纳米材料在生物医学中的应用

HoneycombCarbon:AReviewofGrapheneFigure12.Molecularmodelsshowtheconversionprocessfromgraphitetochemicallyderivedgraphene.(Reprintedwithpermissionfromref101.Copyright2009NaturePublishingGroup.)

thicknesssoastodeliveruniformdeviceperformance.Finally,andforeaseofintegration,anyprocessshouldbecompatiblewithcurrentCMOS(complementarymetal-oxide-semiconductor)processing.

5.1.Oxide

ChemicallyDerivedGraphenefromGraphiteIn2006,Ruoff’sgroupwasthe rsttodemonstrateasolution-basedprocessforproducingsingle-layergraphene(seeFigure12).27,29,96,97Themethodhingedonchemicalmodi cationofgraphitetoproduceawaterdispersibleintermediary,graphiteoxide(GO).AfteroxidationbyHummers’method,GOisalayeredstackofpuckeredsheetswithABstacking,whichcompletelyexfoliatesupontheadditionofmechanicalenergy.98,99Thisisduetothestrengthofinteractionsbetweenwaterandtheoxygen-containing(epoxideandhydroxyl)functionalitiesintroducedintothebasalplaneduringoxidation.Thehydrophilicityleadswatertoreadilyintercalatebetweenthesheetsanddispersethemasindividuals.

AlthoughGOitselfisnonconducting,thegraphiticnetworkcanbesubstantiallyrestoredbythermalannealingorthroughtreatmentwithchemicalreducingagents,anumberofwhichhavebeenexplored.Ruoff’sgroupdetailedtheuseofhydrazinehydratetoeliminateoxidationthroughtheforma-tionandremovalofepoxidecomplexes.29ThiswasdonebyaddinghydrazinedirectlytoaqueousdispersionsofGO.Intheiroriginalreport,thereducedsinglesheetswereusedasanadditiveforpolystyrene-basedcomposites.27,100The2Dgeometryledtoanextremelylowpercolationthresholdofjust0.1%,enhancingboththeconductivityandstrengthofthematrix.

OneproblemwiththeoriginalaqueousreductionofGOwasthattheremovalofoxygengroupscausedthereducedsheetstobecomelesshydrophilicandquicklyaggregateinsolution.GordonWallace,DanLi,andco-workersincollaborationwithourgrouplatershowedthatraisingthepHduringreductionleadstocharge-stabilizedcolloidaldispersions,evenofthedeoxygenatedsheets.30Recentlywe’veimprovedthereductionstepbymakingdispersionsdirectlyinanhydroushydrazine.101-103Notethatuseofhydrazinerequiresgreatcarebecauseitisbothhighlytoxicandpotentiallyexplosive.104

ThemostexcitingadvantagesoftheGOmethodareitslow-costandmassivescalability.Thestartingmaterialissimplegraphite,andthetechniquecaneasilybescaled-uptoproducegramquantitiesorlargerof“chemicallyderivedgraphene”dispersedinaliquid.GOisalsoaninterestingmaterialinitsownrightforcompositesapplications.Ruoff’sgrouphasdemonstratedfree-standing lmswithextremelyhightensilestrengthupto～42GPa(seeFigure13).105,106

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5.1.1.Depositions

Obtaininguniformandreproducibledepositionsisoneforthemostimportantrequirementsforincorporatingasolution-basedtechniqueintodevicefabrication.Furthermore,thetypeofdepositionrequiredcanrangewidelydependingonthedesignspeci csofagivendevice.Well-suitedtothistask,chemicallyconvertedgraphenesuspensionsareversatileandhavepermitteda-large109numberofdepositiontechniques(seeFigure14).101,107Thesehavebeenusedtoproduce lmswithcoverageranginganywherefromevenlyspacedsinglesheetstodenselypackedoverlapping lms.

Theoriginaltechniqueusedbyourgroupfordepositing lmswasspraycoatingfromwaterontoaheatedsubstrate.31Althoughwewereabletoisolateandcharacterizesomesinglesheets,highsurfacetensioncausedsigni cantag-gregationevenifthesubstratewasheatedto ash-drythesuspensionuponcontact.Wehavebeenmoresuccessfulspin-coatingdispersionsmadedirectlyinhydrazine.Themethodallowsforafullrangeofcoveragedensitiesby ne-tuningofspin-speedandapretreatmentappliedtothesurfaceofthesubstrate.

Huang’sgroupatNorthwesternrecentlydemonstratedwonderfulcontroloverdepositionsusingLangmuir-BlodgettassemblyofGO.107Theyshowedthatelectrostaticrepulsionpreventsthesinglelayersfromoverlappingwhencompressedatanair/surfaceinterface.ThisledtodepositionsonSiOthatincludeddilute,close-packed,andoverpacked lms.2Dai’sgroupatStanfordhasdonesimilarworkwithLangmuir-Blodgetttechniquesandalsolayer-by-layeras-semblyusingelectrostaticattractiontobiasedsubstrates.109

5.1.2.DefectDensityinChemicallyDerivedGraphene

Aswithmechanicallyexfoliatedgraphene,itisimportanttocharacterizechemicallyderived akesbeforefabricatingdevices.Thisisespeciallytrueinthechemicalcasebecausethebasalplaneofgrapheneundergoesseriousalterationduringtheprocessofoxidationandreduction.ResidualoxidationwasmadeobviousbyanappreciableDbandnear1350cm-1intheRamanspectrumofchemicallyderivedgraphene.29ThisbandismadeRamanactivebythesigni cantnumberofdefectsandresultingbrokensymmetryofthebasalplane.X-rayphotoelectronspectroscopy(XPS)ofreducedGOindicatesnearlycompleteremovalofoxygen,whichhasledRuoff’sgroupandourowntosurmisethatnonconjugatedsp3carbonconstitutesmostofthedefects.Thesealsolimittheobservationofinterestingphysicsphenomenainchemicallyderivedgrapheneandinhibitmobility.

ThepresenceofalargeDbandalsoprecludesthe ngerprintingtechniquethatisusedtodeterminecrystallitethicknessformechanicallyexfoliatedgraphene.Thepromi-nenceofthebandmakesassigninganyexactpositiontotheGbandnexttoimpossible.Instead,mostgroupshaveturnedtoatomicforcemicroscopyinordertocon rmthethicknessofdeposited akes.Asinothercases,thegraphene-substratestepheightisdif culttoresolveandreportsrangebetween0.4and1.0nmforsinglelayers.30,101,107

5.1.3.Field-EffectDevices

Evenataloweroverallcrystalquality,theavailabilityofchemicallyderivedgraphenehasremovedaseriouslogjamintheengineeringofgraphene-baseddevicesandscientistshavebeeneagertomakeelectricalmeasurements.The rst