High charge flexibility of the surface dangling bonds on the Si(1 1 1 ...


High charge flexibility of the surface dangling bonds on the Si(1 1 1 ...

CPLETT 16674DISK / 12/3/02ARTICLE IN PRESSNo. ong>ofong> pages:6DTD 4.3.1/ SPS-NX. Lu et al. / Chemical Physics Letters xxx (2002) xxx–xxx 3104 ted half ong>ofong> ong>theong> Si(1 1 1)-7 7 unit cell. This model105 including ong>theong> 3rd- and 4th-sublayer atoms is larger106 and should be more reliable than ong>theong> previously107 reported Si 9 H 12 model [31], which consists ong>ofong> at-108 oms from only ong>theong> 1st and 2nd layers. Apart from109 ong>theong> Si 16 H 18 cluster model, we have used a larger110 cluster model consisting ong>ofong> 25 Si atoms to check111 ong>theong> convergence ong>ofong> cluster size. No significant112 differences were found in ong>theong> calculated energies113 and geometries for ong>theong> dissociative chemisorption114 ong>ofong> NH 3 over ong>theong> restatom–adatom pair site. The115 data reported below were obtained with ong>theong>116 Si 16 H 18 cluster model.117 3. Results and discussion118 Fig. 1b gives ong>theong> optimized geometry ong>ofong> ong>theong>119 Si 16 H 18 ong>surfaceong> model. The optimal SiSi bond120 lengths range from 2.33 to 2.46 A. Particularly, ong>theong>121 bond lengths between ong>theong> adatom (Si1) and its122 nearest neighbors (Si3, Si4, and Si5) are about123 2.44 A, and ong>theong> bond lengths between ong>theong> restatom124 (Si2) and its nearest neighbors (Si7, Si8 and Si9) are125 around 2.36 A. These values are in good agreement126 with those predicted for ong>theong> Si(1 1 1)-2 2 ong>surfaceong>127 based on ong>theong> DFT periodic slab model calculations128 [32]. The Si(1 1 1)-2 2 adatom ong>surfaceong> was found129 to be a good, yet computationally feasible, ap-130 proximation ong>ofong> ong>theong> Si(1 1 1)-7 7 ong>surfaceong> in ong>theong>131 previous DFT calculations ong>ofong> hydrogen diffusion132 [33] and ong>theong> initial-stage oxidation by O 2 [34]. De-133 tailed wavefunction analysis based on ong>theong> natural134 bond orbital (NBO) method [35] reveals that in ong>theong>135 Si 16 H 18 cluster ong>theong> two natural DBs at ong>theong> adtaom136 and ong>theong> restatom have ong>theong> occupations ong>ofong> 0.78e and137 0.93e, respectively, with ong>theong> ong>danglingong> bond state at138 ong>theong> restatom being lower in energy. This is quali-139 tatively in agreement with ong>theong> electronic structure140 ong>ofong> ong>theong> Si(1 1 1)-7 7 ong>surfaceong> [4]. It should be noted141 that with such a moderate-size cluster model, one142 cannot discern ong>theong> chemical difference between ong>theong>143 center adatoms and ong>theong> corner adatoms on ong>theong>144 Si(1 1 1)-7 7 ong>surfaceong>. Indeed, ong>theong> periodic145 Si(1 1 1)-2 2 model ong>surfaceong> employed previously146 [32–34] suffers ong>theong> same demerit.147 We have considered two possible pathways for148 ong>theong> reaction ong>ofong> NH 3 with ong>theong> restatom–adatompair site on ong>theong> Si 16 H 18 ong>surfaceong> model, i.e., oneinitially occurs at ong>theong> adatom and anoong>theong>r occursat ong>theong> restatom. The predicted transition states andlocal minima are depicted in Fig. 2 and ong>theong> prong>ofong>ileong>ofong> energy ong>surfaceong>s is presented in Fig. 3.For ong>theong> reaction initiating on ong>theong> adatom, amolecular precursor state LM1a was identifiedwith an optimal Si–NH 3 bond length being 1.96 A and a binding energy ong>ofong> 30:7 kcal mol 1 . It ismetastable and can transform into ong>theong> dissociativestate LM2a after overcoming a low barrier( 2:0 kcal mol 1 ) at ong>theong> transition state TS1a.This results in an NH 2 group bonded to ong>theong> adatomand an H atom bonded to ong>theong> restatom. TheUNCORRECTED PROOFFig. 2. Optimized geometries (bond length in A) and relativeenergies ðkcal mol 1 Þ ong>ofong> ong>theong> stationary points in ong>theong> initialstages ong>ofong> NH 3 reactions with ong>theong> restatom–adatom pair site onong>theong> Si(1 1 1)-7 7 ong>surfaceong>.149150151152153154155156157158159160161162

CPLETT 16674DISK / 12/3/02ARTICLE IN PRESSNo. ong>ofong> pages:6DTD 4.3.1/ SPS-N4 X. Lu et al. / Chemical Physics Letters xxx (2002) xxx–xxxFig. 3. Prong>ofong>ile ong>ofong> energy ong>surfaceong>s for ong>theong> dissociative adsorptionong>ofong> NH 3 on ong>theong> Si 16 H 18 ong>surfaceong> model. Relative energies given inparenong>theong>ses are ZPE-corrected values.163 predicted exoong>theong>rmicity for this reaction is164 68:6 kcal mol 1 .165 A similar reaction pathway was found to initi-166 ate on ong>theong> restatom, but with slightly different re-167 action energetics, as shown in Figs. 2 and 3. That168 is, NH 3 forms a molecular precursor LM1r with169 ong>theong> restatom, ong>theong>n via a transition state TS1r to ong>theong>170 dissociative state LM2r. The final products are171 NH 2 adspecies on ong>theong> restatom and H adspecies on172 ong>theong> adatom. The overall reaction heat is173 66:5 kcal mol 1 , comparable with that initiating174 on ong>theong> adatom. The binding energy for ong>theong> pre-175 cursor state LM1r is 39:6 kcal mol 1 , and ong>theong>176 barrier from LM1r to LM2r is 7:1 kcal mol 1 ,177 indicating ong>theong> precursor state formed on ong>theong> rest-178 atom is slightly more stable than that formed on179 ong>theong> adatom. Neverong>theong>less, both precursor statesare metastable and can hardly be detectable experimentally.The PES given in Fig. 3 shows clearly that ong>theong>dissociation ong>ofong> NH 3 over ong>theong> restatom–adatompair site occurs readily and can initiate on eiong>theong>rong>theong> adatom or ong>theong> restatom, i.e., ong>theong> reactionshows no noticeable site-preference if restrictedwithin a restatom–adatom pair site. This finding isin contradiction with ong>theong> previous experimentalinference that ong>theong> reactivity ong>ofong> ong>theong> restatoms ishigher than ong>theong> adatoms toward NH 3 [4]. Notethat ong>theong> number ong>ofong> adatoms is two times that ong>ofong>restatoms on ong>theong> Si(1 1 1)-7 7 ong>surfaceong>. It meansthat even when all ong>theong> restatoms are consumed byNH 3 chemisorption, half ong>ofong> ong>theong> adatoms remainunreacted. That is why ong>theong> STM and photo-emissionexperiments detected immediate quenching ong>ofong>ong>theong> DB states connected to ong>theong> restatoms uponNH 3 adsorption, and, in contrast, a much slowerquenching ong>ofong> ong>theong> DB states connected to ong>theong> adatoms.Such phenomena do not reflect ong>theong> higherreactivity ong>ofong> ong>theong> restatoms than that ong>ofong> ong>theong> adatoms,but simply show that both ong>theong> restatoms andong>theong> adatoms are involved in ong>theong> initial stages ong>ofong>NH 3 reaction with ong>theong> Si(1 1 1)-7 7 ong>surfaceong>.Anoong>theong>r surprising, yet interesting, finding isong>theong> barrierless formation ong>ofong> ong>theong> molecular precursorstate on ong>theong> restatom site, as it has been generallybelieved that ong>theong> DB at ong>theong> restatom isoccupied due to ong>theong> favorable adatom-to-restatomong>chargeong> transfer on ong>theong> clean ong>surfaceong> and, hence,would be repulsive to ong>theong> lone pair ong>ofong> an approachingNH 3 molecule [4]. For ong>theong> barrierlessformation ong>ofong> ong>theong> molecular precursor state on ong>theong>Table 1Natural occupation ong>ofong> ong>theong> ong>danglingong> bond (DB) states and relative energies ong>ofong> ong>theong> NH 3 =Si 16 H 18 system at different H 3 N–Si distancesR (Si–N) ( A) DE a ðkcal mol 1 Þ DB at adatomoccupation (e)DB at restatomoccupation (e)Free ong>surfaceong> model 0.0 0.78 0.93H 3 N ! restatom 3.4 4:6 1.79 0.553.1 9:0 1.86 0.48LM1r 1.97 39:6 1.89 N/AH 3 N ! adatom 3.4 3:1 0.57 1.243.1 5:8 0.54 1.71UNCORRECTED PROOFLM1a 1.96 30:7 N/A 1.78a DE ¼ EðNH 3 =Si 16 H 18 Þ–EðNH 3 Þ–EðSi 16 H 18 Þ, ZPE-uncorrected.180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213

CPLETT 16674DISK / 12/3/02ARTICLE IN PRESSNo. ong>ofong> pages:6DTD 4.3.1/ SPS-NX. Lu et al. / Chemical Physics Letters xxx (2002) xxx–xxx 5214 restatom site, a facile reverse ong>chargeong> transfer from215 ong>theong> restatom to ong>theong> adatom would be essential [4].216 To confirm this inference, we performed a series ong>ofong>217 NBO analyses on ong>theong> Kohn–Sham wavefunctions218 ong>ofong> ong>theong> model systems at different Si–NH 3 distances219 along ong>theong> potential energy ong>surfaceong>. The results are220 tabulated in Table 1. For a free ong>surfaceong> model, ong>theong>221 DB orbital occupation is 0.78e at ong>theong> adatom and222 0.93e at ong>theong> restatom. When an ammonia molecule223 approaches ong>theong> restatom with its N-end, ong>theong> ong>chargeong>224 transfer from ong>theong> restatom to ong>theong> adatom is found225 to be surprisingly severe even at a very large226 Si–NH 3 distance (e.g. 3.4 A). At ong>theong> Si–N distance227 ong>ofong> 3.4 A, ong>theong> DB orbital occupation at ong>theong> adatom228 increases to 1.79e from 0.78e for ong>theong> free ong>surfaceong>229 model; meanwhile, ong>theong> DB occupation decreases to230 0.55e from 0.93e, showing ong>theong> facile ong>chargeong> trans-231 fer over ong>theong> DB states steered by ong>theong> approaching232 lone-pair ong>ofong> ong>theong> foreign molecule. In ong>theong> molecular233 precursor state LM1r, ong>theong> DB at ong>theong> restatom di-234 minishes, while ong>theong> DB at ong>theong> adatom has an oc-235 cupation ong>ofong> 1.89e. Similar ong>chargeong> transfer is236 observed in ong>theong> process ong>ofong> NH 3 approaching ong>theong>237 adatom with its N-end. Such intriguing lone-pair-238 steering ong>chargeong>-transfer reflects ong>theong> high ong>chargeong>239 ong>flexibilityong> ong>ofong> ong>theong> DB states on ong>theong> Si(1 1 1)-7 7240 ong>surfaceong>, which is ong>theong> very reason why ong>theong>re is no241 noticeable site-preference for NH 3 reaction with242 ong>theong> restatoms and ong>theong> adatoms. Finally, in ong>theong>243 dissociative chemisorption states, ong>theong> N lone pair244 ong>ofong> ong>theong> NH 2 ðaÞ species has a natural occupation ong>ofong>245 1.93e for ong>theong> one adsorbed on ong>theong> restatom site246 and a natural occupation ong>ofong> 1.91e for ong>theong> one ad-247 sorbed on ong>theong> adatom site.248 4. Concluding remarks249 In summary, ong>theong> present DFT cluster model250 calculations have shown that ong>theong> dissociation ong>ofong>251 NH 3 occurs readily on eiong>theong>r ong>theong> restatom or ong>theong>252 adatom via a metastable, molecular precursor.253 Elaborate NBO analyses revealed that upon ong>theong>254 approaching ong>ofong> NH 3 with its N-end to ong>theong> ong>surfaceong>,255 ong>chargeong> transfer from ong>theong> DB at ong>theong> attacked rest-256 atom (or adatom) to ong>theong> DB at ong>theong> adjacent ad-257 atom (or restatom) is quite facile and unexpectedly258 significant even at a very large H 3 N–Si distance(e.g., 3.4 A). Such an intriguing ong>chargeong> transfersteered by ong>theong> lone-pair ong>ofong> a guest molecule reflectsong>theong> high ong>chargeong> ong>flexibilityong> ong>ofong> ong>theong> ong>danglingong> ong>bondsong>tates on ong>theong> Si(1 1 1)-7 7 ong>surfaceong>. To ong>theong> best ong>ofong>our knowledge, ong>theong> above ong>theong>oretical analysesprovide ong>theong> first ong>theong>oretical evidence ong>ofong> ong>theong> highong>chargeong> ong>flexibilityong> ong>ofong> ong>theong> ong>danglingong> bond states on ong>theong>Si(1 1 1)-7 7 ong>surfaceong>, which is ong>ofong> great significanceto our understanding ong>ofong> ong>theong> chemistry ong>ofong> ong>theong>technologically important silicon ong>surfaceong>s.AcknowledgementsThis work was sponsored by ong>theong> Nature ScienceFoundation ong>ofong> China, ong>theong> Ministry ong>ofong> Educationong>ofong> China, Fok Ying-Dung Educational Foundation,Xiamen University and Emory Universitythrough ong>theong> Robert W. Woodruff Prong>ofong>essorship.ReferencesUNCORRECTED PROOF259260261262263264265266267268269270271272273274275[1] H.N. Waltenburg, J.T. Yates Jr., Chem. Rev. 95 (1995) 2761589.277[2] R.J. Hamers, Y. Wang, Chem. Rev. 96 (1996) 1261.[3] M. Nishijima, K. 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