Fundamentals-of-Nanomaterials(3)(1)(纳米材料基础)

FundamentalsofNanomaterialsCarbonnanotubes•1.OverviewofcarbonnanotubesCarbonnanotubes(CNTs;alsoknownasbuckytubes)areallotropesofcarbonwithacylindricalnanostructure.Nanotubeshavebeenconstructedwithlength-to-diameterratioofupto132,000,000:1whichissignificantlylargerthananyothermaterial.•CNTwasfirstdiscoveredandstudiedbyJapanesephysicalscientistSumioIijimain1991.•Thesecylindricalcarbonmoleculeshavenovelpropertiesthatmakethempotentiallyusefulinmanyapplicationsinnanotechnology,electronics,opticsandotherfieldsofmaterialsscience,aswellaspotentialusesinarchitecturalfields.Theyexhibitextraordinarystrengthanduniqueelectricalproperties,andareefficientthermalconductors.Carbonnanotubesaremembersofthefullerenestructuralfamily.Theendsofananotubemightbecappedwithahemisphereofthebuckyballstructure.Theirnameisderivedfromtheirsize,sincethediameterofananotubeisontheorderofafewnanometers(approximately1/50,000thofthewidthofahumanhair),whiletheycanbeupto18centimetersinlength.Carbonnanotubesarecategorizedassingle-walled(SWCNTs)andmulti-walled(MWCNTs).Thenatureofthebondingofananotubeisdescribedbyappliedquantumchemistry,specifically,orbitalhybridization.Thechemicalbondingofnanotubesiscomposedentirelyofsp2bonds,similartothoseofgraphite.Thisbondingstructure,whichisstrongerthanthesp3bondsfoundindiamonds,providesthemoleculeswiththeiruniquestrength.NanotubesnaturallyalignthemselvesintoropesheldtogetherbyVanderWaalsforces1.Single-walledCNTThestructureofaSWCNTcanbeconceptualizedbywrappingaone-atom-thicklayerofgraphitecalledgrapheneintoaseamlesscylinder.Thewaythegraphenesheetiswrappedisrepresentedbyapairofindices(n,m)calledthechiralvector.Theintegersnandmdenotethenumberofunitvectorsalongtwodirectionsinthehoneycombcrystallatticeofgraphene.θ:helicalanglea1,a2:unitvectorindifferentdirectionsCh:Helicalvector(describeshowtorollupthegraphenesheettomakethenanotube)n,mcanbecountedattheendofthenanotube.•n=m,θ=30,armchaircarbonnanotube•n=0orm=0,θ=0,zigzagcarbonnanotube•n≠m≠0,0θ30,chiralcarbonnanotubearmchaircarbonnanotubezigzagcarbonnanotubechiralcarbonnanotube•Single-walledcarbonnanotubesareanimportantvarietyofcarbonnanotubebecausetheyexhibitelectricproperties.Inparticular,theirbandgapcanvaryfromzerotoabout2eVandtheirelectricalconductivitycanshowmetallicorsemiconductingbehavior.•Thecarbonnanotubecanactasmetalorsemiconductorduetothedifferentdiameterandthehelicalangleθ.•n-m=3i(i:integer),metallic;otherwise,semiconducting.•So,allthearmchairSWCNTsaremetallic.•Single-wallednanotubesarethemostlikelycandidateforminiaturizingelectronicsbeyondthemicroelectromechanicalscalecurrentlyusedinelectronics.Themostbasicbuildingblockofthesesystemsistheelectricwire,andSWCNTscanbeexcellentconductors.•OneusefulapplicationofSWCNTsisinthedevelopmentofthefirstintramolecularfieldeffecttransistors(FET).ProductionofthefirstintramolecularlogicgateusingSWCNTFETshasrecentlybecomepossibleaswell.2.Multi-walledCNTs–Multi-wallednanotubes(MWCNT)consistofmultiplerolledlayers(concentrictubes)ofgraphite.–Theinterlayerdistanceinmulti-walledcarbonnanotubesisclosetothedistancebetweengraphenelayersingraphite,approximately3.4Å.•DifferentwithSWCNTs,MWNTsarezero-gapmetals.•Thespecialplaceofdouble-walledcarbonnanotubes(DWCNT)mustbeemphasizedherebecausetheirmorphologyandpropertiesaresimilartoSWCNTbuttheirresistancetochemicalsissignificantlyimproved.•PropertiesofcarbonnanotubesI.Uniquestructure/geometry:Diameter:1-50nm;length:1-10micronClosedhollowtubeStructuralperfection(inprinciple)II.Excellentmechanicalandthermalproperties:Young’smodulus~1TPa(sameasdiamond)Ductile(fracturestrain~5-10%)Highthermalconductivity(inprinciple)III.NovelelectronicpropertiesBecauseofthesymmetryanduniqueelectronicstructureofgraphene,thestructureofananotubestronglyaffectsitselectricalproperties.Intheory,metallicnanotubescancarryanelectricalcurrentdensityof4×109A/cm2whichismorethan1,000timesgreaterthanmetalssuchascopper.•SynthesisTechniqueshavebeendevelopedtoproducenanotubesinsizeablequantities,includingarcdischarge,laserablation,highpressurecarbonmonoxide(HiPCO),andchemicalvapordeposition(CVD).Mostoftheseprocessestakeplaceinvacuumorwithprocessgases.CVDgrowthofCNTscanoccurinvacuumoratatmosphericpressure.Largequantitiesofnanotubescanbesynthesizedbythesemethods;advancesincatalysisandcontinuousgrowthprocessesaremakingCNTsmorecommerciallyviable.TheheatedCarbonnanotubeinthemicroscope.•ApplicationMWCNTsinthebuckpaperLithography•Lithography(fromGreekλίθος-lithos,'stone'+γράφειν-graphein,'towrite')isamethodforprintingusingastone(lithographiclimestone)orametalplatewithacompletelysmoothsurface.'CityofWords',lithographbyVitoAcconci,1999Photolithography•Photolithography(oropticallithography)(orUVlithography)isaprocessusedinmicrofabricationtoselectivelyremovepartsofathinfilmorthebulkofasubstrate.Ituseslighttotransferageometricpatternfromaphotomasktoalight-sensitivechemicalphotoresist,orsimplyresist,onthesubstrate.Principle•Photolithography:processusedtotransferpatternsfromamasktoaSiwaferorothersubstrate,usingaphotosensitiveresistanda