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Materials ExpressPersp

ective

Copyright copy 2011 by American Scientific PublishersAll rights reservedPrinted in the United States of America

2158-584920111010008doi101166mex20111002

wwwaspbscommex

Two-Dimensional Crystals Beyond GrapheneA H Castro Neto1lowast dagger and K Novoselov2lowast

1Graphene Research Centre National University of Singapore 2 Science Drive 3 Singapore 1175422School of Physics and Astronomy University of Manchester Oxford Road Manchester M13 9PL UK

Human progress and development has always beenmarked by breakthroughs in the control of mate-rials Since pre-historic times through the stonebronze and iron ages humans have exploited theirenvironment for materials that can be either useddirectly or can be modified for their benefit to maketheir life more comfortable productive or to givethem military advantage One age replaces anotherwhen the material that is the basis for its sustain-ability runs its course and is replaced by anothermaterial which presents more qualities Multi-taskingspeed versatility and flexibility are at the heart ofmodern technology In recent years a new class ofmaterials that can fulfill these needs have emergedtwo-dimensional (2D) crystals Graphene is probablythe most famous example but there are numer-ous other examples with amazing electronic andstructural properties In this paper we look into thepossible routes for exploration of this new field thatpresents new venues in basic science as well as inapplications

Keywords Graphene Two-Dimensional CrystalsElectronic Properties

CONTENTS

1 Materials Science Goes 2D 102 Growing Nurturing and Taming 2D Crystals 123 Finding Our Way Around in the High Tech Jungle 134 Ultra-Strong Nano-Composite Materials with Controlled

Properties 145 Electro-Mechanical Devices for Ultra-Fast Electronics 146 Novel Photovoltaics 15

lowastAuthors to whom correspondence should be addressedEmails phycastrnusedusg konstantinnovoselovmanchesteracuk

daggerOn leave from Boston University USA

7 Atomically Thin Transparent Tunable FETs 158 Economic Payoff 16

Acknowledgments 16References and Notes 16

1 MATERIALS SCIENCE GOES 2D

Smart advanced materials that are flexible (for transpar-ent wearable electronics) adaptable (that change structuredepending on exterior conditions) multifunctional (thatcan be tuned by application of electric fields magneticfields pressure and strain) and at the same time ldquogreenrdquo(that do not waste energy are low power consuming andecologically friendly) are the ultimate dreams of materialscientists and engineers Such materials hold the key tothe next generation of devices with deep incursion intonew markets (see Fig 1) The isolation of graphene1 andother two-dimensional (2D) crystals2 in 2004 has finallybrought materials with the promise of such properties tolight More importantly the recent breakthrough in theirindustrial scale fabrication3 is paving the way towards anew era in the microelectronic industry4 A shift in sucha key economic sector can provide unprecedented oppor-tunities in transforming the high tech industry This isa strategic area that any economically developed countrycannot shy away from including in its high technologyportfolio given its potential impact in several fundamentalareas energy defense communications electronics artifi-cial intelligence and information technology The develop-ment of a large family of advanced materials which do notexist in nature (and certainly are not yet available on themarket) with functionalities that can have a real economicimpact is key for future technologiesProgress in 2D crystal growth will create a new

paradigm of ldquomaterials on demandrdquo by first identifyingand then constructing the key building blocks of multi-functionality There are three key strategies in this fieldnamely S1 growth S2 functionalization and S3 multi-stacking (see Fig 2) The preparation of the elementary

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Materials ExpressTwo-Dimensional Crystals Beyond GrapheneNeto and Novoselov

Persp

ective

Fig 1 Some of the multiple applications of 2D crystals

blocks (various 2D crystals) their modification (by chem-istry mechanical means etc) and subsequent combinationto create materials with predefined properties can gen-erate a library of materials that will allow for combi-natorial material production (in analogy to combinatorialchemistry) The large variety of starting blocks modifi-cation methods and combination options guarantees notonly a huge range of materials possible but also theirmulti-functionality which will be of crucial importance for

Antonio H Castro Neto received his PhD in Physics in 1994 from University of Illinoisat Urbana-Champaign For the next one year he was a postdoctoral fellow at the Institutefor Theoretical Physics at the University of California at Santa Barbara In 1995 he becameAssistant Professor at University of California at Riverside In 2000 he moved to BostonUniversity as Professor of Physics where he is currently the director of the Quantum Con-densed Matter Theory Visitorrsquos Program In 2003 Professor Castro Neto was elected a fel-low of the American Physical Society He was Divisional Associate Editor for the PhysicalReview Letters and is the Editor for Reviews of Modern Physics Colloquia and co-editorfor EPL (Europhysics Letters) He was awarded the 11th Ross J Martin by the Universityof Illinois at Urbana-Champaign the University of California Regent Fellowship the AlfredP Sloan Research Fellowship and the Miller Professorship by the University of California

at Berkeley Professor Castro Neto has authored more than 180 papers and given hundreds of seminars worldwide Hisresearch has spanned a broad range of topics in condensed matter theory ranging from electronic transport in DNAand decoherence in open quantum systems to superconductivity and quantum magnetism in cuprates and heavy-fermionalloys More recently Prof Castro Neto has been one of the leading theorists in the study of graphene a two-dimensionalallotrope of carbon who is being considered by many as the next generation material for micro-electronics

Konstantin Novoselov received his masterrsquos degree from the Moscow Institute of Physicsand Technology and PhD from the University of Nijmegen the Netherlands In 2001 hemoved to the University of Manchester UK with his doctoral advisor Prof Andre GeimNovoselov has published more than 60 peer-reviewed research papers on topics like meso-scopic superconductivity (Hall magnetometry) sub-atomic movements of magnetic domainwalls the invention of gecko tape and graphene Dr Novoselov is a recipient of Euro-physics Prize (2008) for isolating a single free-standing atomic layer of carbon (graphene)and Knights Commander of the Order of the Netherlands Lion (2010) besides Nobel Prizein Physics (2010) jointly with Andre Geim for groundbreaking discovery of graphene andelucidating its remarkable electronic properties

future applications Furthermore each one of these strate-gies will generate different materials with different costand commercial value leading to a rich environment forbasic science and applicationsTechnological progress is determined to a great extent

by the developments in material science The most surpris-ing breakthroughs are attained when a new type of materialor new combinations of old materials with reduced dimen-sionality and functionality is created Well known exam-ples are for instance the transition from three-dimensional(3D) semiconductors based on Ge and Si to heterostruc-tures that are nowadays the leading platform for themicroelectronic industry Other examples are quantumwells formed at GaAsAlGaAs semiconductor interfacesfor high electron mobility transistors (HEMT)5 magneticthin film multilayers for magnetic hard disc read heads6

and solar cells made out of 2D organic light emittingdiodes (OLED)7 The ultimate goal of modern materialsscience is the development of ldquomaterials on demandrdquo fornovel complex architectures and structures with preciselytailored properties for emerging technological applicationsIt seems clear that graphene is going to play an impor-

tant role in a series of technological applications fromtransparent conducting electrodes to high speed electron-ics The major reason why it took only 6 years for thetransition from the laboratory bench-top to the industrial

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Materials ExpressTwo-Dimensional Crystals Beyond Graphene

Neto and NovoselovPersp

ective

Fig 2 Key strategies for the field of 2D crystals S1 Growth S2 functionalization and S3 combinatorial multi-stacking The various outcomesappear at the bottom of each process

production and realistic applications is because grapheneoffers really unique properties of which the most strikingis the 2D nature Many applications already in place havebeen waiting for a material like this for years Hence whengraphene became available a whole community of scien-tists and technologists reacted immediately The same canbe said about other 2D crystals when they become avail-able they will open doors for a plethora of possibilities

2 GROWING NURTURING AND TAMING2D CRYSTALS

With the fast developments in materials science in thelast few years it is possible to create various raw two-dimensional (2D) crystals by micro-mechanical (or chem-ical) exfoliation andor artificial growth by chemicalvapor deposition (CVD) and molecular beam epitaxy(MBE) Moreover there are methods for the high vacuumplasma beam deposition of crystals at low temperature(lt650 C) which is compatible with the MBE deposi-tion system (examples are graphene h-BN boron carboni-tride BCN on nickel-coated substrate using borazine orborazinemethane as the chemical precursor8)Equally important for the manufacturing of 2D crys-

tals as a bulk commodity is the establishment of largescale transfer facilities eg transfer on thin glass sheetsand (transparent) flexible polymer substrates It is possi-ble to set-up CVD furnaces for producing large sheets(gt30 sheets) of 2D crystals on catalyst foils such as Cuand Ni and a basic research type roll-to-roll transfer facil-ity which allows to transfer such large sheets of 2D crys-tals from the metal substrate to a flexible substrate for largescale applications34 This approach has been successfully

used to produce the first prototypes of graphene-basedtouch-screens It is conceivable to extend this process toform layered structures such that one can transfer CVDgrown graphene sheets to other atomically thin films sup-ported on flexible transparent polymer substrateThe electronic and structural properties of these raw 2D

crystals can be modified by methods such as chemicalfunctionalization9 and strain engineering10 in order to cre-ate artificial 2D materials with new functionalities Eachone of these methods has its pros and cons Chemical func-tionalization has the advantage that one has a direct effecton the properties of carbon However this method usuallyintroduces disorder in the system that can be detrimentalto the electronic properties Strain engineering does notintroduce disorder but it is harder to control given that onehas to find ways to stretch strain or shear 2D materialsin a controllable wayIn the case of graphene for instance chemical sub-

stitution of C by B or N can lead to a semimetal(graphene)11 to semiconductor (BN)12 transition openingdoors for band gap engineering of the material Moreoverhydrogenation13 and fluorination14 can be used for chemi-cal modification of various 2D materials Depending on theapplication one can use either single-sided or double-sidedfunctionalization Some processes (like hydrogenation forinstance) can be done on CVD or MBE grown materialsdirectly in a growth chamber For example it has beendemonstrated that a plasma beam deposition can be usedfor preparing monolayer graphene using graphane (iehydrogenated graphene13) as an intermediate compound15

The two-dimensional sheet-like nature of graphene isinherently compatible with organic composites because itsaromatic framework allows ndash cooperative interactionswith conjugated polymers In addition the work function

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Materials ExpressTwo-Dimensional Crystals Beyond GrapheneNeto and Novoselov

Persp

ective

Fig 3 Stretching graphene on top of a soft substrate using an AFM

of graphene (sim45 eV) enables ohmic hole injection intomost organic materials with comparable HOMO energylevels16 P-N junction type material may be developedfrom graphene-organic hybrids Extending from a host ofdesirable electrical properties the universal absorbance ofgraphene17 improves the broadband absorption propertiesof polymer composite film used as optical elements inlasers1819 When graphene is combined with organic opto-electronic materials it can give rise to enhanced non-linearoptical limiting properties due to photo-induced chargetransfer Moreover using graphene as a scaffold and chem-ically compatibility one can use self-assembly of conduct-ing polymers20 to create new soft crystal structures at thenanoscale with new functionalitiesMetal-insulator transitions can be obtained by uniax-

ial strain in graphene along the correct crystallographicdirection21 for strains fields in the excess of 20 (albeittoo close to the structural instability of the graphene sheet)On the other hand local uniaxial strain can produce trans-port gaps at much smaller values of strain opening doorsfor new concepts in device development which are notbased on a homogenous change of the structure but thecreation of internal interfaces in the material10

Three major methods can be used for strain engineeringglobal strain can be produced either by placing grapheneon stretchable substrate and stretching22 it is possible toexplore the lattice mismatch between different 2D crys-tals to create expandedcontracted lattices in sandwichedstructures23 in order to produce local strain24 one can usegraphene on soft substrate and AFM lithography to strainthe material locally (see Fig 3) In this way one can cre-ate devices that are based on the unique properties of thenovel 2D crystals such as valleytronics applications25

3 FINDING OUR WAY AROUND IN THEHIGH TECH JUNGLE

The ultimate goal of a technology based on 2D crystalsshould be the creation of artificial three-dimensional (3D)

materials by controlled multi-stacking of 2D platformseither raw or functionalized Depending on the applicationone can use one of the following methods(i) making ceramics from suspension of two or more 2Dmaterials(ii) direct CVD growth of monolayers of various materialson top of each other(iii) layer-by-layer transfer of grown 2D crystals to formsandwich structure

In terms of realistic applications this type of approachhas clear strategic advantages since one can use differ-ent starting points for different strategies (see Fig 2) Forexample certain strategies are well-known and have beenalready applied to a certain class of materials as is thecase of the growth of graphene by sublimation of SiC26

or CVD on metal surfaces In such cases material sci-entists and engineers are able to use these strategies andexplore their commercialization from the very beginningEach strategy will produce its own portfolio of productsMoreover it will be possible to apply similar strategies

to new materials (say creation of sandwiches of graphenewith BN to achieve higher electronic mobility) Theoristscan predict the possible outcomes of a strategy and pass itlater to experimental physicists and chemists for develop-ment Such approach unlike many others guarantees thesmooth acquisition of knowledge and the continuous pro-duction of materials and devices (unlike other approacheswhich result if ever in a commercially valuable technol-ogy only at the very end of the process)Using these strategies one can create artificial materi-

als with multiple functionalities that will allow their usein novel multi-tasking (mechanical optical and electronic)applications as for instance ldquosmartrdquo composites and coat-ings for flexible electronic and photovoltaics or photonicdevices for integrated optoelectronic circuits Among thosematerials that can have deep incursion in modern technol-ogy we highlight(1) ldquoSmartrdquo ultra-strong nano-composite materials withcontrolled properties(2) Electro-mechanical devices for ultra-fast electronics(3) Materials with predetermined band-gap and workfunctions for the next generation photovoltaic (solar-cells)applications(4) Atomically thin transparent gate tunable conductingelectrodes and field effect transistors

We stress that the real advantage of the approachdescribed here is that one is able to create materials withpredefined properties which can perform several func-tions (mechanically electronically and optically) simulta-neously Such materials (let alone devices based on them)are not available yet However as more and more function-ality is added to modern portable electronic we alreadysee a huge demand for such multi-functional multi-taskmaterials

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Materials ExpressTwo-Dimensional Crystals Beyond Graphene

Neto and NovoselovPersp

ective

4 ULTRA-STRONG NANO-COMPOSITEMATERIALS WITH CONTROLLEDPROPERTIES

Composite materials are omnipresent in technology andmany existing applications rely on light weight conductive(or insulating) and strong composites The performanceof such materials is however fundamentally limited bythe interaction between the filler and the matrix Henceout of great number of possible combinations only a fewreally work Moreover having a complex structure suchmaterials are subject to unpredictable failure It has beenrecently demonstrated that graphene can improve mechan-ical chemical and electrical properties of composite mate-rials dramatically Graphene is only one atom thick yet thestrongest material known to us As it has been shown onecan produce graphene of suitable dimensions and in largequantities very cheaplyChemical and micro-mechanical cleavage and artificial

growth can be used to obtain 2D crystals (graphene BNMoS2 etc) to be used as fillers in composite materialsUsing materials other than graphene allows one to expandthe functionality of such composites One can make themoptically active in various parts of the optical spectra byusing chemically modified graphene or materials with var-ious band gaps By creating semiconductor-metal andorsemiconductor-semimetal interfaces either in the form ofdispersed hetero-junctions or layered junctions efficientlight collection and charge transfer across the interface canbe achievedmdashgiving rise to the photovoltaic effect Theapplications of such materials include structurally strongplastics for construction engineering which have particularoptical characteristics (color certain optical transparencywindow polarizing effects photovoltaic properties etc)Such composite materials or coatings can be conductiveor insulating or ever have transistor properties (conduc-tivity and optical properties depends on environment likeexternal gating presence of certain gases humidity pHillumination etc)It has also been shown that graphenersquos Raman spectrum

is extremely sensitive to applied strain and that strain trans-fer between the matrix and the graphene is very efficient21

Hence it would be possible to create composite materialswhere accumulated stress could be monitored by contact-less non-invasive optical methods Such techniques canbe of crucial importance in certain areas of engineer-ing where catastrophic material failure is a major issueand where permanent monitoring of the performance ofa material is crucial (avionics electrical grids medicineetc)Surface functionalization can be used to fine-tune the

interaction between the filler and the matrix For instanceit has been shown that hydrophobic graphene can beturned hydrophilic by hydrogenation13 It is possible toapply similar strategies to other 2D materials to pro-duce high-performance composites with a wide range of

functionalities It is well accepted that carbon-based fillersimprove the performance of many composite materialsdramatically However only a small fraction of possiblematrices can bound effectively to carbon fibers nanotubesand graphene thus limiting the number of possible appli-cations By functionalizing graphene and other 2D crystals(which is possible without sacrificing their performance)one can make them work with a much wider range ofmatrices increasing the range of their applicability

5 ELECTRO-MECHANICAL DEVICES FORULTRA-FAST ELECTRONICS

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 mod-ern electronics and technology it also has its limitationsit is very difficult to modify and manipulate (to generatea new functionality one has to create a new crystal) thestructure might be not stable (for instance one can haveelectro-migration) or can be strongly modified by externalenvironmental conditions (such as radiation damage) thesame chemistry that is used to achieve the desired elec-tronic structure (for instance by doping) might cause detri-mental effects on other parameters (such as the decrease ofthe mobility due to scattering by dopants) In contrast 2Dmaterials open a new venue for engineering of the elec-tronic properties It has been demonstrated theoreticallyand experimentally that the electronic structure of 2Dmaterial can be considerably modified by strain shear andbending Moreover one achieves much better control onthose parameters in 2D as compared to 3D systems Thisis a completely new powerful paradigm which never (oronly very limited) has been explored before strong modi-fication of the electronic structure by strain engineering10

It has been shown theoretically that uniaxial and biaxialstrain applied along different crystallographic directionscan tune reversibly graphene from metallic to insulat-ing and can change the optical properties of grapheneMore complicated strain patterns can also be used to pro-duce gaps at smaller strain levels27 Recent experimentsof graphene grown on Platinum have shown that graphenenanobubbles that are under compressive strain can supportsemiconducting behavior with electronic gaps of the orderof a fraction of an electron volt23 Given that there is a one-to-one correspondence between structure and electronicscomputer modeling of these properties can play a funda-mental role in the development of new strain engineeredmaterials for electronics applicationsBand structure is also strongly influenced by stack-

ing A well-known example is graphite depending on thestacking order (Bernal versus rhombohedral versus hexag-onal versus turbostratic) its electronic properties can berather different28 The modification of the electronic bandstructure of twisted bilayer graphene29 is clearly reflected

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Materials ExpressTwo-Dimensional Crystals Beyond GrapheneNeto and Novoselov

Persp

ectivein its Raman spectrum through an electronndashphonon cou-pling and its optical conductivity30 Furthermore intercala-tion of layers of different atoms can change completely theelectronic states For instance graphite intercalated withalkali metals become superconducting31 By introducingdifferent 2D materials with different properties into a 3Dmatrix one is able to modify the final electronic propertiesA completely new field of exploration is the study of

multi-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 Theoretically this can be studied by com-puter modeling in 3D super-cells geometriesHere we would like to make an analogy with the

celebrated spintronics32 Devices which deal with spinrather than charge are extremely successful and allowfor novel modes of operation However the choice ofmaterials is very limited to ferromagnets Strain engineer-ing in 2D materials on the other hand would allow forvalleytronicsmdashwhere devices operate with valley degreeof freedom of quasiparticles rather than spin or chargeBecause of large choice of materials available and a pos-sibility for very precise control of strain such devicescould be much more versatile and allow for much moreadvanced complex electronic architectureIn order to build such systems one can use CVD meth-

ods to grow novel 2D materials which can have interestingelectronics (for instance Heusler alloys based on Ti) Onecan also grow multi-stacked materials (like graphene-BN-graphene) for high-performance transistor applicationsMobility in free-standing graphene or graphene on inertsubstrates can be as high as 106 cm2Vs which is verypromising for high-frequency transistor applications33

By arranging different 2D materials into stacks onewould be able to achieve new composite materials withnovel electronic mechanical and optical properties Wewould like to stress that such materials could be designedto be multifunctional performing several tasks simultane-ously (like harvesting light to feed electronic circuits madefrom the same material for instance) The typical mecha-nisms for the modification of the electronic bands in suchstacks are based on changing the symmetry of the structureand opening gaps in the electronic spectrum

6 NOVEL PHOTOVOLTAICS

It has been demonstrated that due to long mean-free pathand high Fermi velocity graphene can serve as an excel-lent light-to-current converter with quantum efficiencyreaching close to 10034 Its use for solar-cells applica-tions is however limited due to the low absorption of

graphene (the total efficiency is low) However efficientsolar-cells devices can be produced if graphene operateswith other 2D materialsAn example of such hybrid system would be network

(periodic array) of p-n junctions in monolayers of 2Dmaterials in planar geometry This would induce the wholesurface to participate in light-to-current conversion Onecan use either substitution doping during growth (forinstance B- and N-doping for C) or adatoms to create sucha net of p-n junctions In order to increase the efficiencymulti-stacked materials can be usedAnother parallel strategy would be to separate electron

and hole pairs into the neighboring layers of multi-layerstructures This can be achieved either by applying theexternal electric field or by selective doping of the two lay-ers (one n-type another p-type) Selective doping can beimplemented either during growth by substitutional dopingor by doping with adatoms Besides obvious applicationslike solar cells35 one could use such devices as positionsensitive photodetectors photo-voltage generating matri-ces with pixels which are controlled by a gate These mate-rials can be used in applications which allow and requirecombination of transparent electrodes and light harvestingIt has been shown that strain generates an electrostatic

potential36 which leads to the formation of local p-n junc-tions One can use this concept to create an array of p-njunctions This is a novel strategy would allow to useonly one material to form p-n junctions We expect it tobe much more stable than conventional organic solar-cellswhich traditionally use interfaces between different mate-rials Also the conversion level will depend on the strainlevel which might be used as new generation active pho-tovoltaic strain gaugesStacks of various 2D crystals (either the same materi-

als with different doping or completely different materials)can be used to form multi-layers One can use layers ofinsulating material (such as BN) to separate the conductivelayers for more efficient electronndashhole separation (to createweak tunneling barriers) One of the advantages of suchmaterials is the atomically sharp interfaces which wouldallow one to reduce scattering and improve the quantumyield of such devices

7 ATOMICALLY THIN TRANSPARENTTUNABLE FETs

It has been demonstrated that graphene absorbs only 23of light in a wide range of visible spectra17 This number iseven lower for chemically modified graphene and for someother 2D materials Combined with the excellent conduc-tivity properties these materials are extremely promisingfor transparent conductive coating applicationsOne can use local chemical modification to define con-

ductive and non-conductive areas to create atomically thinfilm transistors Another approach can be based on the

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Materials ExpressTwo-Dimensional Crystals Beyond Graphene

Neto and NovoselovPersp

ective

combination of graphene with other 2D sheets such as BNThe importance of BN as a substrate and gate dielectric forgraphene based FETs has been recently demonstrated31

The flatness of BN as well as its chemically inert natureleads to an at least one order of magnitude increase incarrier mobilityThe ability to realize crystalline BNgraphene layer

heterostructures is also of great interest for the estab-lishment of graphene as an efficient transparent conduct-ing electrode material (such as indium tin oxide ITO)since the sheet resistance of graphene will be signifi-cantly reduced when transferred on h-BN-coated glassThis approach can be important for a whole range of trans-parent electrode application including the incorporation ofgrapheneBNglass into solar cell platforms for enhancedpower conversion efficiency The target is to attain powerconversion efficiency that will be competitive with that ofITO-based solar devices h-BN can also be a chemicallytunable platform in terms of its stoichiometry thus lend-ing itself to band gap engineering where the incorporationof carbon in the BN matrix can form the hybrid BCNwith reduced band gap GrapheneBN or grapheneBCNheterostructure interfaces can be exploited for interfacialcharge segregation trapping and light emissionTwo types of approaches can be used to create

transparent conductive coating chemical exfoliation andepitaxial growth of graphene and other conductive materi-als Chemical exfoliation method is rather well developedand can be used for applications straight away The samemethod can be used in other conducting layered materi-als In this way it is possible to develop an infrastructurewhich would allow for large-area production of graphenedoped graphene and other materialsUniaxial strain breaks the symmetry of the lattice and

produces rotation of the planes of polarization37 Thiseffect is enhanced in an applied magnetic field Applica-tion of strain will thus allow for the production of tunablepolarizers in a broad band of optical frequencies By mul-tistacking such crystals one would be able to obtain mate-rials where optical properties are dependent on mechanicalstrain and stress and which can be used for a number ofphotonics applicationsOne can use chemical modification to define conduc-

tive and non-conductive areas for our atomically thin filmtransistors In this way one can prepare the basic elementsto create active displays which would be able to performa number of tasks (for instance combining LCD displaywith touch screens and some basic logic devices)

8 ECONOMIC PAYOFF

While a direct challenge to the existing technology is unre-alistic three key considerations are likely to pave the wayfor a gradual introduction of graphene and other 2D crys-tals as a material of choice for future device applications

cost multi-functionality and new markets We believe that2D material systems will first be viable for niche applica-tions and from there slowly find their way to mass marketconsumer electronics applications and beyond Here thekey to new functionalities will be their mechanical andoptical properties which in contrast to electronic proper-ties have not even remotely competing alternatives in exist-ing semiconducting materials such as Si or for that matterITO We have already discussed earlier the potential ofcomposite materialsAnother example of immediate economic impact will be

the replacement of ITO For this it is worthwhile to lookat the steep increase in ITO price from US$ 60ndash100 per kgonly a few years back to now well beyond US$1000 per kg Even without its mechanical propertiesgraphene is a low cost alternative which is highly desirableand will transform the transparent electrode material mar-ket in the very near future If we add to this the expectedexplosion in demand for flexible electronics we have atext-book-case where a new material leads to a completelynew market Equally important this material will play akey role in a whole range of other applicationsSimilar considerations hold for each strategy discussed

here and will lead to a product that can be commercial-ized Furthermore since each strategy is independent fromeach other one is guaranteed to have products as soon asthe production starts The key point is that as the degreeof sophistication in material preparation increases and onemoves up in complexity so does the risk and the payoffWhile raw 2D crystals can be readily produced chemi-cal functionalization especially with stoichiometric preci-sion depends on delicate balances of energy Moreoverstrain engineering is a field that while already provento work experimentally it is still in its infancy Finallymulti-stacking of functionalized materials was never testedbefore (multi-stacking of raw 2D crystals has already beendone to a certain extent but this is also a new field) Thusthe whole field of 2D crystals is an open territory (ora dense jungle depending on your point of view) Para-phrasing Isaac Newton we can say that we are still in theinfancy of this field of 2D crystals and diverting ourselveswith graphene a material that looks more interesting thanordinary whilst a great field of 2D crystals lay all undis-covered before us

Acknowledgments We would like to thank AndreGeim and our colleagues at NUS especially Yuan PingFeng Kian Ping Loh Barbaros Oumlzyilmaz Vitor Pereiraand Andrew Wee for many enlightening discussions

References and Notes

1 K S Novoselov A K Geim S V Morozov D Jiang Y ZhangS V Dubonos I V Grigorieva and A A Firsov Science 306 666(2004)

16 Mater Express Vol 1 2011

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IP 21021212060Tue 27 Dec 2011 055336

Materials ExpressTwo-Dimensional Crystals Beyond Graphene

Neto and NovoselovPersp

ective

Fig 2 Key strategies for the field of 2D crystals S1 Growth S2 functionalization and S3 combinatorial multi-stacking The various outcomesappear at the bottom of each process

production and realistic applications is because grapheneoffers really unique properties of which the most strikingis the 2D nature Many applications already in place havebeen waiting for a material like this for years Hence whengraphene became available a whole community of scien-tists and technologists reacted immediately The same canbe said about other 2D crystals when they become avail-able they will open doors for a plethora of possibilities

2 GROWING NURTURING AND TAMING2D CRYSTALS

With the fast developments in materials science in thelast few years it is possible to create various raw two-dimensional (2D) crystals by micro-mechanical (or chem-ical) exfoliation andor artificial growth by chemicalvapor deposition (CVD) and molecular beam epitaxy(MBE) Moreover there are methods for the high vacuumplasma beam deposition of crystals at low temperature(lt650 C) which is compatible with the MBE deposi-tion system (examples are graphene h-BN boron carboni-tride BCN on nickel-coated substrate using borazine orborazinemethane as the chemical precursor8)Equally important for the manufacturing of 2D crys-

tals as a bulk commodity is the establishment of largescale transfer facilities eg transfer on thin glass sheetsand (transparent) flexible polymer substrates It is possi-ble to set-up CVD furnaces for producing large sheets(gt30 sheets) of 2D crystals on catalyst foils such as Cuand Ni and a basic research type roll-to-roll transfer facil-ity which allows to transfer such large sheets of 2D crys-tals from the metal substrate to a flexible substrate for largescale applications34 This approach has been successfully

used to produce the first prototypes of graphene-basedtouch-screens It is conceivable to extend this process toform layered structures such that one can transfer CVDgrown graphene sheets to other atomically thin films sup-ported on flexible transparent polymer substrateThe electronic and structural properties of these raw 2D

crystals can be modified by methods such as chemicalfunctionalization9 and strain engineering10 in order to cre-ate artificial 2D materials with new functionalities Eachone of these methods has its pros and cons Chemical func-tionalization has the advantage that one has a direct effecton the properties of carbon However this method usuallyintroduces disorder in the system that can be detrimentalto the electronic properties Strain engineering does notintroduce disorder but it is harder to control given that onehas to find ways to stretch strain or shear 2D materialsin a controllable wayIn the case of graphene for instance chemical sub-

stitution of C by B or N can lead to a semimetal(graphene)11 to semiconductor (BN)12 transition openingdoors for band gap engineering of the material Moreoverhydrogenation13 and fluorination14 can be used for chemi-cal modification of various 2D materials Depending on theapplication one can use either single-sided or double-sidedfunctionalization Some processes (like hydrogenation forinstance) can be done on CVD or MBE grown materialsdirectly in a growth chamber For example it has beendemonstrated that a plasma beam deposition can be usedfor preparing monolayer graphene using graphane (iehydrogenated graphene13) as an intermediate compound15

The two-dimensional sheet-like nature of graphene isinherently compatible with organic composites because itsaromatic framework allows ndash cooperative interactionswith conjugated polymers In addition the work function

12 Mater Express Vol 1 2011

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IP 21021212060Tue 27 Dec 2011 055336

Materials ExpressTwo-Dimensional Crystals Beyond GrapheneNeto and Novoselov

Persp

ective

Fig 3 Stretching graphene on top of a soft substrate using an AFM

of graphene (sim45 eV) enables ohmic hole injection intomost organic materials with comparable HOMO energylevels16 P-N junction type material may be developedfrom graphene-organic hybrids Extending from a host ofdesirable electrical properties the universal absorbance ofgraphene17 improves the broadband absorption propertiesof polymer composite film used as optical elements inlasers1819 When graphene is combined with organic opto-electronic materials it can give rise to enhanced non-linearoptical limiting properties due to photo-induced chargetransfer Moreover using graphene as a scaffold and chem-ically compatibility one can use self-assembly of conduct-ing polymers20 to create new soft crystal structures at thenanoscale with new functionalitiesMetal-insulator transitions can be obtained by uniax-

ial strain in graphene along the correct crystallographicdirection21 for strains fields in the excess of 20 (albeittoo close to the structural instability of the graphene sheet)On the other hand local uniaxial strain can produce trans-port gaps at much smaller values of strain opening doorsfor new concepts in device development which are notbased on a homogenous change of the structure but thecreation of internal interfaces in the material10

Three major methods can be used for strain engineeringglobal strain can be produced either by placing grapheneon stretchable substrate and stretching22 it is possible toexplore the lattice mismatch between different 2D crys-tals to create expandedcontracted lattices in sandwichedstructures23 in order to produce local strain24 one can usegraphene on soft substrate and AFM lithography to strainthe material locally (see Fig 3) In this way one can cre-ate devices that are based on the unique properties of thenovel 2D crystals such as valleytronics applications25

3 FINDING OUR WAY AROUND IN THEHIGH TECH JUNGLE

The ultimate goal of a technology based on 2D crystalsshould be the creation of artificial three-dimensional (3D)

materials by controlled multi-stacking of 2D platformseither raw or functionalized Depending on the applicationone can use one of the following methods(i) making ceramics from suspension of two or more 2Dmaterials(ii) direct CVD growth of monolayers of various materialson top of each other(iii) layer-by-layer transfer of grown 2D crystals to formsandwich structure

In terms of realistic applications this type of approachhas clear strategic advantages since one can use differ-ent starting points for different strategies (see Fig 2) Forexample certain strategies are well-known and have beenalready applied to a certain class of materials as is thecase of the growth of graphene by sublimation of SiC26

or CVD on metal surfaces In such cases material sci-entists and engineers are able to use these strategies andexplore their commercialization from the very beginningEach strategy will produce its own portfolio of productsMoreover it will be possible to apply similar strategies

to new materials (say creation of sandwiches of graphenewith BN to achieve higher electronic mobility) Theoristscan predict the possible outcomes of a strategy and pass itlater to experimental physicists and chemists for develop-ment Such approach unlike many others guarantees thesmooth acquisition of knowledge and the continuous pro-duction of materials and devices (unlike other approacheswhich result if ever in a commercially valuable technol-ogy only at the very end of the process)Using these strategies one can create artificial materi-

als with multiple functionalities that will allow their usein novel multi-tasking (mechanical optical and electronic)applications as for instance ldquosmartrdquo composites and coat-ings for flexible electronic and photovoltaics or photonicdevices for integrated optoelectronic circuits Among thosematerials that can have deep incursion in modern technol-ogy we highlight(1) ldquoSmartrdquo ultra-strong nano-composite materials withcontrolled properties(2) Electro-mechanical devices for ultra-fast electronics(3) Materials with predetermined band-gap and workfunctions for the next generation photovoltaic (solar-cells)applications(4) Atomically thin transparent gate tunable conductingelectrodes and field effect transistors

We stress that the real advantage of the approachdescribed here is that one is able to create materials withpredefined properties which can perform several func-tions (mechanically electronically and optically) simulta-neously Such materials (let alone devices based on them)are not available yet However as more and more function-ality is added to modern portable electronic we alreadysee a huge demand for such multi-functional multi-taskmaterials

Mater Express Vol 1 2011 13

Delivered by Ingenta toGuest User

IP 21021212060Tue 27 Dec 2011 055336

Materials ExpressTwo-Dimensional Crystals Beyond Graphene

Neto and NovoselovPersp

ective

4 ULTRA-STRONG NANO-COMPOSITEMATERIALS WITH CONTROLLEDPROPERTIES

Composite materials are omnipresent in technology andmany existing applications rely on light weight conductive(or insulating) and strong composites The performanceof such materials is however fundamentally limited bythe interaction between the filler and the matrix Henceout of great number of possible combinations only a fewreally work Moreover having a complex structure suchmaterials are subject to unpredictable failure It has beenrecently demonstrated that graphene can improve mechan-ical chemical and electrical properties of composite mate-rials dramatically Graphene is only one atom thick yet thestrongest material known to us As it has been shown onecan produce graphene of suitable dimensions and in largequantities very cheaplyChemical and micro-mechanical cleavage and artificial

growth can be used to obtain 2D crystals (graphene BNMoS2 etc) to be used as fillers in composite materialsUsing materials other than graphene allows one to expandthe functionality of such composites One can make themoptically active in various parts of the optical spectra byusing chemically modified graphene or materials with var-ious band gaps By creating semiconductor-metal andorsemiconductor-semimetal interfaces either in the form ofdispersed hetero-junctions or layered junctions efficientlight collection and charge transfer across the interface canbe achievedmdashgiving rise to the photovoltaic effect Theapplications of such materials include structurally strongplastics for construction engineering which have particularoptical characteristics (color certain optical transparencywindow polarizing effects photovoltaic properties etc)Such composite materials or coatings can be conductiveor insulating or ever have transistor properties (conduc-tivity and optical properties depends on environment likeexternal gating presence of certain gases humidity pHillumination etc)It has also been shown that graphenersquos Raman spectrum

is extremely sensitive to applied strain and that strain trans-fer between the matrix and the graphene is very efficient21

Hence it would be possible to create composite materialswhere accumulated stress could be monitored by contact-less non-invasive optical methods Such techniques canbe of crucial importance in certain areas of engineer-ing where catastrophic material failure is a major issueand where permanent monitoring of the performance ofa material is crucial (avionics electrical grids medicineetc)Surface functionalization can be used to fine-tune the

interaction between the filler and the matrix For instanceit has been shown that hydrophobic graphene can beturned hydrophilic by hydrogenation13 It is possible toapply similar strategies to other 2D materials to pro-duce high-performance composites with a wide range of

functionalities It is well accepted that carbon-based fillersimprove the performance of many composite materialsdramatically However only a small fraction of possiblematrices can bound effectively to carbon fibers nanotubesand graphene thus limiting the number of possible appli-cations By functionalizing graphene and other 2D crystals(which is possible without sacrificing their performance)one can make them work with a much wider range ofmatrices increasing the range of their applicability

5 ELECTRO-MECHANICAL DEVICES FORULTRA-FAST ELECTRONICS

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 mod-ern electronics and technology it also has its limitationsit is very difficult to modify and manipulate (to generatea new functionality one has to create a new crystal) thestructure might be not stable (for instance one can haveelectro-migration) or can be strongly modified by externalenvironmental conditions (such as radiation damage) thesame chemistry that is used to achieve the desired elec-tronic structure (for instance by doping) might cause detri-mental effects on other parameters (such as the decrease ofthe mobility due to scattering by dopants) In contrast 2Dmaterials open a new venue for engineering of the elec-tronic properties It has been demonstrated theoreticallyand experimentally that the electronic structure of 2Dmaterial can be considerably modified by strain shear andbending Moreover one achieves much better control onthose parameters in 2D as compared to 3D systems Thisis a completely new powerful paradigm which never (oronly very limited) has been explored before strong modi-fication of the electronic structure by strain engineering10

It has been shown theoretically that uniaxial and biaxialstrain applied along different crystallographic directionscan tune reversibly graphene from metallic to insulat-ing and can change the optical properties of grapheneMore complicated strain patterns can also be used to pro-duce gaps at smaller strain levels27 Recent experimentsof graphene grown on Platinum have shown that graphenenanobubbles that are under compressive strain can supportsemiconducting behavior with electronic gaps of the orderof a fraction of an electron volt23 Given that there is a one-to-one correspondence between structure and electronicscomputer modeling of these properties can play a funda-mental role in the development of new strain engineeredmaterials for electronics applicationsBand structure is also strongly influenced by stack-

ing A well-known example is graphite depending on thestacking order (Bernal versus rhombohedral versus hexag-onal versus turbostratic) its electronic properties can berather different28 The modification of the electronic bandstructure of twisted bilayer graphene29 is clearly reflected

14 Mater Express Vol 1 2011

Delivered by Ingenta toGuest User

IP 21021212060Tue 27 Dec 2011 055336

Materials ExpressTwo-Dimensional Crystals Beyond GrapheneNeto and Novoselov

Persp

ectivein its Raman spectrum through an electronndashphonon cou-pling and its optical conductivity30 Furthermore intercala-tion of layers of different atoms can change completely theelectronic states For instance graphite intercalated withalkali metals become superconducting31 By introducingdifferent 2D materials with different properties into a 3Dmatrix one is able to modify the final electronic propertiesA completely new field of exploration is the study of

multi-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 Theoretically this can be studied by com-puter modeling in 3D super-cells geometriesHere we would like to make an analogy with the

celebrated spintronics32 Devices which deal with spinrather than charge are extremely successful and allowfor novel modes of operation However the choice ofmaterials is very limited to ferromagnets Strain engineer-ing in 2D materials on the other hand would allow forvalleytronicsmdashwhere devices operate with valley degreeof freedom of quasiparticles rather than spin or chargeBecause of large choice of materials available and a pos-sibility for very precise control of strain such devicescould be much more versatile and allow for much moreadvanced complex electronic architectureIn order to build such systems one can use CVD meth-

ods to grow novel 2D materials which can have interestingelectronics (for instance Heusler alloys based on Ti) Onecan also grow multi-stacked materials (like graphene-BN-graphene) for high-performance transistor applicationsMobility in free-standing graphene or graphene on inertsubstrates can be as high as 106 cm2Vs which is verypromising for high-frequency transistor applications33

By arranging different 2D materials into stacks onewould be able to achieve new composite materials withnovel electronic mechanical and optical properties Wewould like to stress that such materials could be designedto be multifunctional performing several tasks simultane-ously (like harvesting light to feed electronic circuits madefrom the same material for instance) The typical mecha-nisms for the modification of the electronic bands in suchstacks are based on changing the symmetry of the structureand opening gaps in the electronic spectrum

6 NOVEL PHOTOVOLTAICS

It has been demonstrated that due to long mean-free pathand high Fermi velocity graphene can serve as an excel-lent light-to-current converter with quantum efficiencyreaching close to 10034 Its use for solar-cells applica-tions is however limited due to the low absorption of

graphene (the total efficiency is low) However efficientsolar-cells devices can be produced if graphene operateswith other 2D materialsAn example of such hybrid system would be network

(periodic array) of p-n junctions in monolayers of 2Dmaterials in planar geometry This would induce the wholesurface to participate in light-to-current conversion Onecan use either substitution doping during growth (forinstance B- and N-doping for C) or adatoms to create sucha net of p-n junctions In order to increase the efficiencymulti-stacked materials can be usedAnother parallel strategy would be to separate electron

and hole pairs into the neighboring layers of multi-layerstructures This can be achieved either by applying theexternal electric field or by selective doping of the two lay-ers (one n-type another p-type) Selective doping can beimplemented either during growth by substitutional dopingor by doping with adatoms Besides obvious applicationslike solar cells35 one could use such devices as positionsensitive photodetectors photo-voltage generating matri-ces with pixels which are controlled by a gate These mate-rials can be used in applications which allow and requirecombination of transparent electrodes and light harvestingIt has been shown that strain generates an electrostatic

potential36 which leads to the formation of local p-n junc-tions One can use this concept to create an array of p-njunctions This is a novel strategy would allow to useonly one material to form p-n junctions We expect it tobe much more stable than conventional organic solar-cellswhich traditionally use interfaces between different mate-rials Also the conversion level will depend on the strainlevel which might be used as new generation active pho-tovoltaic strain gaugesStacks of various 2D crystals (either the same materi-

als with different doping or completely different materials)can be used to form multi-layers One can use layers ofinsulating material (such as BN) to separate the conductivelayers for more efficient electronndashhole separation (to createweak tunneling barriers) One of the advantages of suchmaterials is the atomically sharp interfaces which wouldallow one to reduce scattering and improve the quantumyield of such devices

7 ATOMICALLY THIN TRANSPARENTTUNABLE FETs

It has been demonstrated that graphene absorbs only 23of light in a wide range of visible spectra17 This number iseven lower for chemically modified graphene and for someother 2D materials Combined with the excellent conduc-tivity properties these materials are extremely promisingfor transparent conductive coating applicationsOne can use local chemical modification to define con-

ductive and non-conductive areas to create atomically thinfilm transistors Another approach can be based on the

Mater Express Vol 1 2011 15

Delivered by Ingenta toGuest User

IP 21021212060Tue 27 Dec 2011 055336

Materials ExpressTwo-Dimensional Crystals Beyond Graphene

Neto and NovoselovPersp

ective

combination of graphene with other 2D sheets such as BNThe importance of BN as a substrate and gate dielectric forgraphene based FETs has been recently demonstrated31

The flatness of BN as well as its chemically inert natureleads to an at least one order of magnitude increase incarrier mobilityThe ability to realize crystalline BNgraphene layer

heterostructures is also of great interest for the estab-lishment of graphene as an efficient transparent conduct-ing electrode material (such as indium tin oxide ITO)since the sheet resistance of graphene will be signifi-cantly reduced when transferred on h-BN-coated glassThis approach can be important for a whole range of trans-parent electrode application including the incorporation ofgrapheneBNglass into solar cell platforms for enhancedpower conversion efficiency The target is to attain powerconversion efficiency that will be competitive with that ofITO-based solar devices h-BN can also be a chemicallytunable platform in terms of its stoichiometry thus lend-ing itself to band gap engineering where the incorporationof carbon in the BN matrix can form the hybrid BCNwith reduced band gap GrapheneBN or grapheneBCNheterostructure interfaces can be exploited for interfacialcharge segregation trapping and light emissionTwo types of approaches can be used to create

transparent conductive coating chemical exfoliation andepitaxial growth of graphene and other conductive materi-als Chemical exfoliation method is rather well developedand can be used for applications straight away The samemethod can be used in other conducting layered materi-als In this way it is possible to develop an infrastructurewhich would allow for large-area production of graphenedoped graphene and other materialsUniaxial strain breaks the symmetry of the lattice and

produces rotation of the planes of polarization37 Thiseffect is enhanced in an applied magnetic field Applica-tion of strain will thus allow for the production of tunablepolarizers in a broad band of optical frequencies By mul-tistacking such crystals one would be able to obtain mate-rials where optical properties are dependent on mechanicalstrain and stress and which can be used for a number ofphotonics applicationsOne can use chemical modification to define conduc-

tive and non-conductive areas for our atomically thin filmtransistors In this way one can prepare the basic elementsto create active displays which would be able to performa number of tasks (for instance combining LCD displaywith touch screens and some basic logic devices)

8 ECONOMIC PAYOFF

While a direct challenge to the existing technology is unre-alistic three key considerations are likely to pave the wayfor a gradual introduction of graphene and other 2D crys-tals as a material of choice for future device applications

cost multi-functionality and new markets We believe that2D material systems will first be viable for niche applica-tions and from there slowly find their way to mass marketconsumer electronics applications and beyond Here thekey to new functionalities will be their mechanical andoptical properties which in contrast to electronic proper-ties have not even remotely competing alternatives in exist-ing semiconducting materials such as Si or for that matterITO We have already discussed earlier the potential ofcomposite materialsAnother example of immediate economic impact will be

the replacement of ITO For this it is worthwhile to lookat the steep increase in ITO price from US$ 60ndash100 per kgonly a few years back to now well beyond US$1000 per kg Even without its mechanical propertiesgraphene is a low cost alternative which is highly desirableand will transform the transparent electrode material mar-ket in the very near future If we add to this the expectedexplosion in demand for flexible electronics we have atext-book-case where a new material leads to a completelynew market Equally important this material will play akey role in a whole range of other applicationsSimilar considerations hold for each strategy discussed

here and will lead to a product that can be commercial-ized Furthermore since each strategy is independent fromeach other one is guaranteed to have products as soon asthe production starts The key point is that as the degreeof sophistication in material preparation increases and onemoves up in complexity so does the risk and the payoffWhile raw 2D crystals can be readily produced chemi-cal functionalization especially with stoichiometric preci-sion depends on delicate balances of energy Moreoverstrain engineering is a field that while already provento work experimentally it is still in its infancy Finallymulti-stacking of functionalized materials was never testedbefore (multi-stacking of raw 2D crystals has already beendone to a certain extent but this is also a new field) Thusthe whole field of 2D crystals is an open territory (ora dense jungle depending on your point of view) Para-phrasing Isaac Newton we can say that we are still in theinfancy of this field of 2D crystals and diverting ourselveswith graphene a material that looks more interesting thanordinary whilst a great field of 2D crystals lay all undis-covered before us

Acknowledgments We would like to thank AndreGeim and our colleagues at NUS especially Yuan PingFeng Kian Ping Loh Barbaros Oumlzyilmaz Vitor Pereiraand Andrew Wee for many enlightening discussions

References and Notes

1 K S Novoselov A K Geim S V Morozov D Jiang Y ZhangS V Dubonos I V Grigorieva and A A Firsov Science 306 666(2004)

16 Mater Express Vol 1 2011

Delivered by Ingenta toGuest User

IP 21021212060Tue 27 Dec 2011 055336

Materials ExpressTwo-Dimensional Crystals Beyond GrapheneNeto and Novoselov

Persp

ective

Fig 3 Stretching graphene on top of a soft substrate using an AFM

of graphene (sim45 eV) enables ohmic hole injection intomost organic materials with comparable HOMO energylevels16 P-N junction type material may be developedfrom graphene-organic hybrids Extending from a host ofdesirable electrical properties the universal absorbance ofgraphene17 improves the broadband absorption propertiesof polymer composite film used as optical elements inlasers1819 When graphene is combined with organic opto-electronic materials it can give rise to enhanced non-linearoptical limiting properties due to photo-induced chargetransfer Moreover using graphene as a scaffold and chem-ically compatibility one can use self-assembly of conduct-ing polymers20 to create new soft crystal structures at thenanoscale with new functionalitiesMetal-insulator transitions can be obtained by uniax-

ial strain in graphene along the correct crystallographicdirection21 for strains fields in the excess of 20 (albeittoo close to the structural instability of the graphene sheet)On the other hand local uniaxial strain can produce trans-port gaps at much smaller values of strain opening doorsfor new concepts in device development which are notbased on a homogenous change of the structure but thecreation of internal interfaces in the material10

Three major methods can be used for strain engineeringglobal strain can be produced either by placing grapheneon stretchable substrate and stretching22 it is possible toexplore the lattice mismatch between different 2D crys-tals to create expandedcontracted lattices in sandwichedstructures23 in order to produce local strain24 one can usegraphene on soft substrate and AFM lithography to strainthe material locally (see Fig 3) In this way one can cre-ate devices that are based on the unique properties of thenovel 2D crystals such as valleytronics applications25

3 FINDING OUR WAY AROUND IN THEHIGH TECH JUNGLE

The ultimate goal of a technology based on 2D crystalsshould be the creation of artificial three-dimensional (3D)

materials by controlled multi-stacking of 2D platformseither raw or functionalized Depending on the applicationone can use one of the following methods(i) making ceramics from suspension of two or more 2Dmaterials(ii) direct CVD growth of monolayers of various materialson top of each other(iii) layer-by-layer transfer of grown 2D crystals to formsandwich structure

In terms of realistic applications this type of approachhas clear strategic advantages since one can use differ-ent starting points for different strategies (see Fig 2) Forexample certain strategies are well-known and have beenalready applied to a certain class of materials as is thecase of the growth of graphene by sublimation of SiC26

or CVD on metal surfaces In such cases material sci-entists and engineers are able to use these strategies andexplore their commercialization from the very beginningEach strategy will produce its own portfolio of productsMoreover it will be possible to apply similar strategies

to new materials (say creation of sandwiches of graphenewith BN to achieve higher electronic mobility) Theoristscan predict the possible outcomes of a strategy and pass itlater to experimental physicists and chemists for develop-ment Such approach unlike many others guarantees thesmooth acquisition of knowledge and the continuous pro-duction of materials and devices (unlike other approacheswhich result if ever in a commercially valuable technol-ogy only at the very end of the process)Using these strategies one can create artificial materi-

als with multiple functionalities that will allow their usein novel multi-tasking (mechanical optical and electronic)applications as for instance ldquosmartrdquo composites and coat-ings for flexible electronic and photovoltaics or photonicdevices for integrated optoelectronic circuits Among thosematerials that can have deep incursion in modern technol-ogy we highlight(1) ldquoSmartrdquo ultra-strong nano-composite materials withcontrolled properties(2) Electro-mechanical devices for ultra-fast electronics(3) Materials with predetermined band-gap and workfunctions for the next generation photovoltaic (solar-cells)applications(4) Atomically thin transparent gate tunable conductingelectrodes and field effect transistors

We stress that the real advantage of the approachdescribed here is that one is able to create materials withpredefined properties which can perform several func-tions (mechanically electronically and optically) simulta-neously Such materials (let alone devices based on them)are not available yet However as more and more function-ality is added to modern portable electronic we alreadysee a huge demand for such multi-functional multi-taskmaterials

Mater Express Vol 1 2011 13

Delivered by Ingenta toGuest User

IP 21021212060Tue 27 Dec 2011 055336

Materials ExpressTwo-Dimensional Crystals Beyond Graphene

Neto and NovoselovPersp

ective

4 ULTRA-STRONG NANO-COMPOSITEMATERIALS WITH CONTROLLEDPROPERTIES

Composite materials are omnipresent in technology andmany existing applications rely on light weight conductive(or insulating) and strong composites The performanceof such materials is however fundamentally limited bythe interaction between the filler and the matrix Henceout of great number of possible combinations only a fewreally work Moreover having a complex structure suchmaterials are subject to unpredictable failure It has beenrecently demonstrated that graphene can improve mechan-ical chemical and electrical properties of composite mate-rials dramatically Graphene is only one atom thick yet thestrongest material known to us As it has been shown onecan produce graphene of suitable dimensions and in largequantities very cheaplyChemical and micro-mechanical cleavage and artificial

growth can be used to obtain 2D crystals (graphene BNMoS2 etc) to be used as fillers in composite materialsUsing materials other than graphene allows one to expandthe functionality of such composites One can make themoptically active in various parts of the optical spectra byusing chemically modified graphene or materials with var-ious band gaps By creating semiconductor-metal andorsemiconductor-semimetal interfaces either in the form ofdispersed hetero-junctions or layered junctions efficientlight collection and charge transfer across the interface canbe achievedmdashgiving rise to the photovoltaic effect Theapplications of such materials include structurally strongplastics for construction engineering which have particularoptical characteristics (color certain optical transparencywindow polarizing effects photovoltaic properties etc)Such composite materials or coatings can be conductiveor insulating or ever have transistor properties (conduc-tivity and optical properties depends on environment likeexternal gating presence of certain gases humidity pHillumination etc)It has also been shown that graphenersquos Raman spectrum

is extremely sensitive to applied strain and that strain trans-fer between the matrix and the graphene is very efficient21

Hence it would be possible to create composite materialswhere accumulated stress could be monitored by contact-less non-invasive optical methods Such techniques canbe of crucial importance in certain areas of engineer-ing where catastrophic material failure is a major issueand where permanent monitoring of the performance ofa material is crucial (avionics electrical grids medicineetc)Surface functionalization can be used to fine-tune the

interaction between the filler and the matrix For instanceit has been shown that hydrophobic graphene can beturned hydrophilic by hydrogenation13 It is possible toapply similar strategies to other 2D materials to pro-duce high-performance composites with a wide range of

functionalities It is well accepted that carbon-based fillersimprove the performance of many composite materialsdramatically However only a small fraction of possiblematrices can bound effectively to carbon fibers nanotubesand graphene thus limiting the number of possible appli-cations By functionalizing graphene and other 2D crystals(which is possible without sacrificing their performance)one can make them work with a much wider range ofmatrices increasing the range of their applicability

5 ELECTRO-MECHANICAL DEVICES FORULTRA-FAST ELECTRONICS

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 mod-ern electronics and technology it also has its limitationsit is very difficult to modify and manipulate (to generatea new functionality one has to create a new crystal) thestructure might be not stable (for instance one can haveelectro-migration) or can be strongly modified by externalenvironmental conditions (such as radiation damage) thesame chemistry that is used to achieve the desired elec-tronic structure (for instance by doping) might cause detri-mental effects on other parameters (such as the decrease ofthe mobility due to scattering by dopants) In contrast 2Dmaterials open a new venue for engineering of the elec-tronic properties It has been demonstrated theoreticallyand experimentally that the electronic structure of 2Dmaterial can be considerably modified by strain shear andbending Moreover one achieves much better control onthose parameters in 2D as compared to 3D systems Thisis a completely new powerful paradigm which never (oronly very limited) has been explored before strong modi-fication of the electronic structure by strain engineering10

It has been shown theoretically that uniaxial and biaxialstrain applied along different crystallographic directionscan tune reversibly graphene from metallic to insulat-ing and can change the optical properties of grapheneMore complicated strain patterns can also be used to pro-duce gaps at smaller strain levels27 Recent experimentsof graphene grown on Platinum have shown that graphenenanobubbles that are under compressive strain can supportsemiconducting behavior with electronic gaps of the orderof a fraction of an electron volt23 Given that there is a one-to-one correspondence between structure and electronicscomputer modeling of these properties can play a funda-mental role in the development of new strain engineeredmaterials for electronics applicationsBand structure is also strongly influenced by stack-

ing A well-known example is graphite depending on thestacking order (Bernal versus rhombohedral versus hexag-onal versus turbostratic) its electronic properties can berather different28 The modification of the electronic bandstructure of twisted bilayer graphene29 is clearly reflected

14 Mater Express Vol 1 2011

Delivered by Ingenta toGuest User

IP 21021212060Tue 27 Dec 2011 055336

Materials ExpressTwo-Dimensional Crystals Beyond GrapheneNeto and Novoselov

Persp

ectivein its Raman spectrum through an electronndashphonon cou-pling and its optical conductivity30 Furthermore intercala-tion of layers of different atoms can change completely theelectronic states For instance graphite intercalated withalkali metals become superconducting31 By introducingdifferent 2D materials with different properties into a 3Dmatrix one is able to modify the final electronic propertiesA completely new field of exploration is the study of

multi-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 Theoretically this can be studied by com-puter modeling in 3D super-cells geometriesHere we would like to make an analogy with the

celebrated spintronics32 Devices which deal with spinrather than charge are extremely successful and allowfor novel modes of operation However the choice ofmaterials is very limited to ferromagnets Strain engineer-ing in 2D materials on the other hand would allow forvalleytronicsmdashwhere devices operate with valley degreeof freedom of quasiparticles rather than spin or chargeBecause of large choice of materials available and a pos-sibility for very precise control of strain such devicescould be much more versatile and allow for much moreadvanced complex electronic architectureIn order to build such systems one can use CVD meth-

ods to grow novel 2D materials which can have interestingelectronics (for instance Heusler alloys based on Ti) Onecan also grow multi-stacked materials (like graphene-BN-graphene) for high-performance transistor applicationsMobility in free-standing graphene or graphene on inertsubstrates can be as high as 106 cm2Vs which is verypromising for high-frequency transistor applications33

By arranging different 2D materials into stacks onewould be able to achieve new composite materials withnovel electronic mechanical and optical properties Wewould like to stress that such materials could be designedto be multifunctional performing several tasks simultane-ously (like harvesting light to feed electronic circuits madefrom the same material for instance) The typical mecha-nisms for the modification of the electronic bands in suchstacks are based on changing the symmetry of the structureand opening gaps in the electronic spectrum

6 NOVEL PHOTOVOLTAICS

It has been demonstrated that due to long mean-free pathand high Fermi velocity graphene can serve as an excel-lent light-to-current converter with quantum efficiencyreaching close to 10034 Its use for solar-cells applica-tions is however limited due to the low absorption of

graphene (the total efficiency is low) However efficientsolar-cells devices can be produced if graphene operateswith other 2D materialsAn example of such hybrid system would be network

(periodic array) of p-n junctions in monolayers of 2Dmaterials in planar geometry This would induce the wholesurface to participate in light-to-current conversion Onecan use either substitution doping during growth (forinstance B- and N-doping for C) or adatoms to create sucha net of p-n junctions In order to increase the efficiencymulti-stacked materials can be usedAnother parallel strategy would be to separate electron

and hole pairs into the neighboring layers of multi-layerstructures This can be achieved either by applying theexternal electric field or by selective doping of the two lay-ers (one n-type another p-type) Selective doping can beimplemented either during growth by substitutional dopingor by doping with adatoms Besides obvious applicationslike solar cells35 one could use such devices as positionsensitive photodetectors photo-voltage generating matri-ces with pixels which are controlled by a gate These mate-rials can be used in applications which allow and requirecombination of transparent electrodes and light harvestingIt has been shown that strain generates an electrostatic

potential36 which leads to the formation of local p-n junc-tions One can use this concept to create an array of p-njunctions This is a novel strategy would allow to useonly one material to form p-n junctions We expect it tobe much more stable than conventional organic solar-cellswhich traditionally use interfaces between different mate-rials Also the conversion level will depend on the strainlevel which might be used as new generation active pho-tovoltaic strain gaugesStacks of various 2D crystals (either the same materi-

als with different doping or completely different materials)can be used to form multi-layers One can use layers ofinsulating material (such as BN) to separate the conductivelayers for more efficient electronndashhole separation (to createweak tunneling barriers) One of the advantages of suchmaterials is the atomically sharp interfaces which wouldallow one to reduce scattering and improve the quantumyield of such devices

7 ATOMICALLY THIN TRANSPARENTTUNABLE FETs

It has been demonstrated that graphene absorbs only 23of light in a wide range of visible spectra17 This number iseven lower for chemically modified graphene and for someother 2D materials Combined with the excellent conduc-tivity properties these materials are extremely promisingfor transparent conductive coating applicationsOne can use local chemical modification to define con-

ductive and non-conductive areas to create atomically thinfilm transistors Another approach can be based on the

Mater Express Vol 1 2011 15

Delivered by Ingenta toGuest User

IP 21021212060Tue 27 Dec 2011 055336

Materials ExpressTwo-Dimensional Crystals Beyond Graphene

Neto and NovoselovPersp

ective

combination of graphene with other 2D sheets such as BNThe importance of BN as a substrate and gate dielectric forgraphene based FETs has been recently demonstrated31

The flatness of BN as well as its chemically inert natureleads to an at least one order of magnitude increase incarrier mobilityThe ability to realize crystalline BNgraphene layer

heterostructures is also of great interest for the estab-lishment of graphene as an efficient transparent conduct-ing electrode material (such as indium tin oxide ITO)since the sheet resistance of graphene will be signifi-cantly reduced when transferred on h-BN-coated glassThis approach can be important for a whole range of trans-parent electrode application including the incorporation ofgrapheneBNglass into solar cell platforms for enhancedpower conversion efficiency The target is to attain powerconversion efficiency that will be competitive with that ofITO-based solar devices h-BN can also be a chemicallytunable platform in terms of its stoichiometry thus lend-ing itself to band gap engineering where the incorporationof carbon in the BN matrix can form the hybrid BCNwith reduced band gap GrapheneBN or grapheneBCNheterostructure interfaces can be exploited for interfacialcharge segregation trapping and light emissionTwo types of approaches can be used to create

transparent conductive coating chemical exfoliation andepitaxial growth of graphene and other conductive materi-als Chemical exfoliation method is rather well developedand can be used for applications straight away The samemethod can be used in other conducting layered materi-als In this way it is possible to develop an infrastructurewhich would allow for large-area production of graphenedoped graphene and other materialsUniaxial strain breaks the symmetry of the lattice and

produces rotation of the planes of polarization37 Thiseffect is enhanced in an applied magnetic field Applica-tion of strain will thus allow for the production of tunablepolarizers in a broad band of optical frequencies By mul-tistacking such crystals one would be able to obtain mate-rials where optical properties are dependent on mechanicalstrain and stress and which can be used for a number ofphotonics applicationsOne can use chemical modification to define conduc-

tive and non-conductive areas for our atomically thin filmtransistors In this way one can prepare the basic elementsto create active displays which would be able to performa number of tasks (for instance combining LCD displaywith touch screens and some basic logic devices)

8 ECONOMIC PAYOFF

While a direct challenge to the existing technology is unre-alistic three key considerations are likely to pave the wayfor a gradual introduction of graphene and other 2D crys-tals as a material of choice for future device applications

cost multi-functionality and new markets We believe that2D material systems will first be viable for niche applica-tions and from there slowly find their way to mass marketconsumer electronics applications and beyond Here thekey to new functionalities will be their mechanical andoptical properties which in contrast to electronic proper-ties have not even remotely competing alternatives in exist-ing semiconducting materials such as Si or for that matterITO We have already discussed earlier the potential ofcomposite materialsAnother example of immediate economic impact will be

the replacement of ITO For this it is worthwhile to lookat the steep increase in ITO price from US$ 60ndash100 per kgonly a few years back to now well beyond US$1000 per kg Even without its mechanical propertiesgraphene is a low cost alternative which is highly desirableand will transform the transparent electrode material mar-ket in the very near future If we add to this the expectedexplosion in demand for flexible electronics we have atext-book-case where a new material leads to a completelynew market Equally important this material will play akey role in a whole range of other applicationsSimilar considerations hold for each strategy discussed

here and will lead to a product that can be commercial-ized Furthermore since each strategy is independent fromeach other one is guaranteed to have products as soon asthe production starts The key point is that as the degreeof sophistication in material preparation increases and onemoves up in complexity so does the risk and the payoffWhile raw 2D crystals can be readily produced chemi-cal functionalization especially with stoichiometric preci-sion depends on delicate balances of energy Moreoverstrain engineering is a field that while already provento work experimentally it is still in its infancy Finallymulti-stacking of functionalized materials was never testedbefore (multi-stacking of raw 2D crystals has already beendone to a certain extent but this is also a new field) Thusthe whole field of 2D crystals is an open territory (ora dense jungle depending on your point of view) Para-phrasing Isaac Newton we can say that we are still in theinfancy of this field of 2D crystals and diverting ourselveswith graphene a material that looks more interesting thanordinary whilst a great field of 2D crystals lay all undis-covered before us

Acknowledgments We would like to thank AndreGeim and our colleagues at NUS especially Yuan PingFeng Kian Ping Loh Barbaros Oumlzyilmaz Vitor Pereiraand Andrew Wee for many enlightening discussions

References and Notes

1 K S Novoselov A K Geim S V Morozov D Jiang Y ZhangS V Dubonos I V Grigorieva and A A Firsov Science 306 666(2004)

16 Mater Express Vol 1 2011

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Materials ExpressTwo-Dimensional Crystals Beyond Graphene

Neto and NovoselovPersp

ective

4 ULTRA-STRONG NANO-COMPOSITEMATERIALS WITH CONTROLLEDPROPERTIES

Composite materials are omnipresent in technology andmany existing applications rely on light weight conductive(or insulating) and strong composites The performanceof such materials is however fundamentally limited bythe interaction between the filler and the matrix Henceout of great number of possible combinations only a fewreally work Moreover having a complex structure suchmaterials are subject to unpredictable failure It has beenrecently demonstrated that graphene can improve mechan-ical chemical and electrical properties of composite mate-rials dramatically Graphene is only one atom thick yet thestrongest material known to us As it has been shown onecan produce graphene of suitable dimensions and in largequantities very cheaplyChemical and micro-mechanical cleavage and artificial

growth can be used to obtain 2D crystals (graphene BNMoS2 etc) to be used as fillers in composite materialsUsing materials other than graphene allows one to expandthe functionality of such composites One can make themoptically active in various parts of the optical spectra byusing chemically modified graphene or materials with var-ious band gaps By creating semiconductor-metal andorsemiconductor-semimetal interfaces either in the form ofdispersed hetero-junctions or layered junctions efficientlight collection and charge transfer across the interface canbe achievedmdashgiving rise to the photovoltaic effect Theapplications of such materials include structurally strongplastics for construction engineering which have particularoptical characteristics (color certain optical transparencywindow polarizing effects photovoltaic properties etc)Such composite materials or coatings can be conductiveor insulating or ever have transistor properties (conduc-tivity and optical properties depends on environment likeexternal gating presence of certain gases humidity pHillumination etc)It has also been shown that graphenersquos Raman spectrum

is extremely sensitive to applied strain and that strain trans-fer between the matrix and the graphene is very efficient21

Hence it would be possible to create composite materialswhere accumulated stress could be monitored by contact-less non-invasive optical methods Such techniques canbe of crucial importance in certain areas of engineer-ing where catastrophic material failure is a major issueand where permanent monitoring of the performance ofa material is crucial (avionics electrical grids medicineetc)Surface functionalization can be used to fine-tune the

interaction between the filler and the matrix For instanceit has been shown that hydrophobic graphene can beturned hydrophilic by hydrogenation13 It is possible toapply similar strategies to other 2D materials to pro-duce high-performance composites with a wide range of

functionalities It is well accepted that carbon-based fillersimprove the performance of many composite materialsdramatically However only a small fraction of possiblematrices can bound effectively to carbon fibers nanotubesand graphene thus limiting the number of possible appli-cations By functionalizing graphene and other 2D crystals(which is possible without sacrificing their performance)one can make them work with a much wider range ofmatrices increasing the range of their applicability

5 ELECTRO-MECHANICAL DEVICES FORULTRA-FAST ELECTRONICS

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 mod-ern electronics and technology it also has its limitationsit is very difficult to modify and manipulate (to generatea new functionality one has to create a new crystal) thestructure might be not stable (for instance one can haveelectro-migration) or can be strongly modified by externalenvironmental conditions (such as radiation damage) thesame chemistry that is used to achieve the desired elec-tronic structure (for instance by doping) might cause detri-mental effects on other parameters (such as the decrease ofthe mobility due to scattering by dopants) In contrast 2Dmaterials open a new venue for engineering of the elec-tronic properties It has been demonstrated theoreticallyand experimentally that the electronic structure of 2Dmaterial can be considerably modified by strain shear andbending Moreover one achieves much better control onthose parameters in 2D as compared to 3D systems Thisis a completely new powerful paradigm which never (oronly very limited) has been explored before strong modi-fication of the electronic structure by strain engineering10

It has been shown theoretically that uniaxial and biaxialstrain applied along different crystallographic directionscan tune reversibly graphene from metallic to insulat-ing and can change the optical properties of grapheneMore complicated strain patterns can also be used to pro-duce gaps at smaller strain levels27 Recent experimentsof graphene grown on Platinum have shown that graphenenanobubbles that are under compressive strain can supportsemiconducting behavior with electronic gaps of the orderof a fraction of an electron volt23 Given that there is a one-to-one correspondence between structure and electronicscomputer modeling of these properties can play a funda-mental role in the development of new strain engineeredmaterials for electronics applicationsBand structure is also strongly influenced by stack-

ing A well-known example is graphite depending on thestacking order (Bernal versus rhombohedral versus hexag-onal versus turbostratic) its electronic properties can berather different28 The modification of the electronic bandstructure of twisted bilayer graphene29 is clearly reflected

14 Mater Express Vol 1 2011

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Materials ExpressTwo-Dimensional Crystals Beyond GrapheneNeto and Novoselov

Persp

ectivein its Raman spectrum through an electronndashphonon cou-pling and its optical conductivity30 Furthermore intercala-tion of layers of different atoms can change completely theelectronic states For instance graphite intercalated withalkali metals become superconducting31 By introducingdifferent 2D materials with different properties into a 3Dmatrix one is able to modify the final electronic propertiesA completely new field of exploration is the study of

multi-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 Theoretically this can be studied by com-puter modeling in 3D super-cells geometriesHere we would like to make an analogy with the

celebrated spintronics32 Devices which deal with spinrather than charge are extremely successful and allowfor novel modes of operation However the choice ofmaterials is very limited to ferromagnets Strain engineer-ing in 2D materials on the other hand would allow forvalleytronicsmdashwhere devices operate with valley degreeof freedom of quasiparticles rather than spin or chargeBecause of large choice of materials available and a pos-sibility for very precise control of strain such devicescould be much more versatile and allow for much moreadvanced complex electronic architectureIn order to build such systems one can use CVD meth-

ods to grow novel 2D materials which can have interestingelectronics (for instance Heusler alloys based on Ti) Onecan also grow multi-stacked materials (like graphene-BN-graphene) for high-performance transistor applicationsMobility in free-standing graphene or graphene on inertsubstrates can be as high as 106 cm2Vs which is verypromising for high-frequency transistor applications33

By arranging different 2D materials into stacks onewould be able to achieve new composite materials withnovel electronic mechanical and optical properties Wewould like to stress that such materials could be designedto be multifunctional performing several tasks simultane-ously (like harvesting light to feed electronic circuits madefrom the same material for instance) The typical mecha-nisms for the modification of the electronic bands in suchstacks are based on changing the symmetry of the structureand opening gaps in the electronic spectrum

6 NOVEL PHOTOVOLTAICS

It has been demonstrated that due to long mean-free pathand high Fermi velocity graphene can serve as an excel-lent light-to-current converter with quantum efficiencyreaching close to 10034 Its use for solar-cells applica-tions is however limited due to the low absorption of

graphene (the total efficiency is low) However efficientsolar-cells devices can be produced if graphene operateswith other 2D materialsAn example of such hybrid system would be network

(periodic array) of p-n junctions in monolayers of 2Dmaterials in planar geometry This would induce the wholesurface to participate in light-to-current conversion Onecan use either substitution doping during growth (forinstance B- and N-doping for C) or adatoms to create sucha net of p-n junctions In order to increase the efficiencymulti-stacked materials can be usedAnother parallel strategy would be to separate electron

and hole pairs into the neighboring layers of multi-layerstructures This can be achieved either by applying theexternal electric field or by selective doping of the two lay-ers (one n-type another p-type) Selective doping can beimplemented either during growth by substitutional dopingor by doping with adatoms Besides obvious applicationslike solar cells35 one could use such devices as positionsensitive photodetectors photo-voltage generating matri-ces with pixels which are controlled by a gate These mate-rials can be used in applications which allow and requirecombination of transparent electrodes and light harvestingIt has been shown that strain generates an electrostatic

potential36 which leads to the formation of local p-n junc-tions One can use this concept to create an array of p-njunctions This is a novel strategy would allow to useonly one material to form p-n junctions We expect it tobe much more stable than conventional organic solar-cellswhich traditionally use interfaces between different mate-rials Also the conversion level will depend on the strainlevel which might be used as new generation active pho-tovoltaic strain gaugesStacks of various 2D crystals (either the same materi-

als with different doping or completely different materials)can be used to form multi-layers One can use layers ofinsulating material (such as BN) to separate the conductivelayers for more efficient electronndashhole separation (to createweak tunneling barriers) One of the advantages of suchmaterials is the atomically sharp interfaces which wouldallow one to reduce scattering and improve the quantumyield of such devices

7 ATOMICALLY THIN TRANSPARENTTUNABLE FETs

It has been demonstrated that graphene absorbs only 23of light in a wide range of visible spectra17 This number iseven lower for chemically modified graphene and for someother 2D materials Combined with the excellent conduc-tivity properties these materials are extremely promisingfor transparent conductive coating applicationsOne can use local chemical modification to define con-

ductive and non-conductive areas to create atomically thinfilm transistors Another approach can be based on the

Mater Express Vol 1 2011 15

Delivered by Ingenta toGuest User

IP 21021212060Tue 27 Dec 2011 055336

Materials ExpressTwo-Dimensional Crystals Beyond Graphene

Neto and NovoselovPersp

ective

combination of graphene with other 2D sheets such as BNThe importance of BN as a substrate and gate dielectric forgraphene based FETs has been recently demonstrated31

The flatness of BN as well as its chemically inert natureleads to an at least one order of magnitude increase incarrier mobilityThe ability to realize crystalline BNgraphene layer

heterostructures is also of great interest for the estab-lishment of graphene as an efficient transparent conduct-ing electrode material (such as indium tin oxide ITO)since the sheet resistance of graphene will be signifi-cantly reduced when transferred on h-BN-coated glassThis approach can be important for a whole range of trans-parent electrode application including the incorporation ofgrapheneBNglass into solar cell platforms for enhancedpower conversion efficiency The target is to attain powerconversion efficiency that will be competitive with that ofITO-based solar devices h-BN can also be a chemicallytunable platform in terms of its stoichiometry thus lend-ing itself to band gap engineering where the incorporationof carbon in the BN matrix can form the hybrid BCNwith reduced band gap GrapheneBN or grapheneBCNheterostructure interfaces can be exploited for interfacialcharge segregation trapping and light emissionTwo types of approaches can be used to create

transparent conductive coating chemical exfoliation andepitaxial growth of graphene and other conductive materi-als Chemical exfoliation method is rather well developedand can be used for applications straight away The samemethod can be used in other conducting layered materi-als In this way it is possible to develop an infrastructurewhich would allow for large-area production of graphenedoped graphene and other materialsUniaxial strain breaks the symmetry of the lattice and

produces rotation of the planes of polarization37 Thiseffect is enhanced in an applied magnetic field Applica-tion of strain will thus allow for the production of tunablepolarizers in a broad band of optical frequencies By mul-tistacking such crystals one would be able to obtain mate-rials where optical properties are dependent on mechanicalstrain and stress and which can be used for a number ofphotonics applicationsOne can use chemical modification to define conduc-

tive and non-conductive areas for our atomically thin filmtransistors In this way one can prepare the basic elementsto create active displays which would be able to performa number of tasks (for instance combining LCD displaywith touch screens and some basic logic devices)

8 ECONOMIC PAYOFF

While a direct challenge to the existing technology is unre-alistic three key considerations are likely to pave the wayfor a gradual introduction of graphene and other 2D crys-tals as a material of choice for future device applications

cost multi-functionality and new markets We believe that2D material systems will first be viable for niche applica-tions and from there slowly find their way to mass marketconsumer electronics applications and beyond Here thekey to new functionalities will be their mechanical andoptical properties which in contrast to electronic proper-ties have not even remotely competing alternatives in exist-ing semiconducting materials such as Si or for that matterITO We have already discussed earlier the potential ofcomposite materialsAnother example of immediate economic impact will be

the replacement of ITO For this it is worthwhile to lookat the steep increase in ITO price from US$ 60ndash100 per kgonly a few years back to now well beyond US$1000 per kg Even without its mechanical propertiesgraphene is a low cost alternative which is highly desirableand will transform the transparent electrode material mar-ket in the very near future If we add to this the expectedexplosion in demand for flexible electronics we have atext-book-case where a new material leads to a completelynew market Equally important this material will play akey role in a whole range of other applicationsSimilar considerations hold for each strategy discussed

here and will lead to a product that can be commercial-ized Furthermore since each strategy is independent fromeach other one is guaranteed to have products as soon asthe production starts The key point is that as the degreeof sophistication in material preparation increases and onemoves up in complexity so does the risk and the payoffWhile raw 2D crystals can be readily produced chemi-cal functionalization especially with stoichiometric preci-sion depends on delicate balances of energy Moreoverstrain engineering is a field that while already provento work experimentally it is still in its infancy Finallymulti-stacking of functionalized materials was never testedbefore (multi-stacking of raw 2D crystals has already beendone to a certain extent but this is also a new field) Thusthe whole field of 2D crystals is an open territory (ora dense jungle depending on your point of view) Para-phrasing Isaac Newton we can say that we are still in theinfancy of this field of 2D crystals and diverting ourselveswith graphene a material that looks more interesting thanordinary whilst a great field of 2D crystals lay all undis-covered before us

Acknowledgments We would like to thank AndreGeim and our colleagues at NUS especially Yuan PingFeng Kian Ping Loh Barbaros Oumlzyilmaz Vitor Pereiraand Andrew Wee for many enlightening discussions

References and Notes

1 K S Novoselov A K Geim S V Morozov D Jiang Y ZhangS V Dubonos I V Grigorieva and A A Firsov Science 306 666(2004)

16 Mater Express Vol 1 2011

Delivered by Ingenta toGuest User

IP 21021212060Tue 27 Dec 2011 055336

Materials ExpressTwo-Dimensional Crystals Beyond GrapheneNeto and Novoselov

Persp

ectivein its Raman spectrum through an electronndashphonon cou-pling and its optical conductivity30 Furthermore intercala-tion of layers of different atoms can change completely theelectronic states For instance graphite intercalated withalkali metals become superconducting31 By introducingdifferent 2D materials with different properties into a 3Dmatrix one is able to modify the final electronic propertiesA completely new field of exploration is the study of

multi-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 Theoretically this can be studied by com-puter modeling in 3D super-cells geometriesHere we would like to make an analogy with the

celebrated spintronics32 Devices which deal with spinrather than charge are extremely successful and allowfor novel modes of operation However the choice ofmaterials is very limited to ferromagnets Strain engineer-ing in 2D materials on the other hand would allow forvalleytronicsmdashwhere devices operate with valley degreeof freedom of quasiparticles rather than spin or chargeBecause of large choice of materials available and a pos-sibility for very precise control of strain such devicescould be much more versatile and allow for much moreadvanced complex electronic architectureIn order to build such systems one can use CVD meth-

ods to grow novel 2D materials which can have interestingelectronics (for instance Heusler alloys based on Ti) Onecan also grow multi-stacked materials (like graphene-BN-graphene) for high-performance transistor applicationsMobility in free-standing graphene or graphene on inertsubstrates can be as high as 106 cm2Vs which is verypromising for high-frequency transistor applications33

By arranging different 2D materials into stacks onewould be able to achieve new composite materials withnovel electronic mechanical and optical properties Wewould like to stress that such materials could be designedto be multifunctional performing several tasks simultane-ously (like harvesting light to feed electronic circuits madefrom the same material for instance) The typical mecha-nisms for the modification of the electronic bands in suchstacks are based on changing the symmetry of the structureand opening gaps in the electronic spectrum

6 NOVEL PHOTOVOLTAICS

It has been demonstrated that due to long mean-free pathand high Fermi velocity graphene can serve as an excel-lent light-to-current converter with quantum efficiencyreaching close to 10034 Its use for solar-cells applica-tions is however limited due to the low absorption of

graphene (the total efficiency is low) However efficientsolar-cells devices can be produced if graphene operateswith other 2D materialsAn example of such hybrid system would be network

(periodic array) of p-n junctions in monolayers of 2Dmaterials in planar geometry This would induce the wholesurface to participate in light-to-current conversion Onecan use either substitution doping during growth (forinstance B- and N-doping for C) or adatoms to create sucha net of p-n junctions In order to increase the efficiencymulti-stacked materials can be usedAnother parallel strategy would be to separate electron

and hole pairs into the neighboring layers of multi-layerstructures This can be achieved either by applying theexternal electric field or by selective doping of the two lay-ers (one n-type another p-type) Selective doping can beimplemented either during growth by substitutional dopingor by doping with adatoms Besides obvious applicationslike solar cells35 one could use such devices as positionsensitive photodetectors photo-voltage generating matri-ces with pixels which are controlled by a gate These mate-rials can be used in applications which allow and requirecombination of transparent electrodes and light harvestingIt has been shown that strain generates an electrostatic

potential36 which leads to the formation of local p-n junc-tions One can use this concept to create an array of p-njunctions This is a novel strategy would allow to useonly one material to form p-n junctions We expect it tobe much more stable than conventional organic solar-cellswhich traditionally use interfaces between different mate-rials Also the conversion level will depend on the strainlevel which might be used as new generation active pho-tovoltaic strain gaugesStacks of various 2D crystals (either the same materi-

als with different doping or completely different materials)can be used to form multi-layers One can use layers ofinsulating material (such as BN) to separate the conductivelayers for more efficient electronndashhole separation (to createweak tunneling barriers) One of the advantages of suchmaterials is the atomically sharp interfaces which wouldallow one to reduce scattering and improve the quantumyield of such devices

7 ATOMICALLY THIN TRANSPARENTTUNABLE FETs

It has been demonstrated that graphene absorbs only 23of light in a wide range of visible spectra17 This number iseven lower for chemically modified graphene and for someother 2D materials Combined with the excellent conduc-tivity properties these materials are extremely promisingfor transparent conductive coating applicationsOne can use local chemical modification to define con-

ductive and non-conductive areas to create atomically thinfilm transistors Another approach can be based on the

Mater Express Vol 1 2011 15

Delivered by Ingenta toGuest User

IP 21021212060Tue 27 Dec 2011 055336

Materials ExpressTwo-Dimensional Crystals Beyond Graphene

Neto and NovoselovPersp

ective

combination of graphene with other 2D sheets such as BNThe importance of BN as a substrate and gate dielectric forgraphene based FETs has been recently demonstrated31

The flatness of BN as well as its chemically inert natureleads to an at least one order of magnitude increase incarrier mobilityThe ability to realize crystalline BNgraphene layer

heterostructures is also of great interest for the estab-lishment of graphene as an efficient transparent conduct-ing electrode material (such as indium tin oxide ITO)since the sheet resistance of graphene will be signifi-cantly reduced when transferred on h-BN-coated glassThis approach can be important for a whole range of trans-parent electrode application including the incorporation ofgrapheneBNglass into solar cell platforms for enhancedpower conversion efficiency The target is to attain powerconversion efficiency that will be competitive with that ofITO-based solar devices h-BN can also be a chemicallytunable platform in terms of its stoichiometry thus lend-ing itself to band gap engineering where the incorporationof carbon in the BN matrix can form the hybrid BCNwith reduced band gap GrapheneBN or grapheneBCNheterostructure interfaces can be exploited for interfacialcharge segregation trapping and light emissionTwo types of approaches can be used to create

transparent conductive coating chemical exfoliation andepitaxial growth of graphene and other conductive materi-als Chemical exfoliation method is rather well developedand can be used for applications straight away The samemethod can be used in other conducting layered materi-als In this way it is possible to develop an infrastructurewhich would allow for large-area production of graphenedoped graphene and other materialsUniaxial strain breaks the symmetry of the lattice and

produces rotation of the planes of polarization37 Thiseffect is enhanced in an applied magnetic field Applica-tion of strain will thus allow for the production of tunablepolarizers in a broad band of optical frequencies By mul-tistacking such crystals one would be able to obtain mate-rials where optical properties are dependent on mechanicalstrain and stress and which can be used for a number ofphotonics applicationsOne can use chemical modification to define conduc-

tive and non-conductive areas for our atomically thin filmtransistors In this way one can prepare the basic elementsto create active displays which would be able to performa number of tasks (for instance combining LCD displaywith touch screens and some basic logic devices)

8 ECONOMIC PAYOFF

While a direct challenge to the existing technology is unre-alistic three key considerations are likely to pave the wayfor a gradual introduction of graphene and other 2D crys-tals as a material of choice for future device applications

cost multi-functionality and new markets We believe that2D material systems will first be viable for niche applica-tions and from there slowly find their way to mass marketconsumer electronics applications and beyond Here thekey to new functionalities will be their mechanical andoptical properties which in contrast to electronic proper-ties have not even remotely competing alternatives in exist-ing semiconducting materials such as Si or for that matterITO We have already discussed earlier the potential ofcomposite materialsAnother example of immediate economic impact will be

the replacement of ITO For this it is worthwhile to lookat the steep increase in ITO price from US$ 60ndash100 per kgonly a few years back to now well beyond US$1000 per kg Even without its mechanical propertiesgraphene is a low cost alternative which is highly desirableand will transform the transparent electrode material mar-ket in the very near future If we add to this the expectedexplosion in demand for flexible electronics we have atext-book-case where a new material leads to a completelynew market Equally important this material will play akey role in a whole range of other applicationsSimilar considerations hold for each strategy discussed

here and will lead to a product that can be commercial-ized Furthermore since each strategy is independent fromeach other one is guaranteed to have products as soon asthe production starts The key point is that as the degreeof sophistication in material preparation increases and onemoves up in complexity so does the risk and the payoffWhile raw 2D crystals can be readily produced chemi-cal functionalization especially with stoichiometric preci-sion depends on delicate balances of energy Moreoverstrain engineering is a field that while already provento work experimentally it is still in its infancy Finallymulti-stacking of functionalized materials was never testedbefore (multi-stacking of raw 2D crystals has already beendone to a certain extent but this is also a new field) Thusthe whole field of 2D crystals is an open territory (ora dense jungle depending on your point of view) Para-phrasing Isaac Newton we can say that we are still in theinfancy of this field of 2D crystals and diverting ourselveswith graphene a material that looks more interesting thanordinary whilst a great field of 2D crystals lay all undis-covered before us

Acknowledgments We would like to thank AndreGeim and our colleagues at NUS especially Yuan PingFeng Kian Ping Loh Barbaros Oumlzyilmaz Vitor Pereiraand Andrew Wee for many enlightening discussions

References and Notes

1 K S Novoselov A K Geim S V Morozov D Jiang Y ZhangS V Dubonos I V Grigorieva and A A Firsov Science 306 666(2004)

16 Mater Express Vol 1 2011

Delivered by Ingenta toGuest User

IP 21021212060Tue 27 Dec 2011 055336

Materials ExpressTwo-Dimensional Crystals Beyond Graphene

Neto and NovoselovPersp

ective

combination of graphene with other 2D sheets such as BNThe importance of BN as a substrate and gate dielectric forgraphene based FETs has been recently demonstrated31

The flatness of BN as well as its chemically inert natureleads to an at least one order of magnitude increase incarrier mobilityThe ability to realize crystalline BNgraphene layer

heterostructures is also of great interest for the estab-lishment of graphene as an efficient transparent conduct-ing electrode material (such as indium tin oxide ITO)since the sheet resistance of graphene will be signifi-cantly reduced when transferred on h-BN-coated glassThis approach can be important for a whole range of trans-parent electrode application including the incorporation ofgrapheneBNglass into solar cell platforms for enhancedpower conversion efficiency The target is to attain powerconversion efficiency that will be competitive with that ofITO-based solar devices h-BN can also be a chemicallytunable platform in terms of its stoichiometry thus lend-ing itself to band gap engineering where the incorporationof carbon in the BN matrix can form the hybrid BCNwith reduced band gap GrapheneBN or grapheneBCNheterostructure interfaces can be exploited for interfacialcharge segregation trapping and light emissionTwo types of approaches can be used to create

transparent conductive coating chemical exfoliation andepitaxial growth of graphene and other conductive materi-als Chemical exfoliation method is rather well developedand can be used for applications straight away The samemethod can be used in other conducting layered materi-als In this way it is possible to develop an infrastructurewhich would allow for large-area production of graphenedoped graphene and other materialsUniaxial strain breaks the symmetry of the lattice and

produces rotation of the planes of polarization37 Thiseffect is enhanced in an applied magnetic field Applica-tion of strain will thus allow for the production of tunablepolarizers in a broad band of optical frequencies By mul-tistacking such crystals one would be able to obtain mate-rials where optical properties are dependent on mechanicalstrain and stress and which can be used for a number ofphotonics applicationsOne can use chemical modification to define conduc-

tive and non-conductive areas for our atomically thin filmtransistors In this way one can prepare the basic elementsto create active displays which would be able to performa number of tasks (for instance combining LCD displaywith touch screens and some basic logic devices)

8 ECONOMIC PAYOFF

While a direct challenge to the existing technology is unre-alistic three key considerations are likely to pave the wayfor a gradual introduction of graphene and other 2D crys-tals as a material of choice for future device applications

cost multi-functionality and new markets We believe that2D material systems will first be viable for niche applica-tions and from there slowly find their way to mass marketconsumer electronics applications and beyond Here thekey to new functionalities will be their mechanical andoptical properties which in contrast to electronic proper-ties have not even remotely competing alternatives in exist-ing semiconducting materials such as Si or for that matterITO We have already discussed earlier the potential ofcomposite materialsAnother example of immediate economic impact will be

the replacement of ITO For this it is worthwhile to lookat the steep increase in ITO price from US$ 60ndash100 per kgonly a few years back to now well beyond US$1000 per kg Even without its mechanical propertiesgraphene is a low cost alternative which is highly desirableand will transform the transparent electrode material mar-ket in the very near future If we add to this the expectedexplosion in demand for flexible electronics we have atext-book-case where a new material leads to a completelynew market Equally important this material will play akey role in a whole range of other applicationsSimilar considerations hold for each strategy discussed

here and will lead to a product that can be commercial-ized Furthermore since each strategy is independent fromeach other one is guaranteed to have products as soon asthe production starts The key point is that as the degreeof sophistication in material preparation increases and onemoves up in complexity so does the risk and the payoffWhile raw 2D crystals can be readily produced chemi-cal functionalization especially with stoichiometric preci-sion depends on delicate balances of energy Moreoverstrain engineering is a field that while already provento work experimentally it is still in its infancy Finallymulti-stacking of functionalized materials was never testedbefore (multi-stacking of raw 2D crystals has already beendone to a certain extent but this is also a new field) Thusthe whole field of 2D crystals is an open territory (ora dense jungle depending on your point of view) Para-phrasing Isaac Newton we can say that we are still in theinfancy of this field of 2D crystals and diverting ourselveswith graphene a material that looks more interesting thanordinary whilst a great field of 2D crystals lay all undis-covered before us

Acknowledgments We would like to thank AndreGeim and our colleagues at NUS especially Yuan PingFeng Kian Ping Loh Barbaros Oumlzyilmaz Vitor Pereiraand Andrew Wee for many enlightening discussions

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Received 17 February 2011 Accepted 25 February 2011

Mater Express Vol 1 2011 17