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The Dynamic Nature of Hydridosilsesquioxane Clusters on Gold Surfaces Kenneth T. Nicholson,† K. Z. Zhang,† Mark M. Banaszak Holl,*,† F. R. McFeely,‡ and U. C. Pernisz§ Chemistry Department, University of Michigan, Ann Arbor, Michigan 48109, IBM T. J. Watson Laboratory, Yorktown Heights, New York 10598, and Dow Corning Corp., Midland, Michigan 48686-0994 Received March 23, 2000. In Final Form: August 15, 2000 The chemisorption of the hydridosilsesquioxane clusters H8Si8O12 and H10Si10O15 onto a freshly evaporated gold surface has been observed by X-ray photoemission and reflection-absorption infrared spectroscopies. On the basis of these analytical techniques and supporting nonlocal density functional calculations, a single Si-Au bond at a cluster vertex is created through a surprising gold-mediated Si-H bond activation. The chemisorbed clusters are stable in a vacuum and when exposed to oxygen or water. Coverage-dependent peak frequency and infrared intensity shifts, characteristic of a chemically dynamic interface, are detected as a function of cluster overpressure. An equilibrium between the clusters and the gold surface is proposed, evidenced by a fairly constant rate of adsorption to gold for several coverage regimes.
Introduction Progress in the areas of microelectronic device packing density, speed, and functionality generally requires smaller device dimensions.1 The ensuing problems associated with smaller line dimensions such as propagation delay, crosstalk noise, and power dissipation must be addressed. As a solution to these problems, new, low dielectric constant (k < 3) materials as well as alternative architectures have been proposed to replace the current Al(Cu) and SiO2 interconnect technology. However, the replacement of SiO2 as the insulator of choice is frequently complicated by the occurrence of undesired chemical reactions at the interface between the new low-k material and the metal lines. As dimensions approach the atomic scale, characteristics of microelectronic devices can become dominated by the chemical structure and dynamics of the interfaces. Hydridosilsesquioxane (HSQ) resin (k ) 2.9) is a siloxane-based polymer consisting primarily of O3SiH entities with an overall stoichiometry of (HSiO1.5)n. High thermal stability, excellent gap fill capability, low electrical leakage, and relatively low outgassing properties are some of the desirable attributes of this polymer.2 However, device failure results when the polymer is used in direct contact with metal lines. To avoid this problem, the first step used during device fabrication involves the formation of a very thin layer of SiO2 directly on the metal lines. The HSQ resin is then spun cast onto the SiO2 followed by curing and testing.2 This procedure is problematic because it adds additional steps to the process and adds nonnegligible amounts of SiO2 (k ∼ 4) to the dielectric layer. Ideally, a low-k dielectric could be applied directly without the formation of the SiO2 barrier layer. In this paper, the reactions that HSQ resin may undergo when in direct contact with metal surfaces are explored. The interface of metal-insulator metal (MIM) devices is also of substantial interest. MIM devices show interest†
University of Michigan. IBM T. J. Watson Laboratory. § Dow Corning Corp. ‡
(1) Lee, W. W.; Ho, P. S. Mater. Res. Soc. Bull. 1997, 22, 19-23. (2) List, R. S.; Singh, A.; Ralston, A.; Dixit, G. Mater. Res. Soc. Bull. 1997, 22, 61-69.
ing electrical properties such as negative differential resistance, electron emission, and switchable, nonvolatile conduction states that may be related to interfacial chemistry.3,4 Recently, a device consisting of HSQ resin sandwiched by two gold electrodes was reported.5,6 As part of exploring this interesting electrical behavior, understanding of the reaction chemistry of HSQ resin with gold metal is needed. Hydridosilsesquioxane clusters, (HSiO1.5)n (n ) 8, 10, 12, 14), are a volatile, well-defined species that can be considered as monodispersed forms of the polymerized resin.7-10 The clusters react with clean, evaporated metal surfaces in ultrahigh vacuum (UHV) conditions permitting high-resolution X-ray photoemission spectroscopy (XPS) and reflection adsorption infrared spectroscopy (RAIRS) experiments to be performed. For these reasons, the hydridosilsesquioxane clusters were selected as a spectroscopically amenable model system for both the low-k dielectric/metal line and dielectric/electrode interfaces. The general method of cluster-based model systems has been fruitfully applied in previous work to further the understanding of Si/SiO2 interfaces as well as metal/metal oxide interfaces.11-18 (3) Dearnaley, G.; Stoneham, A. M.; Morgan, D. V. Rep. Prog. Phys. 1970, 33, 1129-1191. (4) Pagnia, H.; Sotnik, N. Phys. Status Solidi 1988, 108, 11-65. (5) Pernisz, U. C. Electro-formed Thin-Film Silica Device as Oxygen Sensor; Helmers Pub.: Peterborough, NH, and Cleveland, OH, 1994; pp 241-247. (6) Pernisz, U. C. Electronic conduction of nondense silica thin films; Waser, R., Ed.; Verlag der Augustinus Buchhandl: Aachen, Germany, 1994; pp 832-826. (7) Mu¨ller, R.; Ko¨hne, R.; Sliwinski, S. J. Prakt. Chem. 1959, 9, 7173. (8) Agaskar, P. A. Inorg. Chem. 1991, 30, 2707-2708. (9) Agaskar, P. A.; Klemperer, W. G. Inorg. Chim. Acta 1995, 229, 355-364. (10) Baney, R. H.; Itoh, M.; Sakakibara, A.; Suzuki, T. Chem. Rev. 1995, 96, 1409-1430. (11) Lee, S.; Makan, S.; Banaszak Holl, M. M.; McFeely, F. R. J. Am. Chem. Soc. 1994, 116, 11819-11826. (12) Zhang, K. Z.; Greeley, J. N.; Banaszak Holl, M. M.; McFeely, F. R. J. Appl. Phys. 1997, 82, 2298-2307. (13) McFeely, F. R.; Zhang, K. Z.; Banaszak Holl, M. M.; Lee, S.; Bender, J. E., IV J. Vac. Sci. Technol., B 1996, 14, 2824-2831. (14) Klemperer, W. G.; Wall, C. G. Chem. Rev. 1998, 98, 297-306.
10.1021/la0004449 CCC: $19.00 © 2000 American Chemical Society Published on Web 10/06/2000
Hydridosilsesquioxane Clusters on Gold Surfaces
This paper is a report of the chemical structures and dynamics involved during the chemisorption of H8Si8O12 (1) and H10Si10O15 (2) clusters to freshly evaporated gold surfaces. XPS and RAIRS data indicate the H8Si8O12 clusters are stable on the surface to extended UHV, oxygen, and water exposure. Data are also presented for various cluster coverages on the surface. Spectroscopic changes observed as a function of coverage are explained in terms of a surface equilibrium operative at all coverage regimes which involves the facile making and breaking of Si-H bonds. The rate of adsorption as a function of cluster exposure implies that a precursor (or trapped physisorbed state) precedes chemisorption. Experimental Section
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Figure 1. Proposed structure of the H7Si8O12 cluster on the gold surface. The orientation of the cluster is based on a mechanics minimization of the H7Si8O12 on an Au(111) crystal face. Three oxygens attached the gold-bound silicon atom are oriented away from the Au-Au bond vectors and over the triangular holes. Note the remarkable registration of the cluster Si-O 12-membered ring with 6 gold atoms surrounding the binding site.
Results and Discussion
The hydridosilsesquioxane clusters were synthesized by the method of Agaskar and sublimed twice.8 Cluster purity was checked using 1H NMR, IR, and gas chromatography/mass spectroscopy (GC-MS). After the clusters were loaded into a glass or steel UHV compatible sample holder, the samples were sublimed in UHV conditions by heating gently with warm water at ∼50 °C. Gold samples were prepared by evaporating chromium or titanium onto Si(100)-2×1 as an adhesive layer followed by at least 1000 Å of gold. Immediately prior to use, additional gold was evaporated onto the surface in the UHV system and the sample purity was assessed by XPS. Only trace amounts of carbon impurities could be detected. Evaporation of gold was performed in a separate chamber in which exposures to clusters were not performed. The XPS and IR spectrometers used for these experiments have been fully described elsewhere.11,19 Monochromatic synchrotron XPS spectra were collected at beamline U8b at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory using an incident photon energy of 160 eV. The low-frequency infrared (350-700 cm-1) data in Figure 4 were also obtained at the NSLS using the U4IR beamline and a CuGe helium-cooled detector. The detector and apparatus have also been described elsewhere.20 For RAIRS experiments completed at the University of Michigan, the samples were transferred from the analyzer chamber (base pressure 2.0 × 10-10 Torr) into the infrared chamber (base pressure 8 × 10-10 Torr). The sample was aligned and a background taken. The hydridosilsesquioxane clusters were introduced into the chamber through a leak valve. The experiments involving 1 and D8Si8O12 (d-1) were performed at room temperature with dosing pressures of 4 × 10-8 and 1 × 10-8 Torr, respectively. 2 has a lower vapor pressure; therefore, the sample was heated to ∼50 °C by a warm water bath to obtain adequate volatility for a dosing pressure of 1 × 10-8 Torr. Matlab Version 4.0 was employed for curve-fitting the photoemission data. Full coverage data were fit allowing peak height, position, and width to vary. For partial coverage data, peak widths were fixed based upon the value obtained for the full coverage data and peak heights and positions were allowed to change. Nonlocal density functional theory (NL-DFT) was employed to calculate the frequencies and intensities of several model cluster/gold interfaces and compared to experiment. The B3LYP functional with a 6-31G** basis set was used. The calculations were completed using the pseudospectral calculations available in the Jaguar suite of programs, version 3.5 (Schro¨dinger, Inc., Portland, OR).
I. H8Si8O12 (1), D8Si8O12 (d-1), and H10Si10O15 (2) Clusters Chemisorbed to Gold. Exposure of an evaporated gold surface to a saturating dose of 1 results in a chemisorbed layer on the gold surface. XPS and RAIRS (Figures 2a and 3a) data are consistent with the clusters bonding to the surface by a single vertex as shown in Figure 1.21 The XPS spectrum reveals two Si 2p3/2 corelevel features with binding energies (BE) of -101.1 and -102.3 eV with full-widths at half-maximum (fwhm) of 0.57 and 1.14 eV, respectively. The area ratio is 1:7.3. The O 1s core levels are observed as single peak at 531.3 eV with a 2.0 eV fwhm. Similarly, cluster 2 also chemisorbs to evaporated gold in UHV conditions. At saturation coverage, there are two Si 2p3/2 core-level features with BE of -100.9 and -102.1 eV (Figure 2b). The fwhm for the peaks are 0.62 and 1.17 eV, respectively, with an area ratio of 1:8.9. The O 1s core-levels for cluster 2 are observed a as single peak at 531.6 eV with a 2.0 eV fwhm. The Si and O core-level data are also consistent with a single vertex of cluster 2 being attached to the gold surface via an Au-Si bond at saturation coverage. (It must be noted the peak area ratios do not support a single-vertex cluster for 2 for all coverage regimes. This will be discussed in detail in section III.) In both cases, the reaction appears to occur via oxidative addition of a cluster Si-H bond to a gold atom or atoms on the surface followed by reductive elimination of H2. The RAIRS spectra for 1, d-1, and 2 on gold for the energy ranges of 2500-750 cm-1 and 1000-350 cm-1 are illustrated in Figures 3 and 4, respectively. The vibrational mode assignments for the spectra of chemisorbed 1, d-1, and 2 on Au are summarized in Table 1. Experimental and theoretical studies of free clusters as well as several monosubstituted clusters in solution have been used as a basis for the assignments.22 H8Si8O12 clusters in solution have the following characteristic infrared frequencies: 2274, 1140, 878, 560, 468, and 399 cm-1. These have been assigned as ν(Si-H), νas(Si-O-Si), δ(Si-H), νs(Si-OSi), ν(Si-O-Si), and δ(O-Si-O), respectively. Extensive theoretical work, including a normal-mode analysis, has been completed to confirm these assignments.23-27 Several monosubstituted compounds such as C6H13-H7Si8O12, Ph-
(15) Ge, M.; Zhong, B.; Klemperer, W. G.; Gewirth, A. A. J. Am. Chem. Soc. 1996, 118, 5812-5813. (16) Kaba, M. S.; Song, I. K.; Duncan, D. C.; Hill, C. L.; Barteau, M. A. Inorg. Chem. 1998, 37, 398-406. (17) Prokopuk, N.; Shriver, D. F. Chem. Mater. 1999, 11, 12301236. (18) Yeager, L. J.; Saeki, F.; Shelly, K.; Hawthorne, M. F.; Garrell, R. L. J. Am. Chem. Soc. 1998, 120, 9961-9962. (19) Greeley, J. N.; Meeuwenberg, L. M.; Banaszak Holl, M. M. J. Am. Chem. Soc. 1998, 120, 7777-7782. (20) Carr, G. L.; Dumas, P.; Hirschmugl, C. J.; Williams, G. P. Il Nuovo Cimento 1998, 20, 375-395.
(21) Nicholson, K. T.; Zhang, K. Z.; Banaszak Holl, M. M. J. Am. Chem. Soc. 1999, 121, 3232-3233. (22) Calzaferri, G. Octasilsesquioxanes; Corriu, R., Jutzi, P., Eds.; Lengericher Hubert and Co.: Gottingen, 1996; pp 149-169. (23) Bornhauser, P.; Calzaferri, G. Spectrochim. Acta 1990, 46A, 1045. (24) Earley, C. W. J. Phys. Chem. 1994, 98, 8693-8698. (25) Calzaferri, G.; Hoffman, R. J. Chem. Soc., Dalton Trans. 1991, 917-928. (26) Ba¨rtsch, M.; Bornhauser, P.; Calzaferri, G.; Imhof, R. Vib. Spectrosc. 1995, 8, 305. (27) Marcolli, C.; Laine, P.; Buhler, R.; Calzaferri, G. J. Phys. Chem. B 1997, 101, 1171.
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Figure 2. Soft X-ray Si 2p3/2 core level spectra after the chemisorption of H8Si8O12 (panel A) and H10Si10O15 (panel B) to freshly evaporated gold.
Figure 3. RAIRS of the monolayer derived from (A) H8Si8O12, (B) H10Si10O15, and (C) D8Si8O12. These spectra represent 512 scans at 1 cm-1 resolution for the spectral region 750-2400 cm-1 ratioed to background.
Figure 4. Low-frequency RAIRS data (350-1000 cm-1) for the monolayer derived from (A) H8Si8O12, (B) H10Si10O15, and (C) D8Si8O12. These spectra represent approximately 2000 scans at 8 cm-1 resolution.
H7Si8O12, and (CO)4-Co-H7Si8O12 have been synthesized and studied by infrared spectroscopy as well.28-33 The experimental infrared frequencies and intensities for hydridosilsesquioxane clusters chemisorbed to gold closely match those of free clusters and monosubstituted clusters in solution. The observed vibrational features are consistent with those expected upon the lowering of cage symmetry to C3v (for 1) upon surface binding, further supporting a single-vertex attachment of the cluster to gold (Figure 1). The striking similarities in the frequencies of the bands in the RAIRS data for 2 to that for 1 suggest
Table 1. RAIRS Assignments
(28) Marcolli, C.; Calzaferri, G. Appl. Organomet. Chem. 1999, 13, 213. (29) Marcolli, C.; Calzaferri, G. J. Phys. Chem. B 1997, 101, 49254933. (30) Marcolli, C.; Imhof, R.; Calzaferri, G. Mikrochim. Acta 1997, 14, 493. (31) Calzaferri, G.; Imhof, R.; Tornroos, K. W. J. Chem. Soc., Dalton Trans. 1993, 3741. (32) Calzaferri, G.; Imhof, R.; Tornroos, K. W. J. Chem. Soc., Dalton Trans. 1994, 3123. (33) Calazaferri, G.; Marcolli, C.; Imhof, R.; Tornroos, K. W. J. Chem. Soc., Dalton Trans. 1996, 3313.
frequency (cm-1) assignment
H8Si8O12/Au
H10Si10O15/Au
D8Si8O12/Au
ν(Si-H) ν(Si-D) νas(Si-O-Si)
2280 N/A 1180 1111 1075 912 887 N/A
2280 N/A 1177 1112 1077 911 887 N/A
N/A 1662 1176 1108 1074 N/A
555 459 411
557 462 407
δ(Si-H) δ(Si-D) νa(Si-O-Si) νs(O-Si-O) δ(O-Si-O)
708 658 561 462 398
the HSiO3 fragments of the four and five-membered silicon oxide rings of 2 also remain intact. Upon deuteration of 1, the most intense ν(Si-H) and δ(Si-H) features shift ∼618 and ∼179 cm-1, respectively (Table 1, Figures 3 and 4). NL-DFT (Au-H7Si8O12 and Au-D7Si8O12 were employed as model structures) predicts isotopic shifts of 647 and 218 cm-1 whereas the first-order harmonic oscillator
Hydridosilsesquioxane Clusters on Gold Surfaces
approximation suggests shifts of 668 and 260 cm-1. Although the experimental isotopic shifts are less than expected, the frequency shifts observed for the ν(Si-H), ν(Si-D), δ(Si-H), and δ(Si-D) features for 1 and d-1 chemisorbed to gold compare favorably to the 619 and 171 cm-1 shifts observed for 1 and d-1 in solution.23-27 Infrared band positions and intensities have been calculated using nonlocal density functional theory (B3LYP/6-31G**) to test the assignments of the RAIRS spectra and the proposed cluster surface bonding geometry. C3v symmetric Au-H7Si8O12 and Au-D7Si8O12 as well as Cs symmetric Au-H9Si10O15 have been employed as model structures for these calculations. The predicted frequency regions for each vibrational mode of cluster are in accord with the experimental data, further supporting a monovertex attachment of the cluster to gold at saturation coverage (Table 1). The intensity predictions are reasonable given the total dynamic dipole moment is calculated. According to the surface selection rule, only the perpendicular (to the surface) component of the induced change in the net dipole moment will be observed.34-37 In summary, XPS and RAIRS characterization of saturated coverages of hydridosilsesquioxane clusters 1 and 2 on gold indicate the clusters are bonded by a single vertex. The calculated vibrational frequencies and intensities (NL-DFT) of C3v cluster models further support this proposed geometry. The reaction appears to be an effective oxidative addition reaction at the surface wherein a Si-H bond breaks to form an Au-Si linkage at the surface followed by reductive elimination of H2 from the gold surface. II. The Stability of H8Si8O12 (1) Clusters on Gold. The stability of the H8Si8O12 cluster/gold interface toward atmospheric conditions has been investigated by exposing the sample to controlled amounts of oxygen and water. Treatments of greater than 36 000 langmuirs of oxygen cause no observable change by XPS (Si 2p3/2 core levels). A similar lack of reactivity toward water is detected by RAIRS. The monolayer is also stable to >24 h of UHV exposure. The Au-Si linkage appears fairly robust under these conditions. It is interesting to note that molecular species containing Au-Si bonds show similar chemical stability.38 A small increase in the intensity of the ν(Si-H), νas(Si-O-Si), and the δ(Si-H) features (∼5% of original intensity) is observed immediately after a cluster overpressure of 4 × 10-8 Torr is introduced into the vacuum chamber at 20 °C. This intensity increase is accompanied by a small shift in the νas(Si-O-Si) and δ(Si-H) regions (4 and 2 cm-1, respectively). The process can be reversed by re-evacuating the system (Figure 5). After these initial shifts, no additional peak shift or intensity change is observed with increased exposure time. Increasing the overpressure of clusters as high as 6 × 10-7 Torr results in no detectable change. Multilayers of 1 can be formed upon cooling a gold sample to -140 °C and exposing the surface to the clusters. The peak corresponding to the HSiO3 features at ca. -102.3 eV increases in intensity and shifts to higher BE with multilayer coverage as expected due to the decrease in final state stabilization energy.12,39 The feature at (34) Feher, F. J.; Soulivong, D.; Eklund, A. G. J. Chem. Soc., Chem. Commun. 1998, 399-400. (35) Francis, S. A.; Ellison, A. H. J. Opt. Soc. Am. 1959, 49, 131. (36) Greenler, R. G. J. Chem. Phys. 1966, 44, 310. (37) Greenler, R. G. J. Chem. Phys. 1969, 50, 1963. (38) Pearce, H. A.; Sheppard, N. Surf. Sci. 1976, 59, 205. (39) Meyer, J.; Willnecker, J.; Schubert, U. Chem. Ber. 1989, 122, 223-230.
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Figure 5. RAIRS of the νas(Si-O-Si) region after saturation. (A) The spectrum is a set of 100 scans taken 5 min after the removal of H8Si8O12 from the chamber ratioed to a set of 100 scans of a gold surface saturated with H8Si8O12 under a 4 × 10-8 Torr overpressure of cluster. (B) The spectrum is a set of 100 scans taken 5 min after the cluster overpressure is established once again (from A) ratioed to a set of 100 scans taken 5 min after the removal of H8Si8O12 from the system.
-101.1 eV does not shift and decreases in intensity due to the formation of multilayer. When the samples are warmed to 20 °C, all clusters but the chemisorbed layer desorb and the initial spectroscopic features reappear. The formation of hydrogen radicals upon chemisorption of clusters during the reaction process is proposed. Hydrogen atoms do not form a stable chemisorbed layer to gold and rapidly recombine and desorb from gold surfaces to give H2 at room temperature.40 In addition, a mobile hydrogen radical may also recombine with an adsorbed cluster resulting in cluster desorption. Therefore, an equilibrium between adsorbing and desorbing clusters is proposed at all coverage regimes. When the clusters are pumped from the system, the rate of desorption would dominate for a short time as indicated in Figure 5 until all of the hydrogen has recombined and left the surface. When the clusters are reintroduced, the rate of adsorption again dominates until equilibrium is re-established on the surface. III. The Coverage Dependence of H8Si8O12 (1) and H10Si10O15 (2) Chemisorption to Gold. A. H8Si8O12. The formation of a saturated layer of 1 on gold has been examined as a function of coverage (Figures 6 and 7). Two Si 2p3/2 features with binding energies of -101.1 and -102.3 eV with a peak area ratio of 1:7.3 are observed. The relative peak areas and the peak separation measured are independent of percentage of surface coverage. These observations suggest a single species, the monovertex model indicated in Figure 1, is generated regardless of surface coverage. Surface coverage by cluster 1 is estimated to be approximately one cluster for every nine gold atoms based upon the maximum packing that can be expected for space-filling models of cluster 1 on an Au(111) surface. This result also provides strong evidence against the formation of multilayer islands at room temperature. As discussed earlier, both the peak area ratio and binding energy separation are observed to change dramatically upon the formation of multilayers.12,39 (40) Zhang, K. Z.; Banaszak Holl, M. M.; McFeely, F. R. J. Phys. Chem. 1998, 102, 3930-3935.
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Figure 6. Soft X-ray Si 2p3/2 core-level spectra of the chemisorption of H8Si8O12 to gold as a function of coverage. (A) ∼30% of saturated layer: BE -101.0, 102.2 eV. (B) ∼46% of saturated layer: BE -101.1, -102.3 eV. (C) ∼64% of saturated layer: BE -101.1, -102.3 eV. (D) ∼83% of saturated layer BE -101.1, 102.3 eV. (E) Saturated layer: BE -101.1, -102.3 eV. The peak ratio is constant, 7.2:1.
Figure 7. RAIRS of the chemisorption of H8Si8O12 as a function of coverage. Each spectrum represents 100 scans at 8 cm-1 resolution after the exposure of (A) 0.50, (B) ∼1.50, (C) ∼4.0, and (D) ∼5.0 langmuirs of H8Si8O12 ratioed to a gold background. The most intense features in the νas(Si-O-Si) and the δ(Si-H) regions shift ∼15 and ∼8 cm-1, respectively.
Coverage-dependent cluster-cluster interactions are observed by RAIRS during the chemisorption of 1 to gold (Figure 7). The RAIRS spectra indicate a partial Au/cluster interface (∼20% coverage) has formed after ∼1 langmuir dosage. From this, an initial sticking coefficient for chemisorption of ∼0.2 can be estimated.41 For dosages greater than 1 langmuir the νas(Si-O-Si) and the δ(SiH) bands are observed to shift to higher frequency with an increase in surface coverage. When total saturation is obtained, the total shifts are 15 and 8 cm-1. The position (41) Stobinski, L.; Dus, R. Vacuum 1994, 45, 299.
Nicholson et al.
of the ν(Si-H) feature is coverage independent. Much of the shift in peak position is observed at higher coverage regimes. Recall from the stability section that an additional 4 cm-1 in the νas(Si-O-Si) accompanies the previously described ∼5% increase in peak intensities when an overpressure of clusters is present. Frequency shifts as a function of coverage and/or film thickness have been ascribed to several sources.42,43 Some of these include a change in the chemical species or the metal-adsorbate bonding geometry, dipolar coupling between neighboring molecules, change in the local electric field modification leading to anomalous dispersion (optical effects), and vibrational coupling arising from neighboring oscillators.35,44-47 A change in the metal-adsorbate bonding geometry or bonding geometry of the cluster as a function of coverage is not expected to play a role in the peak frequency shift. This proposal is inconsistent with experimental data. As previously discussed, there are only two features detected by XPS whose separation and area ratio are constant for all coverage regimes. If the bonding geometry of the cluster to the surface changes as a function of coverage, significant differences in the number of peaks and/or area ratios would be expected. Dipolar coupling between neighboring molecules is not expected to comprise a large fraction of the frequency shift. A significant change in the bandwidths is expected to accompany the coverage-dependent peak shifting if dipolar coupling is the dominant source. The fwhm for the most intense features in the ν(Si-H), νas(Si-O-Si), and δ(SiH) regions are 20, 28, and 12 cm-1 respectively. These widths remain virtually constant (within 2 cm-1) for all coverage regimes. Anomalous dispersion (AD) and vibrational coupling likely contribute to the coverage-dependent peak frequency shifts. AD is an optical effect that is observed in only reflectance infrared spectroscopy (not transmission). Band distortion (often to higher frequencies) results from a fraction of the incoming beam being reflected off the front of the surface film rather than the metal surface. Therefore, as the film grows as a function of coverage AD is expected to increase proportionally.42,43 On the basis of the work of Kurth and Bein,48 the predicted shift due to AD for a Si-O vibration with a bandwidth of ∼25 cm-1 could be as much as ∼9 cm-1 at saturation coverage. This effect could comprise as much as 50% of the observed frequency shifts. Vibrational coupling arises from overlapping molecular orbitals of adjacent adsorbates. This mechanism is most appropriate for a completely filled monolayer where the adsorbed species are in close contact. For 1 on gold, most of the peak shifting occurs at the moderate to higher coverage regimes. Vibrational coupling has been shown to occur only with similar oscillations of nearest neighbor clusters.42,43 With the lowering of symmetry upon surface binding, there are several, closely spaced bands which represent each of the δ(Si-H) and νas(Si-O-Si) vibrations. These bands may couple with the vibrations with similar energies of neighboring clusters, effectively raising the force constants of the bonds causing at least a portion of (42) Cassuto, A.; King, D. A. Surf. Sci. 1981, 102, 388-404. (43) Willis, R. F.; Lucas, A. A.; Mahan, G. D. Vibrational Properties of Adsorbed Molecules; King, D. A., Woodruff, D. P., Eds.; Elsevier: 1986; Vol. 2, pp 59-163. (44) Moskovits, M.; Hulse, J. E. Surf. Sci. 1978, 78, 397-418. (45) Allara, D. L.; Baca, A.; Pryde, C. A. Macromolecules 1978, 22, 1215. (46) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927. (47) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52. (48) Kurth, D. G. Langmuir 1998, 14, 6987.
Hydridosilsesquioxane Clusters on Gold Surfaces
the observed peak frequency shift. Coupling between the δ(Si-H) and νas(Si-O-Si) vibrational modes must also be considered since they are separated by only ∼150 cm-1. On this basis, however, coupling of the δ(Si-H) or νas(SiO-Si) with the ν(Si-H) is not expected. B. H10Si10O15. The H10Si10O15 cluster surprisingly exhibits a much more complex behavior as it adsorbs to gold as a function of coverage. Two Si 2p3/2 features are observed as function of coverage with binding energies of -100.9 and -102.2 eV. The binding energies and peak separation are constant as a function of coverage. However, unlike 1, the relative peak areas gradually change from 1:5.4 at ∼18% surface saturation to ∼1:9 at complete saturation. The two peaks and their relative positions remain constant as a function of coverage, which suggest only two chemical species of silicon are present on the surface. The 1:5.4 ratio at 18% coverage implies some of the clusters may be bound to the gold surface by two vertexes while others are bound by only a single vertex. A 1:4 ratio would be expected if all clusters were bound by two vertexes. The different dimensions of 2 where there is a larger distance between silicon atoms on the pentagonal face may play a role in its ability to adsorb to two binding sites at low coverage. However, geometric consideration of Au(111) crystal faces and the bond distances for Au-capped cluster faces do not provide a simple rationalization for the observed reactivity difference. The peak ratio eventually becomes ∼1:9 at full coverage. This is indicative of an ongoing change in the bonding geometry of the cluster as a function of coverage. The peak area ratio suggests primarily single-vertex clusters are present at complete saturation. At that point, the clusters initially bound by two vertexes may either have reacted with a hydrogen radical remaining bound by only one vertex or have completely desorbed and been replaced by another single-vertex cluster. As previously mentioned, a significant cluster desorption rate is not surprising due to the presence of hydrogen radicals (formed during the initial reaction). Instead of recombining with another very mobile hydrogen atom, a radical could recombine with chemisorbed 2 bound by one or two vertexes which would lead to desorption or a single-vertex cluster, respectively. As coverage increases, more space constraints from neighboring clusters would be expected, reducing the number of correctly spaced double Si-H activation sites on the gold surface. This may force the clusters to bond by a single-vertex at higher coverages. For the RAIRS data, the shift in peak position for the νas(Si-O-Si) and the δ(Si-H) features of the H10Si10O15 cluster is slightly larger, 23 and 10 cm-1, respectively. The other features of the coverage-dependent data are quite similar to what is observed in Figure 7. The larger peak shift may result from an increase in dipole-dipole or vibrational coupling as a function of coverage for the larger cluster. This larger frequency shift is not likely a result of the proposed change in the bonding geometry of the some of the clusters at low coverage from double to single vertex because the features of the RAIRS spectra as a function of coverage are virtually indistinguishable from the H8Si8O12 experiment. NL-DFT calculations of the infrared frequencies and intensities for double-vertex clusters, Au2-H6Si8O12 and Au2H8Si10O15, do not differ significantly from the predictions for single-vertex clusters. It appears infrared spectroscopy is rather insensitive to a single-vertex vs double-vertex attachment of clusters on gold surfaces. Coverage effects of the chemisorption of hydridosilsesquioxane clusters highlight some of the dynamics of
Langmuir, Vol. 16, No. 22, 2000 8401
Figure 8. H8Si8O12 cluster adsorption as a function of exposure. The rectangles represent the intensity of the major feature of the νas(Si-O-Si) region plotted versus dosage in langmuirs (1 langmuir ) 1 × 10-6 Torr‚s). The triangles are integrated O 1s core-level intensities. Conversion to coverage units has been accomplished by a ratio of the partial dosage intensity to the saturation intensity for both analytical methods. These experiments have been completed in separate vacuum chambers, likely accounting for the discrepancy in saturation dosage between the two methods.
this novel reaction. Several mechanisms have been offered which may contribute at least in part to the apparent peak frequency shifting as a monolayer of clusters forms on the surface. As also discussed, an equilibrium involving a significant rate of desorption may be present on the surface at all coverage regimes. This offers an explanation for the previously discussed loss of ∼5% intensity upon pumping the clusters from the system as well as the apparent change in binding mode from XPS data for some of the H10Si10O15 clusters at low coverage. To explore these dynamics further, the rate of cluster adsorption as a function of exposure has been studied. IV. Mechanistic Considerations The adsorption rate of 1 on gold has been investigated to garner a better understanding of the dynamics of this cluster/gold interface. RAIRS and XPS have been employed for this measurement (Figure 8). Within experimental error, a fairly constant adsorption rate until ∼75% saturation coverage is observed by both analytical methods. For the RAIRS measurement (rectangles), the intensity of the νas(Si-O-Si) feature has been integrated to estimate the coverage. For small molecules, Moskovitz and Hulse have shown that infrared band intensities can be employed to estimate surface coverage as long as the bandwidths are invariant.42,43 (Recall, the ν(Si-H), νas(Si-O-Si), and δ(Si-H) bands have respective widths of ∼20, 28, and 12 cm-1 and do not change (
