FundamentalsofNanomaterials Carbonnanotubes 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 Nanotubesnaturallyalignthemselvesinto ropes heldtogetherbyVanderWaalsforces 1 Single walledCNTThestructureofaSWCNTcanbeconceptualizedbywrappingaone atom thicklayerofgraphitecalledgrapheneintoaseamlesscylinder Thewaythegraphenesheetiswrappedisrepresentedbyapairofindices n m calledthechiralvector Theintegersnandmdenotethenumberofunitvectorsalongtwodirectionsinthehoneycombcrystallatticeofgraphene helicalanglea1 a2 unitvectorindifferentdirectionsCh Helicalvector describeshowto rollup thegraphenesheettomakethenanotube n mcanbecountedattheendofthenanotube n m 30 armchaircarbonnanotuben 0orm 0 0 zigzagcarbonnanotuben m 0 0 30 chiralcarbonnanotube armchaircarbonnanotube zigzagcarbonnanotube chiralcarbonnanotube Single walledcarbonnanotubesareanimportantvarietyofcarbonnanotubebecausetheyexhibitelectricproperties Inparticular theirbandgapcanvaryfromzerotoabout2eVandtheirelectricalconductivitycanshowmetallicorsemiconductingbehavior Thecarbonnanotubecanactasmetalorsemiconductorduetothedifferentdiameterandthehelicalangle n m 3i i integer metallic otherwise semiconducting So allthearmchairSWCNTsaremetallic Single wallednanotubesarethemostlikelycandidateforminiaturizingelectronicsbeyondthemicroelectromechanicalscalecurrentlyusedinelectronics Themostbasicbuildingblockofthesesystemsistheelectricwire andSWCNTscanbeexcellentconductors OneusefulapplicationofSWCNTsisinthedevelopmentofthefirstintramolecularfieldeffecttransistors FET ProductionofthefirstintramolecularlogicgateusingSWCNTFETshasrecentlybecomepossibleaswell 2 Multi walledCNTsMulti wallednanotubes MWCNT consistofmultiplerolledlayers concentrictubes ofgraphite Theinterlayerdistanceinmulti walledcarbonnanotubesisclosetothedistancebetweengraphenelayersingraphite approximately3 4 DifferentwithSWCNTs MWNTsarezero gapmetals Thespecialplaceofdouble walledcarbonnanotubes DWCNT mustbeemphasizedherebecausetheirmorphologyandpropertiesaresimilartoSWCNTbuttheirresistancetochemicalsissignificantlyimproved Propertiesofcarbonnanotubes I 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 Application MWCNTsinthebuckpaper Lithography Lithography fromGreek lithos stone graphein towrite isamethodforprintingusingastone lithographiclimestone orametalplatewithacompletelysmoothsurface CityofWords lithographbyVitoAcconci 1999 Photolithography Photolithography or opticallithography or UVlithography isaprocessusedinmicrofabricationtoselectivelyremovepartsofathinfilmorthebulkofasubstrate Ituseslighttotransferageometricpatternfromaphotomasktoalight sensitivechemical photoresist orsimply resist onthesubstrate Principle Photolithography processusedtotransferpatternsfromamasktoaSiwaferorothersubstrate usingaphotosensitiveresistandaradiationofwavelength Definesthepatte