纳米材料在生物医学中的应用

134ChemicalReviews,2010,Vol.110,No.1nitride4andmolybdenumdisul de.High,in-planeelectrical(～10 -1cm-1)andthermalconductivity(～3000W/mK)enablegraphitetobeusedinelectrodesandasheatingelementsforindustrialblastfurnaces.53,54Highmechanicalstiffnessofthehexagonalnetwork(1060GPa)isalsoutilizedincarbon berreinforcedcomposites.Theseusesandothersgenerateanannualdemandofmorethan1milliontonsofgraphiteworldwide.55

Theanisotropyofgraphite’smaterialpropertiescontinuestofascinatebothscientistsandtechnologists.Thes,patomicorbitalsoneachcarbonhybridizetoformstrongx,andpcovalentysp2bonds,givingriseto120°C-C-Cbondanglesandthefamiliarchicken-wire-likelayers.Theremainingporbitaloneachcarbonoverlapswithitsthreeneighboringzcarbonstoformabandof lledπorbitals,knownasthevalenceband,andabandofemptyπ*orbitals,calledtheconductionband.Whilethreeofthefourvalenceelectronsoneachcarbonformtheσ(single)bonds,thefourthelectronformsone-thirdofaπbondwitheachofitsneighborsproducingacarbon-carbonbondorderingraphiteofoneandone-third.Withnochemicalbondinginthec-direction,out-of-planeinteractionsareextremelyweak.Thisincludesthepropagationofchargeandthermalcarriers,whichleadstoout-of-planeelectricalandthermalconductivitiesthatarebothmorethan103timeslowerthanthoseoftheirin-planeanalogues.56

2.1.ChemistryofGraphite

Graphitehasarichchemistryinwhichitcanparticipateinreactionsaseitherareducingagent(electrondonor)oranoxidizer(electronacceptor).Thisisadirectconsequenceofitselectronicstructure,whichresultsinboth53

anelectronaf nityandanionizationpotentialof4.6eV.Alargenumberofexperimentsforgraphitefocusontheinsertionofadditionalchemicalspeciesbetweenthebasalplanes,orintercalation.Shaffaultiscreditedwiththe rstintercalationcompoundusingpotassium,datingbackto1841.57Graphiteintercalationcompounds(GICs)appeartobetheonlylayeredcompoundssuf cientlyorderedtoexhibit“staging”inwhichthenumberofgraphiticlayersinbetweenadjacentintercalantscanbevariedinacontrolledfashion.Thestageofacompoundreferstothenumberofgraphiticlayersinbetweenadjacentplanesofintercalant.Theinter-layerspacingcanincreasefrom0.34nm(3.4Å)innativegraphitetomorethan1nminsomeGICs,whichfurtherexaggeratestheanisotropyofmanyproperties.56,58

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