Langmuir 2006, 22, 9619-9622
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Formation of Mixed Monolayers of Silsesquioxanes and Alkylsilanes on Gold Thomas M. Owens,† Kenneth T. Nicholson,† Daniel R. Fosnacht,† Bradford G. Orr,‡,§ and Mark M. Banaszak Holl*,†,§ Department of Chemistry, UniVersity of Michigan, Ann Arbor, Michigan 48109-1055, and Department of Physics and The Applied Physics Program, UniVersity of Michigan, Ann Arbor, Michigan 48109-1120 ReceiVed May 24, 2006 The formation of mixed monolayers of hydridospherosilsesquioxane clusters (H8Si8O12) and alkylsilanes (H2n+1CnSiH3) on Au has been investigated using X-ray photoelectron and reflection-absorption infrared spectroscopies and scanning tunneling microscopy. All of the techniques indicate the displacement of the majority of the siloxane clusters from the surface in favor of the alkylsilane.
Introduction The preparation of nonhomogeneous monolayers on metal surfaces offers a method for tailoring the properties of the monolayer to an intended application.1,2 Mixed monolayers of alkanethiols on gold have been synthesized mainly through coadsorption and displacement/exchange reactions.3-11 Control of the final composition of the monolayer can be achieved through variation of the exposure time to the second adsorbate. Additionally, mixed alkanethiol monolayers have been prepared through the adsorption of asymmetric dialkyl sulfides.12 Recently, a mixed monolayer of octahydridosilsesquioxane (H8Si8O12) and H13C6-H7Si8O12 on Au was synthesized through the displacement of the unsubstituted silsesquioxane in ultrahigh vacuum (UHV).13,14 Scanning tunneling microscopy (STM) images indicate formation of adsorbate-specific regions on the surface. Control of the composition of the mixed monolayer was achieved by adjusting the dosing pressure of H13C6-H7Si8O12. The maximum displacement of H8Si8O12 from the surface was found to be ∼60%. A monolayer of H13C6-H7Si8O12 is not displaced from the Au surface by the introduction of H8Si8O12 into the chamber. * To whom correspondence should be addressed. E-mail: mbanasza@ umich.edu. † Department of Chemistry. ‡ Department of Physics. § The Applied Physics Program. (1) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164-1167. (2) Wiencek, K. M.; Fletcher, M. J. Bacteriol. 1995, 177, 1959. (3) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 13301341. (4) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. J. Adhes. Sci. Technol. 1992, 6, 1397-1410. (5) Offord, D. A.; Griffin, J. H. Langmuir 1993, 9, 3015-3025. (6) Dunbar, T. D.; Cygan, M. T.; Bumm, L. A.; McCarthy, G. S.; Burgin, T. P.; Reinerth, W. A.; Jones, L., II; Jackiw, J. J.; Tour, J. M.; Weiss, P. S.; Allara, D. L. J. Phys. Chem. B 2000, 104, 4880-4893. (7) Chen, S.; Li, L.; Boozer, C. L.; Jiang, S. J. Phys. Chem. B 2001, 105, 2975. (8) Dishner, M. H.; Taborek, P.; Hemminger, J. C.; Feher, F. J. Langmuir 1998, 14, 6676. (9) Chung, C.; Lee, M. J. Electroanal. Chem. 1999, 468, 91. (10) Hobara, D.; Ota, M.; Imabayashi, S.-i.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1998, 444, 113-119. (11) Bumm, L. A.; Arnold, J. J.; Charles, L. F.; Dunbar, T. D.; Allara, D. L.; Weiss, P. S. J. Am. Chem. Soc. 1999, 121, 8017-8021. (12) Troughton, E. B.; Bain, C. D.; Whitesides, G. M. Langmuir 1988, 4, 365-385. (13) Nicholson, K. T.; Zhang, K.; Banaszak Holl, M. M.; McFeely, F. R.; Calzaferri, G.; Pernisz, U. C. Langmuir 2001, 17, 7879. (14) Schneider, K. S.; Nicholson, K. T.; Orr, B. G.; Banaszak Holl, M. M. Langmuir 2004, 20, 2250.
The synthesis of a silsesquioxane-alkylsilane mixed monolayer on Au has been achieved via exposure of monolayers of H8Si8O12 on Au to n-octylsilane (H17C8SiH3) in UHV. As in the case for H13C6-H7Si8O12, the silane monolayer cannot be displaced from the Au surface by the introduction of H8Si8O12. Interestingly, the two molecules exhibit strikingly different behavior with respect to mobility on the Au surface after Si-H activation. This can be attributed to the difference in bonding proposed for the two molecules on the gold surface. The mixed monolayer has been characterized by X-ray photoemission and reflection-absorption infrared spectroscopies (XPS and RAIRS, respectively) and STM. Experimental Section All experiments were performed at room temperature in a set of three custom-built UHV chambers. The chambers and their capabilities have been described previously.15-17 Soft X-ray photoemission spectroscopy was performed at beamline U8B at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. Valence band and Si 2p core level data were obtained with an incident photon energy of 160 eV. C 1s core level data were obtained with an incident photon energy of 342.5 eV. Synchrotron XPS spectra have been referenced to the Au 4f7/2 peak at -84.0 eV. Standard methods were employed for curve-fitting the Si 2p core level data. Conventional XPS data were obtained using a Mg KR source (hν ) 1256.3 eV). RAIRS data were acquired with a liquid N2 cooled MCT detector. UHV-STM images were acquired with a sample bias of -1.00 to -1.20 V and a tunneling current of 0.8-1.8 nA. H8Si8O12 was synthesized by the method of Agaskar and sublimed twice.18 The sample was further purified by extended exposure to UHV prior to use. H17C8SiH3 was purchased from Gelest, Inc. (Morrisville, PA) and underwent multiple freeze-pump-thaw cycles prior to use.19 Gold substrates for XPS and RAIRS consisted of Si wafers onto which a thin Cr layer was evaporated followed by ∼1500 Å of Au. A fresh layer of Au was evaporated in vacuo prior to exposure to either H8Si8O12 or octylsilane. Substrate purity was assessed by XPS prior to dosing. STM samples consisted of a piece (15) Lee, S.; Makan, S.; Banaszak Holl, M. M.; McFeely, F. R. J. Am. Chem. Soc. 1994, 116, 11819-11826. (16) Greeley, J. N.; Meeuwenberg, L. M.; Banaszak Holl, M. M. J. Am. Chem. Soc. 1998, 120, 7776-7782. (17) Schneider, K. S.; Nicholson, K. T.; Fosnacht, D. R.; Orr, B. G.; Banaszak Holl, M. M. Langmuir 2002, 18, 8116-8122. (18) Agaskar, P. A. Inorg. Chem. 1991, 30, 2707. (19) Octylsilane is reactive toward O2 and H2O. All manipulations must be carried out under an inert atmosphere.
10.1021/la061477c CCC: $33.50 © 2006 American Chemical Society Published on Web 10/13/2006
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Figure 1. (A) Si 2p core level photoemission spectrum (hν ) 160 eV) of a chemisorbed monolayer of H8Si8O12 on an evaporated Au surface. (B) Spectrum after exposure of the monolayer to 12 000 langmuirs of octylsilane. Spectrum B has been scaled so the H8Si8O12-derived feature has 40% of the intensity of the same feature in spectrum A. The spin-orbit doublet (Si 2p3/2 and Si 2p1/2) has not been removed from either spectrum. of mica onto which was evaporated ∼2500 Å of gold. After evaporation the samples were annealed at ∼673-773 K for 3 h. Sample purity was assessed by STM prior to dosing. All dosing was performed by back-filling the chamber with the reactant of choice via a variable sapphire leak valve. The chamber was evacuated back to UHV conditions before the introduction of the impinging species.
Results and Discussion As the interpretation of the mixed monolayer spectra requires knowledge of the single-component monolayer spectra, the major features of the H8Si8O1213,14,17,20,21 and octylsilane22-25 monolayer are briefly reviewed. The Si 2p core level photoemission spectrum for H8Si8O12 chemisorbed to Au is shown in Figure 1A. The feature is resolved into two distinct peaks at -101.1 and -102.3 eV after mathematical subtraction of the spin-orbit splitting26 (subtraction not shown) with an area ratio of 1:7.2. The fwhm values of these peaks are 0.57 and 1.14 eV, respectively. The area ratio is in good agreement with the predicted ratio of 1:7 for an intact cluster bonded to the surface via a single vertex.20,21 The valence band spectrum of H8Si8O12 on gold consists of several features. The most intense feature arises from the overlap of photoelectrons emitted from the Au 5d, O 2p, Si 3p, and Si 3s orbitals at a binding energy of ∼-3-6 eV. A less intense feature arising from the O 2s core level is observed at ∼-20 eV. The Au 6s photoelectrons are visible with a binding energy of 0 eV. Decomposition of the cluster is not observed on the Au surface. The formation of multilayers is not observed at room temperature. (20) Nicholson, K. T.; Zhang, K. Z.; Banaszak Holl, M. M. J. Am. Chem. Soc. 1999, 121, 3232. (21) Nicholson, K. T.; Zhang, K. Z.; Banaszak Holl, M. M.; McFeely, F. R.; Pernisz, U. C. Langmuir 2000, 16, 8396. (22) Owens, T. M.; Nicholson, K. T.; Banaszak Holl, M. M.; Su¨zer, S. J. Am. Chem. Soc. 2002, 124, 6800-6801. (23) Owens, T. M.; Su¨zer, S.; Banaszak Holl, M. M. J. Phys. Chem. B 2003, 107, 3177-3184. (24) Schneider, K. S.; Owens, T. M.; Fosnacht, D. R.; Orr, B. G.; Banaszak Holl, M. M. ChemPhysChem 2003, 4, 1111-1114. (25) Schneider, K. S.; Lu, W.; Fosnacht, D. R.; Orr, B. G.; Banaszak Holl, M. M. Langmuir 2004, 20, 1258-1268. (26) A ratio of 2:1 and a splitting of 0.6 eV were employed
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Figure 2. (A) RAIRS spectrum (700-3200 cm-1) of H8Si8O12 chemisorbed on Au. (B) Difference RAIRS spectrum after exposure of the H8Si8O12-terminated Au surface to octylsilane, using (A) as the background. The ν(CHx) features (between 2800 and 3000 cm-1) of the alkyl chain are visible in the spectra.
The RAIRS spectrum of H8Si8O12 chemisorbed to Au exhibits features at 2281, 1181, 1111, 1075, and 881 cm-1 (Figure 2A). These peaks are assigned as ν(Si-H), νas(Si-O-Si), νas(SiO-Si), νas(Si-O-Si), and δ(Si-H).20,21 On the basis of RAIRS characterization, ∼5-10% of the H8Si8O12 molecules desorb from the Au surface when the cluster overpressure is removed from the chamber.21 Preferential cluster desorption has been observed to occur at the face-centered cubic (fcc) lattice sites of the Au(111)-23×x3 reconstructed surface by STM.17 The monolayers are stable to O2, H2O, and ambient, though are displaced from the surface by exposure to H13C6-H7Si8O12 in UHV.13 Octylsilane chemisorbs to a clean Au surface via the activation of three Si-H bonds to form a stable monolayer which covers ∼96% of the surface.22,23 Multilayer formation is not observed at room temperature. On the basis of the fwhm of the Si 2p core level, the Si atoms are in an extremely homogeneous chemical environment. The valence band region of octylsilane on Au consists of peaks arising from electrons emitted from the Au 6s, Au 5d, C 2p, C 2s, Si 3p, and Si 3s orbitals. The features arising from the C 2s electrons of the alkyl chains, observed between -12 and -20 eV, exhibit a high degree of correlation with those observed for frozen octane.27 The features arising from the Au 5d, C 2p, Si 3p, and Si 3s orbitals occur between -2 and -12 eV. The RAIRS spectrum of octylsilane on Au exhibits features at 2856, 2879, 2925, and 2967 cm-1 assigned as ν(CHx).22 The most striking feature of the octylsilane monolayer spectrum is the absence of a feature corresponding to ν(Si-H), the most prominent feature in the solution IR spectrum (not shown). Upon exposing the H8Si8O12 monolayer to a saturating dose of octylsilane, a loss of intensity is observed for all of the silsesquioxane features in the RAIRS spectrum and the growth of ν(CHx) features is observed (Figure 2B) between 2850 and 3000 cm-1. The extent of displacement is difficult to extract from either the IR data28 or the photoemission data obtained with (27) Pireaux, J.-J.; Caudano, R. Phys. ReV. B 1977, 15, 2242. (28) The intensities of the IR peaks will be dependent on the strength of the particular dipole and its orientation relative to the Au surface.
Silsesquioxane-Alkylsilane Mixed Monolayers on Au
the synchrotron source. XPS data obtained with a conventional Mg KR source, on the other hand, allow facile determination of the extent of loss of O, and thus H8Si8O12, from the surface. Approximately 60% of the chemisorbed H8Si8O12 is displaced from the Au surface upon exposure to a saturating dose of octylsilane, as judged by the loss of integrated intensity for the O 1s core level spectrum.29 Assuming the octylsilane covers all available regions of the surface, the final C8H17SiH3:H8Si8O12 ratio is approximately 70:30, recalling that H8Si8O12 is not chemisorbed to 10-15% of the Au surface. Employing a synchrotron source with a photon energy of 160 eV, four features are visible in the Si 2p core level spectrum (Figure 1B). The features at -99.8 and -100.4 eV are the Si 2p3/2 and Si 2p1/2 components of the core level for the Si atom of the alkylsilane chemisorbed to the gold surface.22 The dramatic Si 2p spin-orbit splitting observed for the octylsilane-only monolayer on Au is still present. The two remaining Si 2p features are convolutions of the 2p3/2 and 2p1/2 peaks of two features arising from the chemisorbed silsesquioxane. Subtraction of the Si 2p1/2 component confirms the presence of three distinct features in the Si 2p3/2 spectrum, one arising from the silane (-99.8 eV) and two arising from H8Si8O12 (-101.1 and -102.3 eV) on the Au surface. The binding energies of the Si 2p core level features arising from H8Si8O12 are the same as those observed for the silsesquioxane-only monolayer. The feature at -101.1 eV is assigned as the Si atom bonded to the Au surface. The more intense feature at -102.3 eV arises from the remaining seven Si atoms in the silsesquioxane cluster. The fwhm for the Si 2p core level of the silane feature is ∼0.07 eV wider for the mixed monolayer than for the monolayer of octylsilane alone. This may indicate the silane Si atoms are in a slightly less homogeneous chemical environment than in the octylsilane-only monolayer. As this increase is small, the continuing high degree of homogeneity indicated could be interpreted as the result of segregation of octylsilane and silsesquioxane into distinct regions. Phase separation is known to occur in binary self-assembled monolayers of alkanethiols on Au,30 and the formation of species-specific regions on the Au surface has been proposed for the reaction of H13C6-H7Si8O12 with the H8Si8O12 monolayer.13 As will be discussed below, some degree of segregation may be occurring in the H8Si8O12/ octylsilane system. Parts A and B of Figure 3 are constant-current STM images of Au(111)-23×x3 after exposure to 525 langmuirs of H8Si8O12 in UHV. As previously noted, upon adsorption of H8Si8O12, the herringbone reconstruction of the clean Au surface is maintained.17 H8Si8O12 covers ∼85% of the surface with portions of the fcc regions containing unreacted Au. After exposure to octylsilane (Figure 3C,D), the STM image is identical to that of octylsilane adsorbed to clean gold. This indicates that domains of H8Si8O12 do not exist on the gold surface. Experiments using STM to monitor in situ growth of alkanethiol SAMs have been used to demonstrate that the Au reconstruction is maintained in regions of the surface where no alkanethiols are adsorbed.31 Alkanethiols are in fact mobile on the surface, and the surface reconstruction adjusts so as to exist in thiol-free regions. As H8Si8O12 adsorption does not result in the loss of the Au-23×x3 reconstruction and the thiol studies have shown clean Au to be adept at reconstructing, (29) This actually represents 65-70% of the surface being covered by octylsilane due to 5-10% cluster desorption observed when the overpressure of H8Si8O12 is removed from the chamber while the monolayer is being synthesized. (30) Smith, R. K.; Reed, S. M.; Lewis, P. A.; Monnell, J. D.; Clegg, R. S.; Kelly, K. F.; Bumm, L. A.; Hutchison, J. E.; Weiss, P. S. J. Phys. Chem. B 2001, 105, 1119. (31) Yang, G.; Liu, G.-y. J. Phys. Chem. B 2003, 107, 8746-8759.
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the lack of the reconstruction is evidence that no H8Si8O12-only domains exist on the surface. This does not eliminate the possibility of the presence of octylsilane-only domains on the surface. Given the extent to which the silsesquioxane has been displaced, it is likely that octylsilane-only regions exist. The observed increase in the fwhm of the Si 2p photoemission core level for the silane would arise from silane Si atoms in the mixed regions on the surface. Si atoms in the silane-only regions should maintain the 0.4 eV fwhm observed for the silane-only monolayer. In contrast, the H8Si8O12 cluster does not displace the alkylsilane from the Au surface. Neither new features nor loss of intensity of the silane features is observed by either RAIRS or XPS when a monolayer of octylsilane is exposed to a saturating dose of H8Si8O12. This indicates that the alkylsilane is more strongly bonded to the gold surface than is the silsesquioxane. In addition, the silsesquioxane clusters do not chemisorb or physisorb to the alkyl-terminated surface of the octylsilane monolayer at room temperature. The reason for the difference in the displacement of the two molecules from the surface is intriguing. At first glance, the difference suggests that the greater number of Si-Au bonds is responsible. Although reasonable, this neglects the difference in reactivity toward ambient. Upon removal from UHV, octylsilane monolayers undergo oxidation of the Si head to form a siloxane monolayer24,32 while the monolayer of H8Si8O12 is stable.20 More likely, the exchange reaction favors the displacement of the H8Si8O12 molecules from the surface because the intact monolayer does not fully cover the surface. Exposed boundaries exist in the fcc regions where molecular desorption has already occurred in the H8Si8O12 system.17 The reaction of octylsilane with the Au surface creates three H atoms which are mobile on the Au surface. On a clean Au surface, most of the H atoms recombine with another H atom on the surface and desorb as H2. In the confines of the mostly silsesquioxane covered surface, the exposed boundary is nearby and the H atoms can recombine with the H7Si8O12 species and desorb as H8Si8O12. The reaction of one H atom with a chemisorbed octylsilane molecule does not result in desorption as two Si-Au bonds still exist. The molecule can once again undergo Si-H bond cleavage and Si-Au bond formation, releasing the recaptured H atom. A different reaction mechanism for the reaction of H8Si8O12 with the gold surface involving a precursor adsorption has been proposed which minimizes the expected self-desorption.21 The proposed mechanism for the formation of the mixed monolayer suggests that although H8Si8O12-only regions do not exist on the surface, octylsilane-only regions should exist, especially in those regions which previously had fcc packing. Still remaining unanswered is why, with two dissimilar impinging species (H13C6-H7Si8O12 and C8H17SiH3), the final mixture of the molecule/H8Si8O12 monolayer is in a ratio of 60:40 in both systems.13,14 It is convenient to consider the system in two parts: the impinging species and the H7Si8O12/Au monolayer. Considering the impinging molecules, the only obvious trait H13C6-H7Si8O12 and C8H17SiH3 share is the presence of the alkyl chain. Nothing about the alkyl chain would suggest that it should limit the displacement reaction. Therefore, the H7Si8O12/Au half of the system appears to be the more fertile ground to explore for an explanation. As the Au(111)-23 × x3 reconstruction is not lifted by the chemisorption of H8Si8O12, adsorbed H7Si8O12 fragments can be found in three distinct regions of the surface. The Au(111)-23 × x3 surface is characterized by regions of fcc and hexagonal close-packed (hcp) stacking (32) Owens, T. M.; Ludwig, B. J.; Schneider, K. S.; Fosnacht, D. R.; Orr, B. G.; Banaszak Holl, M. M. Langmuir 2004, 20, 9636.
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Figure 3. STM images [(A) 999 × 999 Å and (B) 505 × 505 Å] of Au after exposure to 525 langmuirs of H8Si8O12. The Au-23 × x3 reconstruction can be discerned in each of the images. STM images [(C) 999 × 999 Å and (B) 505 × 505 Å] of the H8Si8O12/Au surface after exposure to 700 langmuirs of H3SiC8H17. After adsorption of octylsilane, features characteristic of H8Si8O12 adsorption have been supplanted by those characteristic of octylsilane on Au.
separated by “bridge” regions. The fcc, hcp, and bridge regions comprise approximately 39%, 26%, and 35% of the Au surface, respectively. H8Si8O12 has been shown to preferentially desorb from regions with fcc stacking under UHV;17 thus, the most plausible site for C8H17SiH3 adsorption is a combination of the fcc and bridge sites, which account for ∼74% of the gold surface. This is in good agreement with the final coverage of adsorbed C8H17SiH3 measured experimentally of 70%.
Conclusions Mixed monolayers of H8Si8O12 and octylsilane on Au have been synthesized in UHV through displacement of the silsesquioxane from the surface. The exchange reaction is self-limiting at ∼60% of H8Si8O12 removed, resulting in final surface area coverage of 70:30 C8H17SiH3/H8Si8O12. Although H8Si8O12 and octylsilane both chemisorb to a clean gold surface at room temperature via Si-H activation, the behaviors of the two molecules on the surface are unique. The silsesquioxane is mobile on the surface once Si-H activation has occurred and the alkylsilane appears to be static. The stability of the Si-Au bonds
formed by the reaction of octylsilane with a gold surface stands in stark contrast to the dynamic nature of the Si-Au bond formed by exposure of H8Si8O12 to a clean Au surface. STM images indicate that the Au-23 × x3 reconstruction does not persist after formation of the mixed monolayer. Acknowledgment. The NSF (Grant DMR-0093641) is thanked for financial support of this work. The research was carried out (in part) at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy (Division of Materials Science and Division of Chemical Sciences of the Office of Basic Energy Sciences) under Contract No. DE-AC02-98CH10886. D.R.F. appreciates the support of an IGERT fellowship from the NSF (Grant DGE9972776). Dr. K. A. Miller is gratefully acknowledged for assistance in sample preparation. J. Kulman is thanked for the initial evaporation of the Au onto the Si. Dr. K. S. Schneider provided illuminating discussions regarding the STM images. LA061477C
