Physica Scripta

Two-dimensional crystals-based heterostructures:materials with tailored propertiesTo cite this article: K S Novoselov and A H Castro Neto 2012 Phys. Scr. 2012 014006

 

View the article online for updates and enhancements.

You may also likeThree-dimensional image reconstructionfor electrical impedance tomographyF Kleinermann, N J Avis, S K Judah et al.

-

Fabrication of Three-Dimensional Cu/CNTComposite Film By ElectrodepositionMasaya Ozawa and Susumu Arai

-

AdS and dS braneworld Kaluza–KleinreductionI Y Park, C N Pope and A Sadrzadeh

-

Recent citationsA review of strain sensors based on two-dimensional molybdenum disulfideMinjie Zhu et al

-

Graphene and graphene oxide for bio-sensing: general properties and the effectsof graphene ripplesGulsah Yildiz et al

-

Defect Agglomeration and Electron-Beam-Induced Local-Phase Transformations inSingle-Layer MoTe2Janis Köster et al

-

This content was downloaded from IP address 89.141.41.232 on 11/01/2022 at 10:15

IOP PUBLISHING PHYSICA SCRIPTA

Phys. Scr. T146 (2012) 014006 (6pp) doi:10.1088/0031-8949/2012/T146/014006

Two-dimensional crystals-basedheterostructures: materials withtailored propertiesK S Novoselov1 and A H Castro Neto2,3

1 School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK2 Graphene Research Centre, National University of Singapore, 2 Science Drive 3, Singapore 117542,Singapore3 Department of Physics Boston University 590 Commonwealth Avenue Boston, MA 02215, USA

E-mail: Konstantin.Novoselov@manchester.ac.uk

Received 3 May 2011Accepted for publication 8 June 2011Published 31 January 2012Online at stacks.iop.org/PhysScr/T146/014006

AbstractGraphene is just one example of a large class of two-dimensional crystals. These crystals caneither be extracted from layered three-dimensional materials or grown artificially by severaldifferent methods. Furthermore, they present physical properties that are unique because of thelow dimensionality and their special crystal structure. They have potential for semiconductingbehavior, magnetism, superconductivity, and even more complex many-body phenomena.Two-dimensional crystals can also be assembled in three-dimensional heterostructures that donot exist in nature and have tailored properties, opening an entirely new chapter in condensedmatter research.

PACS numbers: 81.05.ue, 73.22.Pr, 72.80.Vp, 61.48.Gh

(Some figures may appear in colour only in the online journal)

1. Introduction

Technological progress is determined, to a great extent, bythe developments in materials science. The most surprisingbreakthroughs are attained when a new type of material,with unusual dimensionality and complexity, is created. Someof the most famous examples are the conducting polymers,heavy-fermion magnets, high temperature superconductingcuprates, carbon nanotubes, graphene, topological insulators,iron superconductors, just to name a few. One of theultimate goals of modern materials science is to developnovel complex architectures and structures with tailored andemergent properties. These properties are not necessarily easyto predict due to the structural and electronic complexityinvolved. Hence, beyond new technologies, new materialscreate new challenges in condensed matter research.

The current progress in two-dimensional (2D) crystalisolation [1] and growth [2] can lead to a new paradigmof ‘complex materials on demand’ by first identifying andconstructing the key building blocks and then combiningthem into complex architectures. This approach consists

of three strategies: creation of various 2D materials bymicro-mechanical exfoliation and/or growth; modification ofelectronic, structural, optical and chemical properties of suchmaterials by strain engineering, chemical functionalizationor other means; and development of novel 3D materials bycontrolled multi-stacking of such 2D platforms. The numberof different structures that one can obtain based on thesestrategies is practically unlimited and their physical propertiesare hard to predict a priori. Nevertheless, by understandingthe properties of the 2D components, one should be able tocreate materials that have interesting structural, electronic,optical, mechanical and other properties.

Moreover, these strategies also open doors for amultitude of applications, since it becomes possible to createartificial materials which would combine several uniqueproperties that will allow their use in novel multi-tasking(mechanical, optical and electronic) applications, as, forinstance, ‘smart’ composites and coatings for flexibleelectronic and photovoltaics or photonic devices for integratedoptoelectronic circuits. One can identify already severalkey areas where this kind of approach can have a strong

0031-8949/12/014006+06$33.00 1 © 2012 The Royal Swedish Academy of Sciences Printed in the UK

Phys. Scr. T146 (2012) 014006 K S Novoselov and A H Castro Neto

Figure 1. Micromechanical exfoliation of 2D crystals. (a) Adhesivetape is pressed against a 2D crystal so that the top few layers areattached to the tape (b). (c) The tape with crystals of layeredmaterial is pressed against a surface of choice. (d) Upon peeling off,the bottom layer is left on the substrate.

impact: (i) ‘smart’ ultra-strong nano-composite materials;(ii) electro-mechanical devices for ultra-fast electronics;(iii) materials with predetermined band-gap and workfunctions for next generation photovoltaic (solar-cells)applications; (iv) atomically thin film transistors for photonicapplications.

The real advantage of this approach is that one isable to create materials that can perform several functions(mechanical, electronic, and optical) simultaneously. Suchmaterials (let alone devices based on them) are not availableyet. However, as the functionality of modern portableelectronic equipment grows exponentially, we already seea huge demand for such multi-functional, multi-task,materials.

2. 2D crystals

Since graphene was isolated for the first time [3] severalways of synthesizing this material have been introduced. Thisincludes mechanical exfoliation (see figure 1) [1, 3] epitaxialgrowing on the surface of silicon carbide [4], epitaxialgrowth on metal surfaces [5], chemical exfoliation [6], etc.Depending on the particular application, one or anothermethod can/should be used. There are also a number of waysto modify the properties of graphene obtained by variousmethods. The most common ones are the introduction ofmechanical strain (of various configurations) and chemicalmodification. The latter also leads to the production of other2D materials. Simply using graphene as a scaffolding, one canattach various chemical species to it and end up with a new 2Datomic crystal.

In general, all the methods of graphene production knownto us could be also applied to the production of other2D crystals. Thus, mechanical exfoliation of other layeredmaterials has already been demonstrated [1] as well asepitaxial growth on the surface of a metal [7] or chemicalexfoliation [8] (figure 2). This gives us huge opportunitiesin terms of a variety of different properties covered by thosematerials.

It seems clear that graphene is going to play an importantrole in a series of technological applications, from transparent

Figure 2. Transfer of chemical vapor deposition (CVD) grown 2Dcrystals. (a), (b) 2D crystals are grown by CVD on a surface of ametal. (c) A sacrificial layer is deposited on top of the 2D crystal.(d) The metal is etched away, leaving 2D crystal stuck on thesacrificial layer. (e) The sacrificial layer, together with the 2Dcrystal is transferred onto the substrate of choice. (f) The sacrificiallayer is removed.

conducting electrodes [9] to high speed electronics [10]. Themajor reason why it took only five years for the transitionfrom the laboratory bench-top to industrial production isbecause graphene offers really unique properties of whichthe most striking is the 2D nature [11]. Many applications,already in place, have been waiting for a material likethis for years. Hence, when graphene became available, awhole community of scientists and technologists reactedimmediately. There have been significant efforts to improvethe existing processes and procedures and develop novelmethods of graphene production, as well as investigate other2D materials which might be similarly successful in thefuture. One emerging example is boron-nitride (BN), whichis sometimes called ‘insulating graphite’ and it might bein use everywhere where graphene’s high conductivity isa disadvantage (ultra-thin, high quality, insulating layersfor nano-electronics, non-conductive, ultra-strong, compositematerials). However, there is a huge class of 2D materials,including exotic types such as MoS2, NbSe2, Bi2Te3, thatcan be either isolated by micro-mechanical or chemicalexfoliation or epitaxial growth (figure 3).

The capability of growing different families of 2Dcrystals artificially will enhance tremendously the ‘library’of materials that one can obtain and will also allow theexploration of the unusual physics that occurs in 2D. Unlikethe case of 1D materials, such as conducting polymers(such as polyacetylene) and carbon nanotubes, where phasespace strongly constraints electron–electron interactions,and ordinary 3D solids, where interactions are essentiallysuppressed, 2D crystals are characterized by strong quantumfluctuations and an enhanced phase space for interactions,making the physics in this dimensionality very special.

3. 2D-based, 3D materials

Composite materials are omnipresent in technology and manyexisting applications rely on light, conductive (or insulating),and strong composites. The performance of such materials is,however, fundamentally limited by the interaction between thefiller and the matrix. Hence, out of a great number of possiblecombinations, only a few really work. Moreover, having a

2

Phys. Scr. T146 (2012) 014006 K S Novoselov and A H Castro Neto

Figure 3. Exfoliating other 2D crystals: BN (top panel), MoS2

(middle panel) and NbSe2 (bottom panel).

complex structure, such materials are subject to unpredictablefailure. It has been recently demonstrated that graphene canimprove mechanical, chemical and electrical properties ofcomposite materials dramatically. Graphene is the strongestmaterial known to us [12], but also it is truly 2D (so itcannot be cleaved further and has the ultimate aspect ratio).Moreover, one can produce graphene of suitable dimensionsand in large quantities very cheaply. Thus, one can improvethe quality of such composite materials by fine-tuning theinteraction with the matrix (via controlled chemistry on

Figure 4. Hypothetical example of a 2D-crystals basedheterostructure.

graphene surface, for instance, it was shown that hydrophobicgraphene can be turned hydrophilic by hydrogenation [13]),but also adding additional functionalities, such as the abilityto monitor stress distributions, the control of optical gaps, andelectrical conductivity. One can apply a similar strategy toother 2D materials to produce high-performance compositeswith a wide range of functionalities and physical behavior(figure 4).

Micro-mechanical cleavage and artificial growth can beused to obtain other 2D materials (graphene, BN, MoS2,Bi2Te3, etc) to be used as fillers in composite materials.Using materials other than graphene allows one to expandthe complexity of such composites. One can make themoptically active in various parts of the optical spectraby using chemically modified graphene or materials withvarious band-gaps. By creating semiconductor–metal and/orsemiconducting–semimetal interfaces, either in the form ofdispersed heterojunctions or layered junctions, efficient lightcollection and charge transfer across the interface can beachieved—giving rise to the photovoltaic effect. Previouswork has explored, for instance, electrostatically stackedTiO2-graphene layers [14], where functional segregationof exciton generation and charge transport can be separated.

It has also been shown that graphene’s Raman spectrumis extremely sensitive to applied strain and that strain transferbetween the matrix and graphene is very efficient [15].This property can be used to create composite materialswhere accumulated stress could be monitored by contactless,non-invasive, optical methods. Such techniques can be ofcrucial importance in certain areas of engineering wherecatastrophic material failure is a major issue and where

3

Phys. Scr. T146 (2012) 014006 K S Novoselov and A H Castro Neto

Figure 5. Introducing local strain in graphene by an atomic forcemicroscopy cantilever.

permanent monitoring of the performance of a material iscrucial (such as avionics, electrical grids, medicine, etc).

4. Strain engineering or ‘straintronics’

One of the pillars of solid state physics is that the bandstructure of 3D materials is set by their geometry andchemistry. Although this concept is the force behind modernelectronics and technology, it also has its limitations: itis very difficult to modify and manipulate (to create anew functionality one has to create a new crystal); thestructure might not be stable (for instance, one can haveelectron migration) or can be strongly modified by external,environmental, conditions (such as radiation damage); thesame chemistry that is used to achieve the desired electronicstructure (for instance, by doping) might cause detrimentaleffects on other parameters (such as the decrease of themobility due to scattering by dopants). In contrast, 2Dmaterials open a new avenue for control of electronicproperties. It has been demonstrated that the electronicstructure of 2D materials can be considerably modified bystrain, shear and bending [16]. Moreover, one gets much bettercontrol of those parameters in 2D as compared to 3D systems(for instance, graphene can be stretched up to 20% withoutstructural failure) (figure 5).

Uniaxial and biaxial strain, applied along differentcrystallographic directions, can reversibly tune graphene frommetallic to insulating [17, 18] or significantly change theband structure. Furthermore, usually the two valleys areaffected differently by strain, which gives us a mechanismto differentiate between the valleys and effectively leads tovalleytronics. Just as spin can be used for spintronics wherethe electron spin is manipulated by external magnetic fields,valleytronics uses the orbital degree of freedom (the valleyindex) as a new quantum number that can be manipulatedexternally. Band structure is also strongly influenced bystacking. A well-known example is graphite, dependingon the stacking order (Bernal versus rhombohedral versushexagonal versus turbostratic) its electronic properties can berather different [19]. Furthermore, intercalation of layers ofdifferent atoms can completely change the electronic states.For instance, graphite intercalated with alkali metals becomessuperconducting [20]. By introducing different 2D materials,

with different properties, into a 3D matrix, one is able tomodify the final electronic properties.

A completely new field of exploration is the study ofmulti-stacked materials under strain. Given that the elasticproperties of the 2D layers can be rather different becauseof the atomic bonding (say, graphene in comparison toBN), the final elastic properties of the 3D super-structurecan be engineered as well. The electronic properties ofsuch engineered 3D structures will also be affected bystrain leading to new possibilities in terms of electronicfunctionalities. Even in the simplest case of stacking—bilayergraphene—the electronic structure depends crucially on thestacking order. A small uniaxial strain or a minute shiftbetween the layers would immediately lead to strong changesin the electronic structure.

By arranging different 2D materials into stacks it shouldbe possible to achieve new, composite, materials with novelelectronic, mechanical and optical properties. The typicalmechanisms for the modification of the electronic bands insuch stacks are based on changing the symmetry of thestructure and opening gaps in the electronic spectrum.

5. Optically active materials

It has been demonstrated that, due to the long mean-freepath and high Fermi velocity, graphene can serve as anexcellent light-to-current converter with quantum efficiencyreaching close to 100% [21]. Its use for solar-cell applicationsis however limited, due to the low absorption of graphene(hence, the total efficiency is low) and difficulties withcreation of p–n junctions (necessary for electron–holesseparation) in 2D materials. Nevertheless, the efficiency ofthese devices can be greatly enhanced by exploring differentgeometries, for instance the vertical geometry, where theelectron–hole pair is separated between two neighboringlayers (figure 6).

The strategy would be to separate electron and hole pairsinto the neighboring layers of multi-layer structures. This canbe achieved either by applying an external electric field or byselective doping of the two layers (one n-type, another p-type).Selective doping can be implemented either during growth bysubstitutional doping or by doping with adatoms. It has beenshown that strain generates scalar potential [22], which leadsto the formation of local p–n junctions. It is possible to usethis concept to create an array of p–n junctions. One can usestacks of various 2D crystals (either the same materials withdifferent doping or completely different materials) to formmulti-layers. One can also use layers of insulating material(such as BN) to separate the conductive layers for moreefficient electron–hole separation (to create weak tunnelingbarriers).

It has been demonstrated that graphene absorbs only2.3% of light in the wide range of visible spectra [23].This number is even lower for chemically modifiedgraphene and for some other 2D materials. Combinedwith the excellent conductivity properties, these materialsare extremely promising for transparent conductive coatingapplications. One can use local chemical modificationto define conductive and non-conductive areas to createatomically thin film transistors.

4

Phys. Scr. T146 (2012) 014006 K S Novoselov and A H Castro Neto

Figure 6. Optically active material. Two graphene layers areseparated by several layers of BN, which serve as a tunnelingbarrier. One of the graphene layers is in close proximity to a layerof NbSe2, and gets doped due to the difference between thework-functions of NbSe2 and graphene. Such doping creates abuilt-in electric field between the two graphene layers. Such a fieldseparates an electron–hole pair which is created by an incomingphoton, resulting in photocurrent.

Another large application for 2D materials is coating.The high quality of 2D crystals one can obtain make themperfect gas barriers. Even a single layer of graphene isimpermeable for most gases [24]. Furthermore, such a coatingcan be created by CVD growth directly on an arbitrary shapedsurface—acting as a protective barrier. Another promisingapplication in coating technology is in transparent conductivefilms. Two types of approaches can be used to createtransparent conductive coating: chemical exfoliation andepitaxial growth of graphene and other conductive materials.Uniaxial strain breaks the symmetry of the lattice andproduces rotation of the planes of polarization. This effect isenhanced in an applied magnetic field. Application of strainallows the production of tunable polarizers in a broad bandof optical frequencies. One can use chemical modificationto define conductive and non-conductive areas for atomicallythin film transistors.

6. Conclusions

We have learned in the last few years that 2D crystalscan be obtained by several methods such as exfoliation,molecular bean epitaxy and chemical vapor deposition. Wealso know that the physical and chemical properties of these2D crystals can be modified by chemical or molecular doping,by application of strain, shear or pressure, and by intercalationwith different types of atoms and molecules. We can nowcreate a new class of 2D artificial materials that do not exist innature and whose properties we can control and explore.

It is not hard to imagine that we can take all these 2Dcrystals and pile them into 3D structures. Given that each one

of these 2D crystals can have different physical properties wecan imagine creating a library of 2D crystals and using thoseto make new 3D artificial structures with unique properties.The outcome of such a scheme would be a large portfolioof new materials with different functionalities. In this way,we could develop a ‘materials-on-demand’ strategy for novelcomplex architectures and structures with precisely tailoredproperties for emerging technological applications.

The most important lesson from the graphene story isprobably this one: there is a universe of 2D crystals out therejust waiting to be studied. Each one of them has its own beautyand purpose. Paraphrasing Isaac Newton we can say that weare still in the infancy of a broad field and diverting ourselveswith graphene, a material that looks more interesting thanordinary, whilst a great field of 2D crystals lays undiscoveredbefore us.

References

[1] Novoselov K S, Jiang D, Schedin F, Booth T, Khotkevich V V,Morozov S V and Geim A K 2005 Two dimensional atomiccrystals Proc. Natl Acad. Sci. USA 102 10451

[2] Kim K S et al 2009 Large-scale pattern growth of graphenefilms for stretchable transparent electrodes Nature457 706

[3] Novoselov K S, Geim A K, Morozov S V, Jiang D, Zhang Y,Dubonos S V, Grigorieva I V and Firsov A A 2004 Electricfield effect in atomically thin carbon films Science306 666

[4] van Bommel A J, Crombeen J E and Van Tooren A 1975LEED and Auger electron observations of the SiC(0001)surface Surf. Sci. 48 463

Berger C et al 2004 Ultrathin epitaxial graphite: 2D electrongas properties and a route toward graphene-basednanoelectronics J. Phys. Chem. B 108 19912

[5] Grant J T and Haas T W 1970 A study of Ru(0001) andRh(111) surfaces using LEED and Auger electronspectroscopy Surf. Sci. 21 76

Blakely J M, Kim J S and Potter H C 1970 Segregation ofcarbon to the (100) surface of nickel J. Appl. Phys. 41 2693

Reina A, Jia X, Ho J, Nezich D, Son H, Bulovic V,Dresselhaus M S and Kong J 2009 Large area, few-layergraphene films on arbitrary substrates by chemical vapordeposition Nano Lett. 9 30

Li X et al 2009 Large-area synthesis of high-quality anduniform graphene films on copper foils Science 324 1312

[6] Brodie B C 1859 On the atomic weight of graphite Phil. Trans.R. Soc. A 149 249

Blake P et al 2008 Graphene-based liquid crystal device NanoLett. 8 1704

Hernandez Y et al 2008 High-yield production of graphene byliquid-phase exfoliation of graphite Nature Nanotechnol. 3563

[7] Nagashima A, Tejima N, Gamou Y, Kawai T and Oshima C1995 Electronic structure of monolayer hexagonalboron-nitride physisorbed on metal surfaces Phys. Rev. Lett.75 3918

[8] Coleman J N et al 2011 Two-dimensional nanosheets producedby liquid exfoliation of layered materials Science 331 568

[9] Bae S et al 2010 Roll-to-roll production of 30-inch graphenefilms for transparent electrodes Nature Nanotechnol. 5 574

[10] Liao L, Lin Y C, Bao M Q, Cheng R, Bai J W, Liu Y, Qu Y Q,Wang K L, Huang Y and Duan X 2010 High speed graphenetransistors with a self-aligned nanowire gate Nature467 305

[11] Castro Neto A H, Guinea F, Peres N M R, Novoselov K S andGeim A K 2009 The electronic properties of graphene Rev.Mod. Phys. 81 109

5

Phys. Scr. T146 (2012) 014006 K S Novoselov and A H Castro Neto

[12] Lee C, Wei X, Kysar J W and Hone J 2008 Measurement ofthe elastic properties and intrinsic strength of monolayergraphene Science 321 385

[13] Elias D C et al 2009 Control of graphene’s properties byreversible hydrogenation: evidence for graphene Science323 610

[14] Manga K K, Zhou Y, Yan Y L and Loh K P 2009 Multilayerhybrid films consisting of alternating graphene and titaniananosheets with ultrafast electron transfer andphotoconversion properties Adv. Funct. Mater.19 3638

[15] Mohiuddin T M G et al 2009 Uniaxial strain in graphene byRaman spectroscopy: G-peak splitting, Gruneisenparameters, and sample orientation Phys. Rev.B 79 206433

[16] Pereira V M, Castro Neto A H and Peres N M R 2009 Atight-binding approach to uniaxial strain in graphenePhys. Rev. B 80 045401

Pereira V M, Ribeiro R M, Peres N M R and Castro Neto A H2010 Optical properties of strained graphene Europhys. Lett.92 67001

[17] Guinea F, Katsnelson M I and Geim A K 2010 Energy gapsand a zero-field quantum Hall effect in graphene by strainengineering Nature Phys. 6 30

[18] Levy N, Burke S A, Meaker K L, Panlasigui M, Zettl A,Guinea F, Castro Neto A H and Crommie M F 2010Strain-induced pseudo-magnetic fields greater than 300Tesla in graphene nanobubbles Science 30 544

[19] Guinea F, Castro Neto A H and Peres N M R 2007 Electronicproperties of stacks of graphene layers Solid State Commun.143 116

Guinea F, Castro Neto A H and Peres N M R 2006 Electronicstates and Landau levels in graphene stacks Phys. Rev. B73 245426

[20] Dresselhaus M S and Dresselhaus G 1981 Intercalationcompounds of graphite Adv. Phys. 30 139

[21] Mueller T, Xia F and Avouris P 2010 Graphene photodetectorsfor high-speed optical communications Nature Photonics4 297

[22] Kim E-A and Castro Neto A H 2008 Graphene as an electronicmembrane Europhys. Lett. 84 57007

[23] Nair R R, Blake P, Grigorenko A N, Novoselov K S, Booth TJ, Stauber T, Peres N M R and Geim A K 2010 Finestructure constant defines visual transparency of grapheneScience 320 1308

[24] Bunch J S, Verbridge S S, Alden J S, van der Zande A M,Parpia J M, Craighead H G and McEuen P L 2008 NanoLett. 8 2458

6