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Copyright © 2014 American Scientific PublishersAll rights reservedPrinted in the United States of America

ReviewJournal of

Nanoscience and NanotechnologyVol. 14, 1282–1302, 2014

www.aspbs.com/jnn

Fabrication of One-Dimensional Organic Nanomaterials

and Their Optoelectronic Applications

Hojeong Yu1, Dong Yeong Kim1, Kyung Jin Lee2�∗, and Joon Hak Oh1�∗1School of Nano-Bioscience and Chemical Engineering, KIER-UNIST Advanced Center for Energy, Low Dimensional

Carbon Materials Center, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 689-798, Korea2Department of Fine Chemical Engineering and Applied Chemistry, College of Engineering,

Chungnam National University, Yuseong-gu, Daejeon, 305-764, Korea

This paper reviews the recent research and development of one-dimensional (1D) organic nano-materials synthesized from organic semiconductors or conducting polymers and their applicationsto optoelectronics. We introduce synthetic methodologies for the fabrication of 1D single-crystallineorganic nanomaterials and 1D multi-component organic nanostructures, and discuss their opticaland electrical properties. In addition, their versatile applications in optoelectronics are highlighted.The fabrication of highly crystalline organic nanomaterials combined with their integration into nano-electronic devices is recognized as one of the most promising strategies to enhance charge trans-port properties and achieve device miniaturization. In the last part of this review, we discuss thechallenges and the perspectives of organic nanomaterials for applications in the next generationsoft electronics, in terms of fabrication, processing, device integration, and investigation on thefundamental mechanisms governing the charge transport behaviors of these advanced materials.

Keywords: Organic Nanomaterials, Organic Electronics, Organic Semiconductors, One-Dimensional Nanomaterials, Single Crystals, Organic Nanowires, Optoelectronics.

CONTENTS1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1282

2. Fabrication of 1D Organic Nanomaterials . . . . . . . . . . . . . . . . . 1284

2.1. Self-Assembly in Solution Phase . . . . . . . . . . . . . . . . . . . . 1284

2.2. Self-Assembly in Vapor Phase . . . . . . . . . . . . . . . . . . . . . . 1286

2.3. Template-Assisted Synthesis . . . . . . . . . . . . . . . . . . . . . . . 1288

2.4. Electrospinning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1289

3. Optoelectronic Applications of Organic Nanomaterials . . . . . . . 1293

3.1. Optoelectronic Devices Based on Single-Component

Organic Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1293

3.2. Energy Harvesting Devices Based on Single-Component

Organic Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1296

3.3. Optoelectronics Based on

Multi-Component Nanomaterials . . . . . . . . . . . . . . . . . . . . 1297

4. Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1299

References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1300

1. INTRODUCTIONOrganic semiconducting materials have attracted great

interest because of their various advantages such as

large-scale synthesis, solution processability, and simple

∗Authors to whom correspondence should be addressed.

electronic tunability by molecular design. Furthermore,

their compatibility with flexible and lightweight substrates

enables their versatile implementation to the electronic cir-

cuits, energy and bio-related applications.1–3 In particular,

organic semiconductors based on �-conjugated molecules

can be easily transformed into one-dimensional (1D)

nanostructures that can potentially exhibit novel optical,

electrical, and chemical properties.4–6 Examples of these

assemblies include nanowires (NWs), nanorods, nanorib-

bons, nanobelts, whiskers, and nanotubes. In general, 1D

organic nanostructures can be fabricated with various

methods including solution-phase synthesis, vapor-phase

synthesis, template-assisted synthesis, electrospinning,

nanolithography, soft lithography, and so on. In particular,

1D nanostructures based on organic semiconductors have

a large surface-to-volume ratio compared to bulk mate-

rials and possess a highly crystalline nature that is ben-

eficial to efficient charge transport. To date, 1D organic

nanomaterials based on organic semiconductors and con-

ducting polymers have shown great potential in diverse

applications covering organic light-emitting diodes,7 field-

effect transistors,4�8–11 phototransistors,12�13 photovoltaic

1282 J. Nanosci. Nanotechnol. 2014, Vol. 14, No. 2 1533-4880/2014/14/1282/021 doi:10.1166/jnn.2014.9086

Yu et al. Fabrication of One-Dimensional Organic Nanomaterials and Their Optoelectronic Applications

cells,14–17 nanoscale lasers,18�19 memory devices,20 and

sensors.21 To integrate 1D organic nanostructures into

device applications, they need to be well-aligned and

assembled in desired positions.22–24

In addition to the synthesis of conventional single-

component 1D nanostructures, the concepts of multi-

component nanomaterials such as p–n junctions, core–shell

NWs, composites and multi-shape nanostructures such as

porous, branched structures, hollow structures have been

recently suggested. Progress in nanoscale heterostructures

is important for optoelectronic applications as well as for

Hojeong Yu is currently a graduate student in the School of Nano-Bioscience and Chem-

ical Engineering at Ulsan National Institute of Science Technology (UNIST), Korea. She

received her B.S. degree (2011) from the Department of Polymer Science and Engineer-

ing at Kyungpook National University, Korea. She then joined Professor Joon Hak Oh’s

group (Organic Electronics Laboratory) at UNIST. In 2011 she received a Global Ph.D.

Fellowship funded by the National Research Foundation of Korea (NRF). Her research

interests include single- and multi-component organic nanomaterials and their optoelec-

tronic applications.

Dong Yeong Kim received his B.S. degree (2013) from the School of Nano-Bioscience

and Chemical Engineering at Ulsan National Institute of Science and Technology (UNIST),

Korea. He worked as an undergraduate researcher in Professor Joon Hak Oh’s group

(2011–2012) and is currently a graduate student in the School of Nano-Bioscience and

Chemical Engineering at UNIST. His research interests include the synthesis of novel

organic nanostructures and their applications to versatile organic nanodevices.

Kyung Jin Lee is currently an assistant professor in the Department of Fine Chemical

Engineering and Applied Chemistry at Chungnam National University (Korea) and has

been in this position since 2012. In 2009, he received a Ph.D. degree from the School

of Chemical and Biological Engineering at Seoul National University. His Ph.D. the-

sis focused on the fabrication of one-dimensional nanomaterials using a hard template

approach and their applications. After receiving his Ph.D. degree, he worked with Professor

Lahann as a postdoctoral researcher in the Department of Chemical Engineering at the

University of Michigan. His research interests focus on the preparation and application of

smart organic/inorganic micro/nano building blocks.

Joon Hak Oh is currently an assistant professor at the School of Nano-Bioscience and

Chemical Engineering at Ulsan National Institute of Science Technology (UNIST), Korea.

He received his B.S. degree (1998) from the Department of Chemical Technology at Seoul

National University and his M.S. (2000) and Ph.D. degrees (2004) from the School of

Chemical and Biological Engineering at Seoul National University, Korea. After receiv-

ing his Ph.D. degree, he worked on flexible displays as a senior engineer in Samsung

Electronics (2004–2006). Then, he continued his postdoctoral research (2006–2010) in

the Department of Chemical Engineering at Stanford University (Professor Zhenan Bao’s

group), until he joined the faculty at UNIST in 2010. His research interests include organic

semiconductors, polymer nanomaterials, optoelectronic devices, and graphene.

understanding their charge transport mechanisms as they

typically exhibit different optoelectronic properties due to

the effective functional interplay between two different

materials and their unique structural effects.

This review will focus on the recent advances on the

fabrication of 1D organic nanomaterials, the functionaliza-

tion into multi-shape and multi-component nanostructures,

and their optoelectronic applications. Self-assembly of

small molecules or polymers can lead to the formation of

organic 1D nanostructures in solution or in vapor phase.

Template-assisted fabrication methods can also contribute

J. Nanosci. Nanotechnol. 14, 1282–1302, 2014 1283

Fabrication of One-Dimensional Organic Nanomaterials and Their Optoelectronic Applications Yu et al.

to the growth of 1D nanostructures in both phases.

In the following section, an electrospinning method with

the applications of electrospun fibers will be discussed.

Furthermore, this review will address advantages of opto-

electronic devices based on single-component or multi-

component nanomaterials in comparison with bulk and

thin-film based electronics. Finally, concluding remarks

about 1D organic nanomaterials and their future perspec-

tives in optoelectronic applications will be discussed.

2. FABRICATION OF 1DORGANIC NANOMATERIALS

2.1. Self-Assembly in Solution PhaseSelf-assembly of �-conjugated molecules into 1D nano-

structures is generally dependent upon the balance of

the �–� interaction of �-conjugated backbone and the

hydrophobic interaction between alkyl side chains. Typ-

ically, 1D organic nanostructures can be successfully

obtained via strong co-facial �–� stacking interactions,

while reducing the unfavorable lateral growth driven

by hydrophobic interactions between alkyl side chains.25

Strong intermolecular coupling between the �-stacked

molecules provides an efficient pathway for charge trans-

port along the long axis of NW. 1D organic nanostructures

can be synthesized in various forms such as wires, rods,

ribbons, tubes, and belts via solution-phase self-assembly.

Conjugated organic semiconducting materials with

low solubility in organic solvent can self-assemble

into 1D structures via recrystallization. A representative

n-type organic semiconductor, N ,N ′-bis(2-phenylethyl)-perylene-3,4:9,10-tetracarboxylic diimide (BPE-PTCDI) is

sparingly soluble in a common organic solvent such as

o-dichlorobenzene and benzonitrile at room temperature,

whereas it becomes soluble upon heating with reflux-

ing. BPE-PTCDI molecules tend to self-assemble into

needle-like 1D structures via strong �–� interactions of

�-conjugated backbones upon cooling the hot, homoge-

neous solution. Furthermore, the diameter and the length

of BPE-PTCDI wires can be tuned across a wide range

by controlling the cooling speed and the nucleation den-

sity tunable by the non-solvent (i.e., methyl alcohol) mix-

ing ratio. The size of BPE-PTCDI wires can be easily

tuned, ranging from several tens of nanometers to several

micrometers in diameter and several tens of micrometers

to several millimeters in length (Fig. 1(a)). The structural

characterization with transmission electron microscopy

(TEM) revealed that BPE-PTCDI molecules self-assemble

into 1D nanostructures with the �-planar distance of 3.4 Å

(Figs. 1(b) and (c)).11

The above-mentioned, non-solvent-mediated recrystal-

lization method is also applicable to the preparation of

highly conjugated p-channel organic semiconductors such

as hexathiapentacene (HTP).8 Furthermore, 1-cyano-trans-1,2-bis-(3′�5′-bis-trifluoromethyl-biphenyl)ethylene (CN-

TFMBE) molecules can self-assemble in a mixture solvent

of o-dichlorobenzene and methyl alcohol via recrystalliza-

tion spin coating or drop-casting into highly fluorescent 1D

nanomaterials, which is driven by the interactions between

cyano-stilbene backbone and terminal –CF3 units.26�27 The

CN-TFMBE NWs were synthesized with diameters around

100 nm and showed enhanced fluorescence emission in

the solid state, as shown in Figures 1(d) and (e). The

strong intermolecular �–� stacking was induced by the

planarization of the CN-TFMBE owing to the CN group

and CF3 end groups (Fig. 1(f)).

In the case of organic semiconducting molecules, which

are highly soluble in organic solvents, 1D nanostruc-

tured self-assemblies can be prepared by transferring

the molecules from a good solvent to a non-solvent,

followed by rapid dispersion.28 This method refers to

rapid solution dispersion method. The propoxylethyl-

perylene-3,4,9,10-tetracarboxylic-3,4,9,10-diimide (propo-

xylethyl-PTCDI) molecules have sufficient solubility in

hydrophobic solvents such as chloroform, due to the

flexible propoxyethyl side chain. The concentrated solu-

tion can readily self-assemble to form nanobelt crys-

tals upon injection of a small amount of solution into

methanol.29 Similarly, injection of a small volume of

concentrated 6,13-bis(triisopropylsilylethynyl) pentacene

(TIPS-PEN)/toluene solution into acetonitrile can lead to

the formation of TIPS-PEN microribbon through the self-

assembly on the basis of rapid solution dispersion method

(Figs. 1(g)–(i)).9

When conjugated molecules have different solubility in

two organic solvents, self-assembly can be induced at the

interface between two different solvents through binary

phase transfer. The solvent interface is typically formed

by the difference in the polarity and the density of two

solvents. PTCDI molecules with alkyl side chains (PTCDI-

Cn� can be transformed into 1D nanostructures via the

self-assembly at the methyl alcohol/chloroform interface.8

Moreover, solvent vapor diffusion can be used to

form 1D organic single crystals. Lv et al. reported the

organic single crystal of the tetrachlorinated diperylene

bisimide (C12-4CldiPBI) grown by solvent vapor diffusion

method.30 The C12-4CldiPBI solution dissolved in toluene

was prepared in a small vial and exposed to methyl alcohol

vapor. The nanoribbons with a diameter of around 2 �mwere obtained after 3 days.

Moreover, Goto et al. reported a novel method to fab-

ricate organic single-crystal arrays with a large-scale from

solution-phase synthesis.31 They prepared aligned organic

single-crystal arrays by controlling the supersaturation

region of 3,9-bis(4-ethylphenyl)-peri-xanthenoxanthene

(C2Ph-PXX) solution. In the specific micropatterns, the

supersaturation of organic solution could be changed

locally, leading to the formation of single crystals (Fig. 2).

This simple method allowed the formation of the aligned

organic single crystal arrays. The careful control of the

micropatterns may provide the formation of 1D single

crystals.

1284 J. Nanosci. Nanotechnol. 14, 1282–1302, 2014


Figure 1. (a) Bright field and dark field (inset) OM images of BPE-PTCDI NWs, (b) TEM image and SAED pattern (inset), and (c) schematic

diagram of molecular packing analysis of single BPE-PTCDI NW. (a)–(c) Reprinted with permission from [12], H. Yu, et al., Adv. Funct. Mater. 23, 629(2013). © 2013, Wiley-VCH. (d) Scanning electron microscopy image of CN-TFMBE NWs, (e) fluorescence microscopy image and fluorescence image

(inset) of molecular state (denoted as I) and nanostructured CN-TFMBE (denoted as A), (f) schematic diagram of molecular packing of CN-TFMBE

NW. (d)–(f) Reprinted with permission from [26], B. K. An, et al., J. Am. Chem. Soc. 131, 3950 (2009). © 2009, American Chemical Society. (g) SEM

image of a micro-ribbon of TIPS-PEN, (h) its selected area diffraction (SAED) pattern, and (i) the molecular packing analysis. (g)–(i) Reprinted with

permission from [9], D. H. Kim, et al., Adv. Mater. 19, 678 (2007). © 2007, Wiley-VCH.

A wide range of approaches have also been utilized

to prepare 1D self-assembled crystalline polymer nano-

structures. Poly-(3-alkylthiophene) (P3AT) NWs could be

obtained by the thermally induced crystallization of P3AT

in a poor solvent.32 The crystallization of �-conjugated

polymers, which contain electron-rich backbones and

aliphatic chains, is usually prohibited by the entangled

chain conformations.5 Most of the related reports have

focused on only the fabrication of nanostructured P3ATs

because of their strong self-organizing characteristics.

Highly crystalline 1D polymer NWs with low solubil-

ity in organic solvent can be synthesized in a dilute solu-

tion (0.05∼1%). By heating the mixture of highly diluted

P3AT solution with poor solvent, followed by a cooling

process, conjugated molecules can be stacked via �–�interaction between aromatic backbones without unfavor-

able stacking from aliphatic chains. P3AT NWs can be

obtained in various conditions; controlling the length of

alkyl chain, molecular weight, concentration, crystalliza-

tion temperature, solvent, and regioregularity of polymer

materials in an initial good solvent to a marginal or poor

solvent.6

Kiriy et al. reported the formation of NWs by the aggre-

gation of regioregular head-to-tail P3ATs by using a selec-

tive solvent, hexane, which is a good solvent for alkyl

chains, but a poor solvent for polythiophene backbones.33

The addition of hexane into the chloroform solution of

P3AT induced an ordered aggregation of the P3AT main-

chain driven by a solvophobic interaction. Kim et al.

reported a self-seeding method of poly(3-hexylthiophene)

(P3HT) with a dilute solution in chloroform to fabricate

1D microwires (MWs).34 Slow cooling the solution from

40 �C to 10 �C over three days created crystalline seeds

in solution to facilitate the growth of P3HT MWs in a

solution phase. Figure 3(a) shows the polarized OM image

and SEM images of 1D single-crystalline P3HT MWs. The

width and length of the wires were approximately 1–3 �mand 30–500 �m, respectively. The molecular and crystallo-

graphic structures of P3HT chains in a single-crystal MW

are depicted in Figure 3(b).

Aside from P3AT single crystalline NWs, several

different semiconducting polymers can be utilized to

fabricate 1D nanostructures in solution phase. Dong

et al. fabricated highly crystalline NWs using poly



Figure 2. (a) Schematic diagram of the solubility-supersolubility relationship. Points ABC represent three states of solution at the same temperature

with different solution concentrations; point A is a stable state, where crystallization does not occur. As the concentration increases over solubility

curve, crystallization occurs at points B and C. The spontaneous crystallization can occur at point C because nucleation occurs only in a supersaturated

solution. (b) Schematic diagram of crystal growth in the nucleation control region. (c) Two regions of the micropattern; nucleation control region and

growth control region (up) and the schematic growth model (bottom). (d) Polarized OM image of well-aligned 12× 32 arrays of C2Ph-PXX films.

Reprinted with permission from [31], O. Goto, et al., Adv. Mater. 24, 1117 (2012). © 2012, Wiley-VCH.

(para-phenylene ethynylene) derivatives with thioacetate

end groups (TA-PPE) via slow self-assembly from a

dilute solution (0.05–1.0 mg ml−1� under a volatile sol-

vent pressure in a closed jar.35 Slow evaporation of

the solvent could induce self-assembly of the polymer

molecules with strong intermolecular interactions. Large-

area and well-defined TA-PPE NWs were obtained through

the self-assembly of TA-PPE in tetrahydrofuran (THF)

Figure 3. (a) Polarized OM image of 1D single-crystalline P3HT MWs and SEM images of single P3HT MW with a side-view image showing the

rectangular cross-section with well-defined facets. (b) Molecular and crystallographic structures of P3HT chains with schematic representation of P3HT

chains into single-crystalline MWs along the �–� stacking direction. (a) and (b) Reprinted with permission from [34], D. H. Kim, et al., Adv. Mater.18, 719 (2006). © 2006, Wiley-VCH. (c) SEM images of the growth processes of the self-assembled TA-PPE NWs. (d) Schematic diagram of possible

molecular packing in the 1D TA-PPE NW and the SAED pattern of an individual TA-PPE NW. (c) and (d) Reprinted with permission from [35],

H. Dong, et al., J. Am. Chem. Soc. 131, 17315 (2009). © 2009, American Chemical Society.

solvent. The corresponding SEM images of the self-

assembled TA-PPE NWs and the schematic diagram of

molecular packing in the 1D TA-PPE NW are displayed

in Figures 3(c) and (d).

2.2. Self-Assembly in Vapor PhasePhysical vapor transport (PVT) method has been fre-

quently adopted to grow highly crystalline nanostructures



of organic semiconductors that are insoluble in common

organic solvents. The main advantage of this method is

that the direct growth on any substrates can be realized

with less contamination. In a typical PVT process, quartz

tubes are loaded in a furnace with a temperature gradi-

ent. Then, organic semiconductors are sublimated at the

high-temperature region and the vaporized molecules are

carried by the flow of inert gas to the lower temperature

region. Then, the nucleation of the crystal growth takes

place at the lower temperature region, eventually leading

to the formation of highly purified crystals.

High-purity single crystals can be obtained from the

PVT method without unintentional doping, which is gen-

erally caused by solvent molecules or other impurities.

Moreover, this method can be employed to induce the

direct growth of highly crystalline organic semiconduc-

tor nanomaterials onto a desired substrate. Similarly, the

deposition of thin films or crystals by the condensation of

vaporized target materials onto surfaces is defined as the

physical vapor deposition (PVD) method. Because single

crystals fabricated in a vapor phase are free of defect or

Figure 4. (a) Scheme of vapor phase self-assembly for synthesis of DAAQ nanowires (NWs), (b) SEM image of vertically aligned DAAQ NWs,

(c) TEM image of a single NW with the selected area diffraction pattern (inset), and (d) DAAQ NWs grown on a tungsten tip. (a)–(d) Reprinted with

permission from [39], Y. S. Zhao, et al., ACS Nano 4, 1630 (2010). © 2010, American Chemical Society. (e) Schematic of p–n heterojunction crystal

growth in the one-step PVT process based on CuPc-H2TPyP. Reprinted with permission from [40], Q. H. Cui, et al., Adv. Mater. 24, 2332 (2012).© 2012, Wiley-VCH. (f) TEM image with the corresponding selected area diffraction (SAED) patterns, and optical images of single nanoribbon based

on F16CuPc-CuPc. Reprinted with permission from [41], Y. Zhang, et al., J. Am. Chem. Soc. 132, 11580 (2010). © 2010, American Chemical Society.

impurity, this method provides great potential to fabricate

nanostructures for investigation on their intrinsic electrical

and optical properties of organic nanomaterials.36–38

Recently, NWs or nanostructures based on small

molecule organic semiconductors have been frequently

fabricated in the vapor phase. Vertically aligned NWs

were successfully fabricated in the vapor phase from 1,5-

diaminoanthraquinone (DAAQ). The NWs could be grown

on the sharp tip with high surface energy as shown in

Figures 4(a)–(d).39

Furthermore, fabrication of multi-component nano-

structures is available via the PVT method. Yao and co-

workers fabricated vertical arrays of p–n junction NWs

based on a p-channel semiconductor, copper phthalocya-

nine (CuPc), with an n-channel semiconductor, 5,10,15,20-

tetra(4-pyridyl)-porphyrin (H2TPyP), via the one-step PVT

process (Fig. 4(e)).40 The H2TPyP nanostructures could

provide nucleation sites for the growth of CuPc crys-

tals before the formation of heterojunction structures.

The strong �–� stacking interactions between the two

�-conjugated systems ensured good interfacial connection



at the junction part. Briseno and co-workers reported the

fabrication of F16CuPc-CuPc nanoribbons (Fig. 4(f)) and

found that the selective growth behaviors of vaporized

molecules into crystals are strongly dependent upon the

similarity in molecular structures and lattice parameters at

the interfacial basal planes.41

Furthermore, dendritic organic heterojunctions with alu-

minum tris(8-hydroxyquinoline) (Alq3� MW trunks and

DAAQ NW branches were prepared with the two-step

PVD growth process.42 In this system, the Alq3 MWs were

used as seeds for the vertical growth of DAAQ NWs. Simi-

larly, Yan et al. reported the site-selective growth behaviors

of 4,4′-bis(phenylethynyl)anthracene (BPEA) NWs using

the PVD method.43 The condensation of vaporized BPEA

molecules took place on the sharp tips of silver NWs.

Park et al. demonstrated a selective preparation

method of pentacene crystals into 1D wires and two-

dimensional (2D) disks via a vaporization-condensation-

recrystallization (VCR) process, which can induce the

growth of organic crystals in various geometries by con-

trolling the precursor temperature (Fig. 5(a)).44 In the VCR

process, 1D wires were obtained at a higher temperature

(Fig. 5(b)), while 2D disk crystals were obtained at a lower

temperature (Fig. 5(c)). The crystal-plane-dependent pho-

toluminescence (PL) properties of pentacene 1D wires and

2D disks revealed a correlation between the arrangement

of pentacene molecules and optoelectronic performance

(Figs. 5(f)–(n)). Pentacene 1D wires possessing (010) and

(001) planes exhibited strong PL to the (010) plane when

the light beam was normal to the (010) plane, while PL

Figure 5. (a) Schematic diagram of crystal growth of pentacene 1D wires and 2D disk crystals via the VCR process, (b) 1D pentacene wires fabricated

at a relatively higher temperature region (T = 350 �C) region. The SAED patterns of (d) 1D wire and (e) 2D disk pentacene crystals. (f) TEM, (g) PL,

and (h) SAED pattern images of 1D pentacene wire which possesses (010) plane parallel to substrate. (i) TEM, (j) PL, and (k) SAED pattern images

of 1D pentacene wire which have (001) plane parallel to substrate. (l) TEM, (m) PL, and (n) SAED pattern images of 2D disk pentacene with (001)

plane parallel to substrate. All of the insets are the corresponding TEM images. Reprinted with permission from [44], J. E. Park, et al., Angew. Chem.,Int. Ed. 51, 6383 (2012). © 2012, Wiley-VCH.

was not detected with regard to the (001) plane. Similarly,

pentacene 2D disk crystals having only (001) crystal plane

did not show the PL property.

2.3. Template-Assisted SynthesisTemplate-assisted synthesis has been adopted to prepare 1D

organic nanostructures composed of the oligomer/polymer

materials with less conjugated molecules in the backbone.

The two kinds of templates are broadly defined as hard tem-

plates and soft templates. Particle track-etched membranes

such as polycarbonate membrane are a common hard tem-

plate for the synthesis of 1D nanostructures. The diameters

of nanostructures can be easily modified by controlling the

pore size of the template adopted. Anodic aluminum oxide

(AAO) membranes and mesoporous silica also exemplify

hard templates.45–47

A broad range of techniques have been attempted to

fill the pores of templates. Among them, vapor deposition

polymerization (VDP) based on hard templates is one of

the simplest ways to fill the pores of AAO membrane with

polymeric materials. The diameter and length can be easily

tuned by varying the type of template. It is also possible

to prepare NW arrays with tailored architecture via con-

trol of the pore density. Furthermore, the wall thickness

of nanotube can be easily controlled in nanometer scale

by injecting different amounts of monomers (Fig. 6).48

By using two different monomers, multi-layered nanotubes

can also be fabricated. Capillary force between monomer

and template enables to realize fabrication of nanotubes

with desired thicknesses and 1D nanostructures.49



Figure 6. TEM image of polypyrrole (PPy) nanotubes via the VDP

method. Thickness of the PPy nanotubes can be controlled by chang-

ing the injection amount of monomer as (a) 0.07 ml, (b) 0.14 ml,

(c) 0.21 ml. (a)–(c) Reprinted with permission from [48], J. Jang and

J. H. Oh, Chem. Commun. 882 (2004). © 2004, Royal Chemical Soci-

ety. (d) Multi-cell nanotubes based on PAN/rhodamine B/PAN coaxial

nanotubes structure by using the VDP method. (e) TEM images and

(f) confocal microscope image of the fabricated PAN/rhodamine B/PAN

nanotube. (d)–(f) Reprinted with permission from [49], K. J. Lee, et al.,

Chem. Mater. 18, 5002 (2006). © 2006, American Chemical Society.

Lee et al. fabricated organic NWs by stacking

small molecules such as pyrene and 1,4-bis[2-(5-

phenyloxazolyl)]benzene in the pores of the AAO mem-

brane. They introduced organic molecules into the pores

of the AAO membrane via sublimation. Strong �–� inter-

action between organic dyes makes it possible to maintain

their 1D structure even after removing the AAO template.50

Figure 7. (a) Schematic illustration of fixed-angle centrifugation-assisted growth method. In centrifugation process, an empty AAO template was

loaded with organic solutions. SEM images of (b) AAO template (c) rubrene NWs (d) Alq3 NWs, and (e) PCBM NWs. Reprinted with permission

from [56], K. M. Alam, et al., Adv. Funct. Mater. 22, 3298 (2012). © 2012, Wiley-VCH.

As soft templates, assemblies of block copolymers are

widely used to synthesize various nanomaterials. The mor-

phology of block copolymer assemblies can be modified by

controlling the composition of polymers and their molec-

ular weight.51 Moreover, surfactants,52 liquid crystals,53�54

and polymer nanofibers55 have been used as soft templates.

Moreover, Alam et al. demonstrated a novel and ver-

satile method to fabricate NW arrays based on 5,6,11,12-

tetraphenylnaphthacene (Rubrene), Alq3, and [6,6]-Phenyl

C61 butyric acid methyl ester (PCBM).56 Figure 7(a) shows

the illustration of the fixed-angle centrifugation-assisted

growth method. The AAO template was placed in the bot-

tom of organic solution and organic NWs could be obtained

by the centrifugation of the solution together with AAO

template, as shown in the SEM images in Figures 7(b)–(e).

2.4. ElectrospinningElectrospinning is a straightforward and cost-effective

method to fabricate nanofibers from diverse polymer

solutions or melts under high electric field. Although the

first patent mentioning electrospinning had been reported

as early as the 1900s,57–59 over the last two decades a broad

range of research has been reported to obtain 1D nanoma-

terials such as nanofibers, nanotubes, and nanobelts using

electrospinning due to the recent increasing attention on

nanomaterials.

The basic instrumental setup for electrospinning is as

follows:

(1) prepare a certain polymeric solution,

(2) deliver the polymeric solution with a desired feed rate

through the needle,

(3) apply the high voltage onto the solution and

(4) collect the products in the form of nanofibers.

Numerous experimental parameters should be considered

for precise control of nanofiber quality, such as solution

parameters (including viscosity, surface tension, types of

polymers, concentration, and conductivity) and external



parameters (applied voltage, feed rate, type of collector,

distance between needle to collector and environmental

conditions).

When high voltages are applied onto the solution, the

surface charge will be accumulated on the solution droplet.

Because there is certain limitation on the maximum sur-

face charge per surface area (such as Rayleigh limitation),

the droplet explosion will occur in order to increase their

surface area as increasing surface charges. When the poly-

mers dissolved in the solution contain high enough chain

entanglements, the droplet will be elongated rather than

exploded to increase their surface area, resulting in fiber

thinning. These fiber thinning is the main factor to obtain

nanosized fibers using electrospinning. After jet-launching,

the spun-fiber will be discharged during the flying onto the

collector, implying relaxation of the elongation force of

the thin fiber. If the jet fibers were not fully dried before

losing the surface charges, surface tension of the solution

will again become the main factor to decide the nanofiber

quality, and bead-on-a-string morphology can be obtained.

Therefore, one can say that electrospinning is the result

of the competition between the elongation force and the

surface tension of the polymeric solutions.60

As mentioned previously, enough entanglement of the

polymeric chain is the decisive requirement for elec-

trospinning. Therefore, there are intrinsic limitations to

prepare 1D nanomaterials composed of low-molecular

weight organo-electronic materials. Polymeric materials

such as conducting polymers should be considered for 1D

nanomaterials from electrospinning. However, conducting

polymers have low solubility to get enough chain entangle-

ments in general. In addition, a high degree of crystalliza-

tion of conducting polymers or organo-electronic materials

Figure 8. Fluorescence microscopy images of semiconducting polymer nanofibers fabricated by electrospinning with molecular structures of

the polymers. NWs based on (a) poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(9-hexyl-3,6-carbazole)], (b) poly[(9,9-dioctylfluorenylene-2,7-diyl)-co-

(1,4-diphenylenevinylene-2-methoxy-5-f2-ethylhexyloxyg-benzene)], (c) poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-f2,10,3g-thiadiazole)], and

(d) poly[f2-methoxy-5-(2-ethylhexyloxyg-1,4-(1-cyanovinylenephenylene))-co-f2,5-bis(N ,N0-diphenylamino)-1,4-phenyleneg]. Reprinted with permis-

sion from [62], F. Di Benedetto, et al., Nat. Nanotechnol. 3, 614 (2008). © 2008, Nature Publishing Group.

is required for better performance in electronic devices,

but the rapid crystallization can result in needle block-

ing during the electrospinning. Therefore, in general, it is

difficult to obtain 1D organic nanomaterials for electronic

applications using electrospinning.

Several additional modifications on conventional elec-

trospinning should be followed to obtain 1D nanoma-

terials for electronics and optoelectronics. Chang et al.

reported the preparation of PVDF (a piezo-electric poly-

mer) nanofibers using near-field electrospinning.61 In con-

ventional electrospinning, the product is typically in the

form of a nanofiber web because of whipping insta-

bility. In near-field electrospinning, the spun fiber is

rapidly collected prior to the whipping instability, so

that aligned nanofibers can be obtained. They said

that the aligned spun-fiber contained a high degree

of �-crystals which is important for piezo-properties

because of the electrical stretching during the electro-

spinning, resulting in high performances in polymer-based

nanogenerators.

Figure 8 illustrates typical examples of preparing

semiconducting nanofibers using electrospinning. The

fluorescence micrographs of emitting electrospun fibers

are presented with the molecular structures for the

semiconducting polymers: poly[(9,9-dioctylfluorenyl-2,7-

diyl)-alt-co-(9-hexyl-3,6-carbazole)] (Fig. 8(a)), poly[(9,

9-dioctylfluorenylene-2,7-diyl)-co-(1,4-diphenylenevinylene-

2-methoxy-5-f2-ethylhexyloxyg-benzene)] (Fig. 8(b)), poly

[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-f2,10,3g-thia-

diazole)] (Fig. 8(c)), and poly[f2-methoxy-5-(2-ethyl-

hexyloxyg-1,4-(1-cyanovinylenephenylene))-co-f2,5-bis

(N ,N0-diphenylamino)-1,4-phenyleneg] (Fig. 8(d)),

respectively.62



Figure 9. (a) SEM image of porous Poly-L-lactide (PLLA) nanofibers obtained from selective etching of blended-nanofibers fabricated by electro-

spinning of PLLA and poly(vinyl pyrrolidone) (PVP) mixture solution. Reprinted with permission from [63], M. Bognitzki, et al., Polym. Eng. Sci.41, 982 (2001). © 2001, Society of Plastics Engineers. (b) Porous PLLA nanofiber obtained by electrospinning of PLLA dissolved in dichloromethane.

Reprinted with permission from [64], M. Bognitzki, et al., Adv. Mater. 13, 70 (2001). © 2001, Wiley-VCH. (c) TEM image of TiO2/PVP hollow

nanofibers fabricated by co-spinning of PVP/titanium tetraisopropoxide solution based on mineral oil and ethyl alcohol. Reprinted with permission

from [65], D. Li and Y. N. Xia, Nano Lett. 4, 933 (2004). © 2004, American Chemical Society. (d) TEM image of poly(ethylene oxide) (PEO)–

Poly(dodecylthiophene) (PDT) core–shell nanofiber. Reprinted with permission from [66], Z. C. Sun, et al., Adv. Mater. 15, 1929 (2003). © 2003,

Wiley-VCH. (e) SEM image of aligned PVP nanofibers. Reprinted with permission from [67], D. Li, et al., Adv. Mater. 16, 361 (2004). © 2004,

Wiley-VCH. (f) SEM image of uniaxially aligned PVP nanofiber arrays. Reprinted with permission from [24], D. Li and Y. Xia, Adv. Mater. 16, 1151(2004). © 2004, Wiley-VCH.

The diameter of the electrospun fiber can be tuned by

controlling viscosity, surface tension, conductivity of the

polymer solution, electric field, collection distance, and

so on. By varying the composition of materials, multi-

component or multi-shape nanofibers can be easily fabri-

cated via electrospinning. Nanofibers of binary blends of

two polymers can be produced by electrospinning with the

mixed solution of desired materials. For fabricating multi-

shape nanofibers such as porous nanofibers, both methods

of inducing the phase separation phenomena or collect-

ing electrospun nanofibers, followed by selective removal

of one component, can be utilized. After electrospinning

of the mixture of Poly-L-lactide (PLLA) and poly(vinyl

pyrrolidone) (PVP) solution, either PLLA or PVP can be

selectively removed, and then porous nanofibers can be

obtained (Fig. 9(a)).63 PLLA in dichloromethane solution

can be used to fabricate porous PLLA nanofibers,24�64 due

to the use of a volatile solvent such as dichloromethane,

as shown in Figure 9(b). Furthermore, porous fibers can

be used as templates for the formation of various tubes

or nanostructures by removing the fiber templates under

annealing at elevated temperatures. Moreover, Figure 9(c)

represents hollow nanofibers produced by co-spinning two

immiscible liquids with a coaxial, two-capillary spinneret,

followed by selective removal of the core components.

Electro co-spinning of PVP in mineral oil with ethanol

and titanium tetraisopropoxide liquids was prepared to

fabricate hollow nanofibers from composite containing

amorphous TiO2 and PVP. Octane treatments on the elec-

tro co-spun fibers can induce extraction of oily cores and

fabricate hollow nanofibers.65

Core–shell electrospinning is an excellent pathway

to incorporate organo-electronic materials into electro-

spinning. It is generally known that overall spinning

behavior is totally governed by that of shell, imply-

ing that if one introduces spinnable polymer solution

into the shell, diverse materials including low-molecular

weighted materials can be incorporated into the core.

Figure 9(d) shows the TEM image of poly(ethylene

oxide) (PEO)–Poly(dodecylthiophene) (PDT) core–shell

nanofiber.66 Controllability of the pore size of electro-

spun nanofibers and direct deposition property on substrate

enable versatile applications in fabrication of multi-

component and multi-shape nanofibers. Furthermore, the

patterning of electrospun fibers can be realized with a high

surface-to-volume ratio in afilmstate (Figs. 9(e) and (f)).24�67

Jeong et al. recently described an interesting method

to prepare P3HT nanofibers using core–shell electro-

spinning.68 They introduced additional solvent into the

shell to prevent needle blocking, locating P3HT/chloroform

solution into the core. High-performance organic field-

effect transistors (OFETs) were fabricated with polyelec-

trolyte gate dielectric and electrospun P3HT nanofibers by

utilizing this approach.69 The operational voltage, field-

effect mobility and on/off ratio of the prepared OFET were

2 V, ∼ 2 cm2 V−1 s−1, and 105, respectively.



Beyond core–shell electrospinning, a polymeric blend

or composite of conducting polymer has been adopted

to produce 1D conducting polymer nanomaterials using

electrospinning. Jeong and co-workers have reported on

the fabrication of nanofibers with poly(�-caprolactone)(PCL)/P3HT blend.68 After selective dissolution of the

PCL contents on blended nanofibers, an aligned surface

morphology of P3HT could be obtained, because of the

shear stress of P3HT during electrospinning. They sug-

gested that fine nanofibers composed of P3HT can be pre-

pared by adequate control of the shear stress. Srivastava

et al. has reported on the preparation of nanofibers of

PVP and their composites with PPy.70 They employed a

multi-spinnerette electrospinning device using microchan-

nels cast in polydimethylsiloxane (PDMS) for scale-up.

The pyrrole monomer and initiator for polymerization

were introduced onto PVP solution. The insoluble PPy

compartments were well-dispersed into a PVP solution,

resulting in PPy/PVP composite nanofibers.

Yoon et al. has reported on the electrospinning of

nanofibers with polymer blend containing diacetylene

molecules.71–73 The diacetylene incorporated into poly-

mer nanofibers could be polymerized by UV irradiation

in the matrix. By utilizing the end functional groups

of diacetylene, diverse types of poly(diacetylene) elec-

trospun nanofibers could be prepared. Because these

Figure 10. (a) and (b) SEM images of the rubrene nanofibers with diameters about 600 nm. (c) Photograph of rubrene nanofibers after 10-second

electrospinning. (d) Confocal Raman spectra of rubrene nanofibers indicating both amorphous and crystallized phase of rubrene nanofibers in the

electrospun nanofibers. Reprinted with permission from [74], K. P. Dhakal, et al., J. Appl. Phys. 111, 123504 (2012). © 2012, AIP Publishing.

poly(diacetylene) showed different PL properties under

certain environments, the nanofiber-web prepared could be

used as colorimetric sensors for detecting harmful organic

solvent vapors such as chloroform and toluene.

Aside from polymer materials, small molecules such as

rubrene can also be used as the raw materials of electro-

spun fibers. Dhakal et al. fabricated organic rubrene nano-

fibers via electrospinning.74 The rubrene was mixed with

a minimal amount of PEO to obtain the optimum viscos-

ity required for electrospinning. The electrospun rubrene

nanofibers showed uniform features with a diameter of

around 600 nm as shown in Figures 10(a) and (b). After a

10-second collection, large-scale rubrene nanofibers were

obtained and their confocal Raman spectra revealed that

nanofibers have both amorphous and the crystal phase of

the rubrene molecule within the electrospun nanofibers

(Figs. 10(c) and (d)).

Besides the above-mentioned methods, nano-

lithography75 and soft lithography76–78 are also powerful

techniques for the preparation of tailored-nanostructures.

In addition, the single particle nanofabrication technique

(SPNT) method suggests a great platform to fabricate

NW and nanostructures. Fullerene NWs and p–n junction

NWs based on polyfluorene (PFO)/PC61BM heterojunc-

tion prepared by the SPNT method are illustrated in

Figure 11.79



Figure 11. (a) Fullerene (C60� and fullerene derivatives-based NWs and (b) PFO/PC61BM p–n heterojunction NWs fabricated by single particle

nanofabrication technique (SPNT) method. Reprinted with permission from [79], Y. Maeyoshi, et al., Sci. Rep. 2, 600 (2012). © 2012, Nature Publishing

Group.

3. OPTOELECTRONIC APPLICATIONS OFORGANIC NANOMATERIALS

3.1. Optoelectronic Devices Based onSingle-Component Organic Nanomaterials

Highly crystalline or single-crystalline organic semicon-

ductor NWs typically show higher charge carrier mobil-

ity than that of thin film counterparts because of their

high structural perfection. Their grain-boundary-free fea-

tures are beneficial to efficient charge transport, leading to

high-performance electronic devices.12�80

BPE-PTCDI MWs prepared by non-solvent nucleation

method have shown high charge carrier mobility that

reached up to 1.4 cm2 V−1 s−1 with good air-stability.11

In order to realize NW/MW array with controllable align-

ment and density, the filtration-and-transfer (FAT) align-

ment method has been developed. The organic NWs or

MWs can be aligned by fluid flow through a PDMS

mask mold in a simple vacuum filtration setup. The fil-

trated, aligned NW/MW patterns could be transferred onto

the desired device substrate via hydrophobic interactions.

OFETs based on BPE-PTCDI NW/MW arrays exhibited

superior performance compared to thin film OFETs.

Organic phototransistors (OPTs) based on BPE-PTCDI

NWs were prepared by Oh and co-workers12 to ana-

lyze their optoelectronic properties (Figs. 12(a)–(c)). OPTs

based on a single BPE-PTCDI NW exhibited high pho-

toresponsivity, on/off switching ratio, and substantially

high external quantum efficiency (EQE) compared to

thin-film-based BPE-PTCDI phototransistors. A significant

mobility enhancement was observed when the incident

optical power density increased and the wavelength of

the light source matched the light absorption range of

the photoactive material. The photoswitching ratio was

strongly dependent upon the incident optical power den-

sity, whereas the photoresponsivity was more dependent on

matching the light-source wavelength with the maximum

absorption range of the photoactive material. BPE-PTCDI

NW-OPTs exhibited much higher external quantum effi-

ciency (EQE) values (≈ 7900 times larger) than thin-film

OPTs. This is attributed to the intrinsically defect-free

single-crystalline nature of the BPE-PTCDI NWs. In addi-

tion, compared to the thin-film OPTs, the single-crystalline

BPE-PTCDI NW-OPTs exhibited two orders of magnitude

higher charge accumulation/release rates from deep traps

under on/off switching of external light sources.

Gemayel et al. prepared OPTs based on single or

multi perylenebis(dicarboximide)s (PDIs) NWs.81 They

also observed enhanced phototransistor performance com-

pared to thin-film OPTs (Figs. 12(d) and (e)). The photore-

sponse in OPTs could be tuned by controlling the device

geometry and the morphology of the active semiconduct-

ing layer from thin films to NW structures.

Chou et al. utilized self-assembled BPE-PTCDI NWs

for nonvolatile transistor memory devices (Figs. 12(f)

and (g)).20 The NWs with small diameters resulted in a

large memory window. Their stable current response to

the write-read-erase-read (WRER) cycles revealed high

possibility of organic n-type semiconducting NWs for

applications in nonvolatile OFET memory devices. In addi-

tion, self-assembled NWs based on alkyl-chain-substituted

PTCDI derivatives were used as n-channel semiconductors

in OFETs with an electron mobility of 0.01 cm2 V−1 s−1,



Figure 12. Schematic diagram of BPE-PTCDI single NW phototransistor and (b) their transistor performance under various incident lights (c) EQE as

a function of applied gate voltage. (a)–(c) Reprinted with permission from [12], H. Yu, et al., Adv. Funct. Mater. 23, 629 (2013). © 2013, Wiley-VCH.

(d) Schematic drawing of PDI NW phototransistors and (e) their electrical properties upon illumination of different incident light. (d) and (e) Reprinted

with permission from [81], M. El Gemayel, et al., J. Am. Chem. Soc. 134, 2429 (2012). © 2012, American Chemical Society. (f) Schematic diagram

of BPE-PTCDI NW memory device and (g) the memory response according to the WRER cycles. (f) and (g) Reprinted with permission from [20],

Y. H. Chou, et al., Adv. Funct. Mater. 22, 4352 (2012). © 2012, Wiley-VCH.

and utilized as high-performance complementary inverters

combined with p-channel HTP NWs.8

The charge transport characteristics of polymer NWs

have been intensively investigated.82 Self-assembled poly-

mer NWs based on P3ATs have attracted great interest due

to their high charge carrier mobilities and self-organizing

property. Single component system or blended system

of semiconducting/insulating polymers has been actively

studied.83–88 Lu et al. fabricated poly(3-butylthiophene)

(P3BT)/polystyrene (PS) composite films and found that

the increased mobility of free charge carriers, in partic-

ular hole mobility, contributes to the enhanced electrical

conductivity of this semiconductor/insulator composite.86

Qiu et al. also reported polymer NW/insulator composite

OFETs based on P3HT and PS.87 By using marginal

solvent such as CH2Cl2, a network of highly crystalline

P3HT nanofibers embedded in an insulating PS matrix

was prepared. The P3HT/PS NW OFETs showing rea-

sonably high charge carrier mobilities are promising upon

considering the low-cost, environmentally stable, and flex-

ible device characteristics (Figs. 13(a)–(c)). By optimizing

the composite fabrication conditions, the devices based on

P3HT/PS blend films achieved mobility up to 0.01 cm2

V−1 s−1, which is comparable to the pristine P3HT

film.89

Similarly, Kim et al. prepared self-assembled crystalline

P3BT NWs and their nanocomposites with insulating PS,

and utilized them for high-performance OFETs.90 The

intra-NW carrier mobility reached up to 0.2 cm2 V−1 s−1

(Figs. 13(d) and (e)). The self-organized P3BT NWs

provided efficient charge transport pathways with enhanced

performance compared to the pure P3BT NW films.



Figure 13. Atomic force microscopy (AFM) images for a P3HT/PS (5:95) blend spin-coated from a CH2Cl2 solution; top surface (left), bottom

surface (middle), and interface after selectively dissolving PS (right). (b) Schematic representation of the fabrication process of P3HT/PS nanofibrillar

network in the PS matrix. (c) Transfer and output characteristics of a flexible FET based on a P3HT/PS (10:90) blend spin-cast from CH2Cl2 solution.

(a)–(c) Reprinted with permission from [87], L. Qiu, et al., Adv. Mater. 21, 1349 (2009). © 2009, Wiley-VCH. (d) Schematic diagram of molecular

packing of P3BT NWs and P3BT-NW/PS nanocomposite OFETs. (e) Output and transfer characteristics of P3BT NW FET (left) and P3BT NW/PS

nanocomposite (20 wt% P3BT NWs) thin-film FET (right). (d)–(e) Reprinted with permission from [90], F. S. Kim and S. A. Jenekhe, Macromolecules45, 7514 (2012). © 2012, American Chemical Society.

In addition, Mitsui et al. prepared solution-processed

single-crystal FETs using novel p-type semiconductors,

naphtho[2,1-b:6,5-b′]difuran derivatives (C8-DPNDF),91

which exhibited a hole mobility of up to 3.6 cm2 V−1 s−1

when using a solution sustaining piece to control the crys-

tal growth (Fig. 14). They found that these features orig-

inate from the dense crystal packing and the resulting

large intermolecular �-orbital overlap as well as from the

small reorganization energy, all of which originate from

the small radius of an oxygen atom.

For the large scale and facile synthesis of highly crys-

talline small molecule nanostructures, dip-coating with

a proper controlling of solvent drying speed can be

adopted.92–94 Jang et al. reported a facile one-step growth

of self-aligned acene crystal arrays.95 In the dip-coating

process, solvents with a low boiling point, such as

dichloromethane or chloroform, can be used to pro-

duce continuous and uniform crystals over substrates.

Figure 15(a) shows a schematic illustration for crys-

tal growth of TIPS-PEN or fluorinated 5,11-bis(triethyl-

silylethynyl) anthradithiophene (FTES-ADT) by dipping

with a fast evaporation process. The uniformly grown

TIPS-PEN crystals were utilized to fabricate FETs, achiev-

ing hole mobility up to 1.5 cm2 V−1 s−1 (Figs. 15(b)–(f)).

In order to fabricate dense arrays of single crystals

over large-area substrates with controlled crystal align-

ment, Bao and co-workers developed a droplet-pinned

crystallization (DPC) method.10 The schematic illustra-

tion for growth process of C60 crystals is depicted in

Figure 16(a). The DPC method induced the fabrication of

aligned C60 single crystal arrays along an additional piece

Figure 14. (a) Schematic illustration of crystal growth of C8-DPNDF

with solution sustaining piece. (b) OM image of the device fabricated

with C8-DPNDF crystal. (c) Molecular packing structures in C8-DPNDF

crystal. Reprinted with permission from [91], C. Mitsui, et al., J. Am.Chem. Soc. 134, 5448 (2012). © 2012, American Chemical Society.



Figure 15. (a) Schematic diagram of TIPS-PEN crystal growth by fast evaporation, (b) AFM image of uniformly grown TIPS-PEN crystals, (c) OM

and polarized optical microscope (POM) images of TIPS-PEN crystals grown on an Al wire, (d) schematic description of FTES-ADT and TIPS-PEN

crystals, (e) schematic illustration of TIPS-PEN crystal grown on an Al wire and the corresponding SEM images and (f) FET device based TIPS-PEN

crystal and (left) its typical transfer and output characteristics. Reprinted with permission from [95], J. Jang, et al., Adv. Funct. Mater. 22, 1005 (2012).© 2012, Wiley-VCH.

of silicon wafer and is scalable to a 100-mm wafer sub-

strate, with around 50% of surface coverage with aligned

crystals. The needle-like C60 single crystal FETs revealed

the maximum electron mobility of up to 11 cm2 V−1 s−1,

while the ribbon-like C60 single crystal FETs showed an

electron mobility of ∼ 3 cm2 V−1 s−1 (Figs. 16(b)–(e)).

3.2. Energy Harvesting Devices Based onSingle-Component Organic Nanomaterials

Organic semiconductor nanomaterials have attracted great

interest for their applications to organic photovoltaic cells.

The active layer in organic solar cells is typically based on

bulk heterojunction (BHJ) system in which the photoac-

tive layer consists of a bicontinuous blend of an electron

donor (p-type) and an electron acceptor (n-type). Sincethe exciton diffusion length is approximately 5–10 nm

in most organic semiconductors, the nanoscale control of

the photoactive layer is critical for achieving high power

conversion efficiency (PCE). In order to tune the surface

morphology in nanoscale, thermal annealing.96–98 and addi-

tive processing99–103 have been extensively employed. As

another promising strategy, implementation of NWs in the

active layer as either the donor or acceptor component

has been explored.14�16�17�104–106 Since diameters of NWs

and dimensions can be well matched to the exciton dif-

fusion length, charge separation can be ensured before

recombination occurs in NW solar cells. Furthermore, their

high carrier mobilities, high absorption coefficients,107 and

longer exciton diffusion lengths make them a competitive

alternative to the BHJ layer.6

In the case of the NW solar cells based on

small-molecule organic semiconductors, C60 NWs,79 and

N ,N ′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8�NWs108�109 have been utilized as nanostructured electron

donor (n-type) materials. On the other hand, research on

polymer NW solar cells have mostly focused on the p-typeNWs, such as P3ATs and their block copolymers, while

a small number of n-type polymer NW solar cells were

reported; for instance, n-type poly (benzobisimidazoben-

zophenanthroline) (BBL) nanoribbons were reported by

Briseno et al.82

While most of the NW solar cells have been fabri-

cated with either donor or acceptor NWs, BHJ solar cells

incorporating both donor and acceptor NWs have also

been demonstrated recently. Jenekhe and co-workers110

fabricated a series of oligothiophene-functionalized naph-

thalene diimides (NDI-nTH and NDI-nT) NWs and uti-

lized as electron acceptor layers in all NW BHJ organic

solar cells, NDI-3TH NWs/poly(3-hexylthiophene) NWs.

The NW-based solar cells exhibited PCE of 1.15%.

The device structure was composed of ITO/PEDOT:PSS

(40 nm)/P3HT (15 nm)/active layer (∼ 80 nm)/1,3,5-Tris(1-

phenyl-1H-benzimidazol-2-yl)benzene (TPBI) (3 nm)/Al

(80 nm), where the P3HT and TPBI thin films acted as the



Figure 16. (a) Schematic illustration for growth process of C60 crystals via the DPC method. OM images of (b) needle and (c) ribbon structures

of crystallized C60. Typical transfer characteristics of FETs based on the crystallized (d) C60 needles and (e) C60 nanoribbons. Inset is the schematic

representation of the FET configuration. Reprinted with permission from [10], H. Li, et al., J. Am. Chem. Soc. 134, 2760 (2012). © 2012, American

Chemical Society.

electron-blocking and hole-blocking layers, respectively,

and the active layer consisted of P3HT NWs:NDI NWs

(1:1 wt/wt) blend.

In particular, periodically and vertically aligned BHJ

solar cells are considered to be ideal the architecture

for achieving high efficiency. In this device architecture,

photogenerated excitons can be easily separated, and the

charge carriers have their efficient pathways to each cor-

responding electrode. The efficient charge transport via

vertically aligned NW system has been demonstrated.

Figure 17(a) shows organic solar cells based on verti-

cally aligned [6,6]-phenyl-C61-butyric styryl dendronester

(PCBSD) nanorods. Fabrication of nanorods was real-

ized via the AAO templating method in combination with

the cross-linkable property of PCBSD materials. Li and

co-workers.15 prepared organic/organic ordered nanostruc-

tured heterojunction solar cells with a P3HT:Indene-C60

bisadduct (ICBA) BHJ upper layer that showed enhanced

photocurrent with high PCE of 7.3%.

He et al. fabricated interpenetrating nanostructured poly-

mer heterojunctions by double nanoimprinting method,

and utilized them in the photoactive layer for organic solar

cells (Fig. 17(b)).111 Poly((9,9-dioctylfluorene)-2,7-diyl-

alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2′′-diyl) (F8TBT)/P3HT films were nanoimprinted, resulting

in ordered nanostructured array with about pitch size of 20

to 50 nm. The PCE of nanostructured solar cells reached

1.9%, which is 50% higher than that of the blend devices.

3.3. Optoelectronics Based onMulti-Component Nanomaterials

Aside from the application of single-component organic

nanomaterials, development of multi-component nanoma-

terials and their application in optoelectronics have been

extensively studied. Multi-component organic nanomate-

rials are very promising candidates as building blocks

for high-performance optoelectronic components on a

scale ranging from micro- to nano-scale.112–115 Although

several inorganic NW heterojunctions have been success-

fully obtained by crystal growth, multi-component organic

nanomaterials such as p–n junctions, core–shell NWs,

and organic–inorganic hybrid nanostructures have been far



Figure 17. (a) Illustration of organic solar cells fabricated by cross-linked PCBSD nanostructures as active layers (left) and its photovoltaic per-

formance. Reprinted with permission from [15], C. Y. Chang, et al., Angew. Chem., Int. Ed. 50, 9386 (2011). © 2011, Wiley-VCH. (b) Schematic

diagram of aligned NW F8TBT/P3HT organic solar cells prepared by imprinting method (left) and the corresponding SEM image (right). Reprinted

with permission from [111], X. He, et al., Nano Lett. 10, 1302 (2010). © 2010, American Chemical Society.

less developed and incorporated into nanoscale photonic

devices.116–118 Assembling organic NWs into complex

heterostructures is critical for realizing multi-functional

devices and understanding charge transport and energy

transfer phenomena.

Briseno and co-workers41 fabricated organic single-

crystalline F16CuPc-CuPc p–n junction nanoribbons by

PVD method (Fig. 18). The selective crystallization of

n-type F16CuPc on p-type CuPc single-crystalline nanorib-bons realized ambipolar charge transporting properties in

OFETs with balanced mobilities of 0.05 and 0.07 cm2

V−1 s−1 for F16CuPc and CuPc, respectively. The single

F16CuPc-CuPc nanoribbon solar cells exhibited PCE of

0.007%.

Figure 18. (a) Schematic diagram of preparation for F16CuPc-CuPc nanoribbon by PVT method. (b) Device structure and SEM mage of OFETs

(top), and transfer and output performance of F16CuPc-CuPc nanoribbon transistors (bottom). Reprinted with permission from [41], Y. Zhang, et al.,

J. Am. Chem. Soc. 132, 11580 (2010). © 2010, American Chemical Society.

Multi-component NW building blocks can be pre-

pared from the controlled growth of two different

materials. Figure 19 shows Alq3-DAAQ wire-on-wire

nanostructures and their optical waveguiding properties.42

The preformed Alq3 MWs, made by self-assembly in

the liquid phase, were used as nucleation sites for the

site-specific vapor growth of DAAQ branches to give

dendritic heterostructures. When the trunk was excited

with a focused laser beam, emitted light of various col-

ors was simultaneously channeled from the branched

nanowires via both waveguiding and energy transfer. The

intensity of the out-coupled emissions was modulated

effectively by changing the polarization of the incident

light.



Figure 19. (a) Illustration of Alq3-DAAQ wire-on-wire structure (b) and polarized light (PL) microscope image obtained from ultraviolet (UV) light

regime. (c) PL microscope image obtained from laser with 351 nm wavelength and (d) PL spectra of excited spot. Reprinted with permission from [42],

J. Y. Zheng, et al., J. Am. Chem. Soc. 134, 2880 (2012). © 2012, American Chemical Society.

Figure 20. (a) Schematic illustration for organic NW printing and NW FETs. (b) SEM image of P3HT:PEO blend NW transistors with ion-gel

dielectric, and (c) their output and transfer characteristics. (d) Integrated large-scale NW transistor and inverter devices. Reprinted with permission

from [119], S. Y. Min, et al., Nat. Commun. 4, 1773 (2013). © 2013, Nature Publishing Group.

Lee and co-workers recently reported on the high-speed,

large-area printing of highly aligned individual NWs.119

High-speed electrohydrodynamic printing of organic NW

was utilized to fabricate organic semiconducting NW

arrays on specific substrates. The prepared P3HT:PEO

blend NW FETs exhibited field-effect mobility of up to

9.7 cm2 V−1 s−1. Furthermore, complementary inverter cir-

cuit arrays based on the aligned NWs were also fabricated

(Fig. 20).

4. CONCLUDING REMARKSOrganic nanomaterials are promising building blocks for

soft and flexible electronics. 1D nanomaterials and 1D

multi-component nanostructures based on small molecules

and polymers can be widely employed for use in optoelec-

tronic devices such as OFETs, OPTs, organic memories,

organic solar cells, and optical waveguides. The structural

perfection of single-crystalline organic 1D NWs that

are self-assembled along the �–� stacking direction

enables efficient charge transport; therefore, one can

expect enhanced optoelectronic properties compared to

thin film counterparts. Furthermore, the in-depth study on

the fundamental charge transport mechanism of organic

electronic materials can be effectively performed using

1D tailored organic nanomaterials. Moreover, a multi-

component system composed of more than two different

materials provides unique and multi-functional properties

emerged from the combination of each compound. Inter-

molecular interactions enable efficient energy transfer or

tunable optoelectronic behaviors. The synthetic approaches

of organic nanomaterials and the fabrication techniques of

nanoscale optoelectronics need to be further advanced to

realize fully integrated soft, stretchable, cost-effective, and

energy-efficient nanoelectronics.



ABBREVIATIONS1-D One-dimensional

2-D Two-dimensional

AAO Anodic aluminum oxide

AFM Atomic force microscopy

Alq3 Aluminum tris(8-hydroxyquinoline)

BBL Poly(benzobisimidazobenzophenanthroline)

BHJ Bulk heterojunction

BPE-PTCDI N�N ′-bis(2-phenylethyl)-perylene-3,4:9,10-tetracarboxylic diimide

BPEA 4,4′-bis(phenylethynyl)anthraceneC12-4CldiPBI Tetrachlorinated diperylene bisimide

C2Ph-PXX 3,9-bis(4-ethylphenyl)-peri-

xanthenoxanthene

C8-DPNDF Naphtho[2,1-b:6,5-b′]difuranCN-TFMBE 1-cyano-trans-1,2-bis-(3′,5′-bis-

trifluoromethyl-biphenyl)ethylene

CuPc Copper phthalocyanine

DAAQ 1,5-diaminoanthraquinone

DPC Droplet-pinned crystallization

EQE External quantum efficiency

F8TBT Poly((9,9-dioctylfluorene)-2,7-diyl-

alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-

benzothiadiazole]-2′,2′′-diyl)FAT Filtration-and-transfer

FTES-ADT Fluorinated 5,11-bis(triethylsilylethynyl)

anthradithiophene

H2TPyP 5,10,15,20-tetra(4-pyridyl)-porphyrin

HTP Hexathiapentacene

ICBA Indene-C60 bisadduct

MW Microwire

NW Nanowire

OFET Organic field-effect transistor

OM Optical microscope

OPT Organic phototransistor

P3AT Poly-(3-alkylthiophene)

P3BT Poly(3-butylthiophene)

P3HT Poly(3-hexylthiophene)

PCBM [6,6]-phenyl C61 butyric acid methyl ester

PCBSD [6,6]-phenyl-C61-butyric styryl dendron-

ester

PCE Power conversion efficiency

PCL Poly(�-caprolactone)PDI Perylenebis(dicarboximide)

PDMS Polydimethylsiloxane

PDT Poly(ethylene oxide)

PEO Poly(dodecylthiophene)

PFO Polyfluorene

PL Polarized light microscopy

PLLA Poly-L-lactide

POM Polarized optical microscope

PPy Polypyrrole

PS Polystyrene

PTCDI Perylene-3,4,9,10-tetracarboxylic-

3,4,9,10-diimide

PVD Physical vapor deposition

PVP Poly(vinyl pyrrolidone)

PVT Physical vapor transport

Rubrene 5,6,11,12-tetraphenylnaphthacene

SAED Selected area electron diffraction

SEM Scanning electron microscopy

SPNT Single particle nanofabrication technique

TA-PPE Thioacetate poly(para-phenylene ethyny-

lene)

TEM Transmission electron microscopy

THF Tetrahydrofuran

TIPS-PEN Triisopropylsilylethynyl pentacene

TPBI 1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)

benzene

UV Ultraviolet

VCR Vaporization–condensation–

recrystallization

VDP Vapor deposition polymerization

WRER Write-read-erase-read.

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Received: 10 August 2013. Accepted: 26 August 2013.


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