ReviewsC.N.R.Raoetal.DOI: 10.1002/anie.200901678Graphene: TheNewTwo-DimensionalNanomaterialC.N.R.Rao,*A.K.Sood,K.S.Subrahmanyam,andA.GovindarajKeywords:carbongraphenegrapheneoxidemonolayersnanostructuresAngewandteChemie7752.angewandte.org2009Wiley-VCHVerlagGmbH&Co.KGaA,WeinheimAngew.Chem.Int.Ed.2009,48,77527777 / Edited by Foxit ReaderAngewandteChemieGrapheneCopyright(C) by Foxit Software pany,2005-2008For Evaluation Only.Everyfewyears,anewmaterialwithuniquepropertiesemergesand FromtheContentsfascinatesthescientificmunity,typicalrecentexamplesbeinghigh-temperaturesuperconductorsandcarbonnanotubes.Grapheneisthelatestsensationwithunusualproperties,suchashalf-integerquantumHalleffectandballisticelectrontransport.Thistwo-dimen-sionalmaterialwhichistheparentofallgraphiticcarbonformsisstrictlyexpectedtopriseasinglelayer,butthereisconsiderableinterestininvestigatingtwo-layerandfew-layergraphenesaswell.Synthesisandcharacterizationofgraphenesposechallenges,buttherehasbeenconsiderableprogressinthelastyearorso.Herein,wepresentthestatusofgrapheneresearchwhichincludesaspectsrelatedtosynthesis,characterization,structure,andproperties.1.Introduction7753775477602.Synthesis3.ElectronicStructure4.PhononsandRamanSpectroscopy776277645.EffectsofDoping6.FunctionalizationandSolubilization77677.DecorationwithMetalandMetalOxideNanoparticles77697770777377731.Introduction8.PropertiesGraphene,theparentofallgraphiticforms(Figure1),hasbeeoneofthemostexcitingtopicsofresearchinthelastthree to four years.9.Polymerposites10.Outlook1This two-dimensional material consti-tutes a new nanocarbon prising layers of carbon atomsarrangedinsix-memberedrings.Itisdistinctlydifferentfromcarbonnanotubes(Ts)andfullerenes,andexhibitsuniqueproperties which have fascinated the scientific munity.Typically important properties of graphene are a quantumHall effect at room temperature,fieldeffectalongwithballisticconductionofchargecarriers,tunablebandgap, andhighelasticity. Althoughgrapheneis expected to be perfectly flat, ripples occur because ofthermal fluctuations. Ideally graphene is a single-layerseveralstrategieshavesincebeendevelopedforthesynthesisofgraphenes.Graphenehasbeencharacterized byavarietyof micro-scopic and other physical techniques including atomic force24an ambipolar electric8567microscopy(AFM),transmissionelectronmicroscopy1(TEM), scanning tunneling microscopy (STM), X-ray dif-fraction(XRD),andRamanspectroscopy. Itisinterestingmaterial,butgraphenesampleswithtwoormorelayersarebeinginvestigatedwithequalinterest.Threedifferenttypesof graphenes can be defined: single-layer graphene (SG),bilayergraphene(BG),andfew-layergraphene(FG,numberof layers 10). Although single-layer graphene and bilayer1that single-layer graphene placed on a silicon wafer with a300nm thick layer of SiO2, bees visible in an opticalmicroscope(Figure2aandb). WhileAFMdirectlygivesthe number of layers (Figure2c), STM (Figure2d) and11TEM (Figure2e) images are useful in determining the8108graphenewerefirstobtainedbymicro-mechanicalcleavage,512morphologyandstructureofgraphene.Ramanspectroscopyhasemergedtobeanimportanttoolforthecharacterizationof graphene samples. Herein, we shall discuss various1316aspectsofgraphene,includingsynthesis,structure,properties,functionalization,andpolymerposites.AlthoughwehavecoveredmostoftheimportantfacetsofgraphenepublisheduptoMay2009,wehavegivensomewhatgreaterimportancetothechemicalaspectsandcitedalargenumberofreferencesfrom the rapidly increasing literature. We do hope that the* Prof.Dr.C.N.R.Rao,K.S.Subrahmanyam,Dr.A.GovindarajInternationalCentreforMaterialsScience,NewChemistryUnitandCSIRCentreofExcellenceinChemistry,JawaharlalNehruCentreforAdvancedScientificResearchJakkurP.O., Bangalore560064(India)Fax:(+91)80-2208-2760:rraojncasr.ac.inProf.Dr.A.K.SoodDepartmentofPhysics,IndianInstituteofScienceBangalore560012(India)Figure1. Graphene:theparentofallgraphiticforms.(FromRef.1a.)Angew.Chem.Int.Ed.2009,48,775277772009Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim7753Edited by Foxit ReaderReviewsC.N.R.Raoetal.Copyright(C) by Foxit Software pany,2005-2008For Evaluation Only.on to a silicon substrate. Besides mechanical cleavage ofgraphite,theotherimportantmethodsemployedtoproducegraphenesamplesareepitaxialgrowthonaninsulatorsurface(such as SiC), chemical vapor deposition (CVD) on thesurfacesofsinglecrystalsofmetals(e.g.,Ni),arcdischargeofgraphiteundersuitableconditions,useofintercalatedgraph-ite as the starting material, preparation of appropriatecolloidal suspensions in selected solvents, and reduction ofgrapheneoxidesheets.8Byemployingmechanicalexfoliationofgraphite,mono-layers and bilayers