Honeycomb Carbon A Review of Graphene 石墨烯综述

纳米材料在生物医学中的应用

132Chem.Rev.2010,110,132–145

HoneycombCarbon:AReviewofGraphene

MatthewJ.Allen, VincentC.Tung, andRichardB.Kaner*, ,

DepartmentofChemistryandBiochemistryandCaliforniaNanoSystemsInstitute,andDepartmentofMaterialsScienceandEngineering,University

ofCalifornia,LosAngeles,LosAngeles,California90095

ReceivedFebruary20,2009

Contents

1.Introduction

2.BriefHistoryofGraphene2.1.ChemistryofGraphite3.DowntoSingleLayers

3.1.CharacterizingGrapheneFlakes3.1.1.ScanningProbeMicroscopy3.1.2.RamanSpectroscopy

4.ExtraordinaryDeviceswithPeeledGraphene4.1.High-SpeedElectronics4.2.SingleMoleculeDetection

5.AlternativestoMechanicalExfoliation

5.1.ChemicallyDerivedGraphenefromGraphite

Oxide

5.1.1.Depositions

5.1.2.DefectDensityinChemicallyDerived

Graphene

5.1.3.Field-EffectDevices5.1.4.PracticalSensors

5.1.5.TransparentElectrodes5.2.TotalOrganicSynthesis

5.3.EpitaxialGrapheneandChemicalVapor

Deposition

6.GrapheneNanoribbons7.FutureWork8.Conclusions

9.Acknowledgments10.References

132133134134136136136136137138138139139139139140141141142143143144144144

1.Introduction

Grapheneisthenamegiventoatwo-dimensionalsheetofsp2-hybridizedcarbon.Itsextendedhoneycombnetworkisthebasicbuildingblockofotherimportantallotropes;itcanbestackedtoform3Dgraphite,rolledtoform1Dnanotubes,andwrappedtoform0Dfullerenes.Long-rangeπ-conjugationingrapheneyieldsextraordinarythermal,mechanical,andelectricalproperties,whichhavelongbeentheinterestofmanytheoreticalstudiesandmorerecentlybecameanexcitingareaforexperimentalists.

Whilestudiesofgraphitehaveincludedthoseutilizingfewerandfewerlayersforsometime,1the eldwasdeliveredajoltin2004,whenGeimandco-workersatManchesterUniversity rstisolatedsingle-layersamplesfromgraphite(seeFigure1).2Thisledtoanexplosionofinterest,inpart

DepartmentofChemistryandBiochemistryandCaliforniaNanoSystemsInstitute.

DepartmentofMaterialsScienceandEngineeringandCaliforniaNano-SystemsInstitute.

becausetwo-dimensionalcrystalswerethoughttobether-modynamicallyunstableat nitetemperatures.3,4Quasi-two-dimensional lmsgrownbymolecularbeamepitaxy(MBE)arestabilizedbyasupportingsubstrate,whichoftenplaysasigni cantroleingrowthandhasanappreciablein uenceonelectricalproperties.5Incontrast,themechanicalexfo-liationtechniqueusedbytheManchestergroupisolatedthetwo-dimensionalcrystalsfromthree-dimensionalgraphite.Resultingsingle-andfew-layer akeswerepinnedtothesubstratebyonlyvanderWaalsforcesandcouldbemadefree-standingbyetchingawaythesubstrate.6-9Thismini-mizedanyinducedeffectsandallowedscientiststoprobegraphene’sintrinsicproperties.

Theexperimentalisolationofsingle-layergraphene rstandforemostyieldedaccesstoalargeamountofinterestingphysics.10,11Initialstudiesincludedobservationsofgraphene’sambipolar eldeffect,2thequantumHalleffectatroomtemperature,12-17measurementsofextremelyhighcarriermobility,7,18-20andeventhe rsteverdetectionofsinglemoleculeadsorptionevents.21,22Thesepropertiesgeneratedhugeinterestinthepossibleimplementationofgrapheneinamyriadofdevices.Theseincludefuturegenerationsofhigh-speedandradiofrequencylogicdevices,thermallyandelectricallyconductivereinforcedcomposites,sensors,andtransparentelectrodesfordisplaysandsolarcells.

Despiteintenseinterestandcontinuingexperimentalsuccessbydevicephysicists,widespreadimplementationofgraphenehasyettooccur.Thisisprimarilyduetothedif cultyofreliablyproducinghighqualitysamples,espe-ciallyinanyscalablefashion.23Thechallengeisreally2-foldbecauseperformancedependsonboththenumberoflayerspresentandtheoverallqualityofthecrystallattice.19,24-26Sofar,theoriginaltop-downapproachofmechanicalexfoliationhasproducedthehighestqualitysamples,butthemethodisneitherhighthroughputnorhigh-yield.Inordertoexfoliateasinglesheet,vanderWaalsattractionbetweenexactlythe rstandsecondlayersmustbeovercomewithoutdisturbinganysubsequentsheets.Therefore,anumberofalternativeapproachestoobtainingsinglelayershavebeenexplored,afewofwhichhaveledtopromisingproof-of-conceptdevices.

Alternativestomechanicalexfoliationincludeprimarilythreegeneralapproaches:chemicaleffortstoexfoliateandstabilizeindividualsheetsinsolution,27-32bottom-upmeth-odstogrowgraphenedirectlyfromorganicprecursors,33-36andattemptstocatalyzegrowthinsituonasubstrate.37-43Eachoftheseapproacheshasitsdrawbacks.Forchemicallyderivedgraphene,completeexfoliationinsolutionsofarrequiresextensivemodi cationofthe2Dcrystallattice,whichdegradesdeviceperformance.31,44Alternatively,bot-tom-uptechniqueshaveyettoproducelargeanduniform

10.1021/cr900070d 2010AmericanChemicalSociety

PublishedonWeb07/17/2009

纳米材料在生物医学中的应用

HoneycombCarbon:AReviewofGrapheneMatthewJ.AllenisagraduatestudentintheKanerlaboratoryattheUniversityofCalifornia,LosAngeles(UCLA).HereceivedhisB.S.inphysicsatRiceUniversity,whereheresearchedcarbonnanostructuresinthelaboratoriesofRichardSmalleyandRobertCurl.

VincentC.TungisagraduatestudentintheYanglaboratorycoadvisedbyProf.KanerattheUniversityofCalifornia,LosAngeles(UCLA).HereceivedhisM.S.inchemistryfromtheNationalTsing-HuaUniversityinHsinchu,Taiwan.Hispreviousworkwasonthephotochemistryoforganiclightemittingdiodes(OLEDs).

singlelayers.Totalorganicsyntheseshavebeensizelimitedbecausemacromoleculesbecomeinsolubleandtheoccur-renceofsidereactionsincreaseswithmolecularweight.36Substrate-basedgrowthofsinglelayersbychemicalvapordeposition(CVD)orthereductionofsiliconcarbidereliesontheabilitytowalkanarrowthermodynamictightrope.40Afternucleatingasheet,conditionsmustbecarefullycontrolledtopromotecrystalgrowthwithoutseedingad-ditionalsecondlayersorforminggrainboundaries.

Despitetremendousprogresswithalternatives,mechanicalexfoliationwithcellophanetapestillproducesthehighestqualitygraphene akesavailable.Thisfactshouldnot,however,dampenanyinterestfromchemists.Onthecontrary,therecenttransitionfromtheconsiderationofgrapheneasa“physicstoy”toitstreatmentasalargecarbonmacromoleculeoffersnewpromise.Yearsofcarbonnano-tube,fullerene,andgraphiteresearchhaveproducedamyriadofchemicalpathwaysformodifyingsp2carbonstructures,45-50whichwillundoubtedlybeadaptedtofunctionalizeboththebasalplaneofgrapheneanditsreactiveedges.Thisnotonlypromisestodeliverhandlesforexploitinggraphene’sintrinsicpropertiesbutalsoshouldtoleadtonewpropertiesaltogether.Thisreviewwilldiscussthe eldofgraphenefromamaterialschemistrystandpoint.Afterabriefhistoryofthetopic,theexcitingprogressmadesince2004,inboththeproductionofgrapheneanditsimplementationindevices,

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RichardB.KanerreceivedaPh.D.ininorganicchemistryfromtheUniversityofPennsylvaniain1984.AftercarryingoutpostdoctoralresearchattheUniversityofCalifornia,Berkeley,hejoinedtheUniversityofCalifornia,LosAngeles(UCLA),in1987asanAssistantProfessor.HewaspromotedtoAssociateProfessorwithtenurein1991andbecameaFullProfessorin1993.ProfessorKanerhasreceivedawardsfromtheDreyfus,Fulbright,Guggenheim,andSloanFoundations,aswellastheExxonFellowshipinSolidStateChemistryandtheBuck-WhitneyResearchAwardfromtheAmericanChemicalSocietyforhisworkonrefractorymaterials,includingnewsyntheticroutestoceramics,intercala-tioncompounds,superhardmaterials,graphene,andconductingpolymers.

willbediscussed.Forathoroughdiscussionfocusedonthephysicsofgraphene,seerefs10,11,51,and52.

2.BriefHistoryofGraphene

Tounderstandthetrajectoryofgrapheneresearch,itisusefultoconsidergrapheneassimplythefewestlayerlimitofgraphite.Inthislight,theextraordinarypropertiesofhoneycombcarbonarenotreallynew.Abundantandnaturallyoccurring,graphitehasbeenknownasamineralfornearly500years.Eveninthemiddleages,thelayeredmorphologyandweakdispersionforcesbetweenadjacentsheetswereutilizedtomakemarkinginstruments,muchinthesamewaythatweusegraphiteinpencilstoday.Morerecently,thesesamepropertieshavemadegraphiteanidealmaterialforuseasadrylubricant,alongwiththesimilarlystructuredbutmoreexpensivecompoundshexagonalboron

Figure1.Singlelayergraphenewas rstobservedbyGeimandothersatManchesterUniversity.Hereafewlayer akeisshown,withopticalcontrastenhancedbyaninterferenceeffectatacarefullychosenthicknessofoxide.(ReprintedwithpermissionfromScience(),ref2.Copyright2006AmericanAssociationfortheAdvancementofScience.)

纳米材料在生物医学中的应用

134ChemicalReviews,2010,Vol.110,No.1nitride4andmolybdenumdisul de.High,in-planeelectrical(～10 -1cm-1)andthermalconductivity(～3000W/mK)enablegraphitetobeusedinelectrodesandasheatingelementsforindustrialblastfurnaces.53,54Highmechanicalstiffnessofthehexagonalnetwork(1060GPa)isalsoutilizedincarbon berreinforcedcomposites.Theseusesandothersgenerateanannualdemandofmorethan1milliontonsofgraphiteworldwide.55

Theanisotropyofgraphite’smaterialpropertiescontinuestofascinatebothscientistsandtechnologists.Thes,patomicorbitalsoneachcarbonhybridizetoformstrongx,andpcovalentysp2bonds,givingriseto120°C-C-Cbondanglesandthefamiliarchicken-wire-likelayers.Theremainingporbitaloneachcarbonoverlapswithitsthreeneighboringzcarbonstoformabandof lledπorbitals,knownasthevalenceband,andabandofemptyπ*orbitals,calledtheconductionband.Whilethreeofthefourvalenceelectronsoneachcarbonformtheσ(single)bonds,thefourthelectronformsone-thirdofaπbondwitheachofitsneighborsproducingacarbon-carbonbondorderingraphiteofoneandone-third.Withnochemicalbondinginthec-direction,out-of-planeinteractionsareextremelyweak.Thisincludesthepropagationofchargeandthermalcarriers,whichleadstoout-of-planeelectricalandthermalconductivitiesthatarebothmorethan103timeslowerthanthoseoftheirin-planeanalogues.56

2.1.ChemistryofGraphite

Graphitehasarichchemistryinwhichitcanparticipateinreactionsaseitherareducingagent(electrondonor)oranoxidizer(electronacceptor).Thisisadirectconsequenceofitselectronicstructure,whichresultsinboth53

anelectronaf nityandanionizationpotentialof4.6eV.Alargenumberofexperimentsforgraphitefocusontheinsertionofadditionalchemicalspeciesbetweenthebasalplanes,orintercalation.Shaffaultiscreditedwiththe rstintercalationcompoundusingpotassium,datingbackto1841.57Graphiteintercalationcompounds(GICs)appeartobetheonlylayeredcompoundssuf cientlyorderedtoexhibit“staging”inwhichthenumberofgraphiticlayersinbetweenadjacentintercalantscanbevariedinacontrolledfashion.Thestageofacompoundreferstothenumberofgraphiticlayersinbetweenadjacentplanesofintercalant.Theinter-layerspacingcanincreasefrom0.34nm(3.4Å)innativegraphitetomorethan1nminsomeGICs,whichfurtherexaggeratestheanisotropyofmanyproperties.56,58

TheincreasedinterlayerspacinginGICsalsomeansasigni cantreductioninthevanderWaalsforcesbetweenadjacentsheets,whichleadsonetoconsidertheirexfoliationasapossibleroutetosinglelayersofgraphene.Ourgrouptriedjustthatin2003byviolentlyreactingastage-1potassiumintercalationcompound(KC8)withvarioussol-ventssuchasalcohols,butexfoliationproducedonlymetastableslabsaround30layersthickthathadatendencytoscrollunderhigh-poweredsonication(seeFigure2).53,59,60TheinterlayerspacinginGICscanbefurtherincreasedbythermalshocktoproduce“expanded”graphite,whichhasnowservedasastartingmaterialforrecenttechniques,including53,61

ananoribbonsynthesisdevelopedbyDai(seeFigure3).Asecondfocusofexperimentsongraphitehasbeensubstitutionaldopingbythereplacementofcarbonwithotherelements.ThisincludesworkbyBartlettandco-workersatBerkeleyinwhichsubstitutionofcarbonwithboronand

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Figure2.Schematicdiagramshowingtheintercalationandexfoliationprocesstoproducethinslabsofgraphite.Potassiumisinsertedbetweenthelayersandreactedviolentlywithalcohols.Theexfoliatedslabsare～30layersthick.(Reprintedwithpermis-sionbyTheRoyalSocietyofChemistryfromref60.)

nitrogenresultedinp-andn-typegraphite,respectively.62,63InlightofrecentprogresswithCVDofsinglelayergraphene,suchworkwillalmostcertainlyberevisitedasanalternativetoexternalgatingforcontrollingelectronicbehavioringraphene-baseddevices,orperhapstoformgraphene-onlyp-njunctions.

Itisalsoimportanttomentionafewpointsaboutprogressinthechemistryofcarbonnanotubes.Amongthemostimportantobservationshavebeenofthedifferencesinreactivitybetweenthe-different66crystallographicdirections(zigzagorarmchair).64Thisknowledgeshouldtransferdirectlytothe“unrolled”or“ attened”caseofplanargraphene.Amyriadoftechniqueshavealsobeendevelopedtoselectivelymodifyeitherthesidewallsofcarbonnanotubesortheirend-caps.Suchreactionsareimportantlookingforwardbecausetheycorrespondtomodi cationofthebasalplaneofgrapheneanditsedges.Infact,insituTEMwasrecentlyusedtostudyreactionsongraphene’szigzagedgebyZettlandothersatBerkeley.67

3.DowntoSingleLayers

Researchershaveusedmechanicalexfoliationoflayeredcompoundstoproducethinsamplesforsometime.In1999,Ruoff’sgrouppresentedonesuchapproachforgraphitebyusinganatomicforcemicroscope(AFM)tiptomanipulatesmallpillarspatternedintohighlyorientedpyrolyticgraphite(HOPG)byplasmaetching(seeFigure4).1Thethinnestslabsobservedatthattimeweremorethan200nmthickortheequivalentof～600layers.Kim’sgroupatColumbialaterimprovedthemethodbytransferringthepillarstoatiplesscantilever,whichsuccessivelystampeddownslabsasthinas10nm,or～30layers,onSiOonthethincrystallitesforeshadowed2.68Electricalmeasurementsmadeawealthofworktocome.OtherearlygroupsworkingtowardgrapheneincludedEnoki’sinTokyo,whousedtemperaturesaround1600°Ctoconvertnanodiamondsintonanometer-sizedregionsofgrapheneatopHOPGin2001.69

Whiletheseelegantmethodsproducedthinsamples,itwasultimatelyamuchsimplerapproachthatledtothe rstisolationofsinglelayergraphenein2004byaManchestergroupledbyGeim(seeFigures5).2Initsmostbasicform,the“peeling”methodutilizescommoncelluphenetapetosuccessivelyremovelayersfromagraphite ake.Thetapeisultimatelypresseddownagainstasubstratetodeposita

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Figure3.Scanningelectronmicrographsofnaturalgraphitebefore(a)andafter(b)expansionbyacidintercalationandthermalshock.(ReprintedwithpermissionbyTheRoyalSocietyofChemistryfromref60.)

Figure4.Scanningelectronmicroscopeimagesofearlyattemptsatmechanicalexfoliationusinggraphitepillars.(aandb)Ruoff’sgrouppeeledawaylayerswithanAFMtip.(Reprintedwithpermissionfromref1.Copyright1999InstituteofPhysics.)(candd)Kim’sgrouptransferredthepillarstoatiplesscantileveranddepositedthinslabsontoothersubstratesintappingmode.AseriesofscanningelectronmicroscopeimagesshowthinsamplescleavedontotheSi/SiO2substrateandatypicalmesoscopicdevice.(Reprintedwithpermissionfromref68.Copyright2005AmericanInstituteofPhysics.)

sample(seeFigure1).Althoughthe akespresentonthetapearemuchthickerthanonelayer,vanderWaalsattractiontothesubstratecandelaminateasinglesheetwhenthetapeisthenliftedaway.Themethodrequiresagreatdealofpatience,asdepositionsputdownbyinexperiencedscientistsareoftenamessofthickslabsinwhichlocatingasinglelayercanbeextremelydif cult.Withpractice,thetechniqueresultsinhigh-qualitycrystallites,whichcanbemorethan100µm2insize.

Perhapsthemostimportantpartofisolatingsinglelayergrapheneforthe rsttimewastheabilitytospotanatomicallythinspecimeninsomereadilyidenti ablefashion.Opticalabsorbanceofgraphenehassincebeenmeasuredatjust2.3%,rulingoutdirectvisualobservation(seeFigure6).70,71Inordertovisualizesingle akes,theManchestergrouptookadvantageofaninterferenceeffectataspeciallychosenthickness(300nm)ofSiO2onSitoenhancetheopticalcontrastunderwhite-lightillumination.72Althoughseeminglyasimpleidea,thiswasamajorstepforwardandhascontributedagreatdealtowardprogressinthis eld.

Figure5.Mechanicalexfoliationproducedthevery rstsinglelayergraphene akes.(a)Anatomicforcemicroscopyimageshowsthesubstrate-graphenestepheightof<1nmandafoldedstepheightof0.4nm.(Reprintedwithpermissionfromref9.Copyright2005PNAS.)(b)TEMimageofafree-standinggraphene lmafteretchingoftheunderlyingsubstrate.[ReprintedwithpermissionfromNature(),ref6.Copyright2007NaturePublishingGroup.]

纳米材料在生物医学中的应用

136ChemicalReviews,2010,Vol.110,No.1Figure6.Asingleandbilayersamplesuspendedonaporousmembrane.Opticalabsorbanceismeasuredat2.3%perlayer.Theinsetshowsthesampledesignwithseveralapertures.[ReprintedwithpermissionfromScience),ref70.Copy-right2008AmericanAssociationfortheAdvancementofScience.]

Groupshavesinceadaptedthesameeffecttoimagegrapheneonavariety72-75

ofsubstratesandundernonwhite-lightcondi-tions.3.1.CharacterizingGrapheneFlakes

Withnewaccessto2Dcrystallites,experimentalistsscrambledtocon rmresultslongpredictedbytheory.Beforetheycoulddoso,techniquesneededtobedevelopedforthecharacterizationofdeposited akes.Whileopticalmicros-copyusingtheinterferenceeffectwasagoodmethodforidentifyingthincandidates,itcouldnotprovideconclusiveevidencethatagiven akewassingle,double,ormultilay-ered.Thisisanimportantissuebecausesomeofthemoreinterestingpropertiesofgraphenearedependentoncrystallitethickness.Themostobviousexampleiselectronicbandstructure.Single-layergrapheneisazerobandgapsemi-conductororsemimetalinwhichthehighestoccupiedmolecularorbital(HOMO)touchesthelowestunoccupiedmolecularorbital(LUMO)atasingleDiracpoint.Forthicker akes,stackingofmultiplelayersleadstosomeoverlapoftheircarrierwavefunctionsandtheoverallbehaviorbecomesmetallic.Tomatchobservationswiththeory,reliableiden-ti cationofthenumberoflayerspresentinagivensamplebecameimperative.

3.1.1.ScanningProbeMicroscopy

Scanningprobemicroscopywasperhapsthemostobviouschoiceforveri cationofcrystallitethickness.Themethodisrelativelyslow,butthe0.34nm(3.4Å)stepheightforeachsuccessivelayeriswellwithinthedetectionlimitsformodernatomicforcemicroscopes(AFMs).Resolvingthesubstrate-grapheneheightpro leproveddif cult,however,duetothedifferencesintipattraction/repulsionbetweentheinsulatingsubstrateandsemimetallicgraphene.Thisissuewasexacerbatedunderambientconditionsbythepreferentialadsorptionofathinlayerofwaterongraphene.Withsuchcomplications,reportsofsubstrate-grapheneheightpro lesbyatomicforcemicroscopyhavetypicallyrangedfrom0.6to1.0nmforsinglelayers.2

Thefoldededgesofgraphenehaveoftenprovidedamorereliableandaccuratemeasurementofthicknessunderatomicforcemicroscopybecausethereisnochangeinmaterialassociatedwiththelocationofthestep.Itwassuchafold

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thatallowedtheManchestergrouptocon rmthesingle-layerstepheightof～0.4nmintheiroriginalreport(seeFigure5).Althoughseeminglyunlikely,foldscommonlyoccurduringmechanicalexfoliationbecausevanderWaalsattractionbetweenasheetanditselfissizableanddoublingoversometimesprovidesanenergeticminimum.

Scanningtunnelingmicroscopy(STM)haslongbeenusedtoobservetheelectronictopographyofgraphite.76-78Intheseexperiments,onlythreecarbonsofthesix-memberringsarevisibleduetotheABstackingofgraphite(seeFigure7).79Inthisarrangement,electrondensityisconsiderablyhigherforthethreeR-carbons(thosethateclipsecarbonsinthesheetjustbelow),andhence,theyaretheonlyonesvisiblebySTM.Thisisasopposedtowhatwasexpectedforsinglelayergraphene,inwhichthesixcarbonsarecompletelyequivalentandthusshouldallappearwithequalintensity.Thiswasindeedcon rmedbyultrahighvacuumSTMimagestakenatColumbiabyFlynnandothers.79Theirmeasurementsalsogaveevidenceofthehighcrystalqualityinmechanicallyexfoliatedsamples,whichshowedfew-to-nodefectsovertensofnanometers.

3.1.2.RamanSpectroscopy

Whilegraphene’slayeredstructuremakesitideallysuitedforfurtherstudybyscanningprobemicroscopy,samplepreparationtimeandsubstraterequirementsmeanthatadditionalmethodsarenecessarytoreliablycon rmspeci-menthicknessinahigh-throughputfashion.UltimatelyitwasnotadirectlytopographicaltechniquebutinsteadRamanspectroscopythatemergedasthemostusefulwaytoprobethethicknessofmechanicallyexfoliated akes.Althoughlessthanobvious,thismakesgoodsensebecausethefeaturesofgraphiteandgraphenedirectlyre ectchangesin80-electronicstructurefromthestackingofsuccessivelayers.86Obser-vationsofgradualchangesintheRamanspectrumallowonetoinferthenumberoflayers(uptothescreeninglength)ina“ ngerprint”fashion(seeFigure8).

ThemajorfeaturesoftheRamanspectraofgraphiteandgraphene～2700cmare-1theGbandat～1584cm-1andtheG′bandat.TheGbandisduetotheE2gvibrationalmode,andtheG′bandisasecond-ordertwo-phononmode.Athirdfeature,theDbandat～1350cm-1,isnotRamanactiveforpristinegraphenebutcanbeobservedwheresymmetryisbrokenbyedgesorinsampleswithahighdensityofdefects.ItischangesinthepositionsandrelativepeakheightsoftheGandG′bandsthatservetoindicatethenumberoflayerspresentforagiven ake.ThelocationoftheGpeakforsinglelayergrapheneis3-5cm-1higherthanthatforbulkgraphite,whileitsintensityisroughlythesame.TheG′peakshowsasigni cantchangeinbothshapeandintensityasthenumberoflayersisdecreased.Inbulkgraphite,theG′bandiscomprisedoftwocomponents,theintensitiesofwhichareroughly1/4and1/2thatoftheGpeakforthelowandhighshifts,respectively.Forsinglelayergraphene,theG′bandisasinglesharppeakatthelowershift,withintensityroughly4timesthatoftheGpeak.Itwas ttingtothesetrendsthat nallyenabledscientiststoreliablycon rmtheidentityofmechanicallyexfoliated akes.

4.ExtraordinaryDeviceswithPeeledGraphene

MechanicalexfoliationandtheRaman ngerprintingtechniqueallowedscientiststoforgeaheadwithafullsuite

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Figure7.(a)STMimageofgraphiteshowingonlythethreecarbonsthateclipseaneighborinthesheetdirectlybelow.(b)Incontrast,allsixcarbonsareequivalentandthusvisibleinmechanicallyexfoliatedsingle-layergraphene.(Reprintedwithpermissionfromref79.Copyright2007PNAS.)

Figure8.Ramanspectroscopyisapowerfuldiagnostictoolforthestudyofgraphene.BoththeG(near1584cm-1)andG′(near2700cm-1)bandsundergosigni cantchangesduetothethicknessofABstacked akes,asproducedbymechanicalexfoliation.(Reprintedwithpermissionfromref80.Copyright2006AmericanPhysicalSociety.)

ofexperimentsonsingle-layergraphene.Theseledtoanumberofextraordinaryproof-of-conceptdevices.

4.1.High-SpeedElectronics

Theoreticalpredictionslongsuggestedextremelyhighcarriermobilityandanambipolar eld-effectingraphene.87,88Thismotivatedthevery rstexperimentsthatwired-upmechanicallyexfoliated akesbye-beamlithography.2,18Beyondcon rminganumberofpredictions,thosemeasure-mentsgeneratedsigni cantinterestingrapheneasapossiblematerialforthenextgenerationofsemiconductordevices.Thatattentionmayormaynotbewarranted,butmanyagreethatourabilitytosustainMoore’slawwillultimatelybecomeaquestionofcarriermobility.

Extraordinaryelectronicpropertiesingraphenearereallyduetothehighqualityofits2Dcrystallattice.9,19,89-91Thathighqualityimpliesanunusuallylowdensityofdefects,whichtypicallyserveasthescatteringcentersthatinhibitchargetransport.In2008,Kim’sgroupatColumbiamea-suredacarriermobilityinexcessof200,000cm2/(Vs)forasinglelayerofmechanicallyexfoliatedgraphene(seeFigure9).7Intheirexperiments,substrate-inducedscatteringwasminimizedbycleverlyetchingunderthechanneltoproducegraphenecompletelysuspendedbetweengoldcontacts.Atsuchhighcarriermobility,chargetransportisessentiallyballisticonthemicrometer-scaleatroomtemperature.This

Figure9.Suspendedgrapheneshowsextremelyhighmobilityduetotheminimizationofsubstrate-inducedscattering.(a)SEMimageofasuspendedsheetafteretching.(b)Field-effectmeasurementsindicatemobilitygreaterthan200,000cm2/(Vs).(Reprintedwithpermissionfromref7.Copyright2008Elsevier.)

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138ChemicalReviews,2010,Vol.110,No.1Figure10.Schematicdiagramshowingthebandstructureandresultingambipolar eldeffectingraphene.ConductionandvalencebandsmeetattheDiracpointwithoutanexternal eld.Undergatebias,theFermilevelmovesaboveorbelowtheDiracpointtointroduceasigni cantnumberoffreecarriers.[ReprintedwithpermissionfromNature(),ref10.Copyright2007NaturePublishingGroup.]

hasmajorimplicationsforthesemiconductorindustrybecauseitenables,inprinciple,fabricationofall-ballisticdevicesevenattoday’sintegratedcircuit(IC)channellengths(currentlydownto45nm).

Thesecondimportantpointaboutchargetransportingrapheneisambipolarity.Inthe eld-effectcon guration,thisimpliesthatcarrierscanbetunedcontinuouslybetweenholesandelectronsbysupplyingtherequisitegatebias.Thiscanbeeasilyvisualizedgiventheuniquebandstructureofgraphene(seeFigure10).10Undernegativegatebias,theFermileveldropsbelowtheDiracpoint,introducingasigni cantpopulationofholesintothevalenceband.Underpositivegatebias,theFermilevelrisesabovetheDiracpoint,promotingasigni cantpopulationofelectronsintotheconductionband.

Besidesmotivatingacademicinterest,accesstoatrulyambipolarsemiconductorenablesanumberofnoveldevicestructures.Thesearefundamentallydifferentfromsilicon-basedlogicbecausedopinglevelscanbedynamicallycontrolledentirelybygating.Momentarilyprovidinglocalgatebiasestodifferentpartsofthesame akecanformjunctionsorevenmorecomplicatedlogic.Subsequentlyrearrangingthebiasescanthencompletelyrede nethedevicewithoutmakinganyphysicalchangestothechannelmaterial.

4.2.SingleMoleculeDetection

Thesecondexcitingproof-of-conceptimplementationofmechanicallyexfoliatedgraphenewasinchemicalsensors.Severalimportantfeaturesofgraphenemadeitanexcellentcandidateforthesensingactivearea.Firstandforemost,the2Dstructureofgrapheneconstitutesanabsolutemaximumofthesurfaceareatovolumeratioinalayeredmaterial,whichisessentialforhighsensitivity.Infact,thishasbeenthemajormotivationbehindimplementationofothernano-structuredmaterialsinsensors.Inthecaseoftraditionalmaterials,bulkpropertiessuchasresistivityarenotsubstan-tiallyin uencedbysingleadsorptioneventsontheirsurface.Ingraphene,however,thereisnosuchdistinctionbetweensurfacesitesandthebulkmaterial,soeveryadsorptioneventissigni cant.

Versatilityofgrapheneasthebasisofasensorresultsfromitsuniqueelectronicstructure.Theambipolaritymeansthat

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Figure11.Thelackofsurfacestatesingraphenemakespossiblethedetectionofevensingleadsorbate.Thedirectionofchangeinthe gureindicatesthesignofinducedcarriers(holesforH2OandNO2;electronsforCOandNH(),3).[ReprintedwithpermissionfromNatureref21.Copyright2007NaturePublishingGroup.]

adsorptionofeitherelectronwithdrawingordonatinggroupscanleadto“chemicalgating”ofthematerial,whichcanbeeasilymonitoredinaresistive-typesensorsetup.

In2007,singlemoleculesensitivitytoNObytheManchester21,22groupforthe2andNH rstgraphene-3wasdemonstratedbasedsensor(seeFigure11).Inthecaseofeitheranalyte,adsorptioneventsinducedsomepopulationoffreecarriersandtheresistivityofasinglelayer akedecreaseduponexposure.AHall-typecon gurationcon rmedtheoppositesignofcarriersgeneratedbythetwogases,withanelectron-withdrawingspecies(e.g.,NO2)inducingconductionbyholes(p-type)andanelectron-donor(e.g.,NH3)inducingconduc-tionbyelectrons(n-type).

Withsuchexcitingearlyresults,experimentswithgraphene-basedsensorshavecertainlyjustbegun.Whiletherearefewquestionsaboutthelimitsofsensitivityforthesedevices,therealdrawbackthusfarisalackofselectivity.Asensorisratherimpracticalifitrespondsinasimilarwayuponexposuretoanyanalyte.Thisisanexcellentareaofopportunityforchemists.Modi cationofthebasalplaneoritsedgescouldcertainlyincorporateanalyte-speci clock-and-keytypebindingsites.Suchanapproachwouldnotonlyprovideselectivesensitivitytoalargevarietyofchemicalspeciesbutperhapsalsoenabledetectionofbiologicalagentsaswell.Similarschemeshavebeensuccessfullydemon-stratedwithcarbonnanotubesandquantumwires.92-95

5.AlternativestoMechanicalExfoliation

Excitingprogressinthe eldofgraphene,andespeciallysosoonafteritsinitialdiscovery,begantosuggestabrightfuture.Thelimitingstepformostexperimentswassimplyobtaininggoodsinglelayersbymechanicalexfoliation.Thiswouldhavegreaterimplicationsforreal-worlddevicesbecausetheprocessislowthroughputandunlikelytobeindustriallyscalable.Withthisinmind,thechallengeof ndinganalternateroutetosingle-layergraphenebecamethefocusofagreatdealofresearch.

Oneshouldkeepinmindthreeimportantfactorsbeyondscalabilitywhenconsideringthepro ciencyofanysyntheticroutetographene.Firstandforemost,aprocessmustproducehighqualityinthe2Dcrystallatticetoensurehighmobility.Second,themethodmustprovide necontrolovercrystallite

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HoneycombCarbon:AReviewofGrapheneFigure12.Molecularmodelsshowtheconversionprocessfromgraphitetochemicallyderivedgraphene.(Reprintedwithpermissionfromref101.Copyright2009NaturePublishingGroup.)

thicknesssoastodeliveruniformdeviceperformance.Finally,andforeaseofintegration,anyprocessshouldbecompatiblewithcurrentCMOS(complementarymetal-oxide-semiconductor)processing.

5.1.Oxide

ChemicallyDerivedGraphenefromGraphiteIn2006,Ruoff’sgroupwasthe rsttodemonstrateasolution-basedprocessforproducingsingle-layergraphene(seeFigure12).27,29,96,97Themethodhingedonchemicalmodi cationofgraphitetoproduceawaterdispersibleintermediary,graphiteoxide(GO).AfteroxidationbyHummers’method,GOisalayeredstackofpuckeredsheetswithABstacking,whichcompletelyexfoliatesupontheadditionofmechanicalenergy.98,99Thisisduetothestrengthofinteractionsbetweenwaterandtheoxygen-containing(epoxideandhydroxyl)functionalitiesintroducedintothebasalplaneduringoxidation.Thehydrophilicityleadswatertoreadilyintercalatebetweenthesheetsanddispersethemasindividuals.

AlthoughGOitselfisnonconducting,thegraphiticnetworkcanbesubstantiallyrestoredbythermalannealingorthroughtreatmentwithchemicalreducingagents,anumberofwhichhavebeenexplored.Ruoff’sgroupdetailedtheuseofhydrazinehydratetoeliminateoxidationthroughtheforma-tionandremovalofepoxidecomplexes.29ThiswasdonebyaddinghydrazinedirectlytoaqueousdispersionsofGO.Intheiroriginalreport,thereducedsinglesheetswereusedasanadditiveforpolystyrene-basedcomposites.27,100The2Dgeometryledtoanextremelylowpercolationthresholdofjust0.1%,enhancingboththeconductivityandstrengthofthematrix.

OneproblemwiththeoriginalaqueousreductionofGOwasthattheremovalofoxygengroupscausedthereducedsheetstobecomelesshydrophilicandquicklyaggregateinsolution.GordonWallace,DanLi,andco-workersincollaborationwithourgrouplatershowedthatraisingthepHduringreductionleadstocharge-stabilizedcolloidaldispersions,evenofthedeoxygenatedsheets.30Recentlywe’veimprovedthereductionstepbymakingdispersionsdirectlyinanhydroushydrazine.101-103Notethatuseofhydrazinerequiresgreatcarebecauseitisbothhighlytoxicandpotentiallyexplosive.104

ThemostexcitingadvantagesoftheGOmethodareitslow-costandmassivescalability.Thestartingmaterialissimplegraphite,andthetechniquecaneasilybescaled-uptoproducegramquantitiesorlargerof“chemicallyderivedgraphene”dispersedinaliquid.GOisalsoaninterestingmaterialinitsownrightforcompositesapplications.Ruoff’sgrouphasdemonstratedfree-standing lmswithextremelyhightensilestrengthupto～42GPa(seeFigure13).105,106

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5.1.1.Depositions

Obtaininguniformandreproducibledepositionsisoneforthemostimportantrequirementsforincorporatingasolution-basedtechniqueintodevicefabrication.Furthermore,thetypeofdepositionrequiredcanrangewidelydependingonthedesignspeci csofagivendevice.Well-suitedtothistask,chemicallyconvertedgraphenesuspensionsareversatileandhavepermitteda-large109numberofdepositiontechniques(seeFigure14).101,107Thesehavebeenusedtoproduce lmswithcoverageranginganywherefromevenlyspacedsinglesheetstodenselypackedoverlapping lms.

Theoriginaltechniqueusedbyourgroupfordepositing lmswasspraycoatingfromwaterontoaheatedsubstrate.31Althoughwewereabletoisolateandcharacterizesomesinglesheets,highsurfacetensioncausedsigni cantag-gregationevenifthesubstratewasheatedto ash-drythesuspensionuponcontact.Wehavebeenmoresuccessfulspin-coatingdispersionsmadedirectlyinhydrazine.Themethodallowsforafullrangeofcoveragedensitiesby ne-tuningofspin-speedandapretreatmentappliedtothesurfaceofthesubstrate.

Huang’sgroupatNorthwesternrecentlydemonstratedwonderfulcontroloverdepositionsusingLangmuir-BlodgettassemblyofGO.107Theyshowedthatelectrostaticrepulsionpreventsthesinglelayersfromoverlappingwhencompressedatanair/surfaceinterface.ThisledtodepositionsonSiOthatincludeddilute,close-packed,andoverpacked lms.2Dai’sgroupatStanfordhasdonesimilarworkwithLangmuir-Blodgetttechniquesandalsolayer-by-layeras-semblyusingelectrostaticattractiontobiasedsubstrates.109

5.1.2.DefectDensityinChemicallyDerivedGraphene

Aswithmechanicallyexfoliatedgraphene,itisimportanttocharacterizechemicallyderived akesbeforefabricatingdevices.Thisisespeciallytrueinthechemicalcasebecausethebasalplaneofgrapheneundergoesseriousalterationduringtheprocessofoxidationandreduction.ResidualoxidationwasmadeobviousbyanappreciableDbandnear1350cm-1intheRamanspectrumofchemicallyderivedgraphene.29ThisbandismadeRamanactivebythesigni cantnumberofdefectsandresultingbrokensymmetryofthebasalplane.X-rayphotoelectronspectroscopy(XPS)ofreducedGOindicatesnearlycompleteremovalofoxygen,whichhasledRuoff’sgroupandourowntosurmisethatnonconjugatedsp3carbonconstitutesmostofthedefects.Thesealsolimittheobservationofinterestingphysicsphenomenainchemicallyderivedgrapheneandinhibitmobility.

ThepresenceofalargeDbandalsoprecludesthe ngerprintingtechniquethatisusedtodeterminecrystallitethicknessformechanicallyexfoliatedgraphene.Thepromi-nenceofthebandmakesassigninganyexactpositiontotheGbandnexttoimpossible.Instead,mostgroupshaveturnedtoatomicforcemicroscopyinordertocon rmthethicknessofdeposited akes.Asinothercases,thegraphene-substratestepheightisdif culttoresolveandreportsrangebetween0.4and1.0nmforsinglelayers.30,101,107

5.1.3.Field-EffectDevices

Evenataloweroverallcrystalquality,theavailabilityofchemicallyderivedgraphenehasremovedaseriouslogjamintheengineeringofgraphene-baseddevicesandscientistshavebeeneagertomakeelectricalmeasurements.The rst

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Figure13.Free-standinggraphene lmsshowextremelyhightensilestrength.(a)Cross-sectionalSEMimageofgraphiteoxidestackingina lmproducedby ltration.[ReprintedwithpermissionfromNature(),ref105.Copyright2007NaturePublishingGroup.](b)Chemicalreductionproducesa lmwithshinyluster[ReprintedwithpermissionfromScience(),ref125.Copyright2008AmericanAssociationfortheAdvancementofScience.]

Figure14.Solutionprocessingallowsdepositionofsynthesized/modi edgrapheneinavarietyofdensities.(a)SEMimagesofdifferent lmsspin-coatedfromhydrazine.(b)SEMandatomicforcemicroscopyimagesofagraphiteoxide lmdepositedbyLangmuir-Blodgettassembly.(Reprintedwithpermissionfromref107.Copyright2009AmericanChemicalSociety.)(c)Multilayercoatingsarestillquitetransparent.[ReprintedwithpermissionfromNature(),ref109.Copyright2008NaturePublishingGroup.]

deviceswerefabricatedbye-beamlithographyon akesaround1µm2insize.31Morerecently,Ruoff’sgroupandourownhaveproducedmuchlarger akes,whichenabledustodemonstratescalablearraysof eld-effectdevicesusingconventionalphotolithography(seeFigure15).96,101

Thequalityofthe2Dlatticeinchemicallyderivedgrapheneissacri cedduringoxidationasthehybridizationofmanycarbonschangesfromplanarsp2totetrahedralsp3andthesheetpuckers.Thishasseveralconsequencesindeviceperformance.Unlikethosemadefrommechanicallyderivedgraphene,weobservedp-typecurrentmodulationfortop-contactandback-gated eld-effectdevices.Weattributethistoresidualoxidation,whichprovidesdeeptrapstatesforelectronsandlimitsanygatemodulationtothatofholes.Anotherconsequenceisinhibitedmobility,whichweestimateatlessthan1000cm2/(Vs).

Whilethesepropertieshaveledsometoquestiontheappropriatenessofchemicallyderivedgraphene-basedde-

vices,thematerialhasprovidedanexcellentplatformfortestingnoveldevicestructures.Furthermore,researchershavegainedvaluableexperienceintegratingasolution-basedtechniqueintodevicefabrication.Thosemethodswillcarryovershouldaless-severeroutetocompleteexfoliationbedeveloped.

5.1.4.PracticalSensors

Whilesingle-moleculedetectionfrommechanicallyex-foliatedgraphenewasanexcitingproof-of-principle,thedif cultyofproducingthinspecimensandtherequirementofultrahighvacuumlimitsthepracticalityofthesedevices.Recently,anumberofgroups,includingRobinson’sattheNavalResearchLaboratoryandourown,havedemonstratedgoodsensitivityforNO2,NH3,anddinitrotolueneunderambientconditionsusingchemicallyderivedgraphene(seeFigure16).103,110

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Figure15.(a)SEMimageofalargesinglesheetdepositedonSiO2.(b)Schematicviewofatop-contact,back-gateddevice.(c)Photograph(left),opticalimage(middle),andSEMimage(right)ofaworkingdevicewithachannellengthof7µm.(Reprintedwithpermissionfromref101.Copyright2009NaturePublishingGroup.)

Figure16.Chemicallyderivedgrapheneprovidesapracticalroutetographene-basedresistivesensors.Theresistanceofthep-typematerialdecreasesuponexposuretoelectronwithdrawers(e.g.,NO2)andincreasesuponexposuretoelectrondonors(e.g.,NH3).(Reprintedwithpermissionfromref103.Copyright2009AmericanChemicalSociety.)

Itisinterestingtonotethedifferencesinresponseofchemicallyderivedgraphenesensorsandmechanicallyexfoliatedones.Asdiscussedearlier,electrondonatingorwithdrawinggroupsincreaseelectronorholepopulationsinpristinegrapheneandthusbothleadtoincreasedconductiv-ity.Chemicallyderivedgrapheneisnominallyp-type.Therefore,electronwithdrawinggroupscontributeadditionalcarriers,butelectrondonatorsactuallyservetodepleteholesfromthevalenceband.Hence,NO2andNH3ledtooppositedirectionsofresponseinoursensors.

solarcell,whichhadapowerconversionef ciency(PCE)of0.26%.Chhowalla’sgrouplaterfabricatedapolymersolarcellwithaPCEof0.1%usingasimilar lm.112,113TheperformanceofthesecellswaslessthanthatofthecorrespondingcontroldevicesonITO,buttheyprovideaproof-of-conceptforlow-costtransparentcoatingsbasedongraphene.

5.2.TotalOrganicSynthesis

Althoughgraphiteoxidehasproducedthe rstchemicallyderivedmicrometer-scalegraphene,synthetictechniquesforsmallerplanar,benzene-basedmacromoleculeshavebeenknownforsometime.33,34,114-117Thesegraphene-likepolya-cyclichydrocarbons(PAHs)occupyaninterestingplaceinbetween“molecular”and“macromolecular”structuresandarenowattractingnewinterestasapossiblealternativeroutetographene.

PAHsareattractivebecausetheyarehighlyversatileandcanbesubstitutedwitharangeofaliphaticchainstomodifysolubility.36Thusfar,themajordrawbackofPAHshasbeentheirlimitedsizerange.Thisisduetothefactthatincreasingmolecularweightgenerallydecreasessolubilityandincreasestheoccurrenceofsidereactions.Undertheseconditions,preservationofdispersibilityandaplanarmorphologyforlargePAHshasbeenverychallenging.

Amajoradvancecamein2008,whenMullenandco-workersreportedthesynthesisofnanoribbon-likePAHsupto12nminlength(seeFigure17).35Althoughtheelectronicpropertiesofthesenanoribbonshaveyettobecharacterized,

5.1.5.TransparentElectrodes

Solutionprocessingofchemicallyderivedgrapheneandthedepositionsachievedsoonledresearcherstoconsiderusingthematerialintransparentconductors.Thedemandforsuchcoatingshasgrownrapidlyduetooptoelectronicdevicesincludingdisplays,LEDs,andsolarcells.Whilethecurrentindustrystandardisindiumtinoxide(ITO),carbonnanotubeshavelongbeentoutedasapossiblealternativeduetotheirlowdimensionalityandabilitytoformapercolatingconductivenetworkatextremelylowdensities.Thesamemeritsmakegrapheneanobviouschoice.

Mullenandco-workersdemonstratedthe rstgraphene-basedtransparentconductor.111Filmsweredepositedbydip-coatingwithGOandreducingbythermalannealing.Sheetresistancesaslowas0.9k /)wereobtainedat70%transmittance.WhiletheperformancewasconsiderablylessthanthatofITO(70 /)at90%transmittance),the lmswerelow-costanddidnotrequirevacuumsputtering.Thegroupalsousedthe lmastheanodeinadye-sensitized

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Figure17.Polyacyclicaromatichydrocarbons(PAHs)mayofferaground-upsynthesisofgraphene.(a)ChemicalstructureofPAHsand(b)TEMofananoribbonsynthesizedbyMullen.(Reprintedwithpermissionfromref35.Copyright2008AmericanChemicalSociety.)

theymayindeedexhibitgraphene-likebehavior.Ifresearch-ersinthisareaareabletofurtherextendthesizerangeofPAHsinthecomingyears,thiscouldprovideacleansyntheticroutetographeneforsomeapplications.Inanyevent,theorganictechniquesdevelopedwillhaveimportantimplicationsformodi cationoforadditiontoconjugatedcarbonmacromolecules.

5.3.EpitaxialGrapheneandChemicalVaporDeposition

Whilesolution-basedsyntheticschemesaimtocircumventtheneedforsupportsubstrates,twotechniquestakeadvan-tageofspeciallychosenplatformstoencouragegrowthofhighqualitygraphene.

DeHeerandothersattheGeorgiaInstituteofTechnologypioneeredanepitaxialmethodinwhichgrapheneresultsfromthehightemperaturereductionofsiliconcarbide(seeFigure18).38-40,118-120Theprocessisrelativelystraightforward,assilicondesorbsaround1000°Cinultrahighvacuum.Thisleavesbehindsmallislandsofgraphitizedcarbon,whichwere rstlocatedbySTMandelectrondiffractionexperiments.Morerecently,groupshaveusedphotolithographytopatternepitaxialgrowthinpredeterminedlocationsandtomakedevices.119

Anumberofphysicalpropertiesdifferbetweenepitaxiallygrownandmechanicallyexfoliatedgraphene.37,39Thisisduetothein uenceofinterfacialeffectsinepitaxialgraphene,whichareheavilydependentonboththesiliconcarbidesubstrateandseveralgrowthparameters.Forepitaxialgraphene,differencesintheperiodicityobservedbySTMandLEEDSarenotwellunderstood.121Thesameistruefor

Figure19.Chemicalvapordepositionofgrapheneontransitionmetalsubstrates.Opticalmicroscopeimageof(a)thenickelcatalystand(b)theresultinggraphene lm.TEMimagesshowthenucleationof(c)one,(d)three,or(e)fourlayersduringthegrowthprocess.(Reprintedwithpermissionfromref41.Copyright2009AmericanChemicalSociety.)

theenergygapobservedbyangle-resolvedphotoemissionspectroscopy(ARPES).122

Thesecondsubstrate-basedmethodischemicalvapordeposition(CVD)ofgrapheneontransitionmetal lms(seeFigure19).GroupsatMITandinKoreapioneeredtheprocess,whichreliesonthecarbon-saturationofatransitionmetaluponexposuretoahydrocarbongasathightemperature.41-43Mostoften,nickel lmsareusedwithmethanegas.Uponcoolingthesubstrate,thesolubilityofcarboninthetransitionmetaldecreasesandathin lmofcarbonisthoughttoprecipitatefromthesurface.

Oneofthemajoradvantagesofsubstrate-basedmethodsforgraphenesynthesisistheirhighcompatibilitywithcurrentCMOStechnology.Intheory,bothepitaxialandCVDtechniqueshavetheprospectofproducingasinglesheetofgrapheneoveranentirewafer,whichmaybethesimplestwaytointegratethenewmaterialintocurrentsemiconductorprocessesanddevices.TheremainingchallengeforepitaxialandCVDmethodsisobtaining necontrolover lm

Figure18.Siliconcarbideisreducedtographeneassiliconsublimesathightemperature.(a)SEMimageshowssmallhexagonalcrystallites.(Reprintedwithpermissionfromref120.Copyright2006Elsevier.)(b)STMimageshowslong-rangeorderandalowdensityofdefects.[ReprintedwithpermissionfromScience(),ref38.Copyright2006AmericanAssociationfortheAdvancementofScience.]

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Figure20.Nanoribbonsofferenhancedtransistorbehaviorduetoquantumcon nement.(a)SEMimageofnanoribbonsde nedbyphotolithographyandO2plasmaetching.(b)Kim’sgroupdemonstratedIon/Ioffratiosashighas104atwidthsof～50nm.(Reprintedwithpermissionfromref124.Copyright2007AmericanPhysicalSociety.)

thicknessandpreventingsecondarycrystalformation.Inanidealcase,bothmethodsrelyonthenucleationandgrowthofasinglecrystalwithouttheformationofaboundaryorseedingofasecondlayer.Currently,thebestspecimenshaveavariationinthicknessofperhaps1-3layersandarepolycrystalline.FieldeffectdevicesfabricatedwithepitaxialandCVDgraphenedisplaycarriermobilitiesinexcessof1000cm2/(Vs).42,118

InthecaseofCVDgraphene,etchingoftheunderlyingmetalallowsthecarbon lmstobetransferredtoothersubstrates.This,combinedwiththelargeareaofdepositions,hasgreatpromisefortransparentconductingapplications.Onesuch lmgrownbyCVDandtransferredviaaPDMSstampontoglassshowedasheetresistanceofjust280 /)at80%opticaltransmittance.42

exfoliatedgraphenetomakethe rstsub-50nmnanoribbons(seeFigure20).123,124AlthoughtheprocessyieldedIon/Ioffratiosofupto104,devicesshowedhighvariabilityduetothelackofcontroloveredgetermination.Stencil-likepatterningwasindiscriminateofcrystallographicdirection,andthusedgeeffectswereessentiallydifferenteverytime.Thechallengeofsynthesizingreproduciblegraphenenanoribbonsisaninterestingoneforchemists,whohavesoughttoexploitthedifferencesinreactivityalonggraphene’stwocrystallographicdirections.64In2008,Dai’sgroupatStanforddevelopedthe rsttechniqueforisolatingnano-ribbonsdirectlyfrombulkgraphite(SeeFigure21).61Itinvolvedsonicationofexpandedgraphiteinthepresenceofapolymerknowntoparticipateinπ-stackingwithconjugatedcarbons.Thepolymeractedtononcovalentlyfunctionalizeandconsequentlystabilizenanoribbonsformedbymechan-icalfracture.Atomicforcemicroscopyofthenanoribbonssuggeststhatfracturefollowsnicelyalongthecrystal-lographicdirectionsofgraphene.Inthepresenceofthepolymer,theribbonscanbesuspendedinorganicsolventsandthendepositedbyspin-coating.

ElectricaltestingofDai’snanoribbonsshowedmuchgreaterconsistencythanthosemadebylithography.Aspredictedbytheory,thebandgap(Eg)ofnanoribbonswasfoundtobeinverselyproportionaltotheirwidth,withanEgof～0.4eVforspecimensfewerthan10nmwide.ThisledtoIon/Ioffratiosofupto106forthethinneststrips.Thenextmajorchallengewillbe ndingawaytoreliablydepositthenanoribbonsinprede nedlocationsforscalabledevicefabrication.

6.GrapheneNanoribbons

Amajorissuewithgraphene-basedlogicdevicesistheirpoorIon/Ioffratios.Conductivityingrapheneisminimizedunderzerogatebias,butdevicesareessentiallyimpossibletoturnoffatanyreasonabletemperaturebecausethermalenergyand uctuationsaremorethansuf cienttoproducelargecarrierpopulations.Thishigh“leakage”currentresultsinIon/Ioffratiosthataretypicallyjust1or2ordersofmagnitude,whichisinsuf cientforimplementationinrealdevices.

Whileanumberofapproacheshavebeensuggested,themoststraightforwardwaytominimizetheoffcurrentingraphene-baseddevicesistointroduceanappreciablebandgap.Thishasmotivatedagreatdealofresearchintographenenanoribbons,whicharenolongersemimetallicduetoquantumcon nement.In2007,Kim’sgroupusede-beampatterningandoxygen(O2)plasmaetchingofmechanically

7.FutureWork

Graphenehasaninterestinghistory,butmanynowwonderaboutitsfuture.Thesubjectofconsiderablescholarlydebate,itdoesseemreasonabletoassertafewthingslookingahead.First,thequalityandavailabilityof“synthetic”graphenewillcontinuetoimprove.Whetherhighqualitymaterialcomesintheformofanalternativechemicalroutetothecompleteexfoliationofgraphiteorfromoptimizationofthethermalprocessesrequiredforsubstrate-basedmethods,thereisnosignthatsynthetictechniquesarenearingtheirupperlimit.Thismeansthatdeviceengineerswillhaveampleaccesstoimprovedmaterialsfordevelopingnovelstructuresand ndingwaystointegrategrapheneintopresent-dayelectronicdevices.

Second,chemicalmodi cationofgraphene’sbasalplaneoritsedgeswillsubstantiallyin uencegraphene-baseddevices.Forelectronicapplications,onecanimaginethe

Figure21.Solution-basedmethodforproducinggraphenenanoribbons.(a)Dai’sgroupusedπ-stackingpolymeragentstostabilizenanoribbonsinsolution.(b)Afterspin-coating,ribbonsranginginwidthdownto10nmwerelocatedbyatomicforcemicroscopy.(c)Ion/Ioffratiosupto106weredemonstrated.[ReprintedwithpermissionfromScience(),ref61.Copyright2008AmericanAssociationfortheAdvancementofScience.]

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144ChemicalReviews,2010,Vol.110,No.1attachmentoffunctionalgroupsaimedatself-assemblyofsimplecircuitsortheincorporationofchemicaldopantstolimitleakagecurrentunderzerogatebias.Forsensors,lock-and-keytypebindingsitescouldprovideselectivesensitivitytoawidevarietyofanalytes.Thesemightincludechemicalwarfareagentsorevenbiologicalspecies.

Third,industrialuseofgrapheneasatransparentconductorcouldhavehugeimplicationsforthesolarindustry.Assyntheticroutesimprove,theprospectofreplacingITOwithalow-costcarbon-basedcoatingseemsfeasible.Thiswouldnotonlyremovesigni cantuncertaintyabouttheavailabilityandcostofindiumbutalsoenablenonevaporativeroll-to-rollprocessingoftransparentconductors.

8.Conclusions

The eldofgraphene-relatedresearchhasgrownataspectacularpacesincesingle-layer akeswere anicandmaterialschemistsarebusilyworkingonnewsyntheticroutestohigh-qualitysinglelayers,whileengineersaredesigningnoveldevicestoexploitgraphene’sextraordinaryproperties.Inlightofsuchcollaborations,itisdif culttobelievethatthefutureforgrapheneisanythingbutbright.

9.Acknowledgments

TheauthorswouldliketothanktheNationalScienceFoundation’sNSF-IGERTprogram,theUCLA-basedFunc-tionalEngineeringNanoArchitechtonicsFocusedCenterResearchProgram(FENA-FCRP),theDARPACERAprogram,andNorthrop-Grumman/UCDiscoveryfor nancialsupport.

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