Why_silicon_nanotubes_stably_exist_in_armchair_structure1

Whysiliconnanotubesstablyexistinarmchairstructure?

M.Zhang*,Y.H.Kan,Q.J.Zang,Z.M.Su*,R.S.WangInstituteofFunctionalMaterialChemistry,FacultyofChemistry,NortheastNormalUniversity,Changchun130024,ChinaReceived28May2003;infinalform27July2003Publishedonline:4September2003

AbstractHypotheticsiliconnanotubesarestudiedusingdensityfunctionaltheorycalculationbasedonB3LYP/6-31G(d).Twokindsofprecisestructures,includingarmchairandzigzagsiliconnanotubes,arefirstobtained.Thestructuralandelectronicpropertiesarediscussedindetail.Ourresultalsopredictsthatallthesmalldiametersiliconnanotubesaremetallicregardlessoftheirchiralities.Comparedwithzigzagnanotubes,armchairnanotubesofsiliconarethemostreasonablestructuresduetotheefficientoverlappingofporbitalsanddelocalizedpbonds.Withthedevelopmentofchemicalsynthesis,armchairnanotubesarehopefultobesynthesizedfirstinthefuture.Ó2003ElsevierB.V.Allrightsreserved.

1.Introduction

Onlytwoyearssincecarbonnanotubes(CNT)werefirstdiscoveredbyIijima[1],hadmacro-scopicquantitiesofveryuniformsinglewalledcarbonnanotubes(SWCNTs)beenalmostsimul-taneouslysynthesizedbydifferentgroupsatNECandIBM[2,3].SWCNTsareuniquequasi-one-dimensionalnanostructureswithremarkablystructuralandelectronicproperties,showingsub-stantialpromiseascomposites,catalysts,molecu-larswitches,superstrongfibers,etc.Idealnanotubesareconsideredasgraphitesheetsthatcanberolleduptomakeseamlesshollowcylin-ders.Thesehollowcylindersaretensofmicrome-terslongwiththediameterlessthan1nm.Carbonnanotubescanbepreparedbyavarietyoftech-niques,suchaschemicalvapordeposition(CVD),gasphasecatalyticgrowth,arc-discharge,andla-serablation.Todayitbecomesclearthatthepropertyofbeingfoldedintoclosedassembliesispossessednotonlybygraphitebutalsobyboronnitride,carbonitrides,andtransitionmetalsul-fides,etc[4,5],whichbringsstrongimpetustotheprediction,synthesis,andinvestigationofanclassofnon-carbonorinorganicnanotubes[6].Silicon,anelementresidesinthesamecolumnoftheperiodictablewithcarbonandalsoownsfourvalenceelectrons.Siliconisoftensuggestedasasubstituteforcarbonbecauseofitschemicalsimilaritybasedonthelawofperiodicity.Siliconmakesup25.7%ofthecrustbymassandisthesecondmostabundant.Siliconalsopossessessomeuniquepropertieswhichcarbondoesnot,such

asChemicalPhysicsLetters379(2003)81–86www.elsevier.com/locate/cplett

*Correspondingauthors.Fax:+86-431-5894009(M.Zhang),+86-431-5684009(Z.M.Su).E-mailaddresses:zhangm914@nenu.edu.cn(M.Zhang),zmsu@nenu.edu.cn(Z.M.Su).0009-2614/$-seefrontmatterÓ2003ElsevierB.V.Allrightsreserved.doi:10.1016/j.cplett.2003.08.030highaffinityandstabilitytoacid.Withtheprep-arationofsiliconnanoforms,greatgrowingin-teresthadbeenarousedinnanoscienceandnanotechnology[7].Siliconnanowiresweresyn-thesizedbybothphysicalandchemicalvaporde-position(CVD)[8–11].Sized-selectedSi30andSi39clusterswereproducedbyalaservaporizationclustersource[12].Afewstudieshavebeenre-portedabouttheanalysisoftheelectronicstruc-turesofcagedSi20clustersusingabinitiomethods[13,14].Thesestudiesshowedthateachatomco-ordinatedthreeneighborswithalmosttetrahedralanglesandwasconsideredtohaveanoutwarddanglingbond.TheresultfromNaganoetal.[15]showedthattheendohedralmetalclusterwasmorestablethanthecorrespondinghollowSi20clusterandthechargewastransferredfromtheclustertotheendohedralBaatom.ThestabilityoftheendohedralmetalclusterswasconfirmedbyM@Si16(M¼Ti,Zr,andHf)byKumaretal.[16],butthechargewasoppositelytransferredfromSitometals.Thestudyalsoshowedthatsomeiso-mersofSi16clustershaddoublebondsbetweensomesiliconatomssothateachatomhadtwosinglebondsandadoublebond.Yangetal.[17]firstreportedsiliconnanotubes(SiNT)preparedbyCVDprocessusinganano-channelAl2O3(NCA)substrate.Unlikecarbonnanotubes,nosinglewalledsiliconnanotube(SWSiNT)hasbeenexperimentallyreporteduptonow.Therefore,itisnotclearwhattheactualstructureofsiliconnanotubemightbe.Zhangetal.[18]classifiedsiliconnanowiresandsiliconnanotubesfromchemicalaspectandpointedoutthatsiliconnanotubesmustbecomposedofthreefoldcoordinatedsiliconatoms.Anarmchairð3;3Þstructurehadbeenselectedoutandgeo-metricallyoptimizedwithsemiempiricalPM3method.Motaetal.[19,20]investigatedtheelec-tronic,structural,andthermalpropertiesofthreehypotheticSWSiNTswiththestructureofSWCNTs.Kuang[21,22]studiedthethermalbe-haviorandthestructureofhypotheticsiliconna-notubesusingclassicalmoleculardynamicssimulationsbasedontheTersoffpotential.Seifertetal.[23]studiedthestructuresofsilicateandSiHnanotubesusingdensityfunctionaltheory(DFT)tight-bondingmethod;Andriotisetal.[24]foundoutthattheencapsulationofmetals(M¼NiandV)couldstabilizeSinanotubes,andthesemetal-encapsulatedSinanotubeswhichhadnarrowen-ergygapsweremetallicatinfinitelengthwithtight-bindingmoleculardynamicsmethodology.InthisLetter,itispresentedthattheresultsofthethreesmallestSWSiNTsimulatedwiththestrictdensityfunctionaltheoryB3LYPmethodat6-31G(d)basissetlevel.Itaimsatpredictingtheprecisestructurefromthecalculationandex-plainingwhatthestructureofSWSiNTwouldbe.