The-rise-of-graphene

THERISEOFGRAPHENEA.K.GeimandK.S.NovoselovManchesterCentreforMesoscienceandNanotechnology,UniversityofManchester,OxfordRoadM139PL,UnitedKingdomGrapheneisarapidlyrisingstaronthehorizonofmaterialsscienceandcondensedmatterphysics.Thisstrictlytwo-dimensionalmaterialexhibitsexceptionallyhighcrystalandelectronicqualityand,despiteitsshorthistory,hasalreadyrevealedacornucopiaofnewphysicsandpotentialapplications,whicharebrieflydiscussedhere.Whereasonecanbecertainoftherealnessofapplicationsonlywhencommercialproductsappear,graphenenolongerrequiresanyfurtherproofofitsimportanceintermsoffundamentalphysics.Owingtoitsunusualelectronicspectrum,graphenehasledtotheemergenceofanewparadigmof“relativistic”condensedmatterphysics,wherequantumrelativisticphenomena,someofwhichareunobservableinhighenergyphysics,cannowbemimickedandtestedintable-topexperiments.Moregenerally,graphenerepresentsaconceptuallynewclassofmaterialsthatareonlyoneatomthickand,onthisbasis,offersnewinroadsintolow-dimensionalphysicsthathasneverceasedtosurpriseandcontinuestoprovideafertilegroundforapplications.Grapheneisthenamegiventoaflatmonolayerofcarbonatomstightlypackedintoatwo-dimensional(2D)honeycomblattice,andisabasicbuildingblockforgraphiticmaterialsofallotherdimensionalities(Figure1).Itcanbewrappedupinto0Dfullerenes,rolledinto1Dnanotubesorstackedinto3Dgraphite.Theoretically,graphene(or“2Dgraphite”)hasbeenstudiedforsixtyyears1-3andwidelyusedfordescribingpropertiesofvariouscarbon-basedmaterials.Fortyyearslater,itwasrealizedthatgraphenealsoprovidesanexcellentcondensed-matteranalogueof(2+1)-dimensionalquantumelectrodynamics4-6,whichpropelledgrapheneintoathrivingtheoreticaltoymodel.Ontheotherhand,althoughknownasintegralpartof3Dmaterials,graphenewaspresumednottoexistinthefreestate,beingdescribedasan“academic”material5andbelievedtobeunstablewithrespecttotheformationofcurvedstructuressuchassoot,fullerenesandnanotubes.Allofasudden,thevintagemodelturnedintoreality,whenfree-standinggraphenewasunexpectedlyfoundthreeyearsago7,8and,especially,whenthefollow-upexperiments9,10confirmedthatitschargecarrierswereindeedmasslessDiracfermions.So,thegraphene“goldrush”hasbegun.MATERIALSTHATSHOULDNOTEXISTMorethan70yearsago,LandauandPeierlsarguedthatstrictlytwo-dimensional(2D)crystalswerethermodynamicallyunstableandcouldnotexist11,12.Theirtheorypointedoutthatadivergentcontributionofthermalfluctuationsinlow-dimensionalcrystallatticesshouldleadtosuchdisplacementsofatomsthattheybecomecomparabletointeratomicdistancesatanyfinitetemperature13.TheargumentwaslaterextendedbyMermin14andisstronglysupportedbyawholeomnibusofexperimentalobservations.Indeed,themeltingtemperatureofthinfilmsrapidlydecreaseswithdecreasingthickness,andtheybecomeunstable(segregateintoislandsordecompose)atathicknessof,typically,dozensofatomiclayers15,16.Forthisreason,atomicmonolayershavesofarbeenknownonlyasanintegralpartoflarger3Dstructures,usuallygrownepitaxiallyontopofmonocrystalswithmatchingcrystallattices15,16.Withoutsucha3Dbase,2Dmaterialswerepresumednottoexistuntil2004,whenthecommonwisdomwasflauntedbytheexperimentaldiscoveryofgraphene7andotherfree-standing2Datomiccrystals(forexample,single-layerboronnitrideandhalf-layerBSCCO)8.Thesecrystalscouldbeobtainedontopofnon-crystallinesubstrates8-10,inliquidsuspension7,17andassuspendedmembranes18.Importantly,the2Dcrystalswerefoundnotonlytobecontinuousbuttoexhibithighcrystalquality7-10,17,18.Thelatterismostobviousforthecaseofgraphene,inwhichchargecarrierscantravelthousandsinteratomicdistanceswithoutscattering7-10.Withthebenefitofhindsight,theexistenceofsuchone-atom-thickcrystalscanbereconciledwiththeory.Indeed,itcanbearguedthattheobtained2Dcrystallitesarequenchedinametastablestatebecausetheyareextractedfrom3Dmaterials,whereastheirsmallsize(1mm)andstronginteratomicbondsassurethatthermalfluctuationscannotleadtothegenerationofdislocationsorothercrystaldefectsevenatelevatedtemperature13,14.2Figure1.Motherofallgraphiticforms.Grapheneisa2Dbuildingmaterialforcarbonmaterialsofallotherdimensionalities.Itcanbewrappedupinto0Dbuckyballs,rolledinto1Dnanotubesorstackedinto3Dgraphite.Acomplementaryviewpointisthattheextracted2Dcrystalsbecomeintrinsicallystablebygentlecrumplinginthethirddimensiononalateralscaleof≈10nm18,19.Such3Dwarpingobservedexperimentally18leadstoagaininelasticenergybutsuppressesthermalvibrations(anomalouslylargein2D),whichaboveacertaintemperaturecanminimizethetotalfreeenergy19.BRIEFHISTORYOFGRAPHENEBeforereviewingtheearlierworkongraphene,itisusefultodefinewhat2Dcrystalsare.Obviously,asingleatomicplaneisa2Dcrystal,whereas100layersshouldbeconsideredasathinfilmofa3Dmaterial.Buthowmanylayersareneededtomakea3Dstructure?Forthecaseofgraphene,thesituationhasrecentlybecomereasonablyclear.Itwasshownthattheelectronicstructurerapidlyevolveswiththenumberoflayers,approachingthe3Dlimitofgraphitealreadyat10layers20.Moreover,onlygrapheneand,toagoodapproximation,itsbilayerhavesimpleelectronicspectra:theyarebothzero-gapsemiconductors(canalsobereferredtoaszero-overlapsemimetals)withonetyp