Honeycomb Carbon A Review of Graphene 石墨烯综述(5)

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

ananoribbonsynthesisdevelopedbyDai(seeFigure3).Asecondfocusofexperimentsongraphitehasbeensubstitutionaldopingbythereplacementofcarbonwithotherelements.ThisincludesworkbyBartlettandco-workersatBerkeleyinwhichsubstitutionofcarbonwithboronand

Allenet

al.

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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