乙炔在Ge(001)表面吸附的反应路径

1、MayArticle物理化学学报(WuliHuaxueXuebao)ActaPhys.-Chim.Sin.2012,28(5),1107-1112doi:10.3866/PKU.WHXB乙炔在Ge(001)表面吸附的反应路径范晓丽1,2,*3刘燕1刘2崇1刘焕明2,3,*(1西北工业大学材料科学与工程学院,西安710072;北京计算科学研究中心,北京100084;成都绿色能源与绿色科技研发中心,成都610207)摘要:采用第一性原理方法研究了乙炔分子在Ge(001)表面的吸附反应.通过系统考察0.5和1.0ML覆盖度时形成di-和end-bridge构型的反应路径,研究在表面形成di-和pai

2、red-end-bridge构型的反应几率.除了表面反应以外,本文还涉及了亚表层Ge原子参与的吸附反应,乙炔在亚表层原子上吸附形成的亚稳态结构sub-di-,是形成end-bridge结构的第二条途径,此反应机理对于表面吸附结构的形成起重要的作用.与乙炔分子不同的是,表面以下原子参与时乙烯分子的吸附反应为吸热反应.综合热力学和动力学的分析表明,paired-end-bridge构型是乙炔分子吸附的主要构型,此结论解释了乙炔分子在Ge(001)表面吸附构型的实验结果.对于乙烯和乙炔两分子在Ge(001)表面吸附的分析比较揭示了导致两者之间差异的原因.关键词:密度泛函理论;Ge(001)表面;乙炔

3、分子;形成反应;亚表层;热力学;动力学O641中图分类号:ReactionPathwaysofAcetyleneAdsorptionontheGe(001)SurfaceFANXiao-Li1,2,*LIUYan1LIUChong1LAUWoon-Ming2,3,*(1SchoolofMaterialsScienceandEngineering,NorthwesternPolytechnicalUniversity,Xi'an710072,P.R.China;2BeijingComputationalScienceResearchCenter,Beijing100084,P.R.Chi

4、na;3ChengduGreenEnergyandGreenManufacturingTechnologyResearchandDevelopmentCenter,Chengdu610207,P.R.China)Abstract:TheadsorptionreactionofacetyleneontheGe(001)surfaceisinvestigatedbyfirst-principlescalculations.Inordertounderstandtherelativepopulationsofthedi-andpaired-end-bridgestructures,wecalcula

5、tedtheadsorptionreactionpathsleadingtotheirformationat0.5and1.0MLcoverage.Moreimportantly,westudiedtheadsorptionchannelinvolvingsublayerGeatomsbyformingametastablesub-di-structure.Thissub-di-structurerepresentssecondreactionpathwaythatresultsintheend-bridgestructure,whichplaysanimportantroleinthefor

6、mationoftheadsorptionconfigurations.IncontrasttoC2H2,theadsorptionofC2H4ontheGe(001)surfaceinvolvingsubsurfaceGeatoms,isendothermic.Ourcalculationsshowfrombothkineticandthermodynamicstandpointsthatthepaired-end-bridgestructureistheprimaryadsorptionconfigurationthatexplainstheexperimentalobservations

7、.OurworkalsohelpstounderstandthefundamentaldifferencesbetweentheadsorptionofC2H2andC2H4ontheGe(001)surface.KeyWords:Densityfunctionaltheory;Ge(001)surface;Acetylenemolecule;Sublayer;Thermodynamics;KineticsFormationreaction;Received:December12,2011;Revised:February20,2012;PublishedonWeb:March1,2012.C

8、orrespondingauthors.FANXiao-Li,Email:xlfan;TelLAUWoon-Ming,Email:leolau;Tel:+86-10-82687096.TheprojectwassupportedbytheNationalNaturalScienceFoundationofChina(20903075)andProgramofIntroducingTalentsofDisciplinetoUniversities,China(111Project)(B08040).国家自然科学基金(20903075)及高等学校学科创新引智计划(

9、111)(B08040)资助项目EditorialofficeofActaPhysico-ChimicaSinica1108ActaPhys.-Chim.Sin.2012Vol.281IntroductionTheadsorptionofunsaturatedhydrocarbonmoleculesontheelementalsemiconductorsurfacehasattractedgreatinterestbe-causeofitspotentialintechnologicalapplicationssuchasnon-linearopticaldevices,molecularel

10、ectronicdevices,andchemi-calsensors.1-4Comparingtotheintensivestudiesontheinterac-tionofhydrocarbonmoleculesonSi(001)surface,thedevotiontotheGe(001)surfaceismuchless.Inordertostudythepoten-tialoftheassemblehydrocarbonlayeronGe(001)surfaceinproductionofmolecularmicroelectronicandoptoelectronicde-vice

11、sfabricationandstability,theinteractionofunsaturatedhy-drocarbonmoleculeswithGe(001)surfacehasreceivedmoreattentionrecently.5-7GedimeristhereactivecenterontheGe(001)surface,thus,boththeintradimerandinterdimeradsorptionsarepossibleforC2H4/Ge(001).Theintradimerdi-modelhasthetwoCatomsoftheC2H4bondedtot

12、hetwoGeatomsofasinglesurfacedi-mer,andtheinterdimerend-bridgemodelhasthetwoCatomsbondedtothetwoGeatomsononesideoftwoadjacentdi-mersalongthedimeraxis.Kimetal.5observedtwodistinctde-sorptionfeaturesbytheirtemperature-programmeddesorption(TPD)measurements.Accordingtotheirobservedscanningtunnelingmicros

13、copy(STM)features,theserespectivefeatureswereassignedasthedi-andpaired-end-bridgeconfigurationsinwhichanadditionalC2H4moleculeadsorptionmakestheend-bridgestructuretobepaired.TheSTMimagesshowthatthedi-structuredominatesoverthepaired-end-bridgestruc-ture.Recently,we8studiedthedi-andend-bridgeconfigura

14、-tionsforC2H4adsorptiononGe(001)surfaceat0.5and1.0MLcoveragesusingfirst-principlescalculations.Ourcalculatedto-talenergiesindicatethatdi-andpaired-end-bridgestructuresareenergeticallyfavorable,i.e.,theadsorptionisexothermic.Furthermore,ourcalculationsoftheformationpathsshowthatthereactionrateofdi-st

15、ructureishigher,andtheend-bridgestructureiseasytobepaired.Inadditiontothedi-andend-bridgemodelsinwhichtwoCatomsarebondedtotwoGeatoms,twoadditionaladsorp-tionmodelsofthep-bridgeandr-bridgeconfigurationsarebe-lievedtobeprobableforC2H2/Ge(001).Inbothther-bridgeandp-bridgeconfigurations,thetwoCatomsofC2

16、H2moleculearebondedtothefourGeatomsofthetwoadjacentsurfacedi-mersbyformingfourbonds.TheCCaxisisparalleltotheGe-Gedimerbondinthep-bridgeconfigurationandperpendic-ulartotheC-Cdimerbondinther-bridgeconfiguration.ThepreviousSTMandTPDinvestigationbyKimetal.9observedtwoadsorptionconfigurationsforC2H2onGe(

17、001)surfacela-beledas“FeatureA”and“FeatureB”,whichwereassignedtothedi-andp-bridgeconfigurations,respectively.However,aslistedinTable1,accordingtoourtotalenergycalculation,10aswellasthetheoreticalcalculationsofChoandKleinman,11p-bridgeconfigurationismuchlessstablethanthepaired-end-bridgeconfiguration

18、.Bothcalculationsconfirmthatthetwobondingfeaturesareactuallythedi-andpaired-end-bridgeTable1Calculatedadsorptionenergies(unitineV)perC2H2moleculeonGe(001)surfacedi-end-bridge0.5ML1.0ML0.5ML1.0MLr-bridgep-bridgethisGGA1.591.571.521.880.07-0.23ChoandKleinman111.781.811.641.870.24-0.03configurations,re

19、spectively.Moreover,theoreticalcalculationsofChoandKleinman11foundthatthereactionratetoformtheend-bridgestructurewassmallerthanthatofthedi-structure.Nevertheless,thewell-measuredSTMimagesindicatesthatFeatureBdominatesatallC2H2coverage,andatthesaturatedcoverage,theappear-anceratioofFeatureBtoFeatureA

20、is0.62:0.38.Thisindicatesthereactionratetoformtheend-bridgeandpaired-end-bridgeadsorptionsshouldbelargerthantheratetoformthedi-ad-sorptions,whichisconflictingwithChoandKleinmanscalcu-lationresults.Ontheotherside,theexperimentalresultsonthetwomoleculesareincontrasttoeachother:paired-end-bridgestructu

21、rehasbeenobservedtobedominantoverthedi-struc-tureinthepresentcaseofC2H2,9andthepopulationofdi-structureislargerthanthatofthepaired-end-bridgestructureinthecaseofC2H4,5whichisindeedconsistentwithourcalcu-lationresultsonC2H4/Ge(001).8Inthepresentwork,were-studytheadsorptionanddesorp-tionkineticsofC2H2

22、/Ge(001)byemployingourexperienceinusingthefirst-principlesmethodbasedonthedensityfunction-altheory(DFT)toexaminesmallmoleculesonSiandGesys-tematically,tounderstandthediscrepancybetweentheexperi-mentobservationandtheoreticalcalculation,andtodigouttherootcausetothedifferencebetweentheadsorptionsofC2H2

23、andC2H4onGe(001)surface.2CalculationmethodsTheGe(001)surfacewassimulatedbyaslabcontainingeightGeatomiclayersandavacuumregionof1.294nmspac-ing.Ge(001)surfacehasthesimilarreconstructionastheSi(001)surface.Fourdifferentreconstructions,symmetric(2×1),bucked(2×1),p(2×2),andc(4×2),have

24、beeninvestigated.Amongthemc(4×2)andp(2×2)arethemoststablegeome-tries.Theenergydifferencebetweenthetwogeometriesislessthan0.003eVdimer-1,whichagreeswellwithtwopreviousstudies12,13inthistopic.Thus,theGe(001)surfacewasmodeledbyarepeatedp(2×2)unitcell,whilethedanglingbondsofthebottomGeato

25、msweresaturatedbyHatoms.Thestructureop-timizationwasperformedusingthecalculatedbulklatticecon-stantof0.575nminthetotalenergycalculation,theGeatomsinthebottomlayeroftheslab,aswellastheterminatinghy-drogenatomswerefixedtothebulkposition.TheBrillouinzonewassampledbyaMonkhorst-Packschemewith4×4

26、5;1k-pointgrids.Thegeometryoptimizationinthepresentstudywasper-formedonthebasisofdensityfunctionaltheorywithapplyingtheViennaabinitiosimulationpackage(VASP).14-17ThetotalNo.5FANXiao-Lietal.:ReactionPathwaysofAcetyleneAdsorptionontheGe(001)Surface1109energycalculationswerecarriedoutusingtheVanderbilt

27、ul-trasoftpseudopotential18-20andplane-wavemethodwithcutoffenergyof350eV.Thegeneralizedgradientapproximation(GGA)21,22ofPerdew-Wang(PW91)wasusedfortheelectronicexchange-correlationpotential.Theminimumenergypathsfortheadsorptionreactionsweremappedoutusingthenudgedelasticband(NEB)methoddevelopedbyJ

28、43;nssonandco-work-ers.23,243Resultsanddiscussion3.1Reactionpathwaysat0.5MLAccordingtopreviousstudies8,25-27forC2H4/Si(001),C2H2/Si(001),andC2H4/Ge(001),theconcerted2+2cycloadditionreactionisactuallyallowedbasedonthecalculatedreactionbarrierandtheanalysisofthesymmetry.Therelativeimpor-tancebetweenth

29、esymmetricconcerted2+2additionmecha-nismandthetwo-stepasymmetricadditionmechanismhasbeeninvestigatedfurther.Itturnsoutthattheasymmetricaddi-tionstartingfromathree-atomcomplexstateinwhichthetwoCatomsoftheadsorbedmoleculearebondedtothe“down”-GeatomofaGe-Gedimerwithadativebondisthemoreaccessibleroutebe

30、causethe“down”-GeatomofaGe-Gedimerisknowntobeelectrondeficient,andbecausetheadsorbedmoleculecanaccesstothisprecursorstatewithnostrictalignmentrequirementofthemolecularaxisinitsar-rivaltrajectoryrelativetotheGe-Gedimer.Incontrast,thecon-certedadditionmechanismislessimportantbecausetheincom-ingmolecul

31、araxismustaligninparalleltothesurfacedimer.Thus,statistically,thesignificanceofconcertedadditionpath-waysismuchlessthanthatoftheasymmetricadditionpath-waysandtheroleofconcertedadditioncanbeignored.Inanotherconsiderationofreactionkinetics,westudytheasymmetricadditionpathwaysincludingtheirintermediate

32、pre-cursorstates.ByusingtheNEBmethod,wehavefoundtheminimumenergypathsfromtheprecursorstatestothedi-andend-bridgestructures.PathIdescribestheformationofthedi-configurationfromthePIstateandPathIIleadstothefor-mationoftheend-bridgeconfigurationfromthePIIstate,thecorrespondingatomicgeometriesoftheprecur

33、sor(P),transi-tion(T),andthechemisorptions(C)statesonthereactionpathsIandIIaredisplayedinFig.1,andthecorrespondingadsorptionenergiesaregiveninTable2.AlongPathI,there-spectiveadsorptionenergiesofthePIandTIstatesare0.27and0.08eV;thusthereactionbarrierfromthePIstatetothedi-configurationis0.19eV.Incompa

34、rison,alongPathII,thecorrespondingadsorptionenergiesforthePIIandTIIare0.28and0.05eV,andthereactionbarriertoformtheend-bridgestructureis0.23eV.Itindicatesthatboththedi-andend-bridgeconfigurationsareenergeticallyfavorableovertheprecursorstatesbyabout1.2-1.3eV.Thus,iftheenergyreleasedbytheformationofth

35、eprecursorstateisnotdissipatedawayquickly,itisenoughtodrivethechemisorp-tionsthroughthetransitionstate.Inotherwords,boththedi-andend-bridgeconfigurationscanbeformedviathepre-cursorstateincommonexperimentalconditions,theirforma-tionratesaredecidedbythereactionbarriers.Ourcalculatedadsorptionenergiesf

36、orPIandPIIstatesareconsistentwiththepreviousfirst-principlesstudybyChoandKleinman,11aslistedinTable2.Accordingtotheircalcula-tions,thebarriersare0.09and0.13eVforthedi-andend-bridgestructures,respectively.Althoughourcalculationsraisebothrespectivevaluesto0.19and0.23eV,thedifferencebe-(a)(b)Fig.1Atomi

37、cgeometriesofthe-complexprecursor(P),transition(T),andchemisorption(C)statesfortheadsorptionofC2H2onGe(001)surfacealongdifferentpaths(a)pathI:asymmetricadditionreactionleadingtothedi-structurefromPIstate;(b)pathII:asymmetricadditionreactionleadingtotheformationoftheend-bridgestructurefromthePIIstate

38、1110ActaPhys.-Chim.Sin.2012Vol.28Table2ComparisonbetweentheadsorptionofC2H2andC2H4onGe(001)surface,calculatedadsorptionenergies(Eads)ofthe-complexprecursor(P),transition(T),andthechemisorption(C)states,andenergybarriers(Eb)fromthePtotheCC2H2/Ge(001)C2H4/Ge(001)bConfigurationEads/eVPTCEEads/eVb/eVPTC

39、Eb/eVdi-0.270.081.590.190.400.171.000.230.25a0.16a1.78a0.09aend-bridge0.280.051.520.230.350.160.900.190.26a0.13a1.64a0.13apaired-di-0.240.051.560.970.171.81apaired-end-bridge0.290.161.81a0.29a1.87a0.02aEads=E(mole)+E(Ge(001)-E(mole/Ge(001),whereE(mole)andE(Ge(001)

40、representtheenergiesoftheC2H2(C2H4)moleculeandthebareGe(001)surface,respectively;E(mole/Ge(001)representsthetotalenergyoftheadsorbedC82H2(C2H4)/Ge(001)system.aReference11,bReferencetweenthetwobarriersisagain0.04eV.AsreportedbyChoandKleinman,11theratioofthereactionrateR1toformthedi-configurationtothe

41、reactionrateR2toformtheend-bridgeconfiguration,representedasR1/R2,is1.2ifalltheadsorptionenergyoftheprecursorstateisretained.UsingtheArrhenius-typeactivationprocesswithatypicalvalue(1014s-1)forthepre-exponentialfactorwhichhasbeenadoptedbyChoandKleinman,wehavecalculatedthereactionrateR2fromtheprecurs

42、orstatePIItotheend-bridgestructure;there-sultantR2is1.3×1010s-1.WealsofindthatthereactionrateR1toformthedi-structureis4.7timeslargerthanR2.ThefactorR1/R2becomes1.1ifalltheadsorptionenergiesoftheprecur-sorstateareretained.Therefore,ourcalculationsareagaincon-sistentwiththereportedresultsofChoand

43、Kleinman,withbothpredictingthattheformationrateofdi-structureislarg-erthanthatoftheend-bridgestructure.3.2Reactionpathwaysat1.0MLAfterstudyingtheadsorptionofonesinglemoleculetoformthedi-andend-bridgestructures,wehavefurthercalculatedtherespectivereactionpathwaysofaddingonemoreC2H2mol-eculetoformthea

44、ctualpaired-di-andpaired-end-bridgestructures.AslistedinTable2,comparingtheformationofthepaired-di-structureandthefirstdi-structure,oneseesthattheadsorptionenergyoftheprecursorstatetoformthepaired-di-structureisslightlylowerby0.03eV,andtheformationenergyoftheseconddi-structureisalsolowerby0.02eV.Inc

45、ontrast,theadsorptionenergyoftheprecursorstatetoformthepaired-end-bridgestructureisslightlyhigher,andthefor-mationenergyofthesecondend-bridgestructureishigherbyabout0.4eV.Thesecomparisonsindicatethatthedi-struc-tureisenergeticallyfavorableoverthepaired-di-structure;incontrast,theend-bridgestructurew

46、illbeeventuallypairedup.Alongthereactionpathwaytoformthepaired-di-struc-ture,theenergybarrieris0.19eV.Incomparison,thereactionbarrieris0.13eValongtheformationpathofthepaired-end-bridgestructure.Comparingtheformationreactionofthepaired-end-bridgestructureandthefirstend-bridgestructure,thereac-tionbar

47、rierislowerby0.10eV.Inthecaseofthedi-struc-ture,thereactionbarriertoformthepaired-di-structuredoesnotchangefromthebarrierofthefirstdi-structure.Allthesecomparisonsfromboththermodynamicsandkineticsindicatethattheadsorptionofaseconddi-C2H2isimpeded;incon-trast,theformationofthepaired-end-bridgestructu

48、reiseasyandtheformationrateishigheroncetheend-bridgestructureisformed.Inordertodemonstratetheabovecomparisonsmoreexplic-itly,weestimatethereactionratetoformthepaired-end-bridgestructurebyadoptinganArrhenius-typerateformulaagainandthevalueturnsouttobe6.4×1011s-1,whichis11timeslargerthantheformat

49、ionrateofthepaired-di-structure.ThefactorR1/R2becomes1.3ifalltheadsorptionenergiesoftheprecur-sorstatesareretained.AccordingtotheSTMexperiment,theratioof“FeatureA”imagewhichisassignedasdi-structuretothe“FeatureB”imagewhichisnowassignedaspaired-end-bridgestructurewasobtainedas0.38:0.62atsaturationcov

50、er-age.Hence,afactorof1.3isnotenoughtomakeupthedefi-ciencyintheformationratesoftheend-bridgestructure.Con-sistentwithourcalculations,theformationofthepaired-end-bridgestructureisalsoeasierasshowninthereportbyChoandKleiman,11whichcannothelptheappearanceratioofpaired-end-bridgestructuretothedi-structu

51、retorisetotheexperimentalvalueof0.62:ReactionpathwaysinvolvingthesublayerGeatomThecalculatedadsorptionenergiesoftheprecursorstateandthereactionbarrierstoformtheintradimerdi-andinterdimerend-bridgestructuresforC2H4moleculeonGe(001)surfacearealsolistedinTable2.Theenergyreleasedbytheformationof

52、theprecursorstateisnotdissipatedawayquicklywhenthesurfacetemperatureishigh,whichisthecommonexperimen-talcondition.SincetheadsorptionenergyofthePIIstateislarg-erthanthatofthePIstate,thereactionratetoformthedi-structurewillbelargerthanthereactionratetoformtheend-bridgestructure.Thisisconsistentwiththe

53、presentcasefortheadsorptionofC2H2onGe(001)surface.Also,theadsorptionen-ergiesoftheprecursorstatesandreactionbarrierstoformthepaired-di-andpaired-end-bridgestructuresareconsistentbe-tweenthetwomolecules,bothshowstheend-bridgestructurewillbeeasilypairedup.Accordingtotheexperimentalstudies,5ontheothers

54、ide,thedi-structuredominatesoverthepaired-end-bridgestructureontheC2H4adsorbedGe(001)surface,whileinthepresentcaseofC2H2/Ge(001),asmentionedabove,theappearancera-tioofpaired-end-bridgestructuretodi-structureis0.68:0.32.Inotherwords,incontrasttothetheoreticalstudies,therela-tivepopulationbetweenthedi

55、-structureandpaired-end-bridgestructureinthecaseofC2H2/Ge(001)isoppositetothatinthecaseofC2H4/Ge(001).Inourpreviousstudy27forthead-No.5FANXiao-Lietal.:ReactionPathwaysofAcetyleneAdsorptionontheGe(001)Surface1111sorptionsofC2H2andC2H4moleculeonSi(001)surface,wefoundonemoreimportantreactionsitebesides

56、thesurfacedi-mer,thesublayeradsorptionsite,whichplaysacrucialroleinunderstandingthedifferentadsorptionofC2H2andC2H4onSi(001)surface.Here,inordertodigouttherootcausetothedifferencebetweentheadsorptionsofthetwomoleculesonGe(001)surface,andtounderstandthediscrepancybetweentheexperimentobservationandthe

57、oreticalcalculation,westudythesublayeradsorptionagain.Theformationofthesub-di-structurefromthePIIstateisdescribedasPathIIIdisplayedinFig.2(a).Theformationpro-cessinvolvesthebreakingoftheGe1Ge3bondandthefor-mationoftwoCGebonds,C2Ge1andC1Ge3.TheadsorptionenergyforthetransitionstateTIIIis0.10eVaslisted

58、inTable3,thusthereactionbarriertoformthesub-di-struc-tureis0.18eV.Themorestableend-bridgestructurecanbeformedthroughthissub-di-structureinthereactionalongPathIV.TheatomicstructuresinvolvedinthispathareshowninFig.2(b).Duringtheconversionfromthesub-di-structuretothetransitionstateTIV,theGe3Ge3andC1Ge3bondsarebroken,andtheC1Ge3andGe1Ge3bondsareformed.AslistedinTable3,theadsorptionenergyofthesub-di-structureis0.54eV,andthereactionbarrierfromittothemorestableend-bridgestructureis0.31eV.Incommonexperi-mentalconditions,thereleasedenergy