J. Phys. Chem. B 2005, 109, 5055-5059
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Thermal Decomposition of HSCH2CH2OH on Cu(111): Identification and Adsorption Geometry of Surface Intermediates Kuan-Hung Kuo,† Jain-Jung Shih,† Yung-Hsuan Liao,† Tao-Wei Fu,† Liang-Jen Fan,‡ Yaw-Wen Yang,*,‡ and Jong-Liang Lin*,† Department of Chemistry, National Cheng Kung UniVersity, 1, Ta Hsueh Road, Tainan, Taiwan, R.O.C., and National Synchrotron Radiation Research Center, Hsinchu, Taiwan, R.O.C. ReceiVed: NoVember 2, 2004; In Final Form: December 22, 2004
X-ray photoelectron spectroscopy has been employed to study the surface intermediates from the thermal decomposition of HSCH2CH2OH on Cu(111) at elevated temperatures. On the basis of the changes of the core-level binding energies of C, O, and S as a function of temperature, it is found that HSCH2CH2OH decomposes sequentially to form -SCH2CH2OH and -SCH2CH2O-. Theoretical calculations based on density functional theory for an unreconstructed one-layer copper surface suggest that -SCH2CH2OH is preferentially bonded at a 3-fold hollow site, with an adsorption energy lower than the cases at bridging and atop sites by 15.6 and 47.5 kcal‚mol-1, respectively. Other structural characteristics for the energy-optimized geometry includes the tilted C-S bond (14.1° with respect to the surface normal), the C-C bond titled toward a bridging site, and the C-O bond pointed toward the surface. In the case of -SCH2CH2O- on Cu(111), the calculations suggest that the most probable geometry of the adsorbate has its S and O bonded at hollow and bridging sites, respectively. With respect to the surface normal, the angles of the S-C and O-C are 27.9 and 34.0°.
Introduction Alkanethiols, especially those with long alkyl chains, are commonly employed in forming self-assembled monolayers (SAMs) on coinage metals. The structural unit of the SAMs that binds to the surface is known to be composed of alkanethiolates, evidenced by the H-S bond scission taking place after adsorption. A chemical modification of the metal surfaces, mainly achieved by the outer terminal groups of the SAM, alters the surface properties and effects novel applications in electrochemistry, molecular electronics, lubrication, corrosion, wetting, and etching.1-3 For example, the wettability of H2O increases on an SAM covered with terminal hydroxyl groups compared with the SAM covered with methyl groups.1 It has been reported that hydroxyl-terminated SAMs are a good substrate for covalently binding double-stranded DNAs on Au surfaces.4 Electrochemical genotyping systems can be built based on HSCH2CH2OH SAMs on Au electrodes.5 In the present study, we monitor the sequential thermal decomposition of HSCH2CH2OH on Cu(111) at elevated temperatures and identify the surface intermediates by X-ray photoelectron spectroscopy (XPS). In addition, the bonding geometries for the surface intermediates from HSCH2CH2OH decomposition are investigated by employing density functional theory calculations. HSCH2CH2OH is the simplest molecule of mercaptoalkanethiols terminated with functionally similar groups of -OH and -SH at each end. The relative binding strength between hydroxyl and thiol groups to the copper surface is an interesting point worthy to be assessed. Experimental Section The experiments were performed in a mu-metal ultrahigh vacuum chamber pumped by a 400 L/s turbomolecular pump † ‡
National Cheng Kung University. National Synchrotron Radiation Research Center.
and a 150 L/s ion pump with a chamber base pressure of better than 3 × 10-10 Torr. Major instruments installed in the chamber included a quadrupole mass spectrometer (UTI 100 C) for monitoring residual gases and/or desorption products in the chamber, a low energy electron diffraction apparatus (VG) for checking the Cu(111) surface order, a differentially pumped sputter ion gun for surface cleaning, and a triple-channeltron electron energy analyzer (VG CLAM 2) for measuring photoelectron binding energies. Photoemission measurements were carried out at the widerange spherical grating monochromator beamline (WR-SGM) at the National Synchrotron Radiation Research Center. Total instrumental resolution, including the beamline and energy analyzer, was estimated to be better than 0.3 eV. The photoelectrons were collected at an angle of 50° from the surface normal. All the XPS spectra presented here were first normalized to photon flux by dividing the recorded XPS signal with the photocurrent derived from a gold mesh situated in the beamline. The binding energy scale in all the spectra was referenced to a well-resolved spin-orbit component of the bulk Cu 2p3/2 peak at 75.10 eV. The Cu(111) sample was fastened with tungsten wires to the copper feedthroughs welded to the end of a stainless steel liquid nitrogen Dewar. The sample temperature was measured with a chromel-alumel thermocouple inserted into holes drilled on the sides of the substrate. By a combination of liquid nitrogen cooling and resistive heating, the crystal temperature can be varied between 100 and 900 K. The Cu(111) crystal was cleaned by a standard procedure of repeated argon ion sputtering and ultrahigh vacuum annealing. In the XPS study, a clean Cu(111) surface at 100 K was exposed to 1.6 langmuirs (L) HSCH2CH2OH (4 × 10-8 Torr, 40 s), followed by brief annealing and photoelectron energy analysis at 100 K to monitor the surface reactions. The size of X-ray beam used was 2 × 2 mm2, and the diameter of the Cu(111) was 12 mm. Two photon
10.1021/jp0449859 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/25/2005
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TABLE 1: Comparison of X-ray Photoelectron Binding Energies (eV) C 1s C2H5SH/Mo(110) (ref 7) C2H5SH multilayer C2H5Shydrocarbon fragment SCC2H5SH/Cu(110) (ref 8) C2H5SH C2H5SSCHOC2H4OH/Mo(110) (ref 9) HO1C1H2C2H2O2-OCH2CH2OCH2 and CH3/Cu(111) (ref 10) CH2 CH3 CH3I/Pt(111) (ref 11) CH2 CH3
O 1s
284.5, 285.1 284.9, 285.4 282.9
S 2p3/2 163.1 162.6 161.3
283.0 284.1, 285.2 284.1, 285.2
164.2 162.4 161.5
283.6 287.9 for C1 286.7 for C2 286.7
533.7 for O1 532.6 for O2 532.6
284.9 283.3 283.9 283.4
energies were employed: 620 eV for C 1s, O 1s, and S 2p signals and 320 eV for S 2p, Cu 3s, and Cu 2p signals. The X-ray photoelectron spectra shown here were fitted with Gaussian-Lorentzian functions based on a nonlinear least squares algorithm after Shirley background subtraction. In the present study, the bonding geometries of -SCH2CH2OH and -SCH2CH2O-, which are the surface intermediates of HSCH2CH2OH thermal decomposition on Cu(111), are predicted by theoretical calculations. All of our calculations were performed in the framework of density functional theory using the program package Cerius2-DMol3, in which the Perdew-Wang local exchange and correlation functional and double-numerical plus d-DNP basis (DND) were employed for Cu, C, H, O, and S atoms.6 The size of the DND basis is comparable to that of Gaussian 6-31G* basis sets. However, the numerical basis set is much more accurate than a Gaussian basis set of the same size. The calculations were spinunrestricted and did not include a relativistic effect for the core electrons. In the calculations for the optimized geometry of -SCH2CH2OH and -SCH2CH2O-, the convergence criteria used were 1 × 10-5 hartree for energy, 1 × 10-3 hartree/bohr for gradient, and 1 × 10-3 bohr for atomic displacement. Results and Discussion X-ray Photoelectron Studies of HSCH2CH2OH Thermal Decomposition on Cu(111). An HSCH2CH2OH molecule possesses six different kinds of chemical bonds, i.e., S-H, C-S, C-H, C-C, C-O, and O-H. The dissociation of the molecule on Cu(111) may result in surface intermediates such as thiolates, alkoxides, and hydrocarbon fragments. However, depending on the bond dissociation kinetics and surface temperature, decomposition of HSCH2CH2OH on Cu(111) may proceed sequentially, producing interesting bidentate intermediates and more thoroughly fragmented species. Surface species determination based on XPS data generally relies on chemical shifts in binding energies associated with the atoms of interest. Therefore, previous results of XPS investigations of adsorption and thermal reactions of alkanethiols, ethanol, and alkyl halides (precursors to the generation of thiolates, ethoxides, and hydrocarbon fragments) on transition metals are useful references. Table 1 lists X-ray photoelectron binding energy values for several adsorption systems that are related to the present study of
Figure 1. Variation of C 1s X-ray photoelectron spectra with annealing temperature. The Cu(111) surface was initially exposed to 1.6 L HSCH2CH2OH at 100 K.
HSCH2CH2OH on Cu(111). For the systems of C2H5SH on Mo(110)7 and on Cu(110),8 the S 2p3/2 peaks are shifted by 0.5 and 1.8 eV, respectively, toward lower binding energy as the S-H bond breaks, forming ethyl thiolate on the surfaces. Atomic S on both surfaces has a lower 2p3/2 binding energy (161.3-161.5 eV) than those of C2H5SH and C2H5S-. Similarly, the atomic C 1s peaks decrease down to 283.0-283.6 eV. Clear contrasts of C 1s and O 1s binding energy shifts are observed in the sequential O-H dissociation of HOCH2CH2OH on Mo(110).9 The C 1s and O 1s peaks of the C-O-H moiety appear at 287.9 and 533.7 eV, respectively, but they are reduced by 1.1 and 1.2 eV due to the O-H bond scission forming alkoxide. In the case of hydrocarbon fragments adsorbed on transition metal surfaces,7,10,11 the C 1s binding energies for the carbon atoms directly bonded to the surfaces can be largely affected. On Cu(111),10 CH2 and CH3 have their C 1s peaks determined at 284.9 and 283.3 eV, respectively. On Pt(111), the C 1s peaks of CH2 and CH3 are at 283.9 and 283.4 eV. Figures 1-3 show the thermal evolution of C 1s, O 1s, and S 2p photoelectron spectra for the Cu(111) exposed to 1.6 L HSCH2CH2OH at 100 K, followed by brief heating of the surface to the temperatures indicated. All the spectra were taken at 100 K. In the 123 K spectrum of Figure 1, the C 1s peaks appear at 285.2 and 287.1 eV, indicating two different chemical bonding environments of carbon atoms. In terms of the previous studies of the systems of C2H5SH on Mo(110)7 and Cu(110)8 and HOCH2CH2OH on Mo(110)9, the 285.2 and 287.1 eV peaks are attributed to the carbon atoms bonded to the sulfur and oxygen, respectively. In the 123 K spectrum of Figure 2, the O 1s signal appears at 534.2 eV, similar to the oxygen (533.7 eV) of the C-O-H groups in the HOCH2CH2O/ Mo(110) case, as shown in Table 1. Apparently, the O-H bond of HSCH2CH2OH does not dissociate on Cu(111) at this temperature. In the 123 K spectrum of Figure 3, there are two peaks at 162.6 and 163.8 eV. The 1.2 eV difference in the peak positions and the 2:1 ratio of the integrated area of the 162.6 eV peak to that of 163.8 eV indicates that the 162.6 and 163.8 eV peaks are S 2p3/2 and S 2p1/2 signals, respectively, from the same surface species. Therefore, the following discus-
Thermal Decompostion of HSCH2CH2OH on Cu(111)
Figure 2. Variation of O 1s X-ray photoelectron spectra with annealing temperature. The Cu(111) surface was initially exposed to 1.6 L HSCH2CH2OH at 100 K.
Figure 3. Variation of S 2p X-ray photoelectron spectra with annealing temperature. The Cu(111) surface was initially exposed to 1.6 L HSCH2CH2OH at 100 K.
sion of sulfur binding energy is based on 2p3/2 only. The S 2p3/2 signal peaked at 162.6 eV is attributed to the S atoms of thiolate groups with the support of the XPS studies of C2H5SH on Mo(110) and Cu(110) as shown in Table 1, because it is close to the 162.4 eV peak of C2H5S-. Considering together the binding energies of C 1s, O 1s, and S 2p3/2 at 123 K and their related chemical bonding environments, it can be concluded that HSCH2CH2OH decomposes via S-H bond scission, forming -SCH2CH2OH on Cu(111) at this temperature. It is noted that CH3SH and C2H5SH are reported to dissociate on Cu(110) by S-H scission at 100 K.8 Based on the previously reported intensities of S 2p3/2 and Cu 2p3/2 of the ordered CH3S layers ((2 × 2) and c(6 × 2)) on Cu(100)12, it is estimated that, in the present case of 1.6 L HSCH2CH2OH adsorbed on Cu(111), the -SCH2CH2OH coverage is 0.31, which is close to one adsorbed -SCH2CH2OH molecule for every three surface copper
J. Phys. Chem. B, Vol. 109, No. 11, 2005 5057 atoms in terms of the S 2p3/2 and Cu 2p3/2 areas of the -SCH2CH2OH layer at 123 K, assuming a proportional attenuation effect of CH3 and CH2CH2OH on the S and Cu photoelectrons. In Figure 1, the 173 K spectrum is similar to the 123 K one. The C 1s peak at 287.1 eV, characteristic of the carbon atom in a C-OH group, decreases in intensity above 173 K and vanishes at 373 K. The disappearance of the C 1s signal suggests desorption of reaction products or chemical transformation of the C-OH moiety on the surface. For the latter case, it may be due to C-O or O-H bond scission. In contrast to the decrease of the C 1s peak at 287.1 eV, a new peak at a binding energy of 286.3 eV appears at a temperature >173 K. Referring to the binding energies of -OCH2CH2O-, -CH2, and -CH3 shown in Table 1, the 286.3 eV is attributed to the C 1s of C-O of alkoxide species. Moreover, the signal intensity of the 285.3 eV peak, representing the carbon of C-S, remains about the same between 123 and 303 K. This result suggests that -SCH2CH2OH molecules decompose on Cu(111) at a temperature higher than 173 K by O-H bond breakage, forming adsorbed -SCH2CH2O-. Transformation of -SCH2CH2OH into -SCH2CH2O- does not change the binding energy and intensity of the C 1s of the SCH2 moiety, but the C 1s of the CH2O moiety shifts by 0.8 eV toward lower binding energy after O-H bond scission. The 285.2 and 286.3 eV C 1s signals of -SCH2CH2O- gradually decrease in intensity at a temperature higher than 323 K. Parallel to the reaction pathway from -SCH2CH2OH to -SCH2CH2O-, judged by the variation of the binding energies of C 1s with temperature, the oxygen signal of -COH at 534.2 eV decreases in intensity and that of -COformed after the O-H bond scission grows at 532.2 eV. In the change from -COH to -CO-, the O 1s binding energy is decreased by 2 eV on Cu(111) in the present study, as compared to 1.2 eV in the case of HOCH2CH2OH on Mo(110).9 In Figure 1, the small signals at 284 eV observed from 123 to 253 K are likely due to residual surface impurities. In Figure 3, the change of sulfur signal with temperature is more complicated than those for C 1s and O 1s. In the temperature range of 173-253 K, the observation of the gradual decrease of S 2p3/2 peak at 162.6 eV, characteristic of -SCH2CH2OH, and the concomitant growth of the peak at 163.3 eV seem to correlate with the chemical transformation from -SCH2CH2OH into -SCH2CH2O-. However, because the SCH2CH2 moiety remains intact in this transformation process, the S 2p3/2 binding energy is expected to be about the same, instead of the 0.7 eV shift observed here. The contrasted intensity changes of the 162.6 and 163.3 eV signals between 173 and 253 K in Figure 3 are likely due to other temperature-dependent surface effects. In fact, in the XPS studies of temperature-dependent CH3SH adsorption on Ni(111) and Cu(111) reported previously,13,14 it was found that the S 2p3/2 binding energy of CH3S- depended sensitively on the local bonding of sulfur atoms, possibly altered by surface coverage and by local surface reconstruction. On Ni(111), the S 2p3/2 of methyl thiolate (CH3S), suggested to be adsorbed at a bridging site below 150 K, had a binding energy between 162.3 and 162.7 eV, depending on the surface coverage. Between 150 and 250 K, a fraction of bridging CH3S was relocated to hollow sites with the S 2p3/2 energy increased by 0.6-1.0 eV.13 However, another study of angle-resolved X-ray photoelectron spectroscopy showed that CH3S was adsorbed at 3-fold hollow sites on Ni(111) at 150 K.15 As the temperature was increased to 250 K, CH3S was found to be adsorbed on 4-fold hollow sites of reconstructed Ni surface.15 In the case of Cu(111), the temperature-dependent S 2p binding energy of
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Figure 5. Comparison of bonding geometries and energies of -SCH2CH2OH on Cu(111) predicted by theoretical calculations. ∆E is the energy difference in kcal‚mol-1 for each structure relative to the optimized one.
Figure 4. Relative peak areas of S 2p3/2, C 1s, and O 1s as a function of temperature. The total area of each element is also shown. The carbon total area is multiplied by 1/2.
CH3S- was attributed to surface reconstruction which was induced by the thiolate adsorption at higher temperatures, instead of the change of adsorption site. It was claimed that CH3Swas adsorbed on reconstructed Cu(111) at room temperature and its S 2p3/2 binding energy was larger than that of CH3Son unreconstructured Cu(111) at lower temperatures by ∼0.9 eV.14 Figure 4 shows the variation of each fitted and the total areas of the S 2p3/2, C 1s, and O 1s peaks with temperature after 1.6 L HSCH2CH2OH adsorption on Cu(111) at ∼100 K. This figure also shows the temperature ranges in which the surface species are present. The C 1s curves in Figure 4b suggest that -SCH2CH2OH is the surface predominant species below 223 K, consistent with the variation of O 1s signals with temperature, although the latter already shows the formation of some -SCH2CH2O- molecules. The change from -SCH2CH2OH to -SCH2CH2O- mainly proceeds at a temperature >223 K. At temperatures higher than 350 K, the amounts of -SCH2CH2OH and -SCH2CH2O- decrease substantially, in contrast to the rapid increase of atomic sulfur at 162.0 eV. Apparently, -SCH2CH2O- and -SCH2CH2OH react on the surface, inducing desorption of reaction products containing C and O, but leaving sulfur atoms on the surface. The intensity of atomic sulfur is higher than the total sulfur intensity of -SCH2CH2OH and -SCH2CH2O-. For the latter two species, the S signals can be attenuated by C and O, as suggested
by their adsorption geometries shown later. In Figure 4, the same sample was used for each point, but the spectra were taken at four different spots on the crystal to minimize possible adsorbate damage by X-ray irradiation. The previous study of CH3S on Cu(100) found no X-ray-induced damage of the thiolate films from the spectra taken with increasing CH3SH exposure.12 Theoretical Studies of Bonding Geometries of SCH2CH2OH and -SCH2CH2O- on Cu(111). In our calculations for the adsorption geometries of -SCH2CH2OH and -SCH2CH2O- on Cu(111), obtained by minimizing the total energies, a slab of 10 Cu atoms fixed at their lattice positions was used, but allowing the variations of all the bond lengths and angles of -SCH2CH2OH and -SCH2CH2O-, including the bonds that are attached to the surface. Figure 5a shows the optimized -SCH2CH2OH bonding geometry. There are several important structural features in this bonding geometry, including (1) 3-fold hollow adsorption sites of the S atom, (2) the S-C bond tilted away from the surface normal by an angle of 14.1° and toward a bridging site, and (3) the C-O bond close to the surface. The height between the S atom and the Cu surface is ∼0.55 Å. Figure 5 also compares the difference in the bonding energy for -SCH2CH2OH adsorbed at hollow, bridging, and atop sites. In the calculations of the bonding geometries and energies for the last two cases, the S atoms were fixed at bridging and atop sites, but the heights relative to the surface were variable. -SCH2CH2OH adsorbed at bridging and atop sites has adsorption energy 15.6 and 47.5 kcal‚mol-1 higher, respectively, than the case of hollow-site adsorption. These calculations predict that -SCH2CH2OH is most likely to be adsorbed at a hollow site of Cu(111). Note that, in these calculations, no interactions between adsorbed molecules and surface reconstruction were considered. In the studies of -SCH3 geometry on Cu(111) using the techniques of angle-resolved ultraviolet photoemission and near-edge X-ray absorption fine structure, it has been reported that the -SCH3 can be adsorbed with the S-C bond tilted away from the surface normal by ∼30°.16 In the case of -SCH3 adsorbed at a hollow site of Ni(111), angle-resolved photoelectron spectroscopy shows that the S-C bond is titled away from the surface normal by 35° and toward a bridging site.17 Figure 6a shows the optimized -SCH2CH2O- adsorption geometry, with the O and S atoms bonded at positions very close to a bridging site and a hollow site, respectively. With respect to the optimized structure adsorption, our calculations also find that (1) as both the O and S atoms are fixed at hollow sites, the -SCH2CH2O- adsorption energy is 29.6 kcal‚mol-1 higher (Figure 6b); (2) as the O and S are fixed
Thermal Decompostion of HSCH2CH2OH on Cu(111)
J. Phys. Chem. B, Vol. 109, No. 11, 2005 5059 TABLE 2: Geometric Details of Optimized -SCH2CH2OH and -SCH2CH2O- on Cu(111) Predicted by Theoretical Calculationsa
Figure 6. Comparison of bonding geometries and energies of -SCH2CH2O- on Cu(111) predicted by theoretical calculations. ∆E is the energy difference in kcal‚mol-1 for each structure relative to the optimized one.
at hollow and bridging sites, respectively, the adsorption energy is 4.8 kcal‚mol-1 higher (Figure 6c); (3) as both the O and S atoms are fixed at bridging sites, the adsorption energy is 10.2 kcal‚mol-1 higher (Figure 6d); and (4) as the O atom is fixed at atop site with the S at either bridging or hollow sites, the adsorption energy is 40-50 kcal‚mol-1 higher. On the basis of these calculations without considering intermolecular interactions and surface reconstruction, -SCH2CH2O- is most likely to be adsorbed on Cu(111) with the O and S atoms at bridging and hollow sites, respectively. Table 2 displays more detailed structural information for the optimized geometries of -SCH2CH2OH and -SCH2CH2O-. As suggested by the optimized bonding geometries of -SCH2CH2OH and -SCH2CH2O- on Cu(111) with similar atomic positions relative to the surface, dehydrogenation of -SCH2CH2OH to form -SCH2CH2O- may proceed without causing significant atomic displacements of the SCH2CH2 moiety. Summary HSCH2CH2OH decomposes on Cu(111) at 123 K to form -SCH2CH2OH, which is most likely to be adsorbed at a 3-fold hollow site and has the C-S bond tilted (not perpendicular to the surface). -SCH2CH2OH mainly decomposes to form -SCH2CH2O-, which is most probably adsorbed with the S and O atoms bonded at hollow and bridging sites, respectively, at a temperature higher than 173 K.
-SCH2CH2OH
-SCH2CH2O-
d(C-O) ) 1.43 d(C-C) ) 1.50 d(C-S) ) 1.85 d(S-Cu) ) 2.14-2.17 θ(SCSN) ) 14.1 θ(SCC) ) 109.6 θ(OCC) ) 109.2 h(S) ) 0.55
d(C-O) ) 1.41 d(C-C) ) 1.51 d(C-S) ) 1.86 d(S-Cu) ) 2.22 θ(SCSN) ) 27.9 θ(OCSN) ) 34.0 θ(SCC) ) 106.8 θ(OCC) ) 112.8 h(S) ) 0.60 h(O) ) 0.84
a d, bond length (Å); θ, bond angle (deg); h, height from the Cu surface (Å); SN, surface normal.
Acknowledgment. We gratefully acknowledge the National Center for High-performance Computing and the financial support of the National Science Council of the Republic of China (Grant NSC 92-2113-M-006-016) for this research. References and Notes (1) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (2) Oyamatsu, D.; Nishizaea, M.; Kuwabata, S.; Yoneyama, H. Langmuir 1998, 14, 3298. (3) Vondrak, T.; Wang, H.; Winget, P.; Cramer, C. J.; Zhu, X.-Y. J. Am. Chem. Soc. 2000, 122, 4700. (4) Zhao, Y.-D.; Pang, D.-W.; Hu, S.; Wang, Z.-L.; Cheng, J.-K.; Dai, H.-P. Talanta 1999, 49, 751. (5) Cho, S.; Seong, N.; Park, J. J. Sensors, 2002; Proceedings of IEEE, Orlando, FL, 2002; Vol. 1, pp 233-236. (6) Delley, B. J. Chem. Phys. 2000, 113, 7756 (7) Roberts, J. J.; Friend, C. M. J. Phys. Chem. 1988, 92, 5205. (8) Lai, Y.-H.; Yeh, C.-T.; Cheng, S.-H.; Liao, P.; Hung, W.-H. J. Phys. Chem. B 2002, 106, 5438. (9) Queeney, K. T.; Arumaimayagam, C. R.; Weldon, M. K.; Friend, C. M.; Blumberg, N. Q. J. Am. Chem. Soc. 1996, 118, 3896. (10) Chuang, T. J.; Chan, Y. L.; Chuang, P.; Klauser, R. J. Electron. Spectrosc. Relat. Phenom. 1999, 98, 149. (11) Zaera, F.; Hoffmann, H. J. Phys. Chem. 1991, 95, 6297. (12) Vollmer, S.; Witte, G.; Wo¨ll, C. Langmuir 2001, 17, 7560. (13) Rufael, T. S.; Huntley, D. R.; Mullins, D. R.; Gland, J. L. J. Phys. Chem. 1995, 99, 11472. (14) Kariapper, M. S.; Grom, G. F.; Jackson, G. J.; McConville, C. F.; Woodruff, D. F. J. Phys.: Condens. Matter 1998, 10, 8661. (15) Mullins, D. R.; Huntley, D. R.; Tang, T.; Saldin, D. K.; Tysoe, W. T. Surf. Sci. 1997, 380, 468. (16) Seymour, D. L.; Bao, S.; McConville, C. F.; Crapper, M. D.; Woodruff, D. P. Surf. Sci. 1987, 189/190, 529. (17) Mullins, D. R.; Huntley, D. R.; Tang, T.; Saldin, D. K.; Tysoe, W. T. Surf. Sci. 1997, 380, 468.
