纳米材料在生物医学中的应用

142ChemicalReviews,2010,Vol.110,No.1Allenet

al.

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.]