedgraphene quantum dots@ cellulose nanofibereclay hybrids for white-light emitting di-odes, J. Mater. Chem. C 3 (15) (2015) 3536e3541.

[98] P. Roy, et al., Plant leaf-derived graphene quantum dots and applications for white LEDs,New J. Chem. 38 (10) (2014) 4946e4951.


[99] V. Gupta, et al., Luminscent graphene quantum dots for organic photovoltaic devices,J. Am. Chem. Soc. 133 (26) (2011) 9960e9963.

[100] W. Kwon, et al., Electroluminescence from graphene quantum dots prepared by amida-tive cutting of tattered graphite, Nano Lett. 14 (3) (2014) 1306e1311.

[101] J.K. Kim, et al., Origin of white electroluminescence in graphene quantum dots embeddedhost/guest polymer light emitting diodes, Sci. Rep. 5 (2015) srep11032.

[102] W. Kwon, et al., High color-purity green, orange, and red light-emitting diodes based onchemically functionalized graphene quantum dots, Sci. Rep. 6 (2016) 24205.

[103] Z. Luo, et al., Microwave-assisted preparation of white fluorescent graphene quantumdots as a novel phosphor for enhanced white-light-emitting diodes, Adv. Funct. Mater. 26(16) (2016) 2739e2744.

[104] D.H. Kim, T.W. Kim, Ultrahigh current efficiency of light-emitting devices based onoctadecylamine-graphene quantum dots, Nano Energy 32 (2017) 441e447.


Graphene-based compositeemitter 8Hong Hee Kim, Won Kook ChoiCenter for Optoelectronic Materials and Devices, Post-Si Semiconductor Institute, KoreaInstitute of Science and Technology (KIST), Seoul, Korea

8.1 Grapheneemetal/metal oxide hybrid composite

Graphene is known as a semimetal with a zero bandgap and a single layer of carbonatoms arranged in a hexagonal lattice or an infinite alternant (only six-member carbonring) polycyclic aromatic hydrocarbon [1,2]. Because of its bandgapless property,graphene is not considered useful for absorption or photoemission-based optoelec-tronic applications. Unlike 2-D graphene, the opening of bandgap has been observedin the finite sized 0D graphene quantum dots (GQDs) due to quantum confinementphenomenon. In a similar way, photoluminescence (PL) of poly-aromatic moleculesshowed wavelength shifts toward lower energy with the increase of conjugation length[3,4]. The sharp peak observed at about 275 nm (z4.5 eV) in PL spectrum for GQDswas generally known as a pep* transition indicating the presence of sp2 covalentbonding. On the other hand, the visible light emission from GQDs was attributed tosurface traps and/or the edge states. And absorption peaks in visible range were widelyaccepted to come from the electronic transitions of n-p* in the surface states occurredwith their energy levels between p and p* states of C]C [5,6] induced by variousfunctional groups of CeOH, C]O, OeC]O, and C]N during the preparation ofGQDs. Analogous to absorption spectra, PL of the GQDs is also dependent on boththe physical size, shape, and fraction of sp2 domains of the GQDs, chemical environ-ment, and surface functionality such as pH and solvent. pep* stacking interaction andstrong attraction between graphene sheets directly aggregated two graphene nano-sheets and limited its application. Therefore, graphene hybridization using differentmaterials is a powerful technique for increasing its usage. Recent researches revealedthat numerous metals and metal oxides (MOs) such as platinum, silver, gold, palla-dium, titanium dioxide, ferric oxide, tin dioxide, cadmium selenide, and zinc oxide(ZnO) were utilized to hybridize with pure graphene or graphene oxide (GO) andimprove their capacitance.

In most cases, graphene can act passively as a support to disperse and stabilize thesemetal and MO nanoparticles (NPs). For examples of grapheneemetal composites,SneCo-graphene composites for superior lithium storage capability [7], grapheneemetal hybrid for catalyst using an aromatic amino acid as the reducing agent [8],and Pd-graphene for ethanol oxidation in alkaline media [9], Au on GOeFe3O4 nano-composite support for highly efficient catalyst [10], Pt electro-photo-synergistic


https://doi.org/10.1016/B978-0-08-102482-9.00008-3

catalyst on G-TiO2 for methanol oxidation [11], and noble metaledoped graphenecomposite for photocatalytic hydrogen evolution have been developed [12].

In addition, recently, numerous graphene/MO composites have been synthesizedand tested as highly efficient functional materials to improve photocatalytic behavior,electron transport property, and high rate chargeedischarge reversible storage capa-bility crucial factors determining the performance of the optoelectronic, electrochem-ical, and photoconversion devices such as photocatalyst, supercapacitor, Li ion battery,solar cells, and light-emitting diodes (LEDs). Graphene-MO nanocomposite such asgraphene-CuO (G-CuO) for the electrode material for lithium-ion batteries and highcatalytic activity and stability for CO oxidation [13,14], graphene-Co3O4 (G-Co3O4)for a highly reversible anode for lithium rechargeable batteries [15], graphene-ZnO(G-ZnO) for the electrode materials for electrochemical supercapacitors [16], and bet-ter photocatalytic performance and improved sensing property of ZnO [17], graphene-TiO2 (G-TiO2) for the electrode to increase Li-ion insertion/extraction in TiO2 [18] dueto the increased electrode conductivity in the presence of a percolated graphenenetwork embedded into the MO electrodes, for photocatalysts application in hydrogenevolution from water photocatalytic splitting [19], and an interfacial layer of G-TiO2synthesized by UV-assisted photocatalytic reduction of GO-TiO2 [20] between afluorine-doped tin oxide (FTO) and a nanocrystalline TiO2 film in dye-sensitized solarcells to decrease back-transport reaction of electron and the contact resistance betweenI3- ions in the electrolyte and FTO transparent conductive anode layer [21], flash photostimulation of human neural stem cells on G-TiO2 heterojunction for differentiationinto neurons [22], paper-based label-free photoelectrochemical immunoassay ofG-TiO2 complex nanopaper [23], graphene-SnO2 (G-SnO2) for the application ofhigh rate reversible anode for Li ion battery [24e28], selective and sensitive electro-chemical detection of dopamine [29], electrochemical supercapacitors [30], andgraphene-NiO (G-NiO) as an efficient electrocatalyst for glucose and methanol [31]have been prepared by various methods. Moreover, Upadhyay et al. [32] reviewedthe functionality of graphene/MO hybrid composites played as photocatalysts, adsor-bents, and disinfectants in water treatment [33].

8.2 Graphene-based composite emitter

The hybridization of semiconducting oxide NPs and graphene results in synergisticproperties of the individuals, or even creates new properties. Because large bandgapoxide semiconductors such as ZnO and SnO2 graphene-based composite with band-to-band or defect emission (DE) will be drastically influenced and controlled by thecharge transfer at the interface, these graphene hybrid composites are very promisingin the control of visible light PL and electroluminescence and can play actively as anemitter in the application of LEDs. So the interfacial phenomenon will be very inter-esting and needs to be well understood for further creation and application for moreefficient optoelectronic devices.


8.2.1 GrapheneeCdSe composite

Nanocarbons like C60, CNTs, and graphene have been well known as playing an effec-tive quenching material due to high electrical conductivity in their composites. Thisquenching phenomenon was mainly explained in terms of resonance (F€orster) energytransfer and additional charger transfer, which is much slower. The quenching rate offluorescence depends largely on the distance and chemical composition of nanocar-bons. The theoretical distance (z) dependence [34] of the dipole energy transfer rateto graphene is 1/z4 and the rate of energy transfer was calculated as w4 ns�1 forsingle-layer graphene (SLG) [35]. GrapheneeCdSe (GeCdSe) quantum dot (QDs)NP composites were synthesized for flexible photovoltaic solar cells [36].Fig. 8.1(a) shows the PL spectra of pure CdSe QDs and GeCdSe NP solution.From the PL measurement, it is revealed that the characteristic emission peak of theCdSe QDs is located at around 580 nm and about 75% of CdSe QD emission isquenched after CdSe QDs interacted with graphene sheets. Therefore, it has beendemonstrated that the graphene sheets are expected as the electron acceptor. Moreover,quench factors (QFs) of fluorescence of CdSe QDs for different graphene, reduced gra-phene oxide (rGO), and GO substrates, and few layered graphene (FLG) are analyzed[37]. As shown in Fig. 8.1(b), QF is the highest value of about 14 for SLG, 4 for rGO,1.8 for GO, and 8e9 for FLG. This means that the larger the amounts of sp2 hybridizedC atoms, the higher the QF. Effective quenching by resonance energy transfer seems torequire more free or unbound carriers (electron/hole).

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Figure 8.1 (a) Photoluminescence spectra from CdSe QDs and G-CdSe composites. (b)Calculated QFs for CdSe QDs on GO, rGO, SLG, and FLG.(a) Reprinted from J. Chen, F. Xu, J. Wu, K. Qasim, Y. Zhou, W. Lei, L.T. Suna, Y. Zhang,Flexible photovoltaic cells based on a graphene-CdSe quantum dot nanocomposite, Nanoscale 4(2012) 441e443. (b) Reprinted from X.T. Guo, Z.H. Ni, C.Y Liao, H.Y. Nan, Y. Zhang, W.W.Zhao, W.H. Wang, Fluorescence quenching of CdSe quantum dots on graphene, Appl. Phys.Lett. 103 (2013) 201909 1e4.

Graphene-based composite emitter 153

8.2.2 GrapheneeZnO hybrid composite

Kim et al. [38] reported that ZnO nanostructure could be grown on graphene layer bymetalorganic vaporephase epitaxy (MOVPE) at the substrate temperature up to 750�Cas shown in Fig. 8.2. Temperature-dependent PL spectra of the ZnO nanoneedles onthe graphene layers grown at 750�C were taken from 17 to 200K as depicted inFig. 8.3 and included a free exciton PL peak at 3.367 eV with no carbon-related defectpeak, suggesting that high-quality ZnO nanostructures were grown on the graphenelayers without deterioration of the graphene layers during MOVPE.

Recently, Rauwel et al. [39] reviewed the synthesis and PL properties of hybridZnO and carbon nanomaterials. Khenfouch et al. [40] reported that white light emis-sion from ZnO nanorod (NR) hybridized with FLG was successfully observed asshown in Fig. 8.4. rGO showed blue and red emissions at 482 and 686 nm respec-tively. In case of FLG/ZnO NR composite, green emission observed at 524 nm couldbe attributed to the radiative recombination of simply ionized oxygen vacancies (Vo

þ,Vo). They have attributed the emissions at 482 and 498 nm to isolated sp2 clusterswithin the carboneoxygen sp3 matrix [41]. Moreover, poor dispersion or aggregationof graphene flakes gives rise to emissions at 684 and 686 nm [42,43].

Hwang et al. [44] fabricated p-Si/ZnO/graphene hybrid structure and investigatedsurface plasmoneenhanced ultraviolet emission. In-plane wave number q-dependentdispersion relation of graphene plasmon can be expressed as a random phase approx-imation like

uðqÞ¼ nee2

ε0ð1þ εbÞm� qþ34½v2Fq2

i1=2(8.1)

where ne ¼ 1 � 1013cm2 is an electron concentration in graphene, εo is permittivity invacuum, εb is a dielectric constant of background, m* ¼ 0.077 me is an effective massof electron in graphene, and nF ¼ 1.12 � 106 m/s is Fermi velocity of electron. FromEq. (8.1), the graphene plasmon is resonantly excited at the bandgap energy ca. 3.3 eV

Figure 8.2 FE-SEM images of ZnO nanoneedles grown on the graphene substrate at (a) 400�C,(b) 600�C, and (c) 750�C by MOVPE.Reprinted from Y.J. Kim, J.H. Lee, G.C. Yi, Vertically aligned ZnO nanostructures grown ongraphene layers, Appl. Phys. Lett. 95 (2009) 213101 1e3.


of ZnO for an in-plane momentum q ¼ 2p/1.5 (nm�1). This means that radiationcaused by band-to-band transition in ZnO be enhanced by the coupling with thegraphene plasmon in G/ZnO hybrid system within the surface corrugation specified tothe modulus w1.5 nm of ZnO background as shown in Fig. 8.5. The PL intensityshows dependence on temperature, spacer thickness of SiO2 between ZnO and

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Figure 8.3 Temperature-dependent PL spectra of ZnO nanoneedles/graphene by MOVPE.Reprinted from Y.J. Kim, J.H. Lee, G.C. Yi, Vertically aligned ZnO nanostructures grown ongraphene layers, Appl. Phys. Lett. 95 (2009) 213101 1e3.

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Figure 8.4 PL from RGO and FLG/ZnO NRs.Reprinted from M. Khenfouch, M. Baïtoul, M. Maaza, White photoluminescence from a grownZnO nanorods/graphene hybrid nanostructure, Opt. Mater. 34 (2012) 1320e1326.


graphene, and the number of layer of graphene. Enhancement of band-edge emissionvia coupling with localized surface plasmon (LSP) has been investigated throughmetal-capped ZnO, metallodielectric-capped ZnO, and SWCNTs/ZnO structures.

In another work, Han et al. [45] also observed surface plasmoneenhanced PL ofZnO in a hybrid structure of rGO/ZnO nanorods. The relative ratio (IUV/IDE) of UVto DE was largely increased up to 14 times as the content of (CeC)% bonding inrGO increases. Enhanced feature of UV emission could be tuned by introducing theLSP resonances by controlling of the sp2 bonding in rGO. Suppression of DE fromsurface or intrinsic defects was suggested by the charge transfer of photoexcited elec-trons from ZnO to rGO and the decrease of charge trapped in the defects. IUV/IDE wasalso related to the thickness between rGO and ZnO nanorods and decreased as MgOseparating layer increased up to 20 nm as shown in Fig. 8.6.

Zeng et al. [46] grew ZnO nanowires (NWs) by hydrothermal method on the GOsubstrate and glass for comparison. As shown in PL (Fig. 8.7), near band-edge emis-sion (NBE) and green emission at 378 and 568 nm were observed in ZnO NWs onglass with INBE/IG ¼ 0.37. In PL for GO-ZnO NWs, NBE emission was muchenhanced, whereas green emission was greatly reduced and INBE/IG corresponded to4.33. The reasons for the increase of NBE peak could be ascribed to two facts.

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Figure 8.5 (a) Schematic image of G/ZnO, (b) schematically depicted PL enhancement pathfrom G/ZnO hybrid structure, (c) PL of ZnO, single-layer graphene (SG)/ZnO, bilayer gra-phene (BG)/ZnO, and (d) change of PL intensity ratio as a function of temperature at10e300K.Reprinted from S.W. Hwang, D.H. Shin, C.O. Kim, S.H. Hong, M.C. Kim, J. Kim, K.Y. Lim, S.Kim, S. Choi, K.J. Ahn, G. Kim, S.H. Sim, B.H. Hong, Plasmon enhanced ultravioletphotoluminescence from hybrid structures of graphene/ZnO films, Phys. Rev. Lett. 105 (2010)127403 1e4.


One is related to the improved crystalline quality of ZnO NWs compared to thosegrown on amorphous glass substrate because oxygen functional groups contained inGO surfaces initially gave a large contribution for growing a sound crystalline qualityof ZnO NWs. The other is a charge transfer at the interface between ZnO NWs andGO. As depicted in Fig. 8.7(b), some excited electrons into conduction band (Ec) byUV irradiation would be transited into directly valence band (Ev) to emit band-to-band emission or firstly transferred to defects states located just below Ec and sequen-tially trapped to graphene. In the latter case, radiation would not occur and lead toquenching of visible light emission.

Lee et al. [47] also synthesized both ZnO/unoxidized graphene (UG) nanocompo-site and ZnO/GO and investigated the PL in terms of free exciton emission (FEE) ofband-to-band transition and visible light emission of defect states. A typical FEE and abroad deep-level emission (DLE) were observed at 377 nm (3.26 eV) and 530 nm(2.34 eV) for ZnO/UG. DLE was assigned to the transition between Zni and Vo orbetween conduction band (CB) to Vo. As the amounts of UG increased, the intensityof FEE decreased, but that of DLE increased and was maximum at the contents of5wt% UG, which is not consistent with Hwang et al. [44] On the contrary, ZnO/GOshow reverse behavior. In particular, the blue shift of peak DLE from 530 to475 nm in ZnO/GO indicated the reduction of oxygen vacancy [48]. Decrease ofFEE in ZnO/UG can be explained by the lack of ability to provide oxygen to ZnO,but instead oxygen functional groups on GO made FEE largely enhanced by reducingoxygen vacancy.

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Figure 8.6 PL intensity obtained from ZnO, rGO/ZnO, and rGO/(5e20 nm) MgO/ZnO.Reprinted from F. Han, S. Yang, W. Jing, K. Jiang, Z. Jinag, H. Liu, L. Li, Surface plasmonenhanced photoluminescence of ZnO nanorods by capping reduced graphene oxide sheets, Opt.Express 22 (2014) 11436e11445.


8.2.3 GO-ZnO and TrGO-ZnO

William and Kamat [49] suggested excited-state interactions between ZnO NPs andgraphene. Singh et al. [50] synthesized and investigated PL properties of ZnO-GrOwith the variation of GO/ZnO ratio from 0 to 2wt%, where GO was synthesized byconventional Hummers’ method and GrO was obtained by reduction of GO withLiOH$H2O. It is interesting that PL from ZnO-GrO was not only blue shifted ascompared with ZnO but also greatly quenched as GO concentration increased asshown in Fig. 8.8. The quenching behavior was well explained by interfacial chargetransfer from ZnO to GrO. These results of blue shift and quenching could be attrib-uted to a formation of depletion region at the interface between the p-type conductivityof ZnO NPs and n-type conductivity of graphene.

Kaftelen et al. [51] reported that these two defects in ZnO NPs were suggested to belocated at the surface region and the core from the core-shell model. The core-shellmodel based on PL and electron paramagnetic resonance (EPR) suggests defect-related electronic states in the bandgap belonging to negatively charged Zn vacanciesand positively charged oxygen vacancies. This model well described that blue andgreen emissions are attributed to core-level and red emission to surface region shownin Fig. 8.9.

Recently Pham et al. [52] investigated the charge transfer and surface healing effectwithin ZnO NP decorated graphene oxide (GO-ZnO) and thiol-functionalized reducedgraphene oxide (TrGOeZnO) hybrid materials. As shown in Fig. 8.10, two EPR sig-nals in ZnO NPs were observed at Landé g-factor g1 ¼ 2.0037, which is close to that offree electron (g ¼ 2.0034) and g2 ¼ 1.9600, which were well known to originate fromthe singly ionized oxygen vacancy (Vo

þ) with an unpaired electron and the positivelycharged VZn

� , respectively.

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Figure 8.7 (a) PL spectra taken from ZnO NWs on glass and on GO. (b) Visible lightfluorescence quenching in ZnO NWs/GO due to charge trapping to GO through defect level inZnO.Reprinted from H. Zeng, Y. Cao, S. Xie, J. Yang, Z. Tang, W.X, Synthesis, Optical andelectrochemical properties of ZnO nanowires/graphene oxide heterostrucutres, Nanoscale Res.Lett. 8 (2013) 133e139.


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Figure 8.8 (a) PL spectra GO (curve 1) and GrO (curve 2). (b) PL spectra of ZnO and ofZnOeGrO sheets at different GO concentrations of (a) 0wt%, (b) 0.15wt%, (c) 0.6wt%, (d)1.2wt%, and (e) 2wt% taken at RT at excitation wavelength of 340 nm.Reprinted from G. Singh, A. Choudhary, D. Haranath, A.G. Joshi, N. Singh, R. Pasricha, ZnOdecorated luminescent graphene as a potential gas sensor at room temperature, Carbon 50 (2011)385e394.

Shelldefects

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Figure 8.9 EPR and PL from ZnO NPs after 1 min freeze milling and the assumed core shellfrom shell model consisting of a negatively charged interior and a positively charged outershell.Reprinted from H. Kaftelen, K. Ocakoglu, R. Thomann, S. Tu, S. Weber, E. Erdem, EPR andphotoluminescence spectroscopy studies on the defect structure of ZnO nanocrystals, Phys.Rev. B 86 (2012) 014113 1e9.


But in case of both GO-ZnO and TrGO, the core-related signal disappeared andeven the surface defecterelated signal was not detected in TrGO-ZnO. The electrontrapped in the surface region will be confined due to the high dielectric property ofZnO and thus the surface defect signal was still observed at g3 ¼ 2.0024. But the elec-tron captured in the core will be easily dislocated due to high mobility of core as shownin Fig. 8.11. After Fermi level alignment, captured electrons moved to the surfaceregion from the core and could be transferred to GO and TrGO. Because GO haspoor conductivity, the transferred electron spin into GO from ZnO could not wellcouple with spin within the GO lattice and, therefore, the ERP signal persists. WhereasTrGO was conductive so that the electrons were well transferred and then coupled wellwith localized spin in TrGO and as a consequence, the EPR signal in ZnO NPs wasquenched. When thiol groups were functionalized on rGO, a free S by-product wouldfill oxygen vacancy of ZnO NPs and heal the surface defect and, therefore, the EPRsignal related to surface defect in ZnO NPs was quenched.

From PL spectra as shown in Fig. 8.12, a sharp NBE from band-to-band transitionand a broad green emission related to defect existed in the bandgap of ZnO NPs wereobserved. In case of GO-ZnO, the peak intensity of NBE was much reduced and thebroad DE showed a red shift which could be due to the interaction of surface defectstates and the p electron cloud of the graphene lattice. On the other hand, TrGO-ZnO showed that both NBE and DE were almost quenched together.

g1 = 2.0037g2 = 1.9600g3 = 2.0024

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Figure 8.10 EPR signals of (a) ZnO NPs and GOeZnO hybrid materials and (b) rGO, TrGO,and TrGO.Reprinted from C.V. Pham, S. Repp, R. Thomann, M. Krueger, S. Weber, E. Erdem, Chargetransfer and surface defect healing within ZnO nanoparticle decorated graphene hybridmaterials, Nanoscale 8 (2016) 9682e9688.


After band alignment of energy level between ZnO and GO or TrGO, excited elec-trons above the CB (4.19 eV) were transferred into the Fermi level of GO or TrGO(4.5e4.7 eV), which led to the quenching of the band-to-band emission of bothGO-ZnO and TrGO-ZnO. Afterward, in case of GOeZnO hybrid composite, theseelectrons were subsequently transferred into lower energy level of surface defects

COOH COOH COOHCOOHOH

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Figure 8.11 Schematic diagram of core-shell model of ZnO NPs. Charge transfer mechanism inGO-ZnO and TrGO-ZnO.Reprinted from C.V. Pham, S. Repp, R. Thomann, M. Krueger, S. Weber, E. Erdem, Chargetransfer and surface defect healing within ZnO nanoparticle decorated graphene hybridmaterials, Nanoscale 8 (2016) 9682e9688.

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Figure 8.12 PL spectra for ZnO, ZnO-rGO, and ZnO-TrGO.Reprinted from C.V. Pham, S. Repp, R. Thomann, M. Krueger, S. Weber, E. Erdem, Chargetransfer and surface defect healing within ZnO nanoparticle decorated graphene hybridmaterials, Nanoscale 8 (2016) 9682e9688.


(5.25 eV), still remained in the bandgap, and finally transited toward the valence band(7.39 eV), which enhanced the DE considering the relative intensity ratio between DE(IDE) and NBE (INBE) from IDE/INBE ¼ 1.01 for ZnO NPs to 7.52 for GOeZnO hybridas shown in Fig. 8.12. On the other hand, oxygen vacancy surface defects in TrGOeZnO hybrid were healed by sulfur and therefore both NBE and DE were greatlyquenched.

8.2.4 GrapheneeSnO2 hybrid composite

According to Ding et al. [33], graphene was firstly deposited on Si substrate, and thenSnO2 NPs were formed by oxidation of sputtered Sn metal particles at 400�C for 4 hand finally GeSnO2 composites were successfully synthesized. PL for G-SnO2 as wellas SnO2 shown in Fig. 8.13 were taken and compared. Six distinctive emission bandslocated at 403, 422, 447, 485, 527, and 600 nm were distinctively observed in the PLspectra of G-SnO2 composite. It was noteworthy that the intensity of six emissionbands of G-SnO2 increased at the same sputtering time of Sn NPs compared withSnO2 nanostructures, whereas PL intensity of GeSnO2 composite nanostructuresdecreased as sputtering time of Sn NPs decreased to 5 min. However, no change inthe position of six emission bands was observed through depositing graphene bufferlayer or changing sputtering time of Sn NPs. It can be supposed that the peak at403 nm has no relation with the concentration of oxygen vacancies (Vo), while it isrelated to structural defects or luminescent centers, such as nanocrystals and defects

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Figure 8.13 PL spectra of the SnO2 and G-SnO2 nanostructures sputtered by Sn metal for(a) 10 min and (c) 5 min and for (b) 10 min and (d) 5 min, respectively.Reprinted from J. Ding, X. Yan, J. Li, B. Shen, J. Yang, J. Chen, Q. Xue, Enhancement of fieldemission and photoluminescence properties of graphene-SnO2 composite nanostructures, ACSAppl. Mater. Interfaces 3 (2011) 4299e4305.


in the SnO2 film [53]. The peak at 447 nm is attributed to oxygen-related defects [54],and the peak at 485 nm can be related to the 130� Sn coordinated surface oxygen va-cancies [55]. The origin of the peak at 527 nm can be ascribed to the oxygen vacancies,and the peak at 600 nm can be attributed to the SnO2 emission band related to the crys-talline defects induced [56] during the formation process of the SnO2. However, theobserved peak at 422 nm is obscure and may be caused by other defects or oxygen va-cancies considering the previous result [57].

8.2.5 WOxerGO hybrid composite

Thangavel et al. [58] described the synthesis and physical properties of tungstenoxideereduced graphene oxide (WO3erGO) nanocomposite. In the UV-vis absorp-tion spectra of as-prepared WO3erGO with different rGO concentration, the absorp-tion peak at 270 nm resulted from the electron transfer from WO3 to rGO and wasshifted from 270 to 345 nm as the concentration of rGO increased. Such a red shiftis due to the quenching of photoemission, which increased with the concentration ofrGO. The PL results revealed the emission peak at 389 and 450 nm for rGO andWO3erGO nanocomposite, respectively. PL spectra for WO3, GO, rGO, and WO3erGOnanocomposites were taken at the excitation wavelength of 315 nm. WO3erGOcomposite material shows PL spectra in the visible and blue regions. As-synthesizedrGO shows emission at 389 nm, which was ascribed to crystalline sp2 (CeC) bondingin rGO [59,60].

The PL emission peak observed at wavelength of 450 nm for WO3erGO compositematerial was greatly decreased, and this was due to an electron transfer from the WO3

CB to rGO sheets. As the concentration of rGO in composite increased, that peakintensity was proportionately increased. From UV-vis absorption spectra, the bandgapof the nanocomposite was calculated as 2.6 eV and that of pure WO3 was calculated as2.82 eV which is very similar to previous reports [61].

8.2.6 ZnOegraphene quantum dot LED

Up to now, there have been few reports on the realization of LEDs using graphene-based composite as an emitter. Son et al. [62,63] firstly reported that hybrid quasicore-shell type QDs consisting of ZnO NPs as inner core and graphene as a shellwere successfully synthesized and their nanostructures as well as optical propertieswere carefully analyzed. They also reported the potential use of these hybrid QDsas emission layer through bandgap engineering in QD LEDs. As graphically shownin Fig. 8.14, Zn acetate dehydrate [Zn(CH3COOH)$2H2O] was used as a precursorand an embryo of ZnO was formed in dimethyl formamide (DMF). Graphite powerwas acid-treated by the mixture of H2SO4 and HNO3 acid. Thereafter, embryo ZnONPs were mixed with acid-treated graphite power in DMF again and their surfaces


Graphite oxide (GO)

ZnO/Zn2+

Graphite oxide (GO)

OxygenCarbonHydrogen

GO + ZnO QD/ZN2+ Synthesis fromfunctional group

ZnO QD covered by graphene sheets

ZnO ZnO

ZnO

(a)

(b)

(c)

Figure 8.14 Schematic synthetic process of consolidated quasi core-shell type ZnO-G hybridquantum dots. (a) ZnO embryo and GO, (b) chemical reaction with functional O from GO withZn2þ in ZnO, and (c) ZnO QDs covered with graphene.Reprinted from D.I. Son, B.W. Kwon, H.H. Kim, D.H. Park, B. Angadi, W.K. Choi, Chemicalexfoliation of pure graphene sheets from synthesized ZnOegraphene quasi coreeshell quantumdots, Carbon 59 (2013) 289e295.


were covered with chemically exfoliated GO layer from the outer layer of slightlyoxidized graphite powder. At last, consolidated core-shell ZnOegraphene (ZnOeG)hybrid QDs with the size of ca. 10 nm were obtained.

Fig. 8.15(a) shows the PL spectra of ZnO-G quasi core-shell QDs and ZnO QDreference sample. When they are excited with HeeCd (l ¼ 325 nm), pure ZnO NPsshow only NBE emission at 379 nm (3.27 eV) corresponding to the bandgap ofZnO and clearly attributed to the interband transition from CB to valence band anda negligible emission from defects (Zn interstitial, Zn vacancy, O vacancy, and O inter-stitial). On the other hand, the PL spectra of the ZnO-G quasi QDs show reduction ofNBE peak and concurrent appearance of two additional peaks at the wavelength of 406and 436 nm. The attachment of graphene to the ZnO NPs quenches UV PL emissionup to about 71% along with the occurrence of new blue emissions. This can be attrib-uted to the electron transfer from CB of ZnO to graphene LUMO levels by consideringwork function of ZnO (4.19 eV) and Fermi energy level of graphene (4.4 eV) from thevacuum level. To examine the occurrence of two blue emissions in PL spectra, densityof states and projected density of states of ZnO-G QDs are calculated using simplemodel with density functional theory (DFT) as implemented in Gaussian package[64]. A Becke-style three-parameter LeeeYangePar hybrid functional with the splitbasis set 6-31G* was used to perform the calculation. According to DFT calculation,result for several possible bonding such as center and bridge O epoxy (CeO), carbonyl(C]O), and carboxyl (eCOOe) LUMO level of pristine graphene is split into threelevels assigned as LUMO, LUMOþ1, and LUMOþ2 by bridged O epoxy bondingwith graphene (G-Oepoxy). Among them, LUMO (s:p ¼ 7.1%:92%) and LUMOþ2(s:p ¼ 6.7%:93%) levels consist of both s and p orbital except LUMOþ1 containingonly p orbital. By selection rule, only the transition conserving the angular momentum(Dl ¼ �1) can be possible. Therefore, two new peaks in the PL of ZnO-G quasi QDsas shown in schematic energy band diagram of Fig. 8.15(b) can be ascribed to the elec-tron transition from LUMO and LUMOþ2 molecular orbitals of G-Oepoxy to the ZnOvalence band mainly consisted of O 2p orbital considering the above results of DFT.

Fig. 8.16(a) represents the schematic energy band diagram of ITO/PEDOT:PSS/p-TPD/ZnO-G/Cs2CO3/Al QD LED electronic structure. As shown in Fig. 8.16(b), twoprominent EL peaks are observed at 428 nm (2.89 eV) and 452 nm (2.74 eV). Elec-trons are injected from the Cs2CO3/Al electrode (F ¼ 4.3 eV) to the outer graphenelayer (F ¼ 4.4 eV) rather than to the CB of inner ZnO (F ¼ 4.19 eV) QDs, as theenergy level of the former is lower by 0.21 eV than that of the latter. This result indi-cates that the wavelength of excitonic emission of ZnO QD can be changed by theconjugation with graphene, opening up of a new way to tune the center of the electro-luminescence of a MO semiconductor. The modulated wavelength corresponds to thedifference of energy between modified LUMO levels and valence band of ZnO.Fig. 8.16(c) shows a photograph of the light emission from the QD-LED device atapplied biases of 11, 13, 15, and 17 V, where pixels with the size of 5 mm � 5 mmshowed bluish-white color due to the combination of a series of blue (425 nm) and


Wavelength (nm)

PL

inte

nsity

(arb

.uni

ts)

ZnO-graphene QD

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Binding energy (eV)

(a) (b)

ZnO-graphene QD

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350 400 450 550500

PL

inte

nsity

(arb

.uni

ts)

350 400 450 500 550 600 650

Wavelength (nm)460 nm

432 nm

406 nm

436 nm

ZnO–graphene QDZnO QD

ZnO–graphene QDSumPeak 1Peak 2Peak 3Peak 4

25

20

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8 7 6 5 4 3 2 1 0 –1 –2

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sity

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tate

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lect

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)

HO

MO

HO

MO

LUM

OLU

MO

LUM

O+1

LUM

O+2

Pristine DOSG-Oepoxy DOSO PDOS (x5)

Figure 8.15 (a) PL spectra from ZnO and ZnO-G (Inset: fitted subpeaks of PL for ZnO-G) and (b) density of state (DOS) versus binding energy takenfrom 19 benzene ring using DFT.Reprinted from D.I. Son, B.W. Kwon, D.H. Park, B. Angadi, W.S. Seo, W.K. Choi, Emissive ZnO-graphene hybrid quantum dots, Nat. Nanotechnol. 7(2012) 465e471.

166Graphene

forFlexible

Lighting

andDisplays

1

2

3

4

5

6

7

8

Ene

rgy

(eV

)

ITO 5.3 eV

3.3 eV

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4.8 eV4.19 eV

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PEDOT:PSS

poly-TPD

ZnO

4.3 eV

Graphene

h+

Cs2CO3/Al

e–11V13V15V17V

428 nm

452 nm

475 nm

606 nm

(2.89 eV)

(2.74 eV)

(2.6 eV)2.04 eV

350 400 450 500 550 600 650 700

Wavelength (nm)

EL

inte

nsity

(a.u

)

11 V 13 V

15 V 17 V

Vaccum level(a) (b) (c)

Figure 8.16 (a) Energy band diagram of ITO/PEDOT:PSS/p-TPD/ZnO-G/Cs2CO3/Al, (b) EL spectra, and (c) pictures of ZnO-G QDs LED at theforward bias of 11e17 V, respectively.Reprinted from D.I. Son, B.W. Kwon, D.H. Park, B. Angadi, W.S. Seo, W.K. Choi, Emissive ZnO-graphene hybrid quantum dots, Nat. Nanotechnol. 7(2012) 465e471.

Graphene-based

composite

emitter

167

yellow (606 nm) emissions. The yellow emission 2.04 eV was known to result fromdeteriorated poly-TPD. At 15 V applied bias and with optimal CIE coordinates(0.23, 0.20), the maximum brightness reachedw798 cd/m2, with an external quantumefficiency of 0.04%.

As an extension of ZnO-G QDs for real application, a passive matrix ZnO-GebasedQD LED with 10 � 10 pixels was fabricated on a 5 cm � 5 cm ITO glass as shown inFig. 8.17 [64]. This device was programmed using an external circuit and was checkedto evaluate whether each pixel in the passive mode is working properly. Fig. 8.18presents the pictures displaying the performance of the passive matrix ZnO-GQDs-based LED. From the left, a single pixel (1 � 1), line pixels (1 � 10), and matrixpixels (10 � 10) mode are all successfully operated at the operational voltages of8.0e9.8 V.

Furthermore, a passive matrix ZnO-Gebased QD LED with 10 � 10 pixels wasalso fabricated on a quite large size of flexible 5 cm � 5 cm ITO/PET substrate asshown in Fig. 8.19(a). Very bright white emission was observed in the matrix modefrom the device, while one line pixel did not work (Fig. 8.19(b)), and even in thebending stage (Fig. 8.19(c)), white emission still continued without any change inthe intensity or the substrate configuration. From these results, it was proved thatthe ZnO-Gebased QD LED is quite suitable for flexible planar lighting and is consid-ered as a new candidate compatible with OLED.

Wu et al. [66] synthesized green light emission diode using the combined of ZnONW grown by physical vapor deposition in a tube furnace with graphene. In this vander Waals LED, a Schottky diode junction between n-type ZnO NW and p-type gra-phene was formed and a green electroluminescence centered at 528.8 nm was

Passive matrixZnO-graphene

QD LED structure Al (cathode)

PEDOT:PSSPoly TPD

ITO glass (anode)

Cs2CO3ZnO-grapheneQDs

(a) (b)

Figure 8.17 Passive matrix ZnOegraphene hybrid QD-based LED on ITO glass. (a) Schematiccross-sectional view of the QLED and (b) photographic real image of the 5 cm � 5 cm QLEDwith 10 � 10 pixels.Reprinted from W.K. Choi, ZnO-nanocarbon core-shell type hybrid quantum dots. SpringerBriefs in Applied Science and Technology, Nanoscience and Nanotechnology, H.V. Demir,Series Ed. Springer: Singapore, 2017; pp. 60e62.


observed under the forward bias at 6 V as shown in Fig. 8.20. This emission wasexplained by the recombination of hole injected from graphene and electrons locatedat the defect in relation to oxygen vacancy defect states in the ZnO NWs.

Figure 8.18 Passive matrix ZnO-G QD-based LED (a) single pixel (1 � 1), (b) line pixel(1 � 10), and (c) matrix pixels (10 � 10).Reprinted from W.K. Choi, ZnO-nanocarbon core-shell type hybrid quantum dots. SpringerBriefs in Applied Science and Technology, Nanoscience and Nanotechnology, H.V. Demir,Series Ed. Springer: Singapore, 2017; pp. 60e62.

Figure 8.19 Flexible passive matrix ZnO-G QD-based LED fabricated on ITO/PET substrate.(a) Real images of the device, white light emission from the device (b) without, and (c) withbending state.Reprinted from W.K. Choi, ZnO-nanocarbon core-shell type hybrid quantum dots. SpringerBriefs in Applied Science and Technology, Nanoscience and Nanotechnology, H.V. Demir,Series Ed. Springer: Singapore, 2017; pp. 60e62.


References

[1] K.-I. Ho, M. Boutchich, C.-Y. Su, R. Moreddu, E.S.R. Marianathan, L. Montes, C.-S. Lai,A self-aligned high-mobility graphene transistor: decoupling the channel with fluo-rographene to reduce scattering, Adv. Mater. 27 (2015) 6519e6525.

[2] C.D. Simpson, J.D. Brand, A.J. Berresheim, L. Przybilla, H.J. R€ader, K. M€ullen, Synthesisof a giant 222 carbon graphite sheet, Chemistry 8 (2002) 1424e1429.

[3] M.L. Mueller, X. Yan, J.A. McGuire, L.-S. Li, Triplet states and electronic relaxation inphotoexcited graphene quantum dots, Nano Lett. 10 (2010) 2679e2682.

[4] M. Fox, Optical Properties of Solids, vol. 1, Oxford University Press, New York, 2010,p. 227.

[5] S.J. Zhu, S.J. Tang, J.H. Zhang, B. Yang, Control the size and surface chemistry of gra-phene for the rising fluorescent materials, Chem. Commun. 48 (2012) 4527e4539.

Vacuum level

Vacuum level

Vacuum level

4.35 eV

4.35 eV

4.35 eV

3.3 eV

3.3 eV

3.3 eV

4.8 eV

4.8 eV

4.8 eV

Ec-ZnOEf-ZnO Ef-graphene

Ef-graphene

Ef-graphene

Ev-ZnO

Ec-ZnOEf-ZnO

Ev-ZnO

Ec-ZnOEf-ZnO

Ev-ZnO

(a)

(e)

(d)

(b)

(c)

6

4

2

0

–3 –2 –1 0 1 2 3Voltage/V

Cur

rent

/µA

Inte

grat

ed in

tens

ity

400 600 800 1000Wavelength/nm

Vbias = 6V

Figure 8.20 (a) Energy level of graphene (p-type) and ZnO NW(n-type), (b) Schottky junctionbetween ZnO NW and G, (c) when forward bias is applied to ZnO NW(�) and G(þ), greenemission from this Schottky diode. (d) IeV characteristic curve, and (e) EL from the device atV ¼ 6V.Reprinted from Z. WU, Y. Shen, X. Li, Q. Yang, S. Shisheng Lin, Green light-emitting diodebased on graphene-ZnO nanowire van der Waals heterostructure, Front. Optoelectron. 9 (2016)87e92.


[6] L. Tang, R. Ji, X. Cao, J. Lin, H. Jiang, X. Li, K.S. Teng, C.M. Luk, S. Zeng, J. Hao,S.P. Lau, Deep ultraviolet photoluminescence of water-soluble self-passivated graphenequantum dots, ACS Nano 6 (2012) 5102e5110.

[7] J. Zhu, D. Wang, T. Liu, C. Guo, Preparation of Sn-Co-graphene composites with superiorlithium storage capability, Electrochim. Acta 125 (2014) 347e353.

[8] B. Adhikari, A. Banerjee, Catalytic properties of grapheneemetal nanoparticle hybridprepared using an aromatic amino acid as the reducing agent, Mater. Chem. Phys. 139(2013) 450e458.

[9] M. Zhang, J. Xie, Q. Sun, Z. Yan, M. Chen, J. Jing, A.M.S. Hossain, In situ synthesis ofpalladium nanoparticle on functionalized graphene sheets at improved performance forethanol oxidation in alkaline media, Electrochim. Acta 111 (2013) 855e861.

[10] J. Hu, Y.-L. Dong, X.-J. Chen, H.-J. Zhang, J.-M. Zheng, Q. Wang, X.-G. Chen, A highlyefficient catalyst: in situ growth of Au nanoparticles on graphene oxideeFe3O4 nano-composite support, Chem. Eng. J. 236 (2014) 1e8.

[11] L. Ye, Z. Li, L. Zhang, F. Lei, S. Lin, A green one-pot synthesis of Pt/TiO2/Graphenecomposites and its electro-photo-synergistic catalytic properties for methanol oxidation,J. Colloid Interface Sci. 433 (2014) 156e162.

[12] K. Ullah, S. Ye, L. Zhu, S.B. Jo, W.K. Jang, K.-Y. Cho, W.-C. Oh, Noble metal dopedgraphene nanocomposites and its study of photocatalytic hydrogen evolution, Solid StateSci. 31 (2014) 91e98.

[13] Y.J. Mai, X.L. Wang, J.Y. Xiang, Y.Q. Qiao, D. Zhang, C.D. Gu, J.P. Tu, CuO/graphenecomposite as anode materials for lithium-ion batteries research article, Electrochim. Acta56 (2011) 2306e2311.

[14] Y. Wang, Z. Wen, H. Zhang, G. Cao, Q. Sun, J. Cao, CuO nanorods-decorated reducedgraphene oxide nanocatalysts for catalytic oxidation of CO, Catalysts 6 (2016) 214e221.

[15] H. Kim, D.H. Seo, S.W. Kim, J. Kim, K. Kang, Highly reversible Co3O4/graphene hybridanode for lithium rechargeable batteries, Carbon 49 (2011) 326e332.

[16] T. Lu, Y.P. Zhang, H.B. Li, L.K. Pan, Y.L. Li, Z. Sun, Electrochemical behaviors ofgrapheneeZnO and grapheneeSnO2 composite films for supercapacitors Research article,Electrochim. Acta 55 (2010) 4170e4173.

[17] J. Wu, X.P. Shen, L. Jiang, K. Wang, K.M. Chen, Solvothermal synthesis and charac-terization of sandwich-like graphene/ZnO nanocomposites, Appl. Surf. Sci. 256 (2010)2826e2830.

[18] D.H. Wang, D.W. Choi, J. Li, Z.G. Yang, Z.M. Nie, R. Kou, D.H. Hu, C.M. Wang,L.V. Saraf, J.G. Zhang, I.A. Aksay, J. Liu, Self-assembled TiO2egraphene hybrid nano-structures for enhanced Li-ion insertion, ACS Nano 3 (2009) 907e914.

[19] X.Y. Zhang, H.P. Li, X.L. Cui, Y.H. Lin, Graphene/TiO2 nanocomposites: synthesis,characterization and application in hydrogen evolution from water photocatalytic splitting,J. Mater. Chem. 20 (2010) 2801e2806.

[20] G. Williams, B. Seger, P.V. Kamat, Synthesis of TiO2-graphene nanocomposites by UV-assisted photocatalytic reduction of TiO2-graphene oxide, ACS Nano 2 (2008)1487e1491.

[21] S.R. Kim, M.K. Parvez, M. Chhowalla, UV-reduction of graphene oxide and its appli-cation as an interfacial layer to reduce the back-transport reactions in dye-sensitized solarcells, Chem. Phys. Lett. 483 (2009) 124e127.

[22] O. Akhavan, E. Ghaderi, Flash photo stimulation of human neural stem cells on graphene/TiO2 heterojunction for differentiation into neurons, Nanoscale 5 (2013) 10316e10326.


[23] Y. Zhang, L. Ge, S. Ge, M. Yan, J. Yan, D. Zang, J. Lu, J. Yu, X. Song, TiO2egraphenecomplex nanopaper for paper-based label-free photoelectrochemical immunoassay, Elec-trochim. Acta 112 (2013) 620e628.

[24] J. Yao, X.P. Shen, B. Wang, H.K. Liu, G.X. Wang, In situ chemical synthesis ofSnO2egraphene nanocomposite as anode materials for lithium-ion batteries, Electrochem.Commun. 11 (2009) 1849e1852.

[25] P.C. Lian, X.F. Zhu, S.Z. Liang, Z. Li, W.S. Yang, H.H. Wang, High reversible capacity ofSnO2/graphene nanocomposite as an anode material for lithium-ion batteries, Electrochim.Acta 56 (2011) 4532e4539.

[26] S.M. Paek, E.J. Yoo, I. Honma, Enhanced cyclic performance and lithium storage capacityof SnO2/graphene nanoporous electrodes with three-dimensionally delaminated flexiblestructure, Nano Lett. 9 (2009) 72e75.

[27] Y.M. Li, X.J. Lv, J. Lu, J.H.J. Li, Preparation of SnO2-nanocrystal/graphene-nanosheetscomposites and their lithium storage ability, J. Phys. Chem. C 114 (2010) 21770e21774.

[28] Z.Y. Wang, H. Zhang, N. Li, Z.J. Shi, Z.N. Gu, G.P. Cao, Laterally confined graphenenanosheets and graphene/SnO2 composites as high-rate anode materials for lithium-ionbatteries, Nano Res 3 (2010) 748e756.

[29] A. Yang, Y. Xue, Y. Zhang, X. Zhang, H. Zhao, X. Li, Y. He, Z. Yuan, A simple one-potsynthesis of graphene nanosheet/SnO2 nanoparticle hybrid nanocomposites and theirapplication for selective and sensitive electrochemical detection of dopamine, J. Mater.Chem. B 1 (2013) 1804e1811.

[30] F. Li, J. Song, H. Yang, S. Gan, Q. Zhang, D. Han, A. Ivaska, L. Niu, One-step synthesis ofgraphene/SnO2 nanocomposites and its application in electrochemical supercapacitors,Nanotechnology 20 (2009) 455602e455608.

[31] S.-J. Li, N. Xia, X.-L. Lv, M.-M. Zhao, B.-Q. Yuan, H. Pang, A facile one-step electro-chemical synthesis of graphene/NiO nanocomposites as efficient electrocatalyst forglucose and methanol, Sens. Actuators B Chem. 190 (2014) 809e817.

[32] R.K. Upadhyay, N. Soin, S.S. Roy, Role of graphene/metal oxide composites as photo-catalysts, adsorbents and disinfectants in water treatment: a review, RSC Adv. 4 (2014)3823e3851.

[33] J. Ding, X. Yan, J. Li, B. Shen, J. Yang, J. Chen, Q. Xue, Enhancement of field emissionand photoluminescence properties of graphene-SnO2 composite nanostructures, ACSAppl. Mater. Interfaces 3 (2011) 4299e4305.

[34] R.S. Swathi, K.L. Sebastian, Long range resonance energy transfer from a dye molecule tographene has Z-4 dependence, J. Chem. Phys. 130 (2009) 086101e0861013.

[35] Z. Chen, S. Berciaud, C. Nuckolls, T.F. Heinz, L.E. Brus, Energy transfer from individualsemiconductor nanocrystals to graphene, ACS Nano 4 (2009) 2964e2968.

[36] J. Chen, F. Xu, J. Wu, K. Qasim, Y. Zhou, W. Lei, L.-T. Suna, Y. Zhang, Flexiblephotovoltaic cells based on a graphene-CdSe quantum dot nanocomposite, Nanoscale 4(2012) 441e443.

[37] X.T. Guo, Z.H. Ni, C.Y. Liao, H.Y. Nan, Y. Zhang, W.W. Zhao, W.H. Wang, Fluores-cence quenching of CdSe quantum dots on graphene, Appl. Phys. Lett. 103 (2013) 2019091e4.

[38] Y.J. Kim, J.H. Lee, G.C. Yi, Vertically aligned ZnO nanostructures grown on graphenelayers, Appl. Phys. Lett. 95 (2009) 213101 1e3.

[39] P. Rauwel, M. Salumaa, A. Aasna, A. Galeckas, E. Rauwel, A review of the synthesis andphotoluminescence properties of hybrid ZnO and carbon nanomaterials, J. Nanomater.(2016). https://doi.org/10.1155/2016/5320625.


https://doi.org/10.1155/2016/5320625

[40] M. Khenfouch, M. Baïtoul, M. Maaza, White photoluminescence from a grown ZnOnanorods/graphene hybrid nanostructure, Opt. Mater. 34 (2012) 1320e1326.

[41] G. Eda, M. Chhowalla, Chemically derived graphene oxide: towards large-area thin-filmelectronics and optoelectronics, Adv. Mater. 21 (2009) 1e5.

[42] X. Sun, Z. Liu, K. Welsher, J.T. Robinson, A. Goodwin, S. Zaric, H. Dai, Nano-grapheneoxide for cellular imaging and drug delivery, Nano Res. 1 (2008) 203e212.

[43] Z.T. Luo, P.M. Vora, E.J. Mele, A.T.C. Johnson, J.M. Kikkawa, Photoluminescence andband gap modulation in graphene oxide, Appl. Phys. Lett. 94 (2009) 111909e111911.

[44] S.W. Hwang, D.H. Shin, C.O. Kim, S.H. Hong, M.C. Kim, J. Kim, K.Y. Lim, S. Kim,S. Choi, K.J. Ahn, G. Kim, S.H. Sim, B.H. Hong, Plasmon enhanced ultraviolet photo-luminescence from hybrid structures of graphene/ZnO films, Phys. Rev. Lett. 105 (2010)127403 1e4.

[45] F. Han, S. Yang, W. Jing, K. Jiang, Z. Jinag, H. Liu, L. Li, Surface plasmon enhancedphotoluminescence of ZnO nanorods by capping reduced graphene oxide sheets, Opt.Express 22 (2014) 11436e11445.

[46] H. Zeng, Y. Cao, S. Xie, J. Yang, Z. Tang, W.X. Synthesis, Optical and electrochemicalproperties of ZnO nanowires/graphene oxide heterostrucutres, Nanoscale Res. Lett. 8(2013) 133e139.

[47] E. Lee, J.Y. Kim, B.J. Kwon, E.S. Jang, S.J. An, Vacancy filling effect of graphene onphotoluminescence behavior of ZnO/graphene nanocomposite, Phys. Status Solidi RRL 8(2014) 836e840.

[48] L. Dai, X.L. Chen, W.J. Wang, T. Zhou, B.Q. Hu, Growth and luminescence character-ization of large-scale zinc oxide nanowires, J. Phys. Condens. Matter 15 (2003)2221e2227.

[49] G. William, P.V. Kamat, Graphene-semiconductor nanocomposites: excite-state in-teractions between ZnO nanoparticles and graphene oxide, Langmuir 25 (2009)13869e13873.

[50] G. Singh, A. Choudhary, D. Haranath, A.G. Joshi, N. Singh, R. Pasricha, ZnO decoratedluminescent graphene as a potential gas sensor at room temperature, Carbon 50 (2011)385e394.

[51] H. Kaftelen, K. Ocakoglu, R. Thomann, S. Tu, S. Weber, E. Erdem, EPR and photo-luminescence spectroscopy studies on the defect structure of ZnO nanocrystals, Phys. Rev.B 86 (2012) 014113 1e9.

[52] C.V. Pham, S. Repp, R. Thomann, M. Krueger, S. Weber, E. Erdem, Charge transfer andsurface defect healing within ZnO nanoparticle decorated graphene hybrid materials,Nanoscale 8 (2016) 9682e9688.

[53] B. Wang, Y.H. Yang, C.X. Wang, N.S. Xu, G.W. Yang, Field emission and photo-luminescence of SnO2 nanograss, J. Appl. Phys. 98 (2005) 124303 1e4.

[54] Y.C. Her, J.Y. Wu, Y.R. Lin, S.Y. Tsai, Low-temperature growth and blue luminescenceof SnO2 Nanoblades, Appl. Phys. Lett. 89 (2006) 043115 1e3.

[55] S.T. Jean, Y.C. Her, Synthesis of Sb-additivated SnO2 nanostructures and dependence ofphotoluminescence properties on Sb additivation concentration, J. Appl. Phys. 105 (2009)024310 1e6.

[56] J.M. Wu, Sn-doped starfish-like nanostructures from TiO2�SiO2 Core�Shell nanocablesand SiO2 nanowires: processing, properties, and characterization, J. Phys. Chem. C 112(2008) 13192e13199.


[57] T.W. Kim, D.U. Lee, Y.S. Yoon, Microstructureal, electrical, and optical properties ofSnO2 nanocrystalline thin films on InP (100) substrates for applications as gas sensordevices, J. Appl. Phys. 88 (2000) 3759e3761.

[58] S. Thangavel, M. Elayaperumal, G. Venugopal, Synthesis and properties of tungsten oxideand reduced graphene oxide nanocomposites, Mater. Exp. 2 (2012) 327e334.

[59] J. Yang, M. Heo, H.J. Lee, S.M. Park, J.Y. Kim, H.S. Shin, Reduced graphene oxide (rGO)wrapped fullerene (C60) wires, ACS Nano 5 (2011) 8365e8371.

[60] K. Krishnamoorthy, M. Veerapandian, R. Mohan, S.J. Kim, Investigation of Raman andphotoluminescence studies of reduced graphene oxide sheets, Appl. Phys. A 106 (2012)501e506.

[61] P.P.G. Borrero, F. Sato, A.N. Medina, M.L. Baesso, A.C. Bento, G. Baldissera, C. Persson,G.A. Niklasson, C.G. Granqvist, A.F.D. Silva, Optical band-gap determination of nano-structured WO3 film, Appl. Phys. Lett. 96 (2010) 061909, 1-3.

[62] D.I. Son, B.W. Kwon, H.H. Kim, D.H. Park, B. Angadi, W.K. Choi, Chemical exfoliationof pure graphene sheets from synthesized ZnOegraphene quasi coreeshell quantum dots,Carbon 59 (2013) 289e295.

[63] D.I. Son, B.W. Kwon, D.H. Park, B. Angadi, W.S. Seo, W.K. Choi, Emissive ZnO-graphene hybrid quantum dots, Nat. Nanotechnol. 7 (2012) 465e471.

[64] M.J. Frisch, et al., Gaussian 03, 2004 (Gaussian, 2003), http://gaussian.com/g03citation/.[65] W.K. Choi, ZnO-nanocarbon core-shell type hybrid quantum dots, in: H.V. Series (Ed.),

Springer Briefs in Applied Science and Technology, Nanoscience and Nanotechnology,Demir, Springer, Singapore, 2017, pp. 60e62.

[66] Z. WU, Y. Shen, X. Li, Q. Yang, S. Shisheng Lin, Green light-emitting diode based ongraphene-ZnO nanowire van der Waals heterostructure, Front. Optoelectron. 9 (2016)87e92.


http://gaussian.com/g03citation/

Stretchable graphene electrodes 9Shuyan Qi, Nan LiuCollege of Chemistry, Beijing Normal University, Beijing, P.R., China

9.1 Introduction

China has a very famous mythological figure, Sun Wukong, relying on his ever-changing Golden Hoop to successfully defeat every rival in the world. This ruthlessGolden Hoop is hidden in the deep sea and emits radiant rays. It can either be astall as the Optimus Prime, propping up the heavens and the earth, or it can be ascompact as an embroidery needle, stuffing into the ears. With the rapid developmentof advanced science and technology, it is possible that such a powerful and shiningGolden Hoop is not limited to myths and may be realized with the help of new mate-rials and devices. In the upcoming era of artificial intelligence (AI), electronic devicesare no longer just required to be fast and compact, at the same time, it is also hoped thatthey can be bent, stretched, and contract freely. For example, when electronic devicesare attached to the human body or embedded within the clothes, not only will they bebent but they will also be stressed and deformed. To have an electronic device capableof working under stretching circumstances is one of the biggest challenges in thecurrent field of electronics [1].

To construct stretchable electronic devices, the primary requirement is to have elec-trodes or interconnects that have the capacity to absorb a large level of strain (>>1%)without obvious changes in their electrical performance [2]. Conventional materialsfor electrodes or interconnects in microchips, comprising single-crystal inorganicmetals (heavily doped silicon), polycrystalline films of evaporated metals (e.g., copper,nickel, cobalt, and so on), and metal oxides (e.g., indium tin oxide, ITO) [3,4], are notapplicable in stretchable electronic devices due to their brittle and rigid nature. Tosolve this problem, tremendous research efforts have been carried out in developingstretchable conductors. Basically, there are two main strategies, one is to apply newstructural layouts on conventional electrodes and the other one is to design and synthe-size new materials and composites, which are intrinsically stretchable and can also usespecial structural layouts to enhance the stretchability.

Common structural layouts for stretchable electrodes include “wavy” geometries,percolating networks, serpentine interconnects, coiled structures, and etc. While mak-ing conventional electrodes into such configurations, they are able to sustain enoughconductivity at high strains. However, because the fabrication processes of conven-tional conductors involve extreme conditions, such as high temperature or vacuumevaporation, they are highly dependent on the very expensive tools. Moreover, theadhesion of the conductive layer to elastomer substrate is weak, leading to poor


https://doi.org/10.1016/B978-0-08-102482-9.00009-5

durability. To fabricate large-area stretchable electronic devices, a reliable methodwith fairly simple and low cost techniques is very attractive.

Novel nanomaterial conductors made from carbon nanotubes, graphene, and metalnanowires (mNWs)/nanoparticles are excellent flexible electrodes. However, due tothe in-plane stiffness and large Young’s modulus, it is essential to form networksand composites to enable them as stretchable electrodes. For example, randomlyassembled networks of one-dimensional mNWs or carbon nanotubes are very robustconductors, which are able to maintain conductivity up to 100% strain. Comparedwith metal electrodes, carbon-based electrodes have the advantages of being lightweight, low cost, highly transparent, and work function tunable although the conduc-tivity is slightly lower. Utilizing randomly formed network of carbon nanotubes,stretchable electronic devices such as transistors, sensors, displays, etc., were success-fully fabricated, showing comparable electrical performances as traditional Si-baseddevices.

Graphene is one of the strongest materials in existence. Its theoretical strength isdefined as the maximum stress to sustain in the absence of any defects. It has anin-plane stiffness of 340 Nm�1 and a Young’s modulus of 0.5 TPa, which is intrinsi-cally not an ideal material for stretchable electrodes. The strong carbonecarbonnetwork does not provide any energy dissipation mechanisms for applied strain andtherefore readily cracks at less than 5% strain. To apply the atomically thick, highlytransparent, and highly conductive graphene in stretchable transparent devices, over-coming the mechanical limitations and sustaining its extraordinary properties understrain are desired.

Theoretical calculations show that crumpling and interplay between different layersshould strongly decrease the stiffness. When bi- or trilayer graphene are stretched at30% strain, they exhibit 13 times smaller resistance change than that of monolayer gra-phene [5]. In particular, when graphene is adhered onto certain substrates and appliedshear forces, both adhesion and delamination will occur and their intensities varydepending on the surface conditions, such as roughness, moisture, chemical reactivity,etc. The interaction between graphene and target substrates is believed to be van derWaals interactions but will also be complicated by capillary and contaminationeffects [6]. This is because surface roughness plays a vital role in the interfacial tough-ness with fracture mix-mode of tractioneseparation relations. A variety of adhesion/separation energies of graphene to different substrates have been summarized inTable 9.1. When graphene is adhered onto silicon oxide (SiO2), the separation energyis in the range of 0.151e0.45 J/m2. While graphene is transferred onto polyethyleneterephthalate (PET), the separation energy dropped to be just 0.54 mJ/m2, suggestingthat the friction of graphene/soft polymer is lower than that of graphene/SiO2. There-fore, the maximum strain that can be transferred to graphene by stretching the substrateis dependent on the interfacial shear strength between graphene and the substrate, lead-ing to a higher strain tolerance of graphene on elastomer substrates.

In addition, to weaken the strain by using low modulus elastomer as substrate,similar to conventional materials, constructing structural layouts on graphene and uti-lizing graphene kirigami and origami to enable stretchable graphene electrodes are alsovery effective. On the other hand, increasing the amount of conductive paths and


forming percolation pathway under strain will also improve the conductivity straintolerance.

Below we mainly introduce the recent progress on stretchable graphene electrodesfrom the aspects of preparation and application. The section of preparation is furtherdivided into three parts based on the methodology, including “kirigami,” “origami”of graphene, and “formation of percolation networks.” Kirigami and origami aretwo Japanese terms, originated from Asia paper art of “paper cutting” and “paperfolding.” For the atomically thin graphene film, describing the operation of grapheneby “Kirigami” and “Origami” are quite appropriate. “Kirigami” focuses on cutting sus-pended graphene into certain structural layouts and then enable its stretchability, while“origami” can be applied at three stages of graphene during its fabrication process,which are CVD growth, transfer procedure, and posttransfer. Corresponding “origami”methods to enable graphene stretchable electrodes are prepatterning the growth sub-strate, prestraining the transferred target substrate, and direct engineering the graphenefilm. In the last part of this section, forming graphene-based composites by combiningwith other conductors and/or elastomers is also reviewed. The section of applicationincludes the recent progress of stretchable graphene electrodes in optoelectronics,energy-related area and sensors, which is a fast-growing area. In the final section ofthis chapter, we also provide a comment of current research status and an outlookfor further studies related to the future electronic technology.

9.2 Preparation of stretchable graphene electrodes

The unique properties of graphene make it a strong candidate for the next generation oftransparent conductive electrodes. While graphene has shown promising results forflexible electronics, its application in stretchable electronics has been limited by its

Table 9.1 Measured adhesion/separation energies of exfoliated graphene onto differentsubstrates.

Substrate materials G (J/m2) No. of layers References

SiOx 0.45 1 Ref. [7] (2011)

0.31 2e5 Ref. [7] (2011)

0.24 1 Ref. [8] (2016)

0.14 1 Ref. [9] (2013)

0.15 w5 Ref. [10] (2010)

PET 0.54 � 10�3 1 Ref. [11] (2014)

Reproduced with permission from D. Akinwande, C.J. Brennan, J.S. Bunch, P. Egberts, J.R. Felts, H. Gao, R. Huang, J.-S.Kim, T. Li, Y. Li, K.M. Liechti, N. Lu, H.S. Park, E.J. Reed, P. Wang, B.I. Yakobson, T. Zhang, Y.W. Zhang, Y. Zhou, Y.Zhu, A review on mechanics and mechanical properties of 2D materialsdgraphene and beyond, Extreme Mech. Lett. 13(2017) 42e77. Copyright 2017 Elsevier Ltd.

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mechanical properties. As early as 2009, when Kim et al. [12] for the first time devel-oped chemical vapor deposition technique to grow large-area and high-quality gra-phene film, they already successfully demonstrated stretchable graphene electrodes.They adopted dry-transfer process to transfer graphene on the unstrained/prestrainedelastomer substrate. Surprisingly, the conductivity of transferred graphene on theprestrained substrate keeps stable until w11% stretching (which is w6% stretchingfor transferred graphene on the unstrained substrate), and only one order of magnitudechanges at w25% stretching. It shows that graphene is a promising candidate forstretchable electronics. Then, lots of research efforts have been carried out on howto engineer the graphene structure to improve its stretchability and maintain its remark-able electrical properties. Generally, to make the conductivity of graphene strain toler-ance, there are mainly three types of methods, called graphene kirigami, grapheneorigami, and forming graphene-based composites. Below we will introduce themand discuss their advantages and disadvantages.

9.2.1 Graphene “Kirigami”

Blees et al. [13] first proposed graphene kirigami to achieve the stretchability of gra-phene (Fig. 9.1(aee)). They released graphene from the surface and found it could betreated like a sheet of atom-thick paper. Inspired by the old Chinese and Japanese art,kirigami, they created simple cuts in graphene with lithography techniques. Experi-ments show that graphene kirigami behaves like a spring when stretched, the sameas paper kirigami. It is also worth noting that the graphene kirigamiebased transistorscan be stretched by 240% without the reduction of the electrical performance, whichdemonstrates that graphene kirigami is suitable for flexible and stretchable electrodes.

Theoretical investigations also verify the enhancement in the stretchability of gra-phene through applying the kirigami approach. Qi et al. [14] used classical moleculardynamics simulations (MD) to study the deformation response of graphene kirigami.Different from pristine graphene, there are four distinct stages preceding fracture,including (1) the interior cuts elongating, flipping, and rotating; (2) the carbon bondsbeing stretched; (3) yielding beginning; and (4) fracture occurring. The first step onlyoccurs during graphene kirigami stretching, not during graphene stretching. Therefore,the yield and fracture strains of graphene can be enhanced by about a factor of threeusing kirigami as compared with standard monolayer graphene. The mechanical prop-erties of graphene kirigami can be tuned through tailoring the kirigami geometry. Weiet al. [16] studied the stress distribution on graphene kirigami under different tensilestrain and the strain effect on its thermal conductivity. Mortazavi et al. [15] employedclassical MD simulations to evaluate the mechanical of graphene kirigami with peri-odic and curved cuts (Fig. 9.1(f),(g)). The result showed that linear cuts were morefavorable for stretchable electronics because they could deflect more than curvedcuts in the first stretching stage. These simulation results have been confirmed in anexperimental work that kirigami approach can engineer elasticity in nanocompositesthrough patterned defects [17].

Although the kirigami structure could enhance the stretchability of graphenedramatically, high cost and long time-consuming lithographic fabrication process


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Figure 9.1 (a,b) Paper and graphene in-plane kirigami springs, respectively. (c) Graphene spring stretched by about 70%. (d) Electrical properties inapproximately 10 mM KCl. Conductance G is plotted against liquid-gate voltage VLG at sourceedrain bias VSD ¼ 100 mV before stretching (blue)and when stretched by 240% (orange). The top (orange-boxed) inset is split because the stretched device was larger than the visible area. (e) Three-dimensional reconstruction from a z-scan focal series of a graphene spring. The right side remains stuck to the surface and the left side is lifted. Insetsshow views of sections of the graphene (right) and paper models (left). Top images show side views; bottom images show top views. The thin graylines are the bounding box from the three-dimensional reconstruction. The aspect ratio of the side-view paper model was compressed 1.8�. Scale barsare 10 mm. (f) Schematic of the graphene kirigami, with key geometric parameters labeled. The kirigami is deformed via tensile displacement loadingthat is applied at the two ends in the direction indicated by the arrows. (g) Atomistic and periodic structure of graphene kirigami, to illustrate the keygeometric parameters: the curvature angle (q) and the longitudinal and transverse spacing distances (ls and ts).(e) Reproduced with permission from M.K. Blees, A.W. Barnard, P.A. Rose, S. P. Roberts, K.L. McGill, P.Y. Huang, A.R. Ruyack, J.W. Kevek, B.Kobrin, D.A Muller, P.L. McEuen, Graphene kirigami, Nature 524 (7564) (2015) 204e207. Copyright 2015 Macmillan Publishers Limited. (f)Reproduced with permission from ref Z. Qi, D.K. Campbell, H.S. Park, Atomistic simulations of tension-induced large deformation and stretchabilityin graphene kirigami, Phys. Rev. B 90 (24) (2014) Copyright 2014 American Physical Society. (g) Reproduced with permission from B. Mortazavi,A. Lherbier, Z. Fan, A. Harju, T. Rabczuk, J.C. Charlier, Thermal and electronic transport characteristics of highly stretchable graphene kirigami,Nanoscale 9 (42) (2017) 16329e16341. Copyright The Royal Society of Chemistry 2017.

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makes it unsuitable for industrial manufacturing. Thus, there is still a big need toachieve graphene kirigami by low cost and more feasible techniques, such as thevery controllable and high-throughput chemical etching/cutting.

9.2.2 Graphene “Origami”

Origami is another important method to achieve stretchable graphene. The most pop-ular way of graphene origami is to form wrinkled or crumpled graphene, which is aneffective way to make graphene better in terms of stretchability, electrochemicalbehavior, and hydrophobicity [18]. According to the fabrication process, there arethree methods to form wrinkled graphene, namely prestraining the transferred targetsubstrate, prepatterning the growth substrate, and direct engineering the graphene film.

Prestraining the transferred target substrate: Prestraining substrate refers to pre-strain elastomeric target substrates in the posttransfer step. When the prestrain appliedon the target substrate is released, wrinkles form spontaneously on the resulting gra-phene film. Kim et al., as mentioned above, were the first one to successfully preparewrinkled graphene using the prestrained substrate method. Zang et al. [19] demon-strated via experiments and theoretical calculations that wrinkles and delaminatedbuckles were obtained when the substrate was uniaxially released, but crumpledgraphene was obtained when biaxially released (Fig. 9.2(aef)). Besides, flat graphenecould be unfolded if the relaxed substrate was biaxially stretched back. The crumpling-unfolding process was reversible, which made it possible to achieve a set of unprece-dented morphologies of graphene. Graphene, with different morphologies, exhibiteddifferent mechanical and electrical properties. As an example, its wettability and trans-parency can be tuned by biaxially prestretching substrates with different levels.

Utilizing a balloon-blowing method, Mu et al. [20] demonstrated another exampleof prestraining the target substrate to make wrinkled graphene (Fig. 9.2(gei)). Firstly,they prestrained a polyacrylic ester (PEA) substrate with a large strain of z300% andtightly attached it on a circular glass dish. Then they heated the glass dish using a hotstage. The air pressure inside the dish would increase and the PEA film would furtherexpand via the expansion force of the hot air inside. When the air pressure is stable,reduced graphene oxide (rGO) was next spray-coated on the surface of the expandedPEA substrate. After cooling down the glass dish and removing the PEA substrate, twotypes of wrinkles are generated: (1) short-period wrinkles appeared when the glass dishcooled to room temperature; (2) long-period wrinkles formed when the PEA substratewas released from the glass dish. Apart from more wrinkles, there are two great advan-tages of this balloon-blowing method: (1) the rGO film conformed with the substratevery well and (2) its conductivity is isotropically stable as the relaxation process wasisotropic in the horizontal direction.

Prepatterning the growth substrate: Using the above “prestraining the trans-ferred target substrate” method, more cracks may produce during the transfer process,which probably results in low conductivity. As-obtained Cu foil naturally contains aseries of parallel lines after mechanical processing and metal polishing. And it issurprisingly noticed that the transferred graphene film to some extent maintains themorphology of CVD grown substrates, meaning that the parallel processing lines


can be reproduced from grown substrates onto the surface of resultant graphene film[21]. By carefully modifying the transfer procedure, Liu et al. for the first timeachieved parallel wrinkles on transferred graphene film by copying the morphologyof CVD-grown Cu foil onto the graphene film [22]. Similarly, Chen et al. [23] prepat-terned substrates to prepare wrinkled graphene (Fig. 9.3). They slid a tweezer, whichhad a special structure, over a copper foil. The structure of the copper foil surface wasconsequently the same as that of the tweezer. Then, they used this waved copper as the

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Figure 9.2 (a) Schematic illustration of macroscopic deformation of a graphene sheet on abiaxially prestretched substrate. (bee) SEM images of patterns developed on the graphenesheet: first wrinkles form (b) then delaminated buckles as the substrate is uniaxially relaxed (c)followed by crumples as the substrate is biaxially relaxed (d) which unfold as the substrate isbiaxially stretched back (e). (f) Atomistic modeling results of the crumpling of a single-layergraphene under uniaxial compression, and biaxial compression, followed by a visualization ofthe Mises stress distribution (from left to right). Stress concentrations (visualized in red) areobserved near highly deformed regions. (g,h) Schematic representation of hierarchicallywrinkled elastic transparent conductor (HWETC) preparation. (i) SEM images and illustrationsof typical hierarchical wrinkles (short- and long-period wrinkles) in the N-rGO layer depositedon the released PEA substrate. The fabrication strain was 580%.(f) Reproduced with permission from J. Zang, S. Ryu, N. Pugno, Q. Wang, Q. Tu, M.J. Buehler,X. Zhao, Multifunctionality and control of the crumpling and unfolding of large-area graphene,Nat. Mater. 12 (4) (2013) 321e325. Copyright 2013 Macmillan Publishers Limited. (i)Reproduced with permission from J. Mu, C. Hou, G. Wang, X. Wang, Q. Zhang, Y. Li, H.Wang, M. Zhu, An elastic transparent conductor based on hierarchically wrinkled reducedgraphene oxide for artificial muscles and sensors, Adv. Mater. 28 (43) (2016) 9491e9497.Copyright 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


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Figure 9.3 (a,b) Digital photographic images of the copper foil before (a) and after (b) the wrinkle formation. (c) Typical SEM image of the wrinkledgraphene sheet on the PDMS substrate from the top view. (d) Schematic representation of the procedures for producing wrinkled graphene sheets forthe fabrication of transparent and stretchable supercapacitors.Reproduced with permission from T. Chen, X. Yuhua, A. K. Roy, L. Dai, Transparent and stretchable high-performance supercapacitors based onwrinkled graphene electrodes, ACS Nano 8 (1) (2013) 1039e1046.Copyright 2013 American Chemical Society.

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growth substrate to prepare graphene by CVD method. SEM images showed that theas-prepared graphene was wrinkled, with an interwrinkle distance of about 400 mm.And the transmittance of the as-prepared graphene was 50%e60%, which was com-parable to that of planar graphene. But, its electrical resistance increased highlywhen it was stretched up. To solve this critical problem, the authors coated a layerof polyvinyl alcohol (PVA) onto the surface of wrinkled graphene. It is shown thatthe resistance of the PVA-coated wrinkled graphene only increased by less than twicewhen it was stretched up to 40% strain. Xie et al. [24] used a steel rod to prepattern thenickel foam, a catalyst for growing porous graphene. They also deposited a polyaniline(PANI) thin film on the surface of the wrinkled graphene to improve its stretchabilityand electrochemical properties. In addition to enable uniaxially stretchable graphene,Hong et al. [25] pressed a copper foil with a Fresnel lens to fabricate omnidirectionallystretchable graphene. This indicates that using various patterned grown substrates inprinciple can allow the fabrication of stretchable and transparent graphene electrodeswith mechanical durability and performance reliability.

Prepatterning substrates to generate wrinkled graphene is not only suitable for CVDprocess but also for solution process. Starting from microstructured PDMS, Zhu et al.[26] employed layer-by-layer assembly (LBL) technology to prepare wrinkledgraphene on PDMS. First, they prepared a silicon master with recessed pyramid struc-tures by photolithography and silicon etching technologies. Then, a mixture of PDMSelastomers and cross-linker was cast on the surface of the silicon master to form themicrostructured PDMS film. Subsequently, the PDMS film was peeled off from themaster, and graphene oxide (GO) solution was adsorbed on the film by an LBL assem-bly method. After being reduced under hydrazine vapor, a reduced GO (rGO) sheetwith convex pyramid patterns was finally formed. Together with an ITO/PET film,wrinkled rGO/PDMS could serve as a sensor unit on the artificial hand with high sensi-tivity and stability.

Overall, controlling the morphology of both as-grown substrates and transferredtarget substrates can tune the morphology of resultant graphene film, achieving wrin-kled or crumpled graphene. Although this is an indirect tuning method relying on asecond media, it is very effective, cost-efficient, and potentially applied to be massiveproduction.

Direct engineering the graphene film: Transfer process of CVD grapheneincludes etching the copper substrate in the etchant solution, cleaning the graphenein the deionized water, and transferring onto the target substrate. Chen et al. [27]reported a liquid-phase shrink method to prepare wrinkled graphene, which was simpleand could efficiently control the density of wrinkles (Fig. 9.4). They soaked thecleaned graphene in the organic solution before it was transferred on the target sub-strate, such as ethanol, acetone, and so on. The cleaned graphene shrank fast owingto the reduction of the surface and interface energy of the graphene/solution system.Moreover, changing the concentration of the organic solution could control the densityof wrinkles. The wrinkled graphene prepared by this method was much more stretch-able than the pristine graphene. The sheet resistance only changed a little under 40%stretching, while the transparency of this wrinkled graphene was similar to pristinegraphene. It was even conductive under more than 100% strain, which largely


Figure 9.4 (a) Schematic illustration of the preparation of wrinkled graphene in ethanol solution. (b) 3D schematic of as-prepared wrinkled graphene.(c,d) SEM images of clean graphene and wrinkled graphene (prepared in 1:1 vol % ethanol solution) on a silicon substrate, respectively. Inset showshigh-magnification SEM image of the wrinkled graphene. (e,f) AFM images and surface analysis of the clean graphene and wrinkled graphene,respectively. (g) 3D views of wrinkled graphene.Reproduced with permission from W. Chen, X. Gui, B. Liang, M. Liu, Z. Lin, Y. Zhu, Z. Tang, Controllable fabrication of large-area wrinkledgraphene on a solution surface, ACS Appl. Mater. Interfaces 8 (17) (2016) 10977e10984. Copyright 2016 American Chemical Society.

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expanded its working range. Thus-obtained wrinkled graphene was an ideal materialfor sensitive electrochemical sensors.

Mechanical deformation of GO into 3D hierarchical structures as electrochemicalelectrodes shows high hydrophobicity and can improve the electrochemical reactivityand current density. Chen et al. [18] successfully produced wrinkled graphene utilizingthe special property of polystyrene (Fig. 9.5). Polystyrene, thermoplastic “shrink film,”tends to uniaxially or biaxially shrink at an elevated temperature. GO was first coatedon the pretreated polystyrene and then air-dried and baked at a temperature, which isabove the Tg of polystyrene. GO would shrink with polystyrene due to strong intermo-lecular attraction between the GO and polystyrene. If two sides of the sample wereconstrained, GO would undergo one-dimensional uniaxial deformation. Next, theytransferred wrinkled GO (G1) on another polystyrene and heated them again. G1

became much smaller, and the G2 hierarchy was obtained. After multiple repeated pro-cesses and dissolving the substrates in dichloromethane, different hierarchical andfree-standing wrinkle/crumple GO could be obtained. The hydrophobicity and electro-chemical current density of thus-obtained graphene were systematically improved afterGO was reduced. Combined with other literature, they believed that wrinkled grapheneprocessed by sequential mechanical deformation were highly possible in stretchableelectronics.

Compared with kirigami graphene, the fabrication procedures for origami grapheneare relatively simple and low cost, and they are much more suitable for the large-scaleproduction of stretchable graphene. Nevertheless, disadvantages of this method stillcannot be ignored. The stretchability of wrinkled graphene prepared by such methodis either scalable or reproducible enough for wearable electronic devices. Lots of un-controllable wrinkles appeared on the surface of graphene, and the repeated processesmake it too complex to apply in the actual production.

9.2.3 Graphene-based composites

Composites are very popular among materials scientists, because they always exhibitfascinating characters without changing excellent properties of the original materials.Thanks to its extraordinary electrical and mechanical properties, graphene has beenextensively designed to combine with other materials to achieve new functions. Thestrainetolerance conductivity of some graphene-based composites is enhancedgreatly, which can meet the requirements of soft electronics working for robots, elec-tronic skin, and other flexible/stretchable devices.

Many polymers are excellent stretchable substrates, and graphene in particularwrinkles, ripples, and crumples has strong mechanical interlocking with polymerchains. To enable highly conductive and stretchable graphene-based conductors,Chen et al. [29] initiated from high-quality 3D CVDegraphene grown on Ni foamand combined it with polymers (Fig. 9.6(aej)). They coated PMMA thin film onthe as-prepared 3D graphene, which acted as a supporting layer to keep graphenefoam from collapsing when etching the nickel skeleton. Then, they dissolvedPMMA with acetone and immersed the free-standing graphene foam into PDMS toobtain composites. The graphene sheets are seamlessly interconnected into 3D flexible


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Figure 9.5 (a) Schematic illustration of the fabrication process to generate multiscale GOstructures. (b) Controlling surface morphology of the G2 hierarchy through differentcombinations of shrinking orientation. The G2 structures in this figure are shrunk from 20-nmG0 coatings. SEM images of hierarchical G2 structures formed with (b) 2D-2D, (c) 2D-1D, (e)1D-2D, (f) 1Dt1D, and (g) 1Djj1D shrinking orientations are shown. Yellow arrows indicatethe wrinkled feature from the G1 uniaxial shrinkage. Scale bars in the first row are 20 mm; 2 mmin the second row; and 1 mm in the third row.Reproduced with permission from P.Y. Chen, J. Sodhi, Y. Qiu, T.M. Valentin, R.S. Steinberg,Z. Wang, R.H. Hurt, I.Y. Wong, Multiscale graphene topographies programmed by sequentialmechanical deformation, Adv. Mater. 28 (18) (2016) 3564e3571. Copyright 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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Figure 9.6 (a,b) CVD growth of graphene films (NieG, (b)) using a nickel foam (Ni foam, (a)) as a 3D scaffold template. (c) An as-grown graphenefilm after coating a thin PMMA supporting layer (NieG-PMMA). (d) A graphene foam (GF) coated with PMMA (GF-PMMA) after etching thenickel foam with hot HCl (or FeCl3/HCl) solution. (e) A free-standing GF after dissolving the PMMA layer with acetone. (f) A GF/PDMS compositeafter infiltration of PDMS into a GF. All the scale bars are 500 mm. (g) Photograph of a 170 � 220 mm2 free-standing GF. (h) SEM image of a GF.(i) Low-magnification TEM image of a GF. (j) High-resolution TEM images of graphene sheets with different numbers of layers in a GF; theinterlayer spacing of bilayer (2L) and trilayer (3L) graphene is w0:34 nm.Reproduced with permission from Z. Chen, W. Ren, L. Gao, B. Liu, S. Pei, H.M. Cheng, Three-dimensional flexible and conductive interconnectedgraphene networks grown by chemical vapour deposition, Nat. Mater. 10 (6) (2011) 424e428. Copyright 2011 Macmillan Publishers Limited.

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networks, showing 30% increase of resistance at 50% uniaxial tensile strain. Theunique network structure, high specific surface area, and outstanding electrical and me-chanical properties of grapheneefoamePDMS composites should enable many appli-cations including high-performance electrically conductive polymer composites,elastic and flexible conductors.

In addition to 3D graphene network grown by Ni foam, the highly conductive gra-phene to enable stretchable devices can also be achieved by intercalating graphenescrolls (w1e20 mm long, w 0.1e1 mm wide, and w 10e100 nm high) in betweengraphene layers (Fig. 9.7), referred to as multilayer G/G scrolls (MGG). The scrollswere naturally formed during the transfer process, which is from unprotected backsidegraphene grown on Cu foil and rolled up under the effect of surface tension in etchantsolution. They do not require additional synthesis or process. It is hypothesized thatthese graphene scrolls could provide conductive paths to bridge cracks in the graphenesheets, thus maintaining high conductivity under strain. By using MGG graphenestretchable electrodes (source/drain and gate) and semiconducting CNTs, they wereable to demonstrate highly transparent and highly stretchable all-carbon transistors,which can be stretched to 120% strain (parallel to the direction of charge transport)and retain 60% of its original current output. This is the most stretchable transparentcarbon-based transistor so far, and it provides sufficient current to drive an inorganicLED.

mNWs are another ideal materials for stretchable electrodes by virtue of its lowsheet resistance, high transparency, and outstanding mechanical robustness. However,disadvantages of mNWs, such as typically high NWeNW junction resistance, insta-bility in harsh environment, and so on, have limited its further development. Thus,Lee et al. [31] hybridized Ag nanowires with graphene to prepare novel compositesto lower the junction resistance (Fig. 9.8(a,b)). They spun a suspension of AgNWsonto a CVDegraphene layer, whose interaction in between is van der Waals forces.The integration of two-dimensional graphene and one-dimensional NWs in the hybridfilm can significantly enhance electrical properties, and the formation of percolationpathways allow simultaneous charge transport even at strain up to 100%, presentingnegligible resistance change and great optical transparency of >94% transmittancein the visible range. In addition, due to the thermal oxidation stability of graphene,such-made composites are much more chemically inactive than AgNWs only.

Direct engineering of graphene for stretchable conductors not only applies for CVDgraphene but also works for liquid phase GO/rGO. Yan et al. [32] mixed the crumpledgraphene and nanocellulose to get nanopapers via vacuum filtration (Fig. 9.8(eej)).This nanopaper had certain mechanical strength so that they would not crack anddelaminate from the filter membrane and allowed for further processing into stretch-able form. Embedded it into elastomeric matrix, such as PDMS, these nanopapersbecame more stretchable, which could be stretched up to 100% without mechanicalfailure. The stretchability is determined by the mechanical fracture limit of the elas-tomer used. The relative resistance change of 710% was observed at 100% strain, sug-gesting an excellent piezoresistive effect and an ideal material for strain sensors.

In summary, graphene as a novel electrode candidate has drawn extensive attentionfor the application in stretchable electronics. It is highly conductive, transparent, and


(a)

(b)

(f) (g) (h) (i)

(c) (d) (e)

Figure 9.7 (a) Schematic illustration of the fabrication procedure for MGGs as a stretchable electrode. During the graphene transfer, backside graphene on Cu foil was brokenat boundaries and defects, rolled up into arbitrary shapes, and tightly attached onto the upper films, forming nanoscrolls. The fourth cartoon depicts the stacked MGGstructure. (b,c) High-resolution TEM characterizations of a monolayer MGG, focusing on the monolayer graphene (b) and the scroll (c) region, respectively. The inset of(b) is a low-magnification image showing the overall morphology of monolayer MGGs on the TEM grid. Insets of (c) are the intensity profiles taken along the rectangularboxes indicated in the image, where the distances between the atomic planes are 0.34 and 0.41 nm. (d) Carbon K-edge EEL spectrum with the characteristic graphitic p* ands* peaks labeled. (e) Sectional AFM image of monolayer G/G scrolls with a height profile along the yellow dotted line. (fei) Optical microscopy and AFM images of trilayerG without (f,h) and with scrolls (g,i) on 300-nm-thick SiO2/Si substrates, respectively. Representative scrolls and wrinkles were labeled to highlight their differences.Reproduced with permission from N. Liu, A. Chortos, T. Lei, L. Jin, T.R. Kim, W.G. Bae, C. Zhu, S. Wang, R. Pfattner, X. Chen, R. Sinclair, Z. Bao, Ultratransparent andstretchable graphene electrodes, Sci. Adv. 3 (9) (2017) e1700159. Copyright 2017 The Authors, some rights reserved; exclusive licensee American Association for theAdvancement of Science.

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Figure 9.8 (a) Photograph of graphene�AgNW hybrid film on a PET substrate. The scale bar indicates 2 cm. The inset shows a SEM image of thishybrid (scale bar, 5 mm). (b) Dependence of AgNW density (in the area of 100 � 100 mm2) on spin rate. (c) Optical transmittance spectra of the hybridfilms where AgNWs are coated with different spin rates. (d) Log-scale plots of the sheet resistances as a function of NW density. (e) Schematicillustrations of the fabrication processes for stretchable graphene nanopapers. (fei) Example images of the free-standing flexible nanopaper (f,g) andstretchable nanopaper (h,i). (j) Water adsorption comparison of crumpled graphene paper, planar graphene paper, and commercial graphite paper. Thescale bars in (fej) are 10 mm.(d) Reproduced with permission from M.S. Lee, K. Lee, S.Y. Kim, H. Lee, J. Park, K.H. Choi, H.K. Kim, D.G. Kim, D.Y. Lee, S. Nam, J.U. Park,High-performance, transparent, and stretchable electrodes using graphene-metal nanowire hybrid structures, Nano Lett. 13 (6) (2013) 2814e2821.Copyright 2013 American Chemical Society. (fej) Reproduced with permission from C. Yan, J. Wang, W. Kang, M. Cui, X. Wang, C.Y. Foo, K.J.Chee, P.S. Lee, Highly stretchable piezoresistive graphene-nanocellulose nanopaper for strain sensors, Adv. Mater. 26 (13) (2014) 2022e2027.Copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

190Graphene

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light weight and work function tunable. Graphene kirigami, origami, and combiningwith other conductors or elastomers to form composites are three typical methodsfor fabricating stretchable electrodes. Mostly, graphene obtained by these threemethods can achieve high conductivity at certain strain, but in terms of easy prepara-tion and fracture-strain limit, there is still a big room to improve.

9.3 Applications of stretchable graphene electrodes

From “siri” to self-driving car, AI is developing at a rate that is faster than manyexperts imagine. Intelligent machines that are demanded to work and react like humansmake flexible and wearable materials increasingly important. Stretching is one of themanifestations of flexibility and is the basic mode of muscle activity. Tremendousefforts have been done to make stretchable electronic devices during several decades,which is a core of intelligent machines. Stretchable graphene electrodes are a starmaterial for flexible, wearable smart devices due to their extraordinary mechanicalproperties, electrical properties, transparency, and so on. Nowadays, they have alreadybeen applied in sensors, digital electronics, and energy storage devices.

Sensors: There are many types of sensors, such as tactile sensors, temperature sen-sors, chemical sensors, photodetectors, and so on. Tactile sensors, which translate amechanical deformation into an electrical signal to monitor human body motion, arethe most difficult one to mimic skin functions among various sensors. Here we chosestrain sensors as an example of tactile sensors to discuss the application of stretchablegraphene. According to different electrical signals, strain sensors usually are dividedinto three types: piezo-voltage, piezo-capacitive, and piezo-resistive. Piezo-voltagestrain sensors are much more accurate, which can detect small strains with high straingauge. Nevertheless, it is not suitable for monitoring human body motions due to itslow stretchability. Piezo-capacitive strain sensors possess a more linear responseand a lower hysteresis, but a smaller strain gauge. Relatively, piezo-resistive strainsensors are much more popular because they are simple to produce and test.

Larimi et al. [33] developed a simple and low-cost method to fabricate an ultra-stretchable, sensible strain sensor based on stretchable graphene (Fig. 9.9). Theyinfused a solution of graphene nanoflakes (GNFs), methanol, and water into a hole-rich, rubber-like adhesive pad, which was pretreated in acetone before. After beingdried overnight at room temperature, the GNF-pad was washed to remove residuals.Surprisingly, the GNF-pad exhibited outstanding mechanical properties. It could bestretched up to 350%, twisted from 0 to 180 degrees, bent (0e90 degrees), andpressed, which was due to the flexibility of the adhesive pad and relatively slidebetween GNF. On the other hand, its electrical properties and sensitivity were beforeother strain sensors. Its initial resistance was as low as 8 kU, and gauge factor was ashigh as 161. In addition, its stretchability and sensitivity were robust. After repeatedtests, they did not show obvious change. For practical applications, they changed toanother skin-friendly ultrastretchable substrate (Ecoflex 00e50). As expected, theresults showed that the GNF-pad could not only sense small strains, like the pulse


of the artery, but also large strains, for example, muscle movements during runningand walking.

Monitoring human body motions is just the first step. Combined strain sensors withdata processors, it will have even greater practice value. Larimi et al. found the GNF-pad could control the robot’s movements and gestures, which means this fabricatedGNF-pad was an ideal material for humanemachine interfacing. In another workdone by Wang et al. [34], they expected that after acquiring and recognizing soundsignal with graphene-based strain sensors, utilizing big data analysis, intelligentmachine could help people who had trouble in speaking express their feelings byjust moving relative muscles.

Heartbeat monitoring

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Figure 9.9 (a) Heartbeat monitoring using GNF-Pad implanted in Ecoflex wristband. (b) Fingerpose reading. Three GNF-Pad were assembled on a finger pad made out Ecoflex and used tomeasure the strain caused by change in the angle of each finger joints during bending thefinger. (c) Wireless knee band to track the knee angle changing during different activities,including walking, running, and sitting down and standing up.Reproduced with permission from S.R. Larimi,H. Rezaei Nejad, M. Oyatsi, A. O’Brien, M.Hoorfar, H. Najjaran, Low-cost ultra-stretchable strain sensors for monitoring human motionand bio-signals, Sens. Actuators A Phys. 271 (2018) 182e191. Copyright 2018 Elsevier B.V.


Photodetectors are sensors of light, which can translate light signals into electricalsignals. Flexible photodetectors have been actively applied in many areas, includingsensing biological systems and wearable integrated optoelectronics. “Electricaleyes” have recently been put forward, which are used on robots to simulate humaneyes to sense light or on people who have trouble in watching to help them senseobstacles. Apparently, conventional materials for photodetectors are too rigid to stretchand hardly suitable for “electrical eyes.” Because of the broadband absorption fromultraviolet and terahertz frequency, flexible graphene has been considered as an attrac-tive material for optoelectronics. Kang et al. [35] successfully fabricated stretchable,high photoresponsivity photodetectors with crumpled graphene (Fig. 9.10). The crum-pled graphene was prepared by prestraining the transferred target substrate as we havementioned before and used as photocurrent generation. The resultant extinction spec-trum of the crumpled graphene at varying uniaxial tensile strains (ε tensile, x) rangingfrom 0% to 200% and flat graphene showed that the crumple could not only enhancegraphene stretchability but also its photoabsorption. Photocurrent measurementsshowed that the photoresponsivity (Rph) at ε tensile ¼ 0% was estimated to bew0.11 mA W�1, which was 370% larger than that of a flat graphene photodetectorand two times larger than that at ε tensile, x ¼ 200%. Only small variance has beenfound in measured photocurrents and currentevoltage curves at varying stretchingereleasing cycles. Besides, the rise time (son) of this stretchable photodetectors werenot more than 300 ms. The capacity of strain-tunable photoadsorption enhancementcan be modulated by changing the crumple density, height, and pitch of the 3D crum-pled graphene. This graphene photodetector could be attached on arbitrary substrates,such as the surface of human face and heart models. And the stretchability can beextended to a very large strain in that it is determined by the predesigned strains ofthe elastomeric substrate on which the device is fabricated. To further improve theresponsivity of stretchable graphene photodetectors, hybrid systems based on photonicor plasmonic nanostructures have been introduced [36,37]. They demonstrated thatstretchable graphene photodetectors may find broad applications as conformable andflexible optical sensors.

Digital electronics: Transistor is the fundamental building component in digitalelectronics, which is important in stretchable applications because they enable sophis-ticated sensor readout and signal analysis. Because of the atomic thickness, high trans-parency, and extraordinary electrical performance, graphene has been considered as astrong candidate for the next-generation flexible transistors. A variety of stretchablegraphene prepared by the aforementioned three methods, including graphene kirigami,origami, and composites, have been demonstrated in stretchable transistors [38e42].The most stretchable and strain-tolerance transistor was achieved by kirigami gra-phene, which is gated by a liquid electrolyte and can be stretched to as much as240%. However, kirigami method requires suspended graphene, which complicatesthe fabrication process.

Transistors on PDMS and other stretchable elastomers are relatively easier appliedfor wearable health monitoring sensors and electronic skin. Lee et al. for the first timefabricated stretchable and transparent transistors using multilayer graphene as source/drain electrodes and channel material, which can only maintain electrical function up


Biaxially texturedgraphene

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Figure 9.10 (a) Schematic illustration of a stretchable photodetector and the experiment setup. (b) Fabrication procedures of a stretchable texturedgraphene photodetector. (c) Dynamic photoresponse of the device at varying uniaxial tensile strains (εtensile, x) from 0% to 200%. Measuredphotocurrent is normalized with the photocurrent at ε tensile, x ¼ 0%. (d) Comparison of measured photocurrent of the textured graphenephotodetector at the varying strains over two cycles and measured photocurrent of a flat graphene photodetector. The inset shows an opticalmicroscope image of the textured graphene photodetector. The value of 0.27 indicates the measured photocurrent of the flat graphene photodetectornormalized with that of the textured graphene photodetector at εtensile, x ¼ 0%. (e) Highly stretchable and conformal photodetector on the surface of ahuman brain model. (f) Conformal photodetector on a curved surface. The inset shows a photograph of the fabricated device on an Ecoflex substrate.(g) Dynamic photoresponse of the device at different bending strains to an incident 405 nm illumination. Photocurrent was measured in three oneoffcycles.Reproduced with permission from P. Kang, M.C. Wang, P.M. Knapp, S. Nam, Crumpled graphene photodetector with enhanced, strain-tunable, andwavelength-selective photoresponsivity, Adv. Mater. 28 (23) (2016) 4639e4645. Copyright 2016 WILEY-VCH Verlag GmbH & Co. KGaA,Weinheim.

Stretchable

grapheneelectrodes

195

to 5% strain [41]. By combining graphene with single-walled carbon nanotube(SWNT) as electrodes, Chae et al. fabricated stretchable and transparent field-effecttransistors with SWNT-network channel and a geometrically wrinkled inorganicdielectric layer [43]. The devices exhibited an excellent on/off ratio of w105, a highmobility ofw40 cm2 V�1 s�1, and a low operating voltage of less than 1 V. The tran-sistors can be functioned at 20% strain without appreciable leakage current increases orphysical degradation. No significant performance loss was observed after stretchingand releasing the devices for over 1000 times. However, this stretchability is stillsignificantly below the minimum required value for electronic skin (w50%) [44,45].

Liu et al. fabricated all-carbon stretchable and transparent transistors using MGG asbottom gate and source/drain contacts, polymer-sorted semiconductive CNT as semi-conductor, and SEBS as dielectric layer (Fig. 9.11) [30]. The preparation of MGG hasbeen introduced in the section of “preparation methods of stretchable graphene.” Themeasured on/off ratio is greater than 103 and the mobility of the stretchable transistor isabout 5.6 cm2/Vs, similar to the same polymer-sorted CNT transistors on rigid Si sub-strates with 300 nm SiO2 as a dielectric layer. When the transparent, all-carbon devicewas stretched in the direction parallel to the charge transport direction, minimal degra-dation was observed up to 120% strain. During stretching, the mobility continuouslydecreased from 5.6 cm2/Vs at 0% strain to 2.5 cm2/Vs at 120% strain. Notably, at strainas large as 105%, all these transistors still exhibited high on/off ratio (>103) andmobility (>3 cm2/Vs). As an application of the fully transparent and stretchable tran-sistor, they used it to control a LED’s switching. While stretching the transistor up tow100%, the LED light intensity does not change. This demonstrates that these highlystretchable and transparent graphene transistors could enable sophisticated stretchableoptoelectronics.

Energy-storage devices: With the emergence of intelligent machines, stretchableenergy storage devices are key components for the fabrication of complete and inde-pendent stretchable systems. It is very challenging to fabricate stretchable energy-storage devices, and few work has been reported. Conductive polymers [46], carbonnanotubes [47], and graphene [48] were used in fabricating stretchable supercapacitorelectrodes, and stretchable graphene are particularly prominent.

Zang et al. [49] developed stretchable all-solid-state supercapacitors based on crum-pled graphene (Fig. 9.12). They prepared the crumpled-graphene-papers (CG-papers)by biaxially/uniaxially prestretching the target substrate. The cyclic voltammetry (CV)curves of CG-papers electrodes showed that there was a typical rectangular shape at acertain scan rate, which indicated that it is an ideal double-layer electrochemical capac-itor. The CV curves only changed slightly when the CG-papers electrodes are underlarge deformation. Calculated from the discharge slops at different charge/dischargecurrent densities, CG-paper electrodes present remarkable gravimetric capacitance inthe range of 166e196 F g�1 at the operation rate of 1 A g�1, which are comparableto other high-performance but unstretchable supercapacitor electrodes. Furthermore,cycling stressestrain and galvanostatic charge/discharge tests under large deformationverified the stability of CG-paper electrodes. Integrated the two stretchable CG-paperelectrodes with a stretchable polymer gel, all-solid-state supercapacitors presentextraordinary stretchability and outstanding electrochemical properties.


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Figure 9.11 (a) Scheme of graphene-based stretchable transistor. SWNTs, single-walled carbonnanotubes. (b) Photo of the stretchable transistors made of graphene electrodes (top) and CNTelectrodes (bottom). The difference in transparency is clearly noticeable. (c, d) Transfer andoutput curves of the graphene-based transistor on SEBS before strain. (e, f) Transfer curves, onand off current, on/off ratio, and mobility of the graphene-based transistor at different strains.Reproduced with permission from N. Liu, A. Chortos, T. Lei, L. Jin, T.R. Kim, W.G. Bae, C.Zhu, S. Wang, R. Pfattner, X. Chen, R. Sinclair, Z. Bao, Ultratransparent and stretchablegraphene electrodes, Sci. Adv. 3 (9) (2017) e1700159. Copyright 2017 The Authors, some rightsreserved; exclusive licensee American Association for the Advancement of Science.


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Figure 9.12 (aec) Subjected to uniaxial strains of 0%, 100%, 200%, and 300% and (def)biaxial strains of 0%� 0% and 200%� 200%. (a,d) Cyclic voltammetry curves at 50mV s�1,(b,e) galvanostatic charge/discharge curves at 5A g�1, and (c,f) gravimetric capacitancemeasured at different charge/discharge current densities (Is¼ 0.5, 1.0, 2.0, 5.0, 10, 20, 50, and80 Ag�1). The tests were carried out in 1.0MH2SO4. The thickness of the graphene paper isw2mm measured at dehydrated state. g, A schematic diagram of the supercapacitor usingcrumpled-graphene-paper electrodes with a polymer electrolyte gel as the electrolyte andseparator. (h) The CV curves of the supercapacitor collected at a scan rate of 10mV s�1.(i) Galvanostatic charge/discharge curves at a current density of 1A g�1, under uniaxialstrains of 0%, 50%, 100%, and 150%. The thickness of the graphene paper is w0.8mmmeasured at dehydrated state.( j) Reproduced with permission from J. Zang, C. Cao, Y. Feng, J. Liu, X. Zhao, Stretchable andhigh-performance supercapacitors with crumpled graphene papers, Sci. Rep. 4 (2014) 6492.(m) Reproduced with permission from X. Zang, M. Zhu, X. Li, X. Li, Z. Zhen, J. Lao, K. Wang,F. Kang, B. Wei, H. Zhu, Dynamically stretchable supercapacitors based on graphene wovenfabric electrodes, Nano Energy 15 (2015), 83e91. Copyright 2015 ElsevierLtd.


Another example of stretchable supercapacitor is fabricated with graphene-wovenfabric electrodes (GWFs) done by Zang et al. [50]. GWF was prepared on the coppermesh through chemical vapor deposition method. The CV curves of the supercapacitorassembled with two identical GWF electrodes exhibited nearly rectangular at varyingscan rates increased from 0.06 to 1 V/s and the capacitance was up to 17 mF/cm2 at thescan rate of 0.06 V/s. The galvanostatic charge/discharge curve was nearly linear andsymmetrical. They also transferred GWF on prestretched substrates to make electrodesmore stretchable and added a conducting polymer (PANI) into GWF to enhance theelectrochemical properties of supercapacitors. These stretchable supercapacitorswith GWF-PANI electrodes present good galvanic ability and excellent electrochem-ical properties not only under the static mode but also the dynamic mode.

9.4 Summary and outlook

In this chapter, we mainly introduced the ways to prepare stretchable graphene and itsapplications in sensors, digital electronics, and energy storage devices. Grapheneexhibits highly desirable properties of atomic thickness, high transparency, and highconductivity, but its implementation in stretchable applications has been inhibitedby its tendency to crack at small strains. Overcoming the mechanical limitations of gra-phene could enable new functionality in stretchable transparent devices. Basically,there are three methods to obtain stretchable graphene, which are graphene kirigami,origami, and combining with 1D conductive species and/or polymers to form compos-ites. Among them, kirigami on suspended graphene can maintain most superior prop-erties of graphene, such as atomic thickness and high transparency, but the fabricationprocess is very complicated and impossible to be massive production. Grapheneorigami has been broadly used, in particular folding it into the very simple “wavy”structure. According to the fabrication process, graphene origami has been dividedinto three categories, including prestraining the transferred target substrate, patterningthe growth substrate, and direct engineering the graphene film. Another rapidly devel-oping method to enable stretchable graphene is to form composites by combining gra-phene with conductive species on polymers. The composites take advantages of highconductivity and transparency as well as inert chemical reactivity of graphene andavoid its small fracture strain limit by combining with conductive networks or poly-mers. Utilizing methods of “graphene origami and forming composites,” large-scaleand high-performance stretchable sensors and supercapacitors have beendemonstrated.

As an outlook for future studies, a few topics of interest are summarized as follows.Theoretical understanding of the mechanisms of stretchable graphene: Gra-

phene intrinsically is in-plane stiff and can only be bent due to its atomic thickness.Stretchable graphene has been experimentally achieved through origami, kirigami,and forming composites. Fundamental research on the mechanical properties ofgraphene has made significant progress over the last decade, but there is limited under-standing on the mechanisms of stretchable graphene, such as the lateral forces beyond


van der Waal interactions, the effects of surface roughness, and capillary bridging ofgraphene under strain.

Developing more reliable fabrication methods to achieve stretchable gra-phene: Ideally, stretchable graphene electrode should be a superthin, transparent,and conductive film. So far, most fabricated stretchable graphene film cannot satisfyall those merits at the same time, in particular being superthin which is the main char-acteristic of graphene. Thus, there is still a lot of room to develop fabrication methodsto achieve stretchable graphene as well as being large-scale, robust, and highly repro-duced. After deeper understanding of the mechanisms of current preparation methodsof stretchable graphene, a more reliable method may be put forward.

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in (%

)

35

30

25

20

15

10

5

00 5 10 15 20 25

Time (s)

12%/s (0.2 Hz)Charging discharging0.6

0.4

0.2

0.0

–0.2

–0.4

–0.6–0.6 –0.4 –0.2 0.2 0.4 0.6–0.0

Voltage (V)

0%

30%

0%

30%

PDMS pre-stretched 20%

Gel electrolyte

ReleasingStretching to 30%

Releasing

Stre

tchi

ngStre

tching

Stretching Stretching

Releasing Releasing

Releas

ingR

elea

sing

Stre

tchi

ng

Releas

ingStretching Releasing

Releasing StretchingReleasin

g

Stre

tchi

ng

Cur

rent

den

sity

(mA

/cm

2 )C

urre

nt d

ensi

ty (m

A/c

m2 )

(b)

(a)

(c)

Figure 9.13 (a) Strainetime curve at strain rate of 6%/s (left) and corresponding CV curve(right). (b) Strainetime curve at strain rate of 12%/s (left) and corresponding CV curve (right).(c) Schematic illustration of GWF-based electrode in dynamic stretching process.


Extending to other 2D materials: The family of 2D materials has grown beyondgraphene, and together they hold great promise for a wide range of applications. Intrin-sically, the atomic composition, mechanical structure, and electrical properties of other2D materials are more complicated than graphene. Roads toward stretchable graphenebeyond 2D materials must be very challenging but of more interest.

References

[1] B. Chu, W. Burnett, J.W. Chung, Z. Bao, Bring on the bodyNET, Nature 549 (2017)328e330.

[2] T. Cheng, Y. Zhang, W.Y. Lai, W. Huang, Stretchable thin-film electrodes for flexibleelectronics with high deformability and stretchability, Adv. Mater. 27 (22) (2015)3349e3376.

[3] S. Bae, H. Kim, Y. Lee, X. Xu, J.S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H.R. Kim,Y.I. Song, Y.J. Kim, K.S. Kim, B. Ozyilmaz, J.H. Ahn, B.H. Hong, S. Iijima, Roll-to-rollproduction of 30-inch graphene films for transparent electrodes, Nat. Nanotechnol. 5 (8)(2010) 574e578.

[4] D.R. Cairns, R.P. Witte, D.K. Sparacin, S.M. Sachsman, D.C. Paine, G.P. Crawford,R.R. Newton, Strain-dependent electrical resistance of tin-doped indium oxide on polymersubstrates, Appl. Phys. Lett. 76 (11) (2000) 1425e1427.

[5] S. Won, Y. Hwangbo, S.K. Lee, K.S. Kim, K.S. Kim, S.M. Lee, H.J. Lee, J.H. Ahn,J.H. Kim, S.B. Lee, Double-layer CVD graphene as stretchable transparent electrodes,Nanoscale 6 (11) (2014) 6057e6064.

[6] D. Akinwande, C.J. Brennan, J.S. Bunch, P. Egberts, J.R. Felts, H. Gao, R. Huang,J.-S. Kim, T. Li, Y. Li, K.M. Liechti, N. Lu, H.S. Park, E.J. Reed, P. Wang,B.I. Yakobson, T. Zhang, Y.-W. Zhang, Y. Zhou, Y. Zhu, A review on mechanicsand mechanical properties of 2D materialsdgraphene and beyond, Extreme Mech.Lett. 13 (2017) 42e77.

[7] S.P. Koenig, N.G. Boddeti, M.L. Dunn, J.S. Bunch, Ultrastrong adhesion of graphenemembranes, Nat. Nanotechnol. 6 (9) (2011) 543e546.

[8] N.G. Boddeti, P.K. Steven, R. Long, J. Xiao, J.S. Bunch, M.L. Dunn, Mechanics ofadhered, pressurized graphene blisters, J. Appl. Mech. 80 (4) (2013) 040909.

[9] N.G. Boddeti, X. Liu, R. Long, J. Xiao, J.S. Bunch, M.L. Dunn, Graphene blisters withswitchable shapes controlled by pressure and adhesion, Nano Lett. 13 (12) (2013)6216e6221.

[10] Z. Zong, C.-L. Chen, M.R. Dokmeci, K.-t. Wan, Direct measurement of graphene adhesionon silicon surface by intercalation of nanoparticles, J. Appl. Phys. 107 (2) (2010) 026104.

[11] T. Jiang, R. Huang, Y. Zhu, Interfacial sliding and buckling of monolayer graphene on astretchable substrate, Adv. Funct. Mater. 24 (3) (2014) 396e402.

[12] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, J.H. Ahn, P. Kim, J.Y. Choi,B.H. Hong, Large-scale pattern growth of graphene films for stretchable transparentelectrodes, Nature 457 (7230) (2009) 706e710.

[13] M.K. Blees, A.W. Barnard, P.A. Rose, S.P. Roberts, K.L. McGill, P.Y. Huang,A.R. Ruyack, J.W. Kevek, B. Kobrin, D.A. Muller, P.L. McEuen, Graphene kirigami,Nature 524 (7564) (2015) 204e207.


[14] Z. Qi, D.K. Campbell, H.S. Park, Atomistic simulations of tension-induced large defor-mation and stretchability in graphene kirigami, Phys. Rev. B 90 (24) (2014).

[15] B. Mortazavi, A. Lherbier, Z. Fan, A. Harju, T. Rabczuk, J.C. Charlier, Thermal andelectronic transport characteristics of highly stretchable graphene kirigami, Nanoscale 9(42) (2017) 16329e16341.

[16] N. Wei, Y. Chen, K. Cai, J. Zhao, H.-Q. Wang, J.-C. Zheng, Thermal conductivity ofgraphene kirigami: ultralow and strain robustness, Carbon 104 (2016) 203e213.

[17] T.C. Shyu, P.F. Damasceno, P.M. Dodd, A. Lamoureux, L. Xu, M. Shlian, M. Shtein,S.C. Glotzer, N.A. Kotov, A kirigami approach to engineering elasticity in nanocompositesthrough patterned defects, Nat. Mater. 14 (8) (2015) 785e789.

[18] P.Y. Chen, J. Sodhi, Y. Qiu, T.M. Valentin, R.S. Steinberg, Z. Wang, R.H. Hurt,I.Y. Wong, Multiscale graphene topographies programmed by sequential mechanicaldeformation, Adv. Mater. 28 (18) (2016) 3564e3571.

[19] J. Zang, S. Ryu, N. Pugno, Q. Wang, Q. Tu, M.J. Buehler, X. Zhao, Multifunctionality andcontrol of the crumpling and unfolding of large-area graphene, Nat. Mater. 12 (4) (2013)321e325.

[20] J. Mu, C. Hou, G. Wang, X. Wang, Q. Zhang, Y. Li, H. Wang, M. Zhu, An elastictransparent conductor based on hierarchically wrinkled reduced graphene oxide for arti-ficial muscles and sensors, Adv. Mater. 28 (43) (2016) 9491e9497.

[21] N. Liu, Z. Pan, L. Fu, C. Zhang, B. Dai, Z. Liu, The origin of wrinkles on transferredgraphene, Nano Res. 4 (10) (2011) 996e1004.

[22] Z. Pan, N. Liu, L. Fu, Z. Liu, Wrinkle engineering: a new approach to massive graphenenanoribbon arrays, J. Am. Chem. Soc. 133 (44) (2011) 17578e17581.

[23] T. Chen, X. Yuhua, A.K. Roy, L. Dai, Transparent and stretchable high-performancesupercapacitors based on wrinkled graphene electrodes, ACS Nano 8 (1) (2013)1039e1046.

[24] Y. Xie, Y. Liu, Y. Zhao, Y.H. Tsang, S.P. Lau, H. Huang, Y. Chai, Stretchable all-solid-state supercapacitor with wavy shaped polyaniline/graphene electrode, J. Mater. Chem. A2 (24) (2014) 9142e9149.

[25] J.Y. Hong, W. Kim, D. Choi, J. Kong, H.S. Park, Omnidirectionally stretchable andtransparent graphene electrodes, ACS Nano 10 (10) (2016) 9446e9455.

[26] B. Zhu, Z. Niu, H. Wang, W.R. Leow, H. Wang, Y. Li, L. Zheng, J. Wei, F. Huo, X. Chen,Microstructured graphene arrays for highly sensitive flexible tactile sensors, Small 10 (18)(2014) 3625e3631.

[27] W. Chen, X. Gui, S. Li, L. Yang, B. Liang, H. Zhu, J. She, Z. Tang, Fabrication of wrinkledgraphene based on thermal-enhanced Rayleigh-Bénard convection for field electronemission, Carbon 129 (2018) 646e652.

[28] W. Chen, X. Gui, B. Liang, M. Liu, Z. Lin, Y. Zhu, Z. Tang, Controllable fabrication oflarge-area wrinkled graphene on a solution surface, ACS Appl. Mater. Interfaces 8 (17)(2016) 10977e10984.

[29] Z. Chen, W. Ren, L. Gao, B. Liu, S. Pei, H.M. Cheng, Three-dimensional flexible andconductive interconnected graphene networks grown by chemical vapour deposition, Nat.Mater. 10 (6) (2011) 424e428.

[30] N. Liu, A. Chortos, T. Lei, L. Jin, T.R. Kim, W.-G. Bae, C. Zhu, S. Wang, R. Pfattner,X. Chen, R. Sinclair, Z. Bao, Ultratransparent and stretchable graphene electrodes, Sci.Adv. 3 (9) (2017) e1700159.

[31] M.S. Lee, K. Lee, S.Y. Kim, H. Lee, J. Park, K.H. Choi, H.K. Kim, D.G. Kim, D.Y. Lee,S. Nam, J.U. Park, High-performance, transparent, and stretchable electrodes usinggraphene-metal nanowire hybrid structures, Nano Lett. 13 (6) (2013) 2814e2821.


[32] C. Yan, J. Wang, W. Kang, M. Cui, X. Wang, C.Y. Foo, K.J. Chee, P.S. Lee, Highlystretchable piezoresistive graphene-nanocellulose nanopaper for strain sensors, Adv.Mater. 26 (13) (2014) 2022e2027.

[33] S.R. Larimi, H. Rezaei Nejad, M. Oyatsi, A. O’Brien, M. Hoorfar, H. Najjaran, Low-costultra-stretchable strain sensors for monitoring human motion and bio-signals, Sens. Ac-tuators A Phys. 271 (2018) 182e191.

[34] Y. Wang, T. Yang, J. Lao, R. Zhang, Y. Zhang, M. Zhu, X. Li, X. Zang, K. Wang, W. Yu,H. Jin, L. Wang, H. Zhu, Ultra-sensitive graphene strain sensor for sound signal acquisitionand recognition, Nano Res. 8 (5) (2015) 1627e1636.

[35] P. Kang, M.C. Wang, P.M. Knapp, S. Nam, Crumpled graphene photodetector withenhanced, strain-tunable, and wavelength-selective photoresponsivity, Adv. Mater. 28 (23)(2016) 4639e4645.

[36] C.W. Chiang, G. Haider, W.C. Tan, Y.R. Liou, Y.C. Lai, R. Ravindranath, H.T. Chang,Y.F. Chen, Highly stretchable and sensitive photodetectors based on hybrid graphene andgraphene quantum dots, ACS Appl. Mater. Interfaces 8 (1) (2016) 466e471.

[37] M. Kim, P. Kang, J. Leem, S. Nam, A stretchable crumpled graphene photodetector withplasmonically enhanced photoresponsivity, Nanoscale 9 (12) (2017) 4058e4065.

[38] G. Fisichella, S. Lo Verso, S. Di Marco, V. Vinciguerra, E. Schiliro, S. Di Franco, R. LoNigro, F. Roccaforte, A. Zurutuza, A. Centeno, S. Ravesi, F. Giannazzo, Advances in thefabrication of graphene transistors on flexible substrates, Beilstein J. Nanotechnol. 8 (2017)467e474.

[39] Y.H. Jung, H. Zhang, S.J. Cho, Z. Ma, Flexible and stretchable microwave microelectronicdevices and circuits, IEEE Trans. Electron. Devices 64 (5) (2017) 1881e1893.

[40] B.J. Kim, H. Jang, S.K. Lee, B.H. Hong, J.H. Ahn, J.H. Cho, High-performance flexiblegraphene field effect transistors with ion gel gate dielectrics, Nano Lett. 10 (9) (2010)3464e3466.

[41] S.K. Lee, B.J. Kim, H. Jang, S.C. Yoon, C. Lee, B.H. Hong, J.A. Rogers, J.H. Cho,J.H. Ahn, Stretchable graphene transistors with printed dielectrics and gate electrodes,Nano Lett. 11 (11) (2011) 4642e4646.

[42] J. Liang, K. Tong, H. Sun, Q. Pei, Intrinsically stretchable field-effect transistors, MRSBull. 42 (02) (2017) 131e137.

[43] S.H. Chae, W.J. Yu, J.J. Bae, D.L. Duong, D. Perello, H.Y. Jeong, Q.H. Ta, T.H. Ly,Q.A. Vu, M. Yun, X. Duan, Y.H. Lee, Transferred wrinkled Al2O3 for highly stretchableand transparent graphene-carbon nanotube transistors, Nat. Mater. 12 (5) (2013) 403e409.

[44] N. Lu, C. Lu, S. Yang, J. Rogers, Highly sensitive skin-mountable strain gauges basedentirely on elastomers, Adv. Funct. Mater. 22 (19) (2012) 4044e4050.

[45] T. Yamada, Y. Hayamizu, Y. Yamamoto, Y. Yomogida, A. Izadi-Najafabadi, D.N. Futaba,K. Hata, A stretchable carbon nanotube strain sensor for human-motion detection, Nat.Nanotechnol. 6 (5) (2011) 296e301.

[46] C. Zhao, C. Wang, Z. Yue, K. Shu, G.G. Wallace, Intrinsically stretchable supercapacitorscomposed of polypyrrole electrodes and highly stretchable gel electrolyte, ACS Appl.Mater. Interfaces 5 (18) (2013) 9008e9014.

[47] X. Wang, C. Yang, J. Jin, X. Li, Q. Cheng, G. Wang, High-performance stretchablesupercapacitors based on intrinsically stretchable acrylate rubber/MWCNTs@conductivepolymer composite electrodes, J. Mater. Chem. 6 (10) (2018) 4432e4442.

[48] S. Wang, N. Liu, J. Su, L. Li, F. Long, Z. Zou, X. Jiang, Y. Gao, Highly stretchable andself-healable supercapacitor with reduced graphene oxide based fiber springs, ACS Nano11 (2) (2017) 2066e2074.


[49] J. Zang, C. Cao, Y. Feng, J. Liu, X. Zhao, Stretchable and high-performance super-capacitors with crumpled graphene papers, Sci. Rep. 4 (2014) 6492.

[50] X. Zang, M. Zhu, X. Li, X. Li, Z. Zhen, J. Lao, K. Wang, F. Kang, B. Wei, H. Zhu,Dynamically stretchable supercapacitors based on graphene woven fabric electrodes, NanoEnergy 15 (2015) 83e91.


Conclusions and outlook 10This book has reviewed application of graphene in various display and lighting devices(e.g., organic lighteemitting diodes (OLEDs), inorganic LEDs, quantum dot LEDs(QD-LEDs), perovskite LEDs (PeLEDs)). Graphene has superior electrical, optical,and mechanical properties, and there have been significant advances in synthesis andtransfer of high-quality graphene with large area, so it has been widely used as the elec-trode in various LEDs. Especially, the wide-angle spectral stability in OLED usinggraphene electrodes is a strong merit for its use in transparent electrode for displayapplications [1]. However, pristine graphene has high RS and too low (high) work func-tion (WF) to be used as an anode (cathode); these traits limit the hole (electron) injectionfrom graphene electrode to emitting layers, and thereby increase operating voltage andreduce the luminous efficiency of LEDs. Thus, various researchers have attempted toimprove the electrical properties of graphene, so that graphene electrodes can be usedpractically in electrodes of LEDs. Early studies of chemical graphene doping usedsmall-molecule inorganic acid (e.g., HNO3) or metal chloride (e.g., AuCl3) [2]; thedopants substantially improved the electrical properties of graphene. However, thesedopants are severely unstable in ambient conditions and, therefore, are not practicallyapplicable in graphene electrodes in LEDs. Therefore, fluorinated organic acids [3,4]and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzoimidazole derivatives [5,6] wereused as stable p-type and n-type chemical dopants; they effectively modify the WF ofgraphene with substantially improved doping stability.

Graphene has been successfully used as electrodes in various LEDs. The technicalstandard of OLEDs with graphene electrode exceeds demonstration level and achieveshigh luminous properties that are comparable to or higher than those of OLEDs thatuse an ITO electrode.

As the technology to fabricate graphene-based OLEDs has been advanced,research on graphene-based OLEDs has expanded from developing high-efficiencyOLEDs to demonstration of OLEDs arrays and to panel integration. To proceedfrom proof-of-concept to the commercialization level, graphene-based OLEDsshould be used in active matrix display. In this aspect, it is essential to achievepatterning technology of graphene so that graphene is elaborately pixelized withoutdefect generation.

Patterning using O2 plasma etching with shadow mask usually leaves partial dam-age at the edge of the unexposed region. Patterning using laser ablation still showspoor yields, and the laser-exposed region protrudes severely. Photolithography is apromising patterning process due to its predominance in the display industry, but gra-phene adheres weakly to its substrate, so photolithographic patterning of graphenefilm leaves a nonuniform surface. Graphene’s weak adhesion has been increased


https://doi.org/10.1016/B978-0-08-102482-9.00010-1

using a liquid bridging method; it enables photolithographic patterning and pixelatedarrays of OLEDs with stable operation [7].

Graphene-based materials have also been used as interfacial layer and emittinglayers for OLEDs. For those purpose, solution-processable graphene derivativeswere mostly used. These approaches have been actively studied due to the lowcost, simple synthetic routes of solution process, and they have achieved significantadvances in employment of graphene-based derivatives for interfacial layers andemitting layers in LEDs. A graphene-based interfacial layer acts as an intermediatestep between electrode and emitting layer and improves charge injection to the emit-ting layer and increases the luminous properties of OLEDs. In addition, graphene-based emitters including graphene quantum dots and its composite form were devel-oped, and they exhibited continuous progress in luminous properties and syntheticways ranging from top-down to bottom-up approaches. As a result, LEDs weredeveloped using the graphene-based emitters. Moreover, graphene can be used inencapsulating layers in OLED devices [8]. Because graphene is composed of atom-ically fine carbon lattice, graphene can effectively block the penetration of oxygenand moisture that causes degradation of OLEDs.

Graphene has also been evaluated as an electrode in other LEDs (e.g., inorganicLEDs, QD-LEDs, PeLEDs). In these LEDs also, energy-level engineering and chemicaldoping of the graphene electrode substantially increase the luminous properties. Toachieve inorganic LEDs with graphene that exhibit highly luminous properties, the crys-tal quality of epitaxially grown inorganic semiconductor layer must be maintained. QDshave a deeper VBM than other emitters, and the emitting layer must be thin. Thus, toachieve efficient hole injection from anode to QD layers, considerable design of mate-rials and devices is necessary, and the surface must be smooth before deposition of QDlayers. Halide perovskites have low exciton-binding energies, so to achieve highly lumi-nous PeLEDs, luminescence quenching caused by metallic species must be prevented.Graphene is also promising as an electrode material because of its chemical stability.Graphene does not generate chemical and metallic species that cause luminescencequenching when it was exposed to acidic environments, whereas ITO generates metallicspecies causing quenching in those conditions.

The research direction in graphene-based LEDs has diversified from improvementof luminous properties of LEDs to various other subjects including stretchable LEDs,transparent LEDs, and transferable LEDs. Considering the flourishing progress ingraphene-based LEDs, graphene electrodes will become practically useable in LEDsin near future if (i) a method can be developed to synthesize and transfer high-quality graphene and (ii) safe and inexpensive mass production of graphene can beachieved.

High-quality graphene that has large graphene grains with few defects has superiorelectrical conductivity and would yield further improvement of LEDs that usegraphene electrodes. Productivity of graphene must be increased before commercial-ization of graphene-based LEDs is feasible. To synthesize high-quality graphene,chemical vapor deposition (CVD) method is generally used. During the CVD process,high-purity hydrocarbon gases (e.g., CH4, C2H2) are mostly used for graphene synthe-sis, but they are expensive and explosive; this is a great impediment to inexpensive


mass production of graphene. Graphene has been synthesized using inexpensive car-bon precursors (e.g., coal tar pitch, waste plastics, leaves) that are not explosive [9].Introducing these carbon sources in a synthesis process will be safe and substantiallyreduce the production cost of graphene synthesis; this approach would substantiallyimprove the productivity of graphene. After the quality and productivity of grapheneare improved, it can be practically used in transparent electrodes in LEDs of next-generation displays and lightings.

References

[1] H. Cho, J.-W. Shin, N.S. Cho, J. Moon, J.-H. Han, Y.-D. Kwon, S. Cho, J.-I. Lee, Opticaleffects of graphene electrodes on organic light-emitting diodes, IEEE J. Sel. Top. QuantumElectron. 22 (2016) 7230237.

[2] T.-H. Han, Y. Lee, M.-R. Choi, S.-H. Woo, S.-H. Bae, B.H. Hong, J.-H. Ahn, T.-W. Lee,Extremely efficient flexible organic light-emitting diodes with modified graphene anode,Nat. Photon. 6 (2012) 105e110.

[3] T.-H. Han, S.-J. Kwon, N. Li, H.-K. Seo, W. Xu, K.S. Kim, T.-W. Lee, Versatile p-typechemical doping to achieve ideal flexible graphene electrodes, Angew. Chem. Int. Ed. 55(2016) 6197e6201.

[4] S.-J. Kwon, T.-H. Han, T.Y. Ko, N. Li, Y. Kim, D.J. Kim, S.-H. Bae, Y. Yang, B.H. Hong,K.S. Kim, S. Ryu, T.-W. Lee, Extremely stable graphene electrodes doped with macro-molecular acid, Nat. Commun. 9 (2018) 2037.

[5] P. Wei, N. Liu, H.R. Lee, E. Adijanto, L. Ci, B.D. Naab, J.Q. Zhong, J. Park, W. Chen,Y. Cui, Z. Bao, Tuning the dirac point in CVD-grown graphene through solution processedn-type doping with 2-(2-Methoxyphenyl)-1,3-dimethyl-2,3,-dihydro-1H-benzoimidazole,Nano Lett. 13 (2013) 1890e1897.

[6] S.-J. Kwon, T.-H. Han, Y.-H. Kim, T. Ahmed, H.-K. Seo, H. Kim, D.J. Kim, W. Xu,B.H. Hong, J.-X. Zhu, T.-W. Lee, Solution-processed n-type graphene doping for cathode ininverted polymer light-emitting diodes, ACS Appl. Mater. Interfaces 10 (2018) 4874e4881.

[7] J.-W. Shin, J.-H. Han, H. Cho, J. Moon, B.-H. Kwon, S. Cho, T. Yoon, T.-S. Kim,M. Suemitsu, J.-I. Lee, N.S. Cho, Display process compatible accurate graphene patterningfor OLED application, 2D Materials 5 (2017) 014003.

[8] H.-K. Seo, M.-H. Park, Y.-H. Kim, S.-J. Kwon, S.-H. Jeong, T.-W. Lee, Laminated gra-phene films for flexible transparent thin film encapsulation, ACS Appl. Mater. Interfaces 8(2016) 14725e14731.

[9] S.-J. Kwon, H.-K. Seo, S. Ahn, T.-W. Lee, Value-added recycling of inexpensive carbonsources to graphene and carbon nanotubes, Adv. Sustainable Syst. (2018) 1800016.

Conclusions and outlook 207

Index

Note: ‘Page numbers followed by “f ” indicate figures, “t” indicates tables’.

A

Adhesion-mediated transfer methodbiological graphene coatings, 45e46poly(vinyl alcohol) (PVA) carrier

layer, 47polydimethylsiloxane (PDMS), 45, 47prepatterned silicone/PET film, 45e46, 46fsilicone-based pressure-sensitive adhesive

film (PSAF), 46e47stamping transfer, 45e46thermal release tape (TRT), 45, 47

Amino-functionalized GQDs (af-GQDs),123e125

Annealing process, 9e11, 11fe12fAtomic force microscopy (AFM)

analysis, 7Atomic structure, graphene, 5e7, 6f

B

Balloon-blowing method, 180BeereLambert law, 83Benzene, atomic structure, 119e120, 121fBlock copolymers (BCPs), 130Bubble-free electrochemical delamination

method, 44

C

Carbon dots (CDs), 121e123Carbon nanotubes (CNTs), 59e61Chemical etching, 36e38, 42Chemical vapor deposition (CVD), 7e9,

8fe9f, 12f, 59, 60f, 74, 128e130,177e178, 206e207

adhesion-mediated transfer. See Adhesion-mediated transfer method

applications, 33e35liquid crystal display panel, 35e36

structural integrity and uniformity, 35e36support-dissolving transfer. See Support-

dissolving transfer methodtarget-supported transfer, 48e51, 49f

Continuous fabrication method, 33Conventional electrodes, 175Crumpled-graphene-papers

(CG-papers), 196

D

Density functional theory (DFT), 165Digital electronics, 193Dopingchemical dopingcarbon nanotubes (CNTs), 59e61chemical reactions, 59e61conductivity and carrier concentration,62e63, 63f

electronic structure, 59e61, 61fby metal chloride, 61, 62fnegative Gibbs free energy, 59e61photovoltaic devices, 59e61poly-(methyl methacrylate)(PMMA), 62

single-wall carbon nanotubes(SWCNTs), 62e63

tetracyanoethylene (TCNE), 63, 64fthionyl chloride, reaction mechanism,62e63, 64f

transfer and device fabrication process,59e61, 60f

work function (WF), 59e61chemical vapor deposition (CVD), 59, 60fmetal oxideapplication, 67Fermi-level energies, 66Seebeck coefficient, 65e66

Doping (Continued)thermally evaporated molybdenumtrioxide (MoO3) layers, 65, 65f

zero-gap two band model, 65e66substitution method, 59

E

Electrical eyes, 193Electrical resistance, 22, 23fElectrochemical cutting method, 128e130,

129fElectroluminescence (EL) spectra, 75e76Electroluminescent devices, 140e143,

141fe142f, 144fElectron paramagnetic resonance (EPR), 158Energy band structure, graphene, 11Energy-storage devices, 196Etching-free bubbling transfer process,

42e44, 43fExternal quantum efficiency (EQE), 140

F

Fermi energy, 13e14, 13fFew layered graphene (FLG), 153Few layered graphene (FLG)/ ZnO nanorod

(NR) composite, 154Field effect transistor (FET) devices, 13e14,

16e20, 19fe20fFiltration transferring method, 30e31Flexible photodetectors, 193Fluorine-doped tin oxide (FTO), 152Free exciton emission (FEE), 157

G

Grapheneadhesion/separation energies, 176, 177tin-plane stiffness, 176low modulus elastomer, 176e177surface roughness, 176theoretical calculations, 176theoretical strength, 176

Graphene-based composite emittergraphene oxide (GO)-zinc oxide (ZnO)

composite, 154e157band alignment, 161e162charge transfer mechanism, 161fcore-shell model, 161f

electron paramagnetic resonance (EPR),158, 159f

photoluminescence spectra, 158, 161fgrapheneeSnO2 hybrid composite, 162e163semiconducting oxide NPs hybridization,

152thiol-functionalized reduced graphene

oxide (TrGOeZnO) hybrid materialsband alignment, 161e162charge transfer mechanism, 161felectron paramagnetic resonance (EPR),158, 160f

oxygen vacancy surface defects,161e162

photoluminescence spectra, 161ftungsten oxideereduced graphene oxide

(WO3erGO) nanocomposite, 163zinc oxide (ZnO)egraphene quantum dot

LEDdensity functional theory (DFT), 165energy level, 170fgraphite power, 163e165green light emission diode, 168e169passive matrix, 168, 168fe169fphotoluminescence spectra, 165, 166fschematic energy band diagram,165e168, 167f

synthetic process, 164fZn acetate dehydrate, 163e165

zinc oxide (ZnO) nanoneedles, 154fzinc oxide (ZnO) nanowires (NWs),

156e157Graphene-based compositesCVD growth, 187f3D graphene, 185e188fabrication process, 186fgraphene sheets, 185e188metal nanowires (mNWs), 188multilayer G/G scrolls (MGG), 188, 189fNi foam, 188silver nanowires (AgNWs), 188, 190f

Graphene-based interfacial layer, 206Graphene-based organic lighteemitting

diodes (OLEDs)chemical vapor deposition (CVD),

206e207energy-level engineering and chemical

doping, 206

210 Index

graphene-based interfacial layer, 206luminescence quenching, 206photolithographic patterning,

205e206solution-processable graphene derivatives,

206Graphene-based transparent conductive

films (G-TCFs)continuous fabrication, 33hybrid TCFs, 31lab-scale fabrication methods, 33poly(3, 4-

ethylenedioxythiophene):poly(styrenesulfonate):graphene:ethyl cellulose(PEDOT:PSS:G:EC) hybridelectrodes, 33, 34fe35f

reduced graphene oxide (rGO) nanosheets,32, 32f

roll-to-roll (R2R) process, 33silver nanowires (AgNWs), 31e32, 32fsolution casting, 30e31, 31f

GrapheneeCdSe composite, 153GrapheneeCdSe (GeCdSe) quantum dot

(QDs), 153, 153fGraphene electrodes, 1, 2fchemical vapor deposition (CVD). See

Chemical vapor deposition (CVD)doping, stability

annealing-induced degradation, 68e69,69f

electronegativity, 70energy configuration, 68e69environmental conditions, 67p-type doping, 68e69sheet resistance, 67, 67f

graphene-based transparent conductivefilms (G-TCFs), 27. See alsoGraphene-based transparentconductive films (G-TCFs)

graphene oxide (GO)carbon/oxygen atomic ratio, 27chemical reduction (CR), 28graphene sheets, 27high temperature annealing(HTA), 28, 30

LerfeKlinowski model, 27, 28foptoelectrical property, 28, 29toxygen-containing groups, 28e30

reduced GO (rGO), 27e30van der Waals attraction, 27

organic lighteemitting diodes (OLEDs).See Organic lighteemitting diodes(OLEDs)

Grapheneemetal composites, 151e152Grapheneemetal/metal oxide hybrid

composite, 152Graphene nanoflakes (GNFs), 191e192Graphene origamiballoon-blowing method, 180graphene film engineering, 183e185growth substrate prepatterning, 180e183polyacrylic ester (PEA) substrate, 180prestraining substrate, 180, 181f

Graphene oxide quantum dots (GOQDs),121e123

Graphene quantum dots (GQDs), 151amorphous carbon materials, 119atomic structure, benzene, 119e120, 121fblock copolymers (BCPs), 130bottom-up synthesis, 130e131, 132fcarbon dots (CDs), 121e123C60 molecules, 131e132, 133fDFT calculation, 119e120, 122fDirac cone, 117e118electrical and optical properties, 117electrochemical cutting, 128e130, 129fp-electrons, 117e118emission wavelength, 118e119, 118fenergy levels, 117fluorescencechemical exfoliation, photoluminescencespectra, 134, 135f

controlled oxidation, 136e138, 137fdefects/functional groups, 133electroluminescence (EL) devices, 133optical applications, 133structural models, 134e136, 136f

graphene oxide quantum dots (GOQDs),121e123

graphite intercalation compounds, 127,128f

Hummers’ method, 123hydrothermal/solvothermal cuttingamino-functionalized GQDs (af-GQDs),123e125

chemical reduction, 125e127

Index 211

Graphene quantum dots (GQDs) (Continued)mechanism, 123e125, 124fpreparation of, 125, 126fthermal deoxidization, 123e125

lighting applicationsdown-converting white light-emttingdiodes (WLEDs), 138e140, 139f

electroluminescent devices, 140e143,141fe142f, 144f

properties, 138microwave-assisted cutting, 127e128, 129fphoto-Fenton reaction, 130, 131fphotoluminescence (PL), 119, 120fquantum confinement effect, 118e119semiconductor quantum dot, 117top-down and bottom-up approach, 123UV irradiation, 130

Graphene-TiO2 (G-TiO2), 152Graphene-woven fabric electrodes (GWFs),

199Grapheneezinc oxide (ZnO) hybrid

compositeband-edge emission, 154e156band-to-band transition, 154e156few layered graphene (FLG)/ZnO nanorod

(NR) composite, 154, 155ffree exciton emission (FEE), 157localized surface plasmon (LSP), 154e156metalorganic vaporephase epitaxy

(MOVPE), 154, 155fp-Si/ZnO/graphene hybrid structure,

154e156rGO/ZnO nanorods, 156, 157fschematically depicted photoluminescence

enhancement path, 156fGraphene-ZnO (G-ZnO), 152G-TCFs. See Graphene-based transparent

conductive films (G-TCFs)

H

High temperature annealing (HTA), 28, 30Hummers method, 103, 104f, 123Hydrothermal/solvothermal cutting method

amino-functionalized GQDs (af-GQDs),123e125

chemical reduction, 125e127mechanism, 123e125, 124fpreparation of, 125, 126fthermal deoxidization, 123e125

I

Indium tin oxide (ITO), 1, 14e15

L

Lab-scale fabrication methods, 30e31, 33LangmuireBlodgett film, 108Layer-by-layer doping technique, 74LerfeKlinowski model, 27, 28fLight-emitting diodes (LEDs), 1graphene-based buffer layersatomic-scale features, 99e101composite buffer layer, 110e113,111fe112f, 112t

electrical properties, 101graphite powder, 103hole injection layer (HIL), 99Hummers’s method, 103, 104findium tin oxide (ITO), 99ionization energy, 99optical properties, 101optoelectronics, 103e110, 105fe107f,107te108t, 109fe110f

oxidation and exfoliation process, 99e101structure models, 99e101, 100fthermal reduction, 99e101, 102fe103fwork function (WF), 99

graphene-based quantum dot emitters. SeeGraphene quantum dots (GQDs)

Liquid bridging concept, 89e91, 90f

M

Metal nanowires (mNWs)/nanoparticles, 176Microwave-assisted cutting method,

127e128, 129fMonocrystalline ruthenium catalyst, 9e11

N

Nanocarbon-based transparent conductivefilms, 30

Nitrogen-doped GQDs (N-GQDs),128e130

O

OptoelectronicsLangmuireBlodgett film, 108organic light-emitting diodes (OLED),

103e108, 105fperformance of, 108, 108t

212 Index

polymer light-emitting diodes (PLEDs),103e108, 106fe107f

device performance, 103e108, 107tquantum dot light-emitting diodes

(QLEDs), 108e110, 109fe110fOrganic lighteemitting diodes (OLEDs), 1,

103e108, 105f, 140, 141fBeereLambert law, 83chemical bonding, 73chemical vapor deposition (CVD), 74direct transmittances, 83e84, 84felectrical issues, 81e83, 82fexternal quantum efficiencies, 83e84, 84fgraphene-pixel electrode

fabricated integration panel, 92e94, 93fflexible substrate, 94, 94flayout of, 92, 93fphotolithography process, 91, 92fprocess, 91, 91f

indium tin oxide (ITO), 74e76, 75flayer-by-layer doping technique, 74liquid bridging concept, 89e91, 90fluminance distribution, 83e84, 84foptical issues

angular emission, 78e81, 80fcavity enhancement factor, 76e77dipole oscillation theory, 78hole transport layer (HTL), 78, 79fmicrocavity approach, 78e81multiple interference theory, 78optical contrast, 76e77reflectance of, 76e77, 78fscanning electron microscope (SEM), 76,77f

optoelectronic applications, 73panel-level device array, 73patterning hurdles, 87e88, 88fe89fpatterning process, 74performance of, 108, 108tproof-of-concept level, 87random scattering layer (RSL)

effect of, 85, 86flight extraction structure, 84e85, 85f

surface planarization, 84e85

P

Perovskite lighteemitting diodes (PeLEDs),206

Photodetectors, 193, 195fPhoto-Fenton reaction, 130, 131fPhotolithography, 91, 92f, 205e206Photoluminescence (PL), 119, 120f, 151Piezo-capacitive strain sensors, 191Piezo-resistive strain sensors, 191Piezo-voltage strain sensors, 191Poly(methyl-methacrylate) (PMMA),

36e38, 37f, 42Polyacrylic ester (PEA) substrate, 180Polyaniline (PANI), 180e183Poly(vinyl alcohol) (PVA) carrier layer, 47Polydimethylsiloxane (PDMS), 45, 47Polymer light-emitting diodes (PLEDs),

103e108, 106fe107f, 107t,140e143, 142f

device performance, 103e108, 107tproperties, 110e113, 112t

Poly(methyl methacrylate) (PMMA), 7, 62Poly(3, 4-ethylenedioxythiophene):

poly(styrene sulfonate):graphene:ethyl cellulose (PEDOT:PSS:G:EC)hybrid electrodes, 33, 34fe35f

Polystyrene, 185Polyvinyl alcohol (PVA), 180e183

Q

Quantum dot light-emitting diodes(QLEDs), 108e110, 109fe110f

Quench factors (QFs), 153

R

Raman spectroscopy analysis, 15e16,15fe18f

Random scattering layer (RSL)effect of, 85, 86flight extraction structure, 84e85, 85f

S

Scanning tunneling microscopy (STM)topography, 7

Seebeck coefficient, 65e66Semiconductor quantum dot, 117Sheet resistance, 20e22, 21fSilicone-based pressure-sensitive adhesive

film (PSAF), 46e47Silver nanowires (AgNWs), 31e32, 32f

Index 213

Single-wall carbon nanotubes (SWCNTs),30, 62e63, 193e196, 197f

Solution casting methods, 30e31, 31fSolution-processable graphene

derivatives, 206Stretchable graphene electrodes

carbon nanotubes, 176conventional materials, 175digital electronics, 1932D materials, 201energy-storage devices, 196fabrication process, 175e176mechanisms, 199e200metal nanowires (mNWs)/nanoparticles,

176“origami” methods, 177preparationchemical vapor deposition technique,177e178

dry-transfer process, 177e178graphene-based composites, 185e191graphene kirigami, 178e180, 179fgraphene origami, 180e185

reliable fabrication methods, 200sensors, 191structural layouts, 175e176

Support-dissolving transfer methodannealing, 38e39bubble-free electrochemical delamination

method, 44chemical etching, 36e38, 42Cu foil, 42cyclododecane, 39etching-free bubbling transfer process,

42e44, 43fface-to-face technique, 38hydrogen bubbles, 44

layer-by-layer transfer, 38pentacene-supporting layer, 39, 41fpoly(methyl-methacrylate) (PMMA),

36e38, 37f, 42silicon substrate, 38structural continuity and efficiency, 44e45ultraclean and damage-free transfer, 39UV irradiation, 38e39vacuum evaporation technique, 42van der Waals interaction, 45

Surface-mediated reaction, 9e11, 10fSurface segregation, 9e11, 10f

T

Tactile sensors, 191Target-supported transfer method, 48e51, 49fTetracyanoethylene (TCNE), 63, 64fThermal release tape (TRT), 45, 47Transmission electron microscopy (TEM)

analysis, 7Transparent display electrodes, 14e15Tungsten oxideereduced graphene oxide

(WO3erGO) nanocomposite, 163

U

UV-vis transmittance spectra, 14e15, 14f

V

Vacuum evaporation technique, 42van der Waals interactions, 8e9

W

White light-emitting diodes (WLEDs),138e140, 139f, 143, 144f

Wrinkled graphene, 183e185, 184f

214 Index

contents contributors vii about the editor ix preface xi acknowledgments xiii 1. introduction 1...

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