Honeycomb Carbon A Review of Graphene 石墨烯综述(8)

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

Allenet

al.

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.

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