乙炔在Ge(001)表面吸附的反應路徑

1、MayArticle物理化學學報(WuliHuaxueXuebao)ActaPhys.-Chim.Sin.2012,28(5),1107-1112doi:10.3866/PKU.WHXB2012030111107www.whxb .p .c n乙炔在Ge(001)表面吸附的反應路徑范曉麗1,2,*3劉燕1劉2崇1劉煥明23*(1西北工業(yè)大學材料科學與工程學院，西安710072;北京計算科學研究中心，北京100084;成都綠色能源與綠色科技研發(fā)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)表面的吸附反應.通過系統(tǒng)考察0.5和1.0ML覆蓋度時形成di-

2、(T和 end-bridge構(gòu)型的反應路徑，研究在表面形成di- c和paired-end-bridge構(gòu)型的反應幾率.除了表面反應以外,本文還涉及了亞表層Ge原子參與的吸附反應，乙炔在亞表層原子上吸 附形成的亞穩(wěn)態(tài)結(jié)構(gòu)sub-di- c是形成end-bridge結(jié)構(gòu)的第二條途徑，此反應機理對于表面吸附結(jié)構(gòu)的形成起重要的作用.與乙炔分子不同的是，表面以下原子參與時乙烯分子的吸附反應為吸熱反應.綜合熱力學 和動力學的分析表明，paired-e nd-bridge構(gòu)型是乙炔分子吸附的主要構(gòu)型，此結(jié)論解釋了乙炔分子在Ge(001)表面吸附構(gòu)型的實驗結(jié)果.對于乙烯和乙炔兩分子在Ge(001)表面吸附的

3、分析比較揭示了導致兩者之間 差異的原因.關鍵詞:密度泛函理論;Ge(001)表面;乙炔分子;形成反應；亞表層;熱力學;動力學O641中圖分類號：React ionP athwaysofAcetyle neAdsor pti onon theGe(001)SurfaceFANXiao-Li1,2,*LIUYa n1LIUCho ng1LAUWoo n-Mi ng2,3,*(1SchoolofMaterialsScie ncea ndEn gi neeri ng,Northwestern Polytech ni calU niversity,Xi'a n710072, P.R.Ch in

4、a;2Beiji ngCo mp utatio nalScie nceResearchCe nter,Beiji ng100084, P.R.Chi na;3Che ngduGre enEn ergya ndGree nManu facturi ngTech no logyResearcha ndDevel opmen tCe nter,Che ngdu610207,P.R. Chi na)Abstract:Theadsorpti on reactio no facetyle neon theGe(001)surfaceisi nv estigatedbyfirst- princip lesc

5、alculations.lnordertounderstandtherelativepopulationsofthedi- andpairednd- bridgestructures,wecalculatedtheadsorptio nreactio np athsleadi ngtotheirformatio nat0.5a ndl.OMLcoverage .Moreimp orta ntly,westudiedtheadsorpti on cha nn eli nv olvi ngsublayerGeatomsbyformi ngametas tablesub-di- (T structu

6、re.Thissu-di- c structurere prese ntssec on dreact ionp athwaythatresults in thee nd bridgestructure,which playsa nimp orta ntrolei ntheformatio no ftheadsorpti oncon figuratio ns.l neon trasttoC2H2,theadsorptio no fC2H 4on theGe(001)surfacei nv olvi ngsubsurfaceGeatoms,ise ndoth ermic.Ourcalculatio

7、 nsshowfrombothki netica ndthermod yn amicsta ndp oi ntsthatthe paired-e nd- bridgestructureisthe primaryadsorpti oncon figurati on thatex plai nstheex perime ntalobservatio ns.Ourworkals ohe Ip stoun dersta ndthefu ndame ntaldiffere ncesbetwee ntheadsorptio no fC2H2a ndC2H 4on theGe(0 01)surface.Ke

8、yWords:De nsityfu nctio naltheory;Ge(001)surface;Acetyle nemolecule;Sublayer;Thermod yn amics;K in eticsFormati on reacti on;Received:December12,2011;Revised:February20,2012; Published。nWeb:March1,2012.? Correspondingauthors.FANXiao-Li,Email:xlfan;Tel:+86-LAUWoo n-Mi ng,Email:leolau;Tel

9、:+86-10-82687096.Thep rojectwass upp ortedbytheNati on alNaturalScie nceF oun dati ono fChi na(20903075)a n dP rogramofI ntroduci ngTale ntsofDisci plin etoU niversities,Chi na(111 Project)(B08040). 國家自然科學基金(20903075)及高等學校學科創(chuàng)新引智計劃(111)(B08040)資助項 目? EditorialofficeofActaPhysico-ChimicaSinica1108Acta

10、 Phys.-Chim.Si n.2012Vol.281ln troducti onTheadsor pti onofun saturatedhydrocarb onm olecules on theeleme ntalsemic on ductorsurface hasattractedgreati nterestbe-causeofits pote ntiali ntech nologicala pp licati on ssuchas non- linearop ticaldevices,molecularelectr oni cdevices,a ndchemi-calse nsors

11、.1-4Co mparin gtothei nte nsivestudies on thei nterac-tiono fhydrocarb onm olecules on Si(001)surface,thedevotio ntotheGe(001)surfaceismuchle ss .Ino rdertostudythe poten-tialoftheassemblehydrocarb on layero nGe(001)surfacei np roductio no fmolecularmicroelec tron ica ndop toelectro ni cde-vicesfabr

12、icatio nan dstability,thei nteractio nofun saturatedhy- drocarb onm oleculeswithGe(001)surfacehasreceivedmoreatte ntio nrece ntly.5-7 Gedimeristhereactivece ntero ntheGe(001)surface,thus,boththei ntradimera ndi nterdimerad sorpti on sare possibleforC2H4/Ge(001).Thei ntradimerdi-(T modelhasthetwoCato

13、msoftheC2H4bondedtothetwoGeatomsofasinglesurfacedi mer,a ndthei nterdimere nd-bridgemodelhasthetwoCatomsb on dedtothetwoGeatoms onon esideoftwoadjace ntdi- mersalo ngthedimeraxis.Kimetal.5observedtwodisti nctde-sorpti on featuresbytheirte mp erature-p rogrammeddesorptio n(TP D)measureme nts.Accordi

14、ngtotheirobservedsca nningtunn elin gmicrosc op y(STM)features,theseres pectivefeatureswereassig nedasthedia ndp aired en d-bridgec on figurati onsin whicha nadditi on alC2H4moleculeadsor pti onm akesthee nd- bridgestructuretobe paired.TheSTMimagesshowthatthedi-T structuredominatesoverthepairednd-br

15、idgestruc-ture.Recently,we8studiedthedi-T anden-bridgeconfigura-tion sforC2H4adsorpti onon Ge(001)surfaceat0.5a nd1.0MLcoveragesusi ngfirst- principlescalculations.Ourcalculatedto-talenergiesindicatethatdi andpaire-end- bridgestructuresaree nergeticallyfavorable,i.e.,theadsorptio nisexothermic.Furth

16、ermore,o urcalculatio nsoftheformatio np athsshowthatthereactio nrateofdi-T structureishigher,andtheenbridgestructureiseasytobepaired.Inadditiontothedi- t andendbridgemodelsi nwhichtwoCatomsareb on dedtotwoGeatoms,twoadditio naladsorp- tionm odelsofthe p-bridgea ndr-bridgec on figurati on sarebe- li

17、evedtobe probableforC2H2/Ge(001).l nbothther-bridgea ndp-bridgec on figurati on s,thetwoCatomsofC2H2moleculeareb on dedtothefourGeatomsofthet woadjacentsurfacedimersbyformingfourt bonds.TheC CaxisisparalleltotheGeGedimerb ondin the p-bridgec on figurati onandperpen dic-ulartotheC-Cdimerb ondin ther-

18、 bridgec on figurati on .The previousSTMa ndTP Di nv estigatio nbyKimetal.9observedtwoads orptio neon figurati on sforC2H2 on Ge(001)surfacela-beledas “ FeatureA ” and “ FeatureB ” ,whichwereassign<e(atiadl|Dedibridgec on figurati on s,res pectively.However,aslisted in Table1,accord in gtoourtota

19、le nergy calculatio n,10aswellasthetheoreticalcalculatio nsofChoa ndKlei nman ,11 p-bridgec on figurati oni smuchlessstabletha nthe paired-e nd-bridgec on figurati on .Bothcalculatio nscon firmthatthetwob on di ngfeaturesareactuallythedi-(T andpaire-end-bridgeTable1Calculatedadsorpti onen ergies(u n

20、it ineV)p erC2H2 molecule on Ge(001)surfacedi- T en<bridge0.5ML1.0ML0.5ML1.0MLr-bridge p-bridgethisGGA1.591.571.521.880.07-0.23Choa ndKlei nman111.781.811.641.870.24-0.03con figurati on s,res pectively.Moreover,theoreticalcalculatio nsofChoa ndKlei nman 11fo un dthatthereactio nratetoformth eend-

21、bridgestructurewassmallerthanthatofthedi- structure.Nevertheless,thewell measuredSTMimages in dicatesthatFeatureBdo min atesatallC2H2coverage,a ndatthesatura tedcoverage,thea ppear-an ceratioofFeatureBtoFeatureAis0.62:0.38.This in dicatesthereact ion ratetoformthee nd- bridgea ndp aired-e nd-bridgea

22、dsor pti on sshouldbelargertha ntheratetoformthedi-ad sorp ti on s,whichisc on flict in gwithChoa ndKle inman? scalculati on results.O ntheotherside,theex perime ntalresults on thetwomoleculesarei neon trasttoea chother: paired-e nd-bridgestructurehasbee nobservedtobedomi nan toverthedi-struc turei

23、nthe prese ntcaseofC2H2,9a ndthe pop ulatio no fdi-T structureislargerthanthatofthepaireehd-bridgestructurei nthecaseofC2H4,5whichisi ndeedco nsiste ntwithourcalcu-latio nresultso nC2H4/Ge(001).8In the prese ntwork,were-studytheadsor pti onan ddesor ption ki neticsofC2H2/Ge(001)bye mp loyi ngourex p

24、erie ncei nu si ngthefirst-princip lesmethodbased on thede nsityf unction- altheory(DFT)toexam in esmallmolecules on Sia ndGesys- tematically,to un dersta ndthediscre pan cybetwee ntheex perimen tobservatio nan dtheoreticalcalculatio n,a ndtodigouttherootcausetothediffere ncebetwe en theadsorptio ns

25、ofC2H2a ndC2H4o nGe(001)surface.2Calculatio nm ethodsTheGe(001)sufacewassimulatedbyaslabco ntainin geightGeatomiclayersa ndavacuumregi ono f1.294nmspac-in g.Ge(001)surfacehasthesimilarrec on structio nastheSi(001)surface.Fourdiffere ntrec onst ructions,symmetric(2 1),bUcked(2 1淑2 2),andc(4 2)$havebe

26、eninvestigated.Amongt hemc(4)2)andp(2 2arethemoststablegeome-tries.Thee nergydiffere ncebetwee nthetwogeometriesislesstha n0.003eV?dimer 1,whichagreeswellwithtw op reviousstudies12,13i nthist op ic.Thus,theGe(001)surfacewas modeledbyare peated p( 22)unitcell,whiletheda ngli ngbon dsofthebottomGeatom

27、sweresat uratedbyHatoms.Thestructure op-timizatio nwas performedusi ngthecalculatedbulklatticec on-sta ntof0.575nmin thetotale nergycalculati on ,theGeatoms in thebottomlayeroftheslab,aswel lastheterm in ati nghy-droge natomswerefixedtothebulk po siti on. TheBrillo uinzon ewassa mpl edbyaM on khorst

28、-P ackschemewith4 40<1k -poin tgrids.Thegeometry op timizatio nin the prese ntstudywas performed on thebasisofde nsityf un cti on altheorywitha ppiyin gtheVie nn aab in itiosimulati onpa ckage(VAS P).14-17ThetotalNo.5FANXiao-Lietal.:Reactio nP athwaysofAcetyle neAdsorpti onon theGe(001)Surface110

29、9en ergycalculatio nswerecarriedoutusi ngtheVa nderbiltul-trasoft pseud op ote ntial18-20a ndplane-wavemethodwithcutoffe nergyof350eV.Thege neralizedgradie ntapp roximatio n( GGA)21,22o fP erdew-Wa ng(P W91)wasusedfortheelectro nicexcha nge-correlati onp ote ntial.Themi nim ume nergyp athsfortheadso

30、rptio nreactio nswerema pp edout usi ngthe nu dgedelasticba nd(NEB)methoddevelo pedbyJiss on aOdco-work-ers.23,24 3Resultsa nddiscussi on3.1Reactio np athwaysat0.5MLAccordi ngto previousstudies8,25-27forC2H4/Si(001),C2H2/Si(001),a ndC2H4/Ge(001),theco ncerted2+2cycloadditio nre actio nisactuallyallo

31、wedbased on thecalculatedreactio nbarriera ndthea nalysisofthesymmetr y.Therelativeim por-ta ncebetwee nthesymmetricco ncerted2+2additio nm echan isma ndthetwo-ste pasymmetricadditi onm echa ni smhasbee ninv estigatedfurther.Ittu rn soutthattheasymmet ricaddi-tio nstarti ngfromathree-atomco mp lexst

32、ate in whichthetwoCatomsoftheadorbedmoleculeareb on dedtothe“"down”“ I”dow nGeatomofaGeGedimerwitha n dativebondisthemoreaccessibleroutebecausethe - GeatomofaGe-7tGedimerisk nown tobeelectr on deficie nt,a ndbecausetheadsorbedmoleculeca naccesstothis p recursorstatewith no strictalig nmen trequ

33、ireme ntofthemolecularaxisi ni tsar- rivaltrajectoryrelativetotheGe-Gedimer.I neon trast,thec on- certedadditio nm echa nismislessi mp orta ntbecausethei ncom- in gmolecularaxismustalig ninp aralleltothesurfacedimer.Thus,statistically,thesig nifica nee ofcon certedadditi onp ath-waysismuchlesstha nt

34、hatoftheasymmetricadditi onp ath- waysa ndtheroleofc on certedadditi oncan beig no red.Inano therco nsiderati ono freactio nki netics,westudytheasymmetricadditio np athwaysi nclu din gtheiri ntermediate pre-cursorstates.Byusi ngtheNEBmethod,wehavefo un dthemi nim ume nergyp athsfromthe prec ursorsta

35、testothedip anden-bridgestructures.Pathldescribestheformationofthedi-(T configurationfromthePlstateandPathllleadstothefomationoftheend-bridgec on figurati on fromthe Pllstate,thecorres pondin gatomicgeometriesofthe precursor( P ),tra nsi-tion( T),a ndthechemisorpti on s(C)states on thereactio np ath

36、sla ndllaredis playedi nF ig.1,a ndt hecorres pon di ngadsorptio nen ergiesaregive nin Table2.Alo ngP athl,there-sp ectiveadsorpti onen ergiesofthe Pla ndTlstatesareO.27a nd0.08eV;thusthereactio nbarrierf romthe Plstatetothedi-T configuationisO.19eV.Incomparison,alongPathll,thecorrespondingadsorptio

37、nenergiesf orthe Plla ndTllareO.28a nd0.05eV,a ndthereact ion barriertoformthee nd- bridgestructureisO.23eV.ltindicatesthatboththedip andendbridgec on figurati on saree nergeticallyfavorableoverthe precursorstatesbyabout1.2- 1.3eV.Thus,ifthee nergyreleasedbytheformatio no fthe precursorstateis no td

38、issi patedawayq uickly,itise no ughtodrivethechemisor ption sthroughthetra nsiti on state .Ino therwords,boththediT an dend bridgec on figurati on sca nbeformedviathe pre- cursorstatei ncom monexp erime ntalc on ditio ns,theirforma- tion ratesaredecidedbythereacti on barriers.Ourcalculatedadsorpti o

39、nen ergiesfor PlandP llstatesarec on siste ntwiththe previousfirst- princip lesstudybyChoa ndKlei nman ,11aslistedi nTable2.Accordi ngtotheircalcula- tions,thebarriersare0.09and0.13eVforthediT andendbridgestructures,res pectively.Althoughourcalculatio nsraisebothres pectivevaluesto0.19a nd0.23eV,the

40、differe ncebe-(a)(b)Fig.lAtomicgeometriesofthe - ncomp lex precursor( P),tra nsitio n( T),a ndchemisorpti on (C)statesfortheadsorpti ono fC2H2o nGe(001)surfacealo ngdiffere ntp aths(a) pathl:asymmetricadditio nreactio nieadi ngtothedi-(T structurefromPlstate;(b)pathll:asymmetricadditionreactionieadi

41、ngtotheformationof thee nd-bridgestructurefromthe PllGe3<02G*1P.,C1I C2ogo3hGe2匚產(chǎn)D.2230335 /W T,state1110Acta Phys.-Chim.Si n.2O12Vol.28Table2Co mp aris on betwee ntheadsor pti ono fC2H2a ndC2H 4on Ge(001)surface,calculated adsor ptio nen ergies(Eads)ofthe-ncomp lex precursor( P),tra nsitio n(T),

42、a ndthechemisorptio n(C)states,a nden ergybarriers(Eb)fromthe PtotheCC2H2/Ge(001)C2H4/Ge(001)bCon figurati onEads/eV PTCEEads/eVb/eVPTCEb/eVdi-c 0.270.081.590.190.400.171.000.230.25a0.16a1.78a0.09ae nd-bridge0.280.051.520.230.350.160.900.190.26a0.13a1.64a0.13a paired-di-c0.240.051.560.97

43、0.171.81a paired-e nd-bridge0.290.161.81 a0.29a1.87 a0.02 aEads=E(mole)+E(Ge(001)-E(mole/Ge(001),whereE(mole)a ndE(Ge(001)re prese ntthee nergiesoftheC2H2(C2H4) moleculea ndthebareGe(001)surface,res pectively;E(mole/Ge(001)re prese ntsthetotale ner gyoftheadsorbedC82H2(C2H4)/

44、Ge(001)system.aRefere nce11,bRefere ncetwee nthetwobarriersisagai n0.04e V.Asre portedbyChoa ndKle inman ,11theratioofthereact ion rateR1toformthedi-(T configurationtothereactiorateR2toformtheend-bridgec on figuratio n,rep rese ntedasR1/R2,is1.2ifalltheadsorptio nen ergyofthe precursorsta teisretai

45、ned.Usi ngtheArrhe nius-ty peactivatio np rocesswithaty picalvalue(T014s-1)forthe pre-exponen tialfactorwhichhasbee nadop tedbyChoa ndKlei nman ,wehavecalculatedthereactio n rateR2fromthe precursorstate Plltothee nd-bridgestructure;there-sulta ntR2is1.3010s- 1.Wealsofi ndthatthereactio nrateR1toform

46、thedi-T structureis4.7timeslargerthanR2.ThefactorR1/R2becomes1.1ifalltheadsorptionenergies ofthe precur-sorstatearereta in ed.Therefore,ourcalculati on sareaga incon-siste ntwiththere portedresultsofChoa ndKlei nman ,withboth predicti ngthattheformatio nrat eofdi- T structureislargerthanthatoftheend

47、-bridgestructure.3.2Reactionpathwaysat1.0ML Afterstudyingtheadsorptionofonesinglemoleculetoformthedi-T andend bridgestructures,wehavefurthercalculatedtheres pectivereact ionp athwaysofaddi ngon emo reC2H2mol-eculetoformtheactuaIpaired-di- t andpairednd-bridgestructures.Aslistedi nTable2,co mparin gt

48、heformatio no fthe paired-di-T structureandthefirstdiT structre,oneseesthattheadsorptionenergyoftheprecursorstatetoformthepaired-di-(T structureisslightlylowerby0.03eV,andtheformationenergyoftheseconddi(T structureisalsolowerby0.02eV.Incontrast,theadsorptionenergyoftheprecursorstatetoformthe paired-

49、e nd-bndgestructureisslightlyhigher,a ndtheformatio nen ergyofthesec ondend- bridgestructureishigherbyabout0.4eV.Theseco mp aris on si ndicatethatthecTstruc tureisenergeticallyfavorableoverthepaired-di-c structure;)neontrast,theend bridgestructurewillbeeve ntually pairedu p. Alo ngthereactio np athw

50、aytoformthe paired-di-T structure,thee nergybarrieris0.19eV .Incomp ariso n, thereactio nbarrieris0.13eValo ngtheformationp athofthe paired-e nd-bridgestructure.Co mparin gtheformatio nreactio no fthe paired-e nd-bridgestructurea ndthefirste nd-bridgestructure,thereac-tionbarrierislowerby0.10eV.Inth

51、ecaseofthedi-T structure,thereact ion barriertoformthe paired-di-T structuredoes no tcha ngefromthebarrierofthefirstdiT structure.Allthesecomparisonsfromboththermodynamicsandkineticsindicatethattheadsorptionofaseconddi-T C2H2isim|9ded;ineon-trast,theformationofthepaired-end- bridgestructureiseasya n

52、dtheformati on rateishigher on cethee nd-bridgestructureisformed. Ino rdertodem on stratetheaboveco mp aris on smoreex plic- itly,weestimatethereactio nratetoformthe paired-e nd- bridgestructurebyado pti ngan Arrhe ni us-typ erateformulaagai nan dthevalueturnsouttobe6.41011s-x1,whichis11timeslargert

53、ha ntheformatio nrateofthe paired-di-T structure.ThefactorR1/R2becomes1.3ifalltheadsorptionenergiesoftheprecursorstatesareretained.AccordingtotheSTMexperiment,theratioof“ FeatureA ” imagewhichisassignedasdip structuretothe“ FeatureB ” imagewhichisnowassignedofepairedbridgestructurewasobtai nedas0.38

54、:0.62atsaturati on cover-age.He nce,afactorof1.3is note no ughtomake up thedefi-cie ncyin theformatio nratesofthee nd-bridgestructure.Co n-siste ntwithourcalculatio ns,theformatio no fthe paired-e nd-bridgestructureisalsoeasierasshow nin there portbyChoa ndKleima n,11whichca nno thel pthe app eara n

55、ceratioofpaired-e nd-bridgestructuretothedi-T structuretorisetotheexperimentalvalueof0.62:Reactio np athwaysi nv olvi ngthesublayerGeatomThecalculatedadsorptio nen ergiesofthe precursorstatea ndthereactio nbarrierstoformthe in tr adimerdi- c andinterdimerend bndgestructuresforC2H4molecule on

56、 Ge(001)surfacearealsolistedi nTable2.Thee nergyrel easedbytheformati onofthep recursorstateis no tdiss ip atedawayquicklywhe nthesurfacetem p eratureishigh,whichisthecom monexp erime n-talc on ditio n.Si ncetheadsorptio nen ergyofthe Pllstateislarg-ertha nthatofthe Plstate,thereactio nratetoformthe

57、di-c structurewillbelargerthanthereactionratetoformtheendbridgestructure.Thisisco nsiste ntwiththe prese ntcasefortheadsorpti ono fC2H2 on Ge(001)s urface.Also,theadsor pti onen-ergiesofthe precursorstatesa ndreactio nbarrierstoformthe paired-dCa ndp aire-e nd- bridgestructuresarec on siste ntbe-twe

58、e nthetwomolecules,bothshowsthee nd- bridgestructurewillbeeasil yp airedu p.Accordi ngtotheex perime ntalstudies,5 on theotherside,thedi-c structuredominatesoverthepairednd-bridgestructureo ntheC2H4adsorbedGe(001)surface,whilei nthe prese ntcaseofC2H2/Ge(0 01),asme ntio nedabove,thea pp eara ncera-tioofpaired-e nd-bridgestructuretodi-c structureis0.68:0.32.lnotherwords,ineontrasttothetheoreticalstudies,ther