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1、FundamentalsofNanomaterials Carbonnanotubes 1 OverviewofcarbonnanotubesCarbonnanotubes CNTs alsoknownasbuckytubes areallotropesofcarbonwithacylindricalnanostructure Nanotubeshavebeenconstructedwithlength to diameterratioofupto132 000 000 1whichissignificantlylargerthananyothermaterial CNTwasfirstdis
2、coveredandstudiedbyJapanesephysicalscientistSumioIijimain1991 Thesecylindricalcarbonmoleculeshavenovelpropertiesthatmakethempotentiallyusefulinmanyapplicationsinnanotechnology electronics opticsandotherfieldsofmaterialsscience aswellaspotentialusesinarchitecturalfields Theyexhibitextraordinarystreng
3、thanduniqueelectricalproperties andareefficientthermalconductors Carbonnanotubesaremembersofthefullerenestructuralfamily Theendsofananotubemightbecappedwithahemisphereofthebuckyballstructure Theirnameisderivedfromtheirsize sincethediameterofananotubeisontheorderofafewnanometers approximately1 50 000
4、thofthewidthofahumanhair whiletheycanbeupto18centimetersinlength Carbonnanotubesarecategorizedassingle walled SWCNTs andmulti walled MWCNTs Thenatureofthebondingofananotubeisdescribedbyappliedquantumchemistry specifically orbitalhybridization Thechemicalbondingofnanotubesiscomposedentirelyofsp2bonds
5、 similartothoseofgraphite Thisbondingstructure whichisstrongerthanthesp3bondsfoundindiamonds providesthemoleculeswiththeiruniquestrength Nanotubesnaturallyalignthemselvesinto ropes heldtogetherbyVanderWaalsforces 1 Single walledCNTThestructureofaSWCNTcanbeconceptualizedbywrappingaone atom thicklayer
6、ofgraphitecalledgrapheneintoaseamlesscylinder Thewaythegraphenesheetiswrappedisrepresentedbyapairofindices n m calledthechiralvector Theintegersnandmdenotethenumberofunitvectorsalongtwodirectionsinthehoneycombcrystallatticeofgraphene helicalanglea1 a2 unitvectorindifferentdirectionsCh Helicalvector
7、describeshowto rollup thegraphenesheettomakethenanotube n mcanbecountedattheendofthenanotube n m 30 armchaircarbonnanotuben 0orm 0 0 zigzagcarbonnanotuben m 0 0 30 chiralcarbonnanotube armchaircarbonnanotube zigzagcarbonnanotube chiralcarbonnanotube Single walledcarbonnanotubesareanimportantvarietyo
8、fcarbonnanotubebecausetheyexhibitelectricproperties Inparticular theirbandgapcanvaryfromzerotoabout2eVandtheirelectricalconductivitycanshowmetallicorsemiconductingbehavior Thecarbonnanotubecanactasmetalorsemiconductorduetothedifferentdiameterandthehelicalangle n m 3i i integer metallic otherwise sem
9、iconducting So allthearmchairSWCNTsaremetallic Single wallednanotubesarethemostlikelycandidateforminiaturizingelectronicsbeyondthemicroelectromechanicalscalecurrentlyusedinelectronics Themostbasicbuildingblockofthesesystemsistheelectricwire andSWCNTscanbeexcellentconductors OneusefulapplicationofSWC
10、NTsisinthedevelopmentofthefirstintramolecularfieldeffecttransistors FET ProductionofthefirstintramolecularlogicgateusingSWCNTFETshasrecentlybecomepossibleaswell 2 Multi walledCNTsMulti wallednanotubes MWCNT consistofmultiplerolledlayers concentrictubes ofgraphite Theinterlayerdistanceinmulti walledc
11、arbonnanotubesisclosetothedistancebetweengraphenelayersingraphite approximately3 4 DifferentwithSWCNTs MWNTsarezero gapmetals Thespecialplaceofdouble walledcarbonnanotubes DWCNT mustbeemphasizedherebecausetheirmorphologyandpropertiesaresimilartoSWCNTbuttheirresistancetochemicalsissignificantlyimprov
12、ed Propertiesofcarbonnanotubes I Uniquestructure geometry Diameter 1 50nm length 1 10micronClosedhollowtubeStructuralperfection inprinciple II Excellentmechanicalandthermalproperties Young smodulus 1TPa sameasdiamond Ductile fracturestrain 5 10 Highthermalconductivity inprinciple III Novelelectronic
13、propertiesBecauseofthesymmetryanduniqueelectronicstructureofgraphene thestructureofananotubestronglyaffectsitselectricalproperties Intheory metallicnanotubescancarryanelectricalcurrentdensityof4 109A cm2whichismorethan1 000timesgreaterthanmetalssuchascopper SynthesisTechniqueshavebeendevelopedtoprod
14、ucenanotubesinsizeablequantities includingarcdischarge laserablation highpressurecarbonmonoxide HiPCO andchemicalvapordeposition CVD Mostoftheseprocessestakeplaceinvacuumorwithprocessgases CVDgrowthofCNTscanoccurinvacuumoratatmosphericpressure Largequantitiesofnanotubescanbesynthesizedbythesemethods
15、 advancesincatalysisandcontinuousgrowthprocessesaremakingCNTsmorecommerciallyviable TheheatedCarbonnanotubeinthemicroscope Application MWCNTsinthebuckpaper Lithography Lithography fromGreek lithos stone graphein towrite isamethodforprintingusingastone lithographiclimestone orametalplatewithacomplete
16、lysmoothsurface CityofWords lithographbyVitoAcconci 1999 Photolithography Photolithography or opticallithography or UVlithography isaprocessusedinmicrofabricationtoselectivelyremovepartsofathinfilmorthebulkofasubstrate Ituseslighttotransferageometricpatternfromaphotomasktoalight sensitivechemical photoresist orsimply resist onthesubstrate Principle Photolithography processusedtotransferpatternsfromamasktoaSiwaferorothersubstrate usingaphotosensitiveresistandaradiationofwavelength Definesthepatte
