Formation of Mixed Layers Derived from Functional Silicon Oxide

several sizes of the formula (HSiO1.5)n (n ) 8, 10, 12, .... The WIN-IR program for Microsoft Windows ... 25 °C. In addition, a mobile hydrogen radic...
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Langmuir 2001, 17, 7879-7885

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Formation of Mixed Layers Derived from Functional Silicon Oxide Clusters on Gold Kenneth T. Nicholson,† Kangzhan Zhang,† Mark M. Banaszak Holl,*,† F. Read McFeely,‡ Gion Calzaferri,§ and Udo C. Pernisz| Chemistry Department, University of Michigan, Ann Arbor, Michigan 48109, IBM T. J. Watson Laboratory, Yorktown Heights, New York 10598, Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland, and Dow Corning Corp., Midland, Michigan 48686-0994 Received August 3, 2001. In Final Form: September 18, 2001 Single and multicomponent mixed layers of silsesquioxane clusters on freshly evaporated gold surfaces have been investigated by X-ray photoelectron spectroscopy and reflection-absorption infrared spectroscopy. Approximately 5-10% of the cluster layers (e.g., H8Si8O12 and H10Si10O15) on gold desorb upon evacuation of the adsorbate from the reaction chamber. These open adsorption sites are an avenue for cluster displacement reactions that yield mixed monolayers (e.g., H8Si8O12/D8Si8O12 and H8Si8O12/C6H13-H7Si8O12) of several compositions on gold. This dynamic behavior is not observed for the C6H13-H7Si8O12 cluster layer on gold. Rather, this molecule acts as a poison to these reported displacement processes at the gold surface.

Introduction The formation of structurally and chemically welldefined self-assembled monolayers (SAMs) is potentially useful in a number of technologies spanning corrosion prevention,1 chemical sensing,2 and microelectronic device fabrication.3 With these goals in mind, the self-assembly of alkanethiols on gold has been the subject of intense research for more than a decade.4 More recently, multicomponent mixed monolayers of self-assembling molecules on gold surfaces have been reported in the literature.5 Often the assembly of mixed layers involves different endgroup functionalities such as electroactive groups.6 In fact, many of the applications of monolayer assembly require the ability to prepare mixed layers for which the quantities of the two (or more) adsorbing molecules on a surface can be systematically varied and controlled. This feature may †

University of Michigan. IBM T. J. Watson Laboratory. § University of Bern. | Dow Corning Corp. ‡

(1) Zamborini, F. P.; Campbell, J. K.; Crooks, R. M. Langmuir 1998, 14, 640. Zamborini, F. P.; Crooks, R. M. Langmuir 1998, 14, 3279. Jennings, G. K.; Munro, J. C.; Yong, T.-H.; Laibinis, P. E. Langmuir 1998, 14, 4. (2) Crooks, R. M.; Ricco, A. J. Acc. Chem. Res. 1998, 31, 219. Ishihara, T.; Higuchi, M.; Takagi, T.; Ito, M.; Nishiguchi, H.; Takita, Y. J. Mater. Chem. 1998, 8, 2037. (3) Hickman, J. J.; Ofer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991, 252, 688. Abbott, N. L.; Rolison, D. R.; Whitesides, G. M. Langmuir 1994, 10, 2672. Gardner, J. J.; Frisbie, C. D.; Wrighton, M. S. J. Am. Chem. Soc. 1995, 117, 6927. (4) Ulman, A. Chem. Rev. 1996, 96, 1553; Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. Jourdan, J.; Cruchon-Dupeyrat, S. J.; Huang, Y.; Kuo, P. K.; Liu, G.-Y. Langmuir 1999, 15, 6495. Xu, S.; Laibinis, P. E.; Liu, G.-Y. J. Am. Chem. Soc. 1998, 120, 9356. Xu, S.; Liu, G.-Y. Langmuir 1997, 12, 127. (5) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M.; Deutch, J. J. Phys. Chem. 1994, 98, 563-571. Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M., Langmuir 1992, 8, 1330-1341. Shon, Y.-S.; Lee, T. R. J. Phys. Chem. B 2000, 104, 8192-8200. Tirado, J. D.; Abruna, H. D. J. Phys. Chem. 1996, 100, 4556-4563. Hayes, W. A.; Shannon, C. Langmuir 1998, 14, 1099-1102. Stevens, F.; Beebe, J. Langmuir 1999, 15, 6884-6889. Finnie, K. R.; Haasch, R.; Nuzzo, R. G. Langmuir 2000, 16, 6968-6976. (6) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301-4306.

eventually provide viable routes for the preparation of molecular superstructures with the ultimate goal of rendering specific chemical sensing and molecular electronic functions.7 The monolayer assembly of silicon oxide clusters (H8Si8O12, H10Si10O15, etc.) on freshly evaporated gold surfaces in ultrahigh vacuum (UHV) has been previously characterized by soft X-ray photoelectron (XPS) and reflection-absorption infrared spectroscopies (RAIRS).8,9 These adsorbates, commonly known as hydridosilsesquioxane clusters, are a volatile, well-defined chemical species that are easily synthesized and can be separated into several sizes of the formula (HSiO1.5)n (n ) 8, 10, 12, 14).10-12 Both XPS and RAIRS suggest the formation of a Si-Au bond rendering a monolayer of single-vertex clusters by a novel Si-H bond activation on gold surfaces. Calzaferri and co-workers have synthesized several derivatives of these clusters as well. For example, monosubstituted clusters have been reported such as C6H13H7Si8O12,13 Co(CO)4H7Si8O12,14 Ph-H7Si8O12,15 and many others.16 Multisubstituted clusters where the Si8O12 core (7) Creager, S. E.; Rowe, G. K. J. Electroanal. Chem. 1994, 370, 203211. Song, S.; Clark, R. A.; Bowden, E. F.; Tarlov, M. J. J. Phys. Chem. 1993, 97, 6564. Nakashima, N.; Abe, K.; Hirohashi, T.; Hamada, K.; Kunitake, M.; Manabe, O. Chem. Lett. 1993, 1021. (8) Nicholson, K. T.; Zhang, K. Z.; Banaszak Holl, M. M. J. Am. Chem. Soc. 1999, 121, 3232-3233. (9) Nicholson, K. T.; Zhang, K. Z.; Banaszak Holl, M. M.; McFeely, F. R.; Pernisz, U. C. Langmuir 2000, 16, 8396-8403. (10) Agaskar, P. A. Inorg. Chem. 1991, 30, 2707-2708. (11) Bornhauser, P.; Calzaferri, G. Spectrochim. Acta 1990, 46A, 1045-1056. Bornhauser, P.; Calzaferri, G. J. Phys. Chem. 1996, 100, 2035-2044. Bartsch, M.; Bornhauser, P.; Calzaferri, G.; Imhof, R. Vib. Spectrosc. 1995, 8, 305-308. Bartsch, M.; Bornhauser, P.; Calzaferri, G.; Imhof, R. J. Phys. Chem. 1994, 98, 2817-2831. (12) Marcolli, C.; Laine, P.; Buhler, R.; Tomikinson.; Calzaferri, G. J. Phys. Chem. B 1997, 101, 1171-1179. (13) Calzaferri, G.; Imhof, R.; Tornroos, K. W. J. Chem. Soc., Dalton Trans. 1994, 3123-3128. (14) Calzaferri, G.; Imhof, R.; Tornroos, K. W. J. Chem. Soc., Dalton Trans 1993, 3741-3748. (15) Calzaferri, G.; Marcolli, C.; Imhof, R.; Tornroos, K. W. J. Chem. Soc., Dalton Trans. 1996, 3313-3322. (16) Marcolli, C.; Calzaferri, G. J. Phys. Chem. B 1997, 101, 49254933. Marcolli, C.; Imhof, R.; Calzaferri, G. Mikrochim. Acta 1997, 14, 493-496.

10.1021/la015522m CCC: $20.00 © 2001 American Chemical Society Published on Web 11/06/2001

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remains intact including Cl8Si8O12 and (CH3)8Si8O12 have also been reported in the literature.17 Some of the dynamic properties of the silicon oxide cluster monolayer have also been previously discussed.9 For example, an adsorption mechanism involving a precursor adsorption state is proposed to precede chemisorption. This mechanism suggests the chemisorbed layer is very attractive to incoming clusters, even though they do not adsorb to form multilayers at 25 °C.18 It also implies there is a constant equilibrium between the adsorbed cluster layer and those clusters interacting with the adsorbed layer in an “extrinsic” precursor state. At all coverage regimes, the adsorbed clusters may desorb and be replaced by other clusters from an extrinsic precursor state. This hypothesis is supported by the XPS and RAIRS observation that 5-10% of the monolayer desorbs immediately after removal of the cluster overpressure from the UHV chamber. The results reported in this paper demonstrate these open adsorption sites are a route for cluster displacement in the presence of a new adsorbate. With these processes monitored by XPS and RAIRS while the dosing pressures are adjusted accordingly, the relative percentage of each component may also be controlled. In some cases, where the aforementioned equilibrium is still present, the formation of the mixed layer is completely reversible to the initial single layer. Otherwise, the second (or third, etc.) molecule “poisons” the displacement reactions and the resultant mixed monolayer is stable in the presence of other adsorbates Experimental Details Gold samples were prepared by evaporating chromium or titanium onto SiO2/Si(100) as an adhesive layer followed by at least 100 nm of gold. Prior to use, additional gold was evaporated onto the surface in the UHV system and sample purity was assessed by XPS. Only trace amounts of carbon impurities could be detected. Evaporation of gold was performed in a separate chamber (base pressure 4 × 10-10 Torr) in which exposures to clusters were not performed. The photoemission chamber (base pressure 1 × 10-10 Torr) and RAIRS apparatus (base pressure 7 × 10-10 Torr) have been fully described elsewhere.19 H8Si8O12 clusters were synthesized by the method of Agaskar and sublimed twice.10 The D8Si8O12 and C6H13-H7Si8O12 clusters were also prepared by literature procedures.12,13 Cluster purity was checked using 1H NMR, IR, and gas chromatography mass spectrometry (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. The clusters were introduced into the chamber through a leak valve. The experiments involving H8Si8O12 and D8Si8O12 were performed at room temperature with a dosing pressure of 4 × 10-8 and 1 × 10-8 Torr, respectively. The C6H13-H7Si8O12 cluster has a lower vapor pressure; therefore, the sample was heated to ∼50 °C in a warm water bath to obtain adequate volatility for a dosing pressure of ∼1 × 10-8 Torr. For RAIRS experiments, the samples were transferred from the photoemission chamber into the infrared chamber. The sample was aligned and a “clean gold” background taken. After the surface was dosed to saturation coverage, two types of “sample sets” were collected. One set occurred while clusters were still (17) Calzaferri, G. Tailor-made silicon oxygen compounds: from molecules to materials; Corriu, R., Jutzi, P., Eds.; Lengericher Hubert & Co.: Gottingen, 1996; pp 149-169. (18) Schonhammer, K. Surf. Sci. 1979, 83, L633-L636. Kisliuk, P. J. Phys. Chem. Solids 1958, 5, 78-84. Kisliuk, P. J. Phys. Chem. Solids 1957, 3, 95-101. King, D. A. Surf. Sci. 1977, 64, 43. (19) Greeley, J. N.; Meeuwenberg, L. M.; Banaszak Holl, M. M. J. Am. Chem. Soc. 1998, 120, 7776-7782. Lee, S.; Banaszak Holl, M. M.; Hung, W. H.; McFeely, F. R. Appl. Phys. Lett. 1996, 68, 1081-1083. Lee, S.; Makan, S.; Banaszak Holl, M. M.; McFeely, F. R. J. Am. Chem. Soc. 1994, 116, 11819-11826.

Nicholson et al. present in the vacuum chamber (∼1.0 × 10-8 Torr), the other after the clusters were evacuated from the chamber (∼1.0 × 10-9 Torr). Each scan set consisted of 100 scans at 8 cm-1 resolution. For cluster displacement experiments, the initial adsorbate was pumped (∼1.0 × 10-9 Torr) from the chamber before the new cluster was introduced. This was often accomplished within 5-25 min, depending on the volatility of the cluster. The sample was not moved and another RAIRS background taken. After the new adsorbate was introduced, the displacement was observed by collecting consecutive sets of scans while the adsorbate was still present in the chamber. After the completion of the reaction, as evident by the ratio of consecutive sets of scans, the second adsorbate was pumped from the chamber and a final sample scan was collected. Since each sample scan may also be used as a reference point during these several step processes, the sample and background scan will be explicitly indicated in the text and figure captions. The sample was transferred in UHV to the photoemission chamber to further analyze the modified gold surface, where appropriate. The Si 2p, C 1s, and O 1s core levels were collected. Matlab Version 4.2c 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 saturation coverage while peak heights and positions were allowed to change. The WIN-IR program for Microsoft Windows 1993 3.0 was employed to curve-fit and integrate IR data. Estimations of coverage with integrated intensities of the ν(SiH) and ν(Si-D) have been standardized using their values for a saturated monolayer of H8Si8O12 and D8Si8O12 respectively.

Results and Discussion I. H8Si8O12/D8Si8O12 Cluster Displacement. H8Si8O12 and H10Si10O15 clusters chemisorb to freshly evaporated gold surfaces in UHV at room temperature.8 XPS and RAIRS data provide strong evidence for the formation of a new Au-Si bond leading to a monolayer of single-vertex clusters. H8Si8O12 is likely oriented in a C3v symmetric fashion on the surface.12,13,16,17 The splitting of the νas(Si-O-Si) and δ(Si-H) features upon adsorption of H8Si8O12 and H10Si10O15 onto the gold surface is consistent with the solution infrared spectra for several of the monosubstituted clusters in solution. The reaction appears to involve an oxidative addition of a cluster Si-H bond to the gold surface followed by reductive-elimination of H2. This reaction likely proceeds by the mechanism previously proposed by Calzaferri and Hoffmann for substitution reactions of H8Si8O12.20 The dynamic nature of these clusters on gold has also been addressed.9 Upon evacuation of H8Si8O12 from the system, a 5-10% loss of IR signal is detected in all vibrational regions suggesting a small portion of the chemisorbed monolayer has desorbed because H8Si8O12 does not form multilayers on gold at 25 °C (Figure 1). This observation is accompanied by a 4 cm-1 frequency shift in the most intense νas(Si-O-Si) feature. Since the frequency position of the νas(Si-O-Si) is coverage dependent, this affords yet another piece of evidence for the cluster desorption.9 These observations are reversible by simply reintroducing clusters to the vacuum chamber. The presence of hydrogen atoms, released upon the initial adsorption of H8Si8O12, is proposed to bring about this desorption reaction. Hydrogen atoms do not chemisorb to gold, but rather rapidly recombine and desorb as H2 at 25 °C. In addition, a mobile hydrogen radical may also recombine with an adsorbed cluster resulting in cluster desorption. When the chamber is evacuated and a small (20) Marcolli, C.; Calzaferri, G. Appl. Organomet. Chem. 1999, 13, 213. Hoffman, R. J.; Calzaferri, G. J. Chem. Soc., Dalton. Trans. 1991, 917.

Mixed Monolayers from Functional Silicon Oxide

Figure 1. RAIRS of the overall change in the layer of H8/Au after the cluster overpressure has been removed. This spectrum is a set of scans collected after the removal of H8Si8O12 from the chamber ratioed to a gold surface saturated with H8Si8O12 in the midst of a 4 × 10-8 Torr overpressure of cluster.

Figure 2. (A) RAIRS of a chemisorbed layer of H8Si8O12 on gold. The background consists of a freshly evaporated gold sample. (B) RAIRS of a mixed layer comprising ∼40% H8Si8O12 and ∼60% D8Si8O12 ratioed to a clean gold surface. (C) Overall change after the introduction of D8Si8O12 to the layer of H8Si8O12 on gold. This spectrum represents one set of scans collected 5 min after the removal of D8Si8O12 from the chamber ratioed to a gold surface saturated with H8Si8O12.

portion of the monolayer has desorbed, unfilled adsorption sites are likely present on the gold surface. This opens an avenue for displacement/exchange reactions at the surface in the presence of a new adsorbate. The IR intensity diminution and νas(Si-O-Si) frequency shift upon the reintroduction of H8Si8O12 to the vacuum chamber is reversible. This is fairly strong evidence for equilibrium between adsorbing and desorbing clusters at the surface while an overpressure of H8Si8O12 (>1.0 × 10-8 Torr) is still present inside the reaction chamber. This conclusion is completely consistent with the proposed Kisliuk precursor mediated adsorption mechanism.18 D8Si8O12 has been observed to undergo displacement reactions with H8Si8O12 on gold surfaces (Figure 2). Note the appearance of the feature at 1662 cm-1 assigned to ν(Si-D).9 The intensity of the ν(Si-H) at 2271 cm-1 and the δ(Si-H) at 888 cm-1 has also decreased considerably (Figure 2b). After integrating the intensities of the ν(Si-

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Figure 3. Stepwise displacement of a H8Si8O12 layer with D8Si8O12 after (A) ∼2, (B) ∼4, and (C) ∼7 langmuir exposures of D8Si8O12. Each spectrum represents one set of scans taken after the sample had been exposed to the listed amounts of D8Si8O12 ratioed to a clean gold background.

H) and ν(Si-D), one can estimate that a chemisorbed layer comprising ∼40% H8Si8O12 and ∼60% D8Si8O12 is present on the gold surface after the displacement has concluded. A combined RAIRS and XPS analysis of the O 1s core levels of the adsorption of H8Si8O12 as a function of dosage illustrates the intensity of the IR bands is a fair estimate of surface coverage.21 Since the bandwidths of the ν(SiH) and ν(Si-D) do not change as a function of coverage, this is not surprising. This displacement reaction can be reversed by exposing additional H8Si8O12 to the ∼40% H8Si8O12 and ∼60% D8Si8O12 chemisorbed layer after the D8Si8O12 has been evacuated from the chamber. With the intensity and frequency position of the νas(Si-O-Si) remaining constant, the displacement occurs until all of the D8Si8O12 is displaced by H8Si8O12. The proposed Kisliuk mechanism offers some insight into the displacement dynamics. Given one chemisorbed layer of clusters remains on the surface during the entire cluster displacement reaction, one can infer that the incoming cluster may interact with several adsorbed clusters (as an extrinsic precursor) until trapped into a satisfactory adsorption site. When the new cluster adsorbs, the hydrogen atom released from the scission of the Si-H bond could encounter an adsorbed cluster of the initial layer of H8Si8O12, recombine with it, and desorb. This would open a new adsorption site, continuing the displacement reaction beyond the initial ∼5-10% of the layer that desorbs upon pumping the cluster from the chamber. Regardless of the desorption mechanism, ∼60% of the cluster layer appears to be continuously displaced as long as there is an overpressure of clusters present in the chamber, characteristic of the dynamic equilibrium at the surface. When the clusters are removed from the chamber, the source of the displacing cluster is no longer present to interact with the adsorbed layer, ending the displacement reaction abruptly. The process is gradual as opposed to immediate for both desorption and adsorption steps. This characteristic does afford some control over the specific quantities of each cluster employed up to the displacement limit (Figure 3). However, the 60% displacement limit is both surprising and puzzling. One would expect based on solution deposited organothiol reactions on gold that the initial chemisorbed layer would either be stable to the incoming adsorbate or be entirely displaced (21) Moskovitz, M.; Hulse, J. E. Surf. Sci. 1978, 78, 397.

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by the new molecule. The other 40% must be stabilized or protected in some way that prevents the reaction. A diffusion barrier may exist that is dependent upon the contours of the atomically rough gold surface. The chemisorption and displacement behavior of a monosubstituted cluster, C6H13-H7Si8O12, has been investigated to further address this issue. II. C6H13-H7Si8O12/H8Si8O12 Displacement. The monosubstituted C6H13-H7Si8O12 also chemisorbs to freshly evaporated gold surfaces at 25 °C. XPS and RAIRS data are consistent with the clusters assembling as a chemisorbed layer on the surface, similarly to the unsubstituted H8Si8O12, D8Si8O12, and H10Si10O15 clusters.9 The XPS spectrum reveals two Si 2p3/2 core-level features with binding energies (BEs) of -101.2 and -102.5 eV, which have full-widths at half-maximum (fwhm) of 0.57 and 1.14 eV, respectively. These two features comprise the three nominal types of silicon atoms on the surface, H-SiO3, C-SiO3, and Au-SiO3. For H8Si8O12 and H10Si10O15 layers on gold, where there are only two silicon environments, H-SiO3 and Au-SiO3, have binding energies of -101.1 and -102.4 eV, respectively. Therefore, the less intense peak at -101.2 eV for C6H13-H7Si8O12 can be assigned as Au-SiO3. The two features for C6H13H7Si8O12 have a peak area ratio of 1:9.8. If one assumes the C-SiO3 and the HSiO3 fragments have the same Si 2p3/2 binding energies, this ratio may still imply a singlevertex structure even though the expected area ratio is ∼1:7. This assumption is supported by the formation of multilayers of C6H13-H7Si8O12 at lowered temperature. Similar to experiments with the unsubstituted clusters, the peak at -101.2 eV disappears as a function of the number of cluster layers.9 Assuming the alkyl chain is upright on the surface (vida infra), the presence of the long alkyl chain will attenuate the number of emitted electrons from the buried Au-SiO3 fragments that are actually detected. This would explain the higher peak area ratio than initially expected for a single-vertex cluster bonding geometry. The O 1s core levels for C6H13-H7Si8O12 on gold are observed as a single feature at 531.4 eV with a fwhm of 1.8 eV. The integrated area of the O 1s levels for saturation coverage for C6H13-H7Si8O12 is actually ∼10% greater than H8Si8O12, D8Si8O12, and H10Si10O15 (vide infra). This suggests the number of C6H13-H7Si8O12 clusters present on the surface at saturation coverage is similar to that of the previously studied unsubstituted clusters. This also implies the hexyl chain of C6H13-H7Si8O12 is not oriented parallel to the surface if one assumes a rather close packing for H8Si8O12, D8Si8O12, and H10Si10O15 clusters on gold. The C 1s core levels for C6H13-H7Si8O12 are detected as a single peak at 283.6 eV with a fwhm of 1.5 eV. The area ratio between the O 1s and the C 1s core levels is ∼6:5 while the expected ratio is 2:1 if electron attenuation is not considered. The observed area ratio further suggests the alkyl chain of C6H13-H7Si8O12 is upright and not parallel to the surface, attenuating the O 1s electrons from the cluster cage. The RAIRS spectrum for C6H13-H7Si8O12 on gold in the energy range (650-3500 cm-1) is illustrated in Figure 4B. The regions at 2916, 2279, 1165, and 888 cm-1 are assigned as ν(C-H), ν(Si-H), νas(Si-O-Si), and δ(SiH), respectively. The νas(Si-O-Si) and δ(Si-H) regions have clearly resolved shoulder features as well. These assignments have been made based on the published normal-mode analysis for this cluster.13 The frequencies and intensities of the features are fairly close to what is observed for C6H13-H7Si8O12 in hexane and, therefore, consistent with intact clusters on the surface. If the cluster

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Figure 4. (A) Soft X-ray Si 2p3/2 core level spectra after the chemisorption of C6H13-H7Si8O12 to freshly evaporated gold. (B) RAIRS of a chemisorbed layer of C6H13-H7Si8O12 on gold. This spectrum comprises one set of C6H13-H7Si8O12 ratioed to a clean gold background.

decomposed during reaction with the gold, gross discrepancies in the frequencies as well as the intensity ratio between the ν(Si-H) and νas(Si-O-Si) features are expected because the O3Si-H fragments would be broken. The shoulder νas(Si-O-Si) feature at ∼1045 cm-1 is far more intense than that observed in the solution spectrum of C6H13-H7Si8O12, which suggests a further lowering of symmetry upon adsorption to gold. The desorption behavior of C6H13-H7Si8O12 is markedly different from H8Si8O12, D8Si8O12, and H10Si10O15. Recall, ∼5-10% of the initial chemisorbed layer of H8Si8O12 desorbs upon evacuation of the cluster from the reaction chamber (Figure 5A). This is not observed for C6H13H7Si8O12 (Figure 5B). The ν(Si-H) and the δ(Si-H) features remain virtually constant after the cluster is evacuated. A very small shift is observed in the νas(SiO-Si) region for C6H13-H7Si8O12 but is negligible in comparison to the detected shift for H8Si8O12 (Figure 5A). Since 5-10% of the layer of C6H13-H7Si8O12 does not desorb in a dynamic vacuum, there appears to be no obvious avenue for a displacement reaction at the surface. No cluster displacement reaction is detected by RAIRS after the exposure of H8Si8O12 to the monolayer of C6H13H7Si8O12 on gold (Figure 6). Furthermore, the integrated values of the C 1s and O 1s core levels also remain constant after the exposure, additional evidence of a very stable layer of C6H13-H7Si8O12, resistant to cluster displacement.

Mixed Monolayers from Functional Silicon Oxide

Figure 5. (A) RAIRS of the overall change in the layer of H8Si8O12 on gold after the cluster overpressure has been removed. This spectrum is a set of scans collected after the removal of the cluster from the chamber ratioed to a gold surface saturated with H8Si8O12 in the midst of a 4 × 10-8 Torr overpressure of cluster. (B) RAIRS of the overall change in the layer of C6H13-H7Si8O12 on gold after the cluster overpressure has been removed. This spectrum is a set of scans collected after the removal of the cluster from the chamber ratioed to a gold surface saturated with C6H13-H7Si8O12 in the midst of 1 × 10-8 Torr overpressure of this cluster.

Figure 6. (A) Overall change after a chemisorbed layer of C6H13-H7Si8O12 on gold is introduced to H8Si8O12 (4 × 10-8 Torr pressure). (B) Overall change after a layer of H8Si8O12 on gold is introduced to C6H13-H7Si8O12 (1 × 10-8 Torr pressure). For these spectra, the initial layer on gold is the reference.

On the other hand, the reaction is observed by RAIRS upon the addition of C6H13-H7Si8O12 clusters to a chemisorbed layer of H8Si8O12 (Figure 6). Similar control to the H8Si8O12/D8Si8O12 system has also been detected by RAIRS (Figure 7). Specifically, note the δ(Si-H) feature at 857 cm-1 that is unique to the monosubstituted cluster C6H13H7Si8O12 on gold (Figures 6 and 7). Integrating the C 1s core-level feature and comparing to a layer of clusters solely containing C6H13-H7Si8O12 suggests ∼65% of the mixed layer is comprised of C6H13-H7Si8O12. This observation is not surprising given the previously discussed H8Si8O12/D8Si8O12 displacement reaction because there are open adsorption sites within the layer of H8Si8O12 (Figure 5). However, the process is irreversible, contrary to the H8Si8O12/D8Si8O12 system. The “active” adsorption

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Figure 7. Stepwise displacement of H8Si8O12 with C6H13H7Si8O12 after (A) ∼1, (B) ∼2, and (C) ∼4 langmuir exposures of C6H13-H7Si8O12. Each spectrum represents one set of scans (overpressure of the C6H13-H7Si8O12 cluster) ratioed to an intial layer of H8Si8O12. For both A and B, the 7 and 4 langmuir exposures represent the end of the displacement reaction.

sites that may be necessary for the displacement reactions must have become blocked or poisoned. Both XPS and RAIRS data suggest those adsorption sites are occupied by C6H13-H7Si8O12. For example, the intensity increase of the νas(Si-O-Si) feature in the RAIRS data implies a higher quantity of clusters on the surface for the mixed C6H13-H7Si8O12/H8Si8O12 layer than for the original H8Si8O12 layer on gold (Figure 6). Consistent with this observation, the O 1s core-level integrated intensity is ∼15% greater for the mixed C6H13-H7Si8O12/H8Si8O12 layer than for the layer of H8Si8O12. As previously stated, the increased number of clusters results from the filling of the empty gold adsorption sites within the initial layer of H8Si8O12 that had been evacuated by the initial pumping of the system (before C6H13-H7Si8O12 was ever introduced). In other words, adsorption sites appear to be occupied for the mixed C6H13-H7Si8O12/H8Si8O12 layer when the sample reaches the XPS analyzer chamber (no overpressure of clusters) that are vacant when a chemisorbed layer of H8Si8O12 is analyzed. These displacement processes have been further investigated by exposing C6H13-H7Si8O12 to a chemisorbed layer of a partially deuterated cluster, HxD8-xSi8O12. Cluster displacement is observed as evidenced by an intensity increase in the ν(Si-H) and δ(Si-H) features coupled with a loss of intensity in the ν(Si-D) feature (Figure 8). By use of the intensity changes in the ν(Si-H) and ν(Si-D) to estimate the composition of the mixed monolayer, a 62% C6H13-H7Si8O12/38% HxD8-xSi8O12 is calculated, consistent with the previously discussed C6H13-H7Si8O12/H8Si8O12 and H8Si8O12/D8Si8O12 displacement reactions. The reaction is irreversible which is not surprising because C6H13-H7Si8O12 occupies the avenues for displacement (empty adsorption sites within the layer of H8Si8O12, H10Si10O15, D8Si8O12, or HxD8-xSi8O12). Subsequently, HxD8-xSi8O12 does not displace C6H13-H7Si8O12 on gold either. A schematic summary of the displacement processes is illustrated in Figure 9. When freshly evaporated gold is exposed to H8Si8O12, a chemisorbed layer of clusters forms. After the layer has assembled, the cluster is evacuated from the chamber and ∼5-10% of the layer desorbs in UHV, opening adsorption sites for another adsorbate. The new cluster, D8Si8O12, HxD8-xSi8O12, or C6H13-H7Si8O12,

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Figure 8. (A) RAIRS of a chemisorbed layer of HxD8-xSi8O12 on gold. This spectrum comprises one set of scans of HxD8-xSi8O12 on gold ratioed to a clean gold background. (B) RAIRS of a mixed layer made up of ∼40% HxD8-xSi8O12 and ∼60% C6H13H7Si8O12 ratioed to a clean gold surface. (C) Overall change after introduction of C6H13-H7Si8O12 to the layer of HxD8-xSi8O12 on gold. This spectrum represents one set of scans collected after the removal of C6H13-H7Si8O12 from the chamber ratioed to a gold surface modified with a layer of HxD8-xSi8O12.

Figure 9. Schematic diagram of cluster exchange on freshly evaporated gold surfaces in UHV. This figure serves as an illustrative summary of these displacement reactions. No implications should be made regarding the sizes and shapes of the various color-coded regions of the surface. The color code for the scheme is as follows: white ) clean gold surface; black ) C6H13-H7Si8O12 chemisorbed layer; gray ) H8Si8O12 chemisorbed layer.

is then admitted into the chamber. It interacts (as an extrinsic precursor) with the layer until a satisfactory adsorption site is found and chemisorbs. A hydrogen atom, a byproduct of the chemisorption process, may recombine with an adsorbed H8Si8O12 and desorb, opening a new site for the second molecule. The process apparently continues until ∼60% of the initial layer is displaced. It is reversible if D8Si8O12 or HxD8-xSi8O12 is the second adsorbate; however, it is irreversible if C6H13-H7Si8O12 is the second adsorbate. The ∼60% cluster displacement may be correlated with the location of the open adsorption sites that may be necessary for the reaction, those formed when the initial cluster is removed from the UHV chamber (Figure 1). These “active sites”, locations where the displacement reactions begin, may be located on finite size terraces where H (D) diffusion can occur. For example, consider the case of an initial layer of H8Si8O12 exposed to D8Si8O12 clusters (Figure 9). At first, each D8Si8O12 cluster that reacts at the active sites would generate a deuterium atom that would likely react with a nearby chemisorbed cluster

Nicholson et al.

of H8Si8O12. As the displacement process continues, however, the area around the open adsorption site eventually becomes solely occupied by chemisorbed D8Si8O12 clusters. The diffusion length of deuterium atoms across the surface is not sufficient to dissociate chemisorbed H8Si8O12 distant from the active site. At this point, only degenerate D8Si8O12/D8Si8O12 exchange would occur until the D8Si8O12 is pumped from the chamber and the total amount of cluster exchange would be limited to ∼60%. When gold is initially exposed to C6H13-H7Si8O12, the chemisorbed layer is unaffected by a dynamic vacuum (Figure 9). Unlike the behavior observed for H8Si8O12, 5-10% of the layer of C6H13-H7Si8O12 does not desorb thus prohibiting the displacement reaction. The incoming H8Si8O12 or D8Si8O12 likely interact with the initially adsorbed layer as an extrinsic precursor. However, there are no available adsorption sites on the gold surface, preventing displacement or exchange processes. No hydrogen atoms are released as byproducts that could continue the reaction beyond the initial readsorption site. In summary, C6H13-H7Si8O12 blocks these displacement processes. It acts as a poison to further modification of the initial layer on the gold surface. Several mechanisms for this phenomenon are worthy of consideration. It is possible that the alkyl modification of the cage structure causes a large enough change in electronic characteristics of the Au-Si linkage to influence the bonding equilibrium. One could also propose an important interaction between the hexyl side chain and the gold surface. However, there is no evidence of such an interaction in either the RAIRS or XPS data. A particularly intriguing mechanism for this behavior is suggested by the molecular packing of C6H13H7Si8O12 in the crystal.13 In this instance, the hexyl chains pack and it is possible that chain-packing forces are sufficient to prevent the exchange. Ordered chain packing is not required for this hypothesis, and it is quite possible that chain entanglement may be a better description than chain packing. Regardless of the specific mechanism, the single and multicomponent layers containing C6H13H7Si8O12 clusters described herein cannot be further modified even though hydrogen atoms are expected to be generated during the initial adsorption of this cluster. Summary and Conclusions The dynamic nature of silicon oxide cluster monolayers on freshly evaporated gold surfaces has been further explored. This nature is most evident in the desorption of a small fraction (5-10%) of a chemisorbed monolayer of H8Si8O12, D8Si8O12, or H10Si10O15 upon the evacuation of the clusters from the UHV system. These open adsorption sites are avenues for displacement reactions with other volatile, monosubstituted clusters yielding mixed monolayers on gold. The synthesis of these novel mixed layers has several features. For example, the surface modification can be accomplished in UHV where potential for contamination is significantly reduced. Surface analytical techniques such as XPS and RAIRS permit the researcher to observe the mixed layer assembly in situ. This affords some control over the relative amounts of each component comprising the layer. Since derivatives of the silicon oxide clusters have much literature precedence, the synthesis of several mixed monolayers can be easily envisioned from this research. Furthermore, one could even employ a cluster capped with a functional group in the synthesis of the initial mixed monolayer, introduce a volatile selective reagent, and modify only a portion of the surface. This class of displacement/exchange reactions, where chemical bonds are continuously broken and created at

Mixed Monolayers from Functional Silicon Oxide

specific regions, also adds a new interesting dynamic to the modification of a surface at the molecular level. For example, the chemisorbed layers of H8Si8O12, D8Si8O12, and H10Si10O15 on gold surfaces contain empty adsorption sites in dynamic vacuum (no clusters present). However, monosubstituted clusters such as C6H13-H7Si8O12, although likely containing small open spaces within the chemisorbed layer on surface, do not appear to have the same kind of adsorption sites as the layers of H8Si8O12, D8Si8O12, and H10Si10O15. It is these adsorption sites that set the stage for the future reactivity of the initially adsorbed layer on the gold surface.

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Acknowledgment. Dow Corning and the NSF (DMR9727166) are thanked for financial support of this research. Portions of this research were performed at the National Synchrotron Light Source at Brookhaven National Laboratory that is supported by the Department of Energy (Division of Materials Sciences and the Division of Chemical Sciences). Professor Mark Banaszak Holl is also grateful for a Sloan Fellowship (1999-2002). Finally, J. Kulman is thanked for the initial evaporation of the gold onto the silicon. LA015522M