Chemisorption and Decomposition of Thiophene and Furan on the Si(100)-2 1 Surface: A Quantum Chemical Study

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1 J. Phys. Chem. B 2001, 105, Chemisorption nd Decomposition of Thiophene nd Furn on the Si(100)-2 1 Surfce: A Quntum Chemicl Study Xin Lu,*, Xin Xu, Nnqin Wng, Qiner Zhng, nd M. C. Lin*, Stte Key Lbortory for Physicl Chemistry of Solid Surfces nd Deprtment of Chemistry, Ximen UniVersity, Ximen , Chin, nd Deprtment of Chemistry nd Cherry L. Emerson Center for Scientific Computtion, Emory UniVersity, Atlnt, Georgi ReceiVed: June 13, 2001 The chemisorption nd decomposition of thiophene (C 4 H 4 S) nd furn (C 4 H 4 O) on the reconstructed Si(100)-2 1 surfce hs been investigted by mens of the hybrid density functionl (B3LYP) method in combintion with cluster model pproch. Two chemisorption mechnisms, i.e., [4 + 2] nd [2 + 2] cyclodditions of C 4 H 4 X(X) S,O) onto surfce dimer site, hve been considered comprtively. The clcultions reveled tht the former process is brrierless nd fvorble over the ltter, which requires smll ctivtion energy (2.6 kcl/mol for thiophene nd 1.2 kcl/mol for furn). The di-σ bonded surfce species formed by [4 + 2] cycloddition-type chemisorption cn either undergo further [2 + 2] cycloddition with neighboring SidSi dimer site, giving rise to tetr-σ bonded surfce species, or undergo deoxygention (desulfuriztion) by trnsferring the heterotom to neighboring SidSi dimer site, leding to six-member ring metllocyclic C 4 H 4 Si 2 surfce species. The ltter process ws found to be slightly more fvorble thn the former, especilly in the cse of thiophene. 1. Introduction The interction of π-conjugted, romtic compounds with silicon surfces is not only of fundmentl interest, but lso of prcticl importnce to the development of highly ordered thin-films of conducting or semiconducting polymers on silicon surfces. 1,2 Growing efforts devoted to this topic include the experimentl studies of the dsorption of benzene (C 6 H 6 ), 3-5 pyridine (C 5 H 5 N), 6-8 furn (C 4 H 4 O), 9-12 thiophene (C 4 H 4 S), 9,10,13,14 nd pyrrole 10 on the Si(111) nd its reconstructed surfces nd the dsorption of benzene, pyrrole, 19 nd thiophene 20,21 on the Si(100)-2 1, s well s the theoreticl studies of benzene nd pyrrole 28 dsorption on the Si(100)- 2 1 surfce. In the present pper, we report the results of first-principles density functionl cluster model clcultions for the chemisorption nd decomposition of thiophene nd furn on the Si(100)-2 1 surfce. The reconstructed Si(100)-2 1 surfce hs bonding motif tht pirs of toms form dimers with strong σ-bond nd quite wek π bond. Accordingly, these surfce dimers show high rectivity towrd in-coming unsturted orgnic compounds, including simple lkenes, dienes, s well s benzene, s reveled by previous experimentl nd theoreticl studies. 29 The chemistry of unsturted orgnic compounds on the Si(100)- 2 1 surfce cn be summrized s follows: (i) simple lkenes, e.g., ethylene nd cyclopentene, undergo [2 + 2] cycloddition rection with the surfce dimer; 23,29 (ii) conjugted dienes, e.g., 1,3-butdiene nd 1,3-cyclohexdiene, undergoes both [4 + 2] cycloddition rection nd [2 + 2] cycloddition rection with the surfce dimer, with the former process being kineticlly preferble over the ltter; (iii) chemisorption of highly π-conjugted romtics such s benzene on the Si(100)-2 1 * Corresponding uthors. E-mil: xinlu@xmu.edu.cn (for Xin Lu), nd E-mil: chemmcl@emory.edu (for M. C. Lin). Ximen University. Emory University. surfce follows exclusively the [4 + 2] cycloddition mechnism with the initil product being di-σ bonded, 1,4-cyclohexdienelike surfce species ,22-27 Both thiophene nd furn re fivemember ring, π-conjugted compounds with heterotom. They differ from common conjugted dienes by showing considerble romticity. They re lso unlike the romtic benzene, due to its inhomogeneous electron distribution within the whole ring system. As such, they might disply somewht different, yet interesting, chemisorption behvior from tht of common dienes nd benzene on the Si(100)-2 1 surfce. The dsorbed stte of thiophene on the Si(100)-2 1 ws first investigted by Jeong et l. using LEED (low-energy electron diffrction), AES (Auger electron spectroscopy), nd UPS (ultrviolet photoelectron spectroscopy). 21 The (2 1) LEED pttern t 300 K ws sustined fter sturted exposure of thiophene, nd the sturtion coverge ws estimted to be 0.6 by AES, suggesting tht thiophene molecules is chemisorbed moleculrly on the Si(100) surfce most likely by σ bonds between C nd Si toms. UPS spectrum for the chemisorbed thiophene showed not only the π orbitl shift but lso the σ orbitl shift. The uthors lso performed semiempiricl PM3 cluster model clcultions to study the chemisorption mechnism nd proposed tht thiophene is di-σ bonded onto surfce dimer with its 2,3 C toms, i.e., [2 + 2] cycloddition mechnism. 21 This proposl is contrdicted very recently by Qio et l., 20 who studied the sme surfce rection by mens of UPS, XPS (X-ry photoelectron spectroscopy), nd HREELS (high-resolution electron energy loss spectroscopy). They concluded tht thiophene is chemisorbed in the form of 2,5- dihydrothiophene-like species following [4 + 2] cycloddition mechnism. In their experiments, two dsorption sttes were identified t 120 K nd ssigned them to physisorbed nd chemisorbed thiophene, respectively. The physisorbed one desorbs below 200 K. Above 400 K, the chemisorbed species were found to either desorb moleculrly or decompose possibly vi R-thiophenyl nd Si-H through H-bstrction mechnism, /jp012254s CCC: $ Americn Chemicl Society Published on Web 09/22/2001

2 10070 J. Phys. Chem. B, Vol. 105, No. 41, 2001 Lu et l. TABLE 1: Adsorption Energies nd Selected Geometric Prmeters of the C 4 H 4 X/Si 9 O 12 (X ) O,S) Model Systems from B3LYP/6-31G(d) Clcultions C 4H 4S/Si 9O 12 C 4H 4O/ Si 9O 12 rects. LM1 LM2 TS1 rects. LM1 LM2 TS1 Si A-Si B (Å) C 2-Si A (Å) C 3-Si B (Å) C 5-Si B (Å) C 2-C 3 (Å) C 3-C 4 (Å) C 4-C 5 (Å) C 2-X (Å) C 5-X (Å) Si B-Si A-C 2 ( ) Si A-C 2-C 5 ( ) Si A-C 2-C 3 ( ) E (kcl/mol) (0.0) (-36.0) (-23.6) (2.5) (0.0) (-31.6) (-22.9) (1.1) Adsorption energy E ) E(C 4H 4X/Si 9H 12) - E(Si 9H 12) - E(C 4H 4X). Energies without zero-point energy corrections re given in prentheses. nd vi metllocyclic-like intermedite nd tomic S through S-bstrction mechnism. 22 The sme uthors lso exmined the chemisorption of thiophene on the Si(111)-7 7 surfce, nd proposed tht thiophene undergoes [4 + 2] cyclodditionlike chemisorption onto n dtom-rest tom pir site with the formtion of 2,5-dihydrothiophene-like dspecies. 14 Consequently, whether thiophene undergoes [2 + 2] cycloddition-like or [4 + 2] cycloddition-like chemisorption on the Si(100)-2 1 is debtble. One of the objectives of the present theoreticl study is to provide cler-cut nswer to this question nd, in ddition, to investigte the decomposition pthwys of chemisorbed thiophene upon heting. So fr neither experimentl nor theoreticl work could be found in the literture regrding furn dsorption on the Si(100)-2 1 surfce. One expects tht the dsorptive behvior of furn on the Si(100)-2 1 surfce would be similr to tht of thiophene, considering their similrity in geometric nd electronic structures. However, recent TDS (therml desorption spectroscopy) nd HREELS experiments reveled the dimeriztion of furn on the Si(111)-7 7 surfce, 12 wheres no dimeriztion ws found for thiophene on the sme surfce. 14 Another purpose of the present theoreticl study is, therefore, to compre the chemisorption behviors of furn nd thiophene on the Si(100)-2 1 surfce nd exmine the extent of their similrity in surfce rections. Density functionl theory (DFT) in combintion with cluster model pproch hs been used for the bove-mentioned purposes. This pper is orgnized s follows. In Section 2, we introduce the computtionl method nd the cluster model used. In Section 3, the clculted results re presented, long with the chemisorption mechnisms of C 4 H 4 X(X) S,O) on the Si(100)-2 1 surfce s well s the different pthwys of therml isomeriztion nd decomposition of the chemisorbed C 4 H 4 X. Concluding remrks will be given in Section Computtionl Detils We hve used two surfce models to represent the reconstructed Si(100)-2 1 surfce. The first one is single-dimer Si 9 H 12 cluster, where the top lyer is dimer consisting of two Si toms, ech with one dngling bond. 23,27,28,31-34 The second model is Si 15 H 16 cluster, where the top lyer consists of two djcent dimers in the sme dimer row The double-dimer model llows the study of chemisorption nd dissocition of dsorbtes cross the dimer pirs. All clcultions were performed with the GAUSSIAN94 pckge. 37 The hybrid Figure 1. Locl minim nd trnsition sttes for the C 4H 4X/Si 9H 12 (X ) S,O) systems predicted t the B3LYP/6-31G(d) level of theory. density functionl method including Becke s 3-prmeter nonlocl-exchnge functionl 38 with the correltion functionl of Lee-Yng-Prr 39 (B3LYP) s well s nlyticl grdients ws employed. The bsis set used is the stndrd ll-electron splitvlence bsis set 6-31G(d) including the polriztion d-function on non-hydrogen toms. 40 Geometry optimiztions nd vibrtionl nlyses were performed without ny constrint. Finl energy prmeters include the unscled zero-point-energy (ZPE) corrections clculted t the B3LYP/6-31G (d) level of theory. 3. Results nd Discussion 3.1. C 4 H 4 X (X ) S,O) Chemisorption on Si 9 H 12. As mentioned bove, n erlier experimentl nd theoreticl study suggested tht thiophene chemisorption on the Si(100)-2 1 occurred by the [2 + 2] cycloddition mechnism, 21 wheres recent HREELS experiment suggested tht the dsorption occurred by the [4 + 2] cycloddition mechnism. 20 We hve considered both rection pthwys in our clcultions using the Si 9 H 12 cluster model. Figure 1 presents the optimized geometries of the locl minim nd trnsition sttes for the C 4 H 4 X/Si 9 H 12 model systems. The reltive energies nd selected geometricl prmeters of vrious sttionry points re given in Tble 1.

3 Chemisorption nd Decomposition of Thiophene nd Furn J. Phys. Chem. B, Vol. 105, No. 41, The [4 + 2] cycloddition of thiophene onto the dimer site gives rise to di-σ bonded, 2,5-dihydrothiophene-like dspecies (LM1), s shown in Figure 1. This process is exothermic by kcl/mol predicted t the B3LYP/6-31G(d) level of theory. Effort hs been pid to serch for the trnsition stte responsible for the [4 + 2] cycloddition process. The results reveled tht this process is concerted nd brrierless. In the [4 + 2] product (LM1), the length of two equivlent C-Si bonds thus-formed is Å; the Si-Si bond length is elongted by Å with respect to the unrected one; the C 3 -C 4 bond length is shortened to Å, which is typicl for CdC double bond. The [2 + 2] cycloddition of thiophene onto the dimer site of the Si 9 H 12 cluster leds to di-σ bonded, 2,3-dihydrothiophenelike dspecies (LM2), s shown in Figure 1b. This process is exothermic by kcl/mol predicted t the B3LYP/6-31G(d) level of theory, nd is thermodynmiclly less fvorble thn the [4 + 2] cycloddition process. In LM2, the two C-Si bond lengths re nd Å; the Si-Si bond length is elongted by Å with respect to the unrected one; the C 2 -C 3 bond length is elongted to Å, which is typicl forc-c single bond. Unlike the [4 + 2] process, which is concerted nd brrierless, the [2 + 2] process hs trnsition stte (TS1) with brrier height of 2.6 kcl/mol predicted t the B3LYP/6-31G(d) level. At TS1, the two forming C-Si bond lengths re nd Å, showing the [2 + 2] process is not concerted. It is cler tht for thiophene chemisorption onto the Si(100)-2 1 surfce, the [4 + 2] process is both kineticlly nd thermodynmiclly fvorble over the [2 + 2] process, nd the brrierless [4 + 2] process cn redily occur t rther low temperture, e.g., 90 K. Similr chemisorption behvior hs been predicted for furn, s shown in Figure 1 nd Tble 1. A di-σ bonded, 2,5- dihydrofurn-like surfce species is formed by [4 + 2] cycloddition, wheres the [2 + 2] process results in di-σ bonded, 2,3-dihydrofurn-like dspecies. The [2 + 2] process requires n ctivtion energy of 1.2 kcl/mol predicted t the B3LYP/ 6-31G(d) level, nd is kineticlly less fvorble thn the [4 + 2] process. Both processes re exothermic with the predicted rection energies of nd kcl/mol for the [4 + 2] nd [2 + 2] processes, respectively. At this conjuncture, it is interesting to compre the chemisorptive behvior of thiophene nd furn with tht of other conjugted orgnic compounds such s benzene nd 1,3- cyclohexdiene on the sme surfce. For benzene nd 1,3- cyclohexdiene, previous experimentl nd theoreticl studies reveled tht both π-conjugted molecules preferentilly undergo [4 + 2] cycloddition-like chemisorption on the Si(100)-2 1 surfce, 15-18,22-27,32 but the dsorption energy of benzene is by fr lower thn tht of 1,3-cyclohexdiene. 23 This intriguing phenomenon cn be scribed to the romtic stbiliztion in benzene. Indeed, previous B3LYP/6-31G** cluster model clcultions 23 did predict difference of 32.6 kcl/mol in binding energy between benzene nd 1,3-cyclohexdiene, which is comprble to the difference ( 28.3 kcl/mol) 41,42 in resonnce energy between benzene nd 1,3-cyclohexdiene. It is known tht both thiophene nd furn re romtic compounds, but less romtic thn benzene. 41 Hence it is plusible tht if thiophene nd furn disply similr chemisorptive behvior s benzene does, the binding energies of thiophene nd furn would be lrger thn tht of benzene. This inference is confirmed by the present B3LYP/6-31G(d) cluster model clcultions. At the B3LYP/6-31G(d) level, the predicted binding energies for the [4 + 2] cycloddition products of thiophene nd furn on the Si 9 H 12 model surfce re 34.3 nd 30.0 kcl/mol, respectively, which re lrger thn tht of benzene (21.8 kcl/mol predicted t the sme level of theory). Aprt from the [4 + 2] nd [2 + 2] products, we hve lso found for thiophene S-linked physisorbed stte, PS (see Figure 1d), nd for furn O-linked physisorbed stte, PO (see Figure 1e). In the PS stte of thiophene, the S tom of thiophene is linked with the buckled-down Si tom of the surfce dimer, with S-Si distnce of Å; the moleculr plne of thiophene is tilted wy from the surfce norml. The binding energy is 2.9/3.3 kcl/mol with/without ZPE corrections predicted t the B3LYP/6-31G(d) level. The lrge S-Si seprtion s well s the low binding energy implies tht the interction between thiophene nd the surfce is primrily electrosttic, s the buckled-down Si tom of the surfce dimer is positively chrged. In the PO stte of furn, the moleculr plne of furn is coplnr with the surfce dimer with its O heterotom linked with the buckled-down Si tom of the surfce dimer. The binding energy is 6.4/7.3 kcl/mol with/without ZPE corrections predicted t the B3LYP/6-31G(d) level. With lrge O-Si distnce (2.171 Å), the interction between furn nd the surfce is minly electrosttic. It is noteworthy tht the bonding configurtion of furn dsorbed in the PO stte resembles one of the severl bonding configurtions of pyridine on the sme surfce site, for which strong N-Si bond ws reveled with binding energy of 28.4 kcl/mol t the sme level of theory Isomeriztion nd Decomposition of Chemisorbed C 4 H 4 X(X) S,O). We hve shown tht both thiophene nd furn preferentilly undergo [4 + 2] cycloddition onto the dimer site of the Si(100)-2 1 surfce using the Si 9 H 12 cluster model. On the rel Si(100)-2 1 surfce, the thus-formed di-σ bonded dspecies (LM1) my undergo either [2 + 2] cycloddition between the rest >CdC< bond (in the dspecies) nd neighboring SidSi site, forming tetr-σ bonded surfce species, or undergo migrtion of the heterotom to neighboring SidSi site, resulting in six-member ring metllocycle nd dsorbed heterotom. Herefter, these two processes re referred to s the isomeriztion process nd the decomposition process, respectively. We hve investigted these two processes by using double-dimer model, Si 15 H 16. This lrger model lso enbles us to investigte the effect of cluster size. The optimized geometries of the key points in the two rection pthwys re depicted in Figure 2. The reltive energies nd some selected geometricl prmeters of the key points re given in Tble 2 for thiophene nd Tble 3 for furn, respectively. We first consider the thiophene/si 15 H 16 model system. The [4 + 2] cycloddition of thiophene onto one of the dimer site of the Si 15 H 16 cluster cn led to two products, LM1_d nd LM1 _d, s shown in Figure 2, nd d, respectively. These two isomers, however, should be identicl on the Si(100)-2 1 surfce. Indeed, our B3LYP/6-31G(d) clcultions predicted similr vlues of binding energy nd geometries for the two isomers; e.g., the binding energy of 33.4 kcl/mol for LM1_d nd 34.0 kcl/mol for LM1 _d. Both re in good greement with those for LM1 obtined by using the Si 9 H 12 cluster. This indictes tht the effect of cluster size is negligible from Si 9 H 12 to Si 15 H 16. For the isomeriztion process strting from LM1_d, further [2 + 2] cycloddition of the 2,5-dihydrothiophene-like dspecies with neighboring SidSi site proceeds through trnsition stte, TS2_d (see Figure 2b). The predicted brrier height t TS2_d is 20 kcl/mol with respect to LM1_d. However, TS2_d is lower thn the initil rectnts (thiophene nd Si 15 H 16 ) by 13.4 kcl/ mol, suggesting tht this isomeriztion process cn tke plce

4 10072 J. Phys. Chem. B, Vol. 105, No. 41, 2001 Lu et l. Figure 2. Locl minim nd trnsition sttes for the C 4H 4X/Si 15H 16 (X ) S,O) systems predicted t the B3LYP/6-31G(d) level of theory. TABLE 2: Adsorption Energies nd Selected Geometric Prmeters of the Locl Minim nd Trnsition Sttes in the C 4 H 4 S/ Si 15 H 16 Model System from B3LYP/6-31G(d) Clcultions rects. LM1_d LM1 _d TS2_d LM3_d TS3_d LM4_d Si A-Si B (Å) Si C-Si D (Å) C 2-Si A (Å) C 5-Si B (Å) C 3-Si C (Å) C 4-Si D (Å) S-Si C (Å) S-Si D (Å) C 2-C 3 (Å) C 3-C 4 (Å) C 4-C 5 (Å) C 2-S (Å) C 5-S (Å) E (kcl/mol) (0.0) (-35.5) (-35.1) (-14.7) (-55.5) (-18.1) (-82.0) Adsorption energy E ) E(C 4H 4S/Si 15H 16) - E(Si 15H 16) - E(C 4H 4S). Energies without zero-point energy corrections re given in prentheses. TABLE 3: Adsorption Energies nd Selected Geometric Prmeters of the Locl Minim nd Trnsition Sttes in the C 4 H 4 O/ Si 15 H 16 Model System from B3LYP/6-31G(d) Clcultions rects. LM1_d LM1 _d TS2_d LM3_d TS3_d LM4_d Si A-Si B (Å) Si C-Si D (Å) C 2-Si A (Å) C 5-Si B (Å) C 3-Si C (Å) C 4-Si D (Å) O-Si C (Å) O-Si D (Å) C 2-C 3 (Å) C 3-C 4 (Å) C 4-C 5 (Å) C 2-O (Å) C 5-O (Å) E (kcl/mol) (0.0) (-30.8) (-30.8) (-9.4) (-51.0) (-9.1) (-76.9) Adsorption energy E ) E(C 4H 4O/Si 15H 16) - E(Si 15H 16) - E(C 4H 4O). Energies without zero-point energy corrections re given in prentheses. t elevted temperture. In TS2_d, the forming C 4 -Si D nd C 3 - Si C bond lengths re nd Å, respectively, showing tht this [2 + 2] cycloddition is nonconcerted. The product, tetr-σ bonded, tetrmethylene sulfide-like surfce species (LM3_d in Figure 2c) is 19.1 kcl/mol lower thn LM1_d, nd its formtion energy is kcl/mol with respect to free thiophene nd Si 15 H 16. It should be mentioned tht previous theoreticl work reveled the formtion of similr tetr-σ bonded dspecies for benzene chemsorbed on the Si(100)-2 1 surfce t elevted tempertures.

5 Chemisorption nd Decomposition of Thiophene nd Furn J. Phys. Chem. B, Vol. 105, No. 41, Figure 3. Profile of energy surfce (PES) for the rection pthwys of C 4H 4X/Si 15H 16 (X ) S,O) model systems predicted t the B3LYP/6-31G(d) level of theory. For furn, energies re given in prentheses. (Units in kcl/mol) TABLE 4: Comprison of the Clculted Vibrtionl Frequencies (cm -1 ) for Free Thiophene nd Furn with Experiment b thiophene furn modes clc. expt. clc. expt. dc-h str. 3143, 3142, 3103, , 3125, 3098, , 3171, 3146, , 3154, 3149, 3129 in-plne ring 1517, 1410, , 1409, , 1468, 1375, 1127, , 1491, 1384, 1140, 1040 dc-h bend. 1238, 1075, 1073, , 1085, 1083, , 1174, , 1180, 1066 Wvenumbers lrger thn 1000 cm -1 re presented here. b Experimentl dt extrcted from ref 44. For the decomposition process strting from LM1 _d, the bstrction of S tom from the 2,5-dihydrothiophene-like dspecies by neighboring SidSi site proceeds vi trnsition stte, TS3_d (see Figure 2e), producing six-member ring metllocyclic nd tringulr C 2 S surfce species (see LM4_d in Figure 2f). The brrier height t TS3_d is predicted to be 16.6 kcl/mol with respect to LM1 _d. The trnsition stte TS3_d is 17.4 kcl/mol lower thn the initil rectnts, free thiophene nd Si 15 H 16. At TS3_d, the two broken C-S bond lengths re nd Å; the two forming Si-S bond lengths re nd Å (cf. Tble 2). In the product LM4_d, the six-member ring metllocyclic surfce species hs the C 2 -C 3 nd C 4 -C 5 bond lengths of Å, which is typicl forcdc double bond, s well s two C-Si bond lengths of Å nd Si-Si bond length of Å. The tringulr Si 2 S surfce species hs two S-Si bond lengths of Å nd Si-Si bond length of Å. This decomposition (desulfuriztion) process is exothermic with rection het of -48 kcl/mol with respect to LM1 _d nd of kcl/mol with respect to the initil rectnts, free thiophene nd Si 15 H 16. LM4_d is lso 29.5 kcl/mol lower thn the tetr-σ bonded, tetrmethylene sulfide-like surfce species produced by the isomeriztion process. From the predicted energetics, it cn be seen tht strting from the di-σ bonded, dihydrothiophene-like chemisorbed thiophene, the desulfuriztion process producing metllocyclic surfce species nd S() is both kineticlly nd thermodynmiclly fvorble over the isomeriztion process tht leds to the tetr-σ bonded, tetrmethylene sulfide-like surfce species. In the cse of furn, similr isomeriztion nd decomposition processes hve been found for the di-σ bonded, dihydrofurnlike chemisorbed furn, s shown in Figure 2 nd Tble 3. The results cn be summrized s follows: (i) the isomeriztion process gives rise to tetr-σ bonded, tetrhydrofurn-like surfce species, while the decomposition (deoxygention) process leds to six-member ring metllocycle nd n epoxy surfce species; (ii) the deoxygention process is thermodynmiclly fvorble over the isomeriztion process; (iii) both processes, however, hve equivlent ctivtion energy, unlike the thiophene cse, in which the decomposition process is found to be kineticlly more fvorble over the isomeriztion process. The profile of energy surfce for the chemisorption, isomeriztion, nd decomposition of thiophene nd furn on the doubledimer Si 15 H 16 cluster model is depicted in Figure 3, which schemticlly demonstrtes their similr chemisorptive behvior on the Si(100)-2 1 surfce Vibrtionl Frequencies of Chemisorbed C 4 H 4 X (X ) S,O). The vibrtionl frequencies of the vrious dspecies in the C 4 H 4 X/Si 15 H 16 (X ) S,O) model systems hve been clculted t the B3LYP/6-31G(d) level nd scled by fctor of For free thiophene nd furn, the predicted vibrtionl frequencies lrger thn 1000 cm -1 re presented in Tble 4, long with the experimentl vlues, 44 showing tht the theoreticl clcultions fithfully reproduce the experimentl dt. In Tble 5, the clculted vibrtionl frequencies (>900 cm -1 ) for the di-σ bonded, 2,5-dihydrothiophene-like surfce species re tbulted. For comprison, the experimentl dt 20 for the C 4 H 4 S/Si(100) chemisorption system re lso listed in Tble 5.

6 10074 J. Phys. Chem. B, Vol. 105, No. 41, 2001 Lu et l. TABLE 5: Comprison of the Clculted Vibrtionl Frequencies (cm -1 ) for Adspecies in the C 4 H 4 S/Si 15 H 16 Model System with Experiment clc. expt. c mode di-σ bonded (LM1_d) tetr-σ bonded (LM3_d) metllocycle (LM4_d b ) 120 K dc-h str. 3100, , 3022, 3007, C-H str. 3019, , 2992, 2962, CdC str , 1530 dc-h in-plne bend. 1300, , 1339, 1202, , 1088 dc-h out-of-plne bend. 991, 973 -C-H bend. 1176, 1162, 1051, , 1223, 1151, 1124, 1089, 1065, 1044, 1031 C-C str. 935, , Wvenumbers lrger thn 900 cm -1 re given here. b The frequency of the SiS stretching mode is 550 cm -1. c Reference 20. TABLE 6: Clculted Vibrtionl Frequencies (cm -1 ) for Adspecies in the C 4 H 4 O/Si 15 H 16 Model System mode di-σ bonded (LM1_d) tetr-σ bonded (LM3_d) metllocycle (LM4_d) dc-h str. 3127, , 3022, 3008, C-H str. 3011, , 2998, 2963, 2949 CdC str , 1529 dc-h in-plne bend. 1302, , 1339, 1202, 1178 dc-h out-of-plne bend. 991, 974 -C-H bend. 1272, 1220, 1117, , 1216, 1211, 1209, 1165, 1069, 1163, 1049 C-C str. 973, , 936 Wvenumbers lrger thn 900 cm -1 re given here. b The frequency of the SiO stretching mode is 789 cm -1. In their HREELS experiments, Qio et l. observed severl bnds round 3054, 2920, 1180, 1088, nd 946 cm -1 for submonolyer thiophene on the Si(100)-2 1 surfce t 120 K, nd ssigned them to dc-h stretching mode, -C-H stretching mode, dc-h bending mode (two bnds), nd C-C stretching mode, respectively, of the di-σ bonded, 2,5-dihydrothiophenelike surfce species. 20 The ssignment is prtilly supported by our clcultions. On the bsis of our results, the bnd round 1180 cm -1 should be ssigned to the -C-H bending mode, not to the dc-h bending mode, of the di-σ bonded, 2,5- dihydrothiophene-like surfce species. The CdC bond stretching mode of the di-σ bonded, 2,5-dihydrothiophene-like surfce species could not be detected by HREELS, becuse the CdC bond is prllel to the surfce. Besides the clculted vibrtionl frequencies of the di-σ bonded, 2,5-dihydrothiophene-like surfce species, the clculted vibrtionl frequencies of the tetr-σ bonded tetrmethylene sulfide-like surfce species nd of the six-member ring metllocyclic surfce species re lso given in Tble 5. At 700 K, Qio et l. observed brod bnd round 2948 cm -1 wek bnd t 1014 cm -1 in HREELS spectr. 20 On the bsis of our clcultions, these bnds cn be ssigned to the -C-H stretching mode nd to the -C-H bending mode of the tetr-σ bonded, tetrmethylene sulfidelike surfce species, respectively. For the six-member ring metllocyclic surfce species, its CdC stretching mode my be detectble by HREELS s the moleculr plne is perpendiculr to the Si(100) surfce. We noticed tht in the HREELS reported by Qio et l., shoulder pek t bout 3100 cm -1 is discernible t tempertures higher thn 450 K, 20 which cn be ssigned to the CdC stretching mode of the six-member ring metllocycle-like surfce species. The clculted vibrtionl frequecies for vrious dspecies in the C 4 H 4 O/Si 15 H 16 model system re given in Tble 6. Unfortuntely, no experimentl dt re vilble for comprison. Nevertheless, the dt presented in Tble 6 should be informtive to those who re interested in performing experimentl vibrtionl nlysis of the furn/si(100) chemisorption system. 4. Concluding Remrks The chemisorption nd decomposition of thiophene (C 4 H 4 S) nd furn (C 4 H 4 O) on the reconstructed Si(100)-2 1 surfce hve been investigted by mens of hybrid density functionl B3LYP method in combintion with the cluster model pproch. Two chemisorption mechnisms, [4 + 2] nd [2 + 2] cyclodditions of C 4 H 4 X(X) S,O) onto surfce dimer site, hve been exmined comprtively. The clcultions reveled tht the former process is brrierless nd fvorble over the ltter one, which requires n ctivtion energy of 2.6 kcl/mol for thiophene nd 1.2 kcl/mol for furn. The di-σ bonded surfce species formed by [4 + 2] cycloddition-type chemisorption cn either undergo further [2 + 2] cycloddition with neighboring SidSi dimer site, giving rise to tetr-σ bonded surfce species, or undergo deoxygention (desulfuriztion) by trnsferring the heterotom to neighboring SidSi dimer site, leding to six-member ring metllocyclic C 4 H 4 Si 2 surfce species. The ltter process is found to be slightly more fvorble thn the former, especilly in the cse of thiophene. Acknowledgment. This work is supported by the Nturl Science Foundtion of Chin, the Ministry of Eduction of Chin, Ximen University nd Emory University through the Robert W. Woodruff professorship. 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