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Insights into the Formation Mechanisms of Si-OR Monolayers from the Thermal Reactions of Alcohols and Aldehydes with Si(111)-H1 Rabah Boukherroub, Sylvie Morin,† Paula Sharpe, and Danial D. M. Wayner* Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario, K1A 0R6, Canada
Philippe Allongue Physique des Liquides et E Ä lectrochimie, UPR CNRS No. 15, conventionne´ e avec l’Universite´ Paris VI, 4 Place Jussieu, T 22, 75005 Paris, France Received December 23, 1999. In Final Form: June 19, 2000 Hydrogen-terminated Si(111) reacts thermally at moderate temperatures with alcohols (RCH2OH) and aldehydes (RCHO) to form the corresponding Si-OCH2R films. The films are characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). While FTIR and XP spectroscopies suggest that films of similar composition are formed, AFM and the relative chemical stabilities of the organic films show that the two reactions do not result in structurally identical films. A mechanism for the reaction of the aldehyde with Si(111)-H is proposed which is analogous to the well-known hydrosilylation of aldehydes. The reaction proceeds either by nucleophilic addition/ hydride transfer or by a radical chain mechanism via adventitious radical initiation. The alcohol reaction is similar to the chemical etching of Si(111)-H by water and short-chain alcohols. This reaction proceeds by nucleophilic attack followed by loss of dihydrogen. Traces of ammonium fluoride or water on the surface result in etching of the terraces on a time scale which is much faster than the reaction of the alcohol but not of the aldehyde. This etching can be completely suppressed by the addition of chlorotrimethylsilane to the reaction mixture. This reagent quickly scavenges both water and fluoride from the surface and reaction mixture. It is suggested that this may be a useful reagent to scavenge undesirable nucleophiles during wet chemical modification of Si(111)-H.
The covalent attachment of monolayers to semiconductor surfaces is of growing interest as a potential route to surface passivation and to the incorporation of chemical/ biochemical functionality at interfaces. Chidsey and coworkers2-5 used free radical or photochemical initiation to form close-packed alkyl monolayers on the Si(111) surface by reaction with the hydrogen-terminated surface, Si(111)-H. In this case, the proposed mechanism is a surface propagated chain reaction in which an alkyl radical formed by the addition of an alkene to a surface silicon radical abstracts a hydrogen atom from an adjacent site. Lewis and co-workers6-8 formed Si(111)-C linkages in a two-step process by first chlorinating the Si(111)-H surfaces and then reacting the Si(111)-Cl surface with alkyl Grignard or lithium reagents. Recently, we showed that surfaces with similar characteristics can be obtained in a direct thermal reaction of alkylmagnesium bromides † Current address: Petrie Science Building 351, Department of Chemistry, York University, 4700 Keele St, Toronto, Ontario, Canada M3J 1P3.
(1) Issued as NRCC publication Number 43846. (2) Cicero, R. L.; Wagner, P.; Linford, M. R.; Hawker, C. J.; Waymouth, R. M.; Chidsey, C. E. D. Polym. Prepr. 1997, 38, 904. (3) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631. (4) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (5) Wagner, P.; Nock, S.; Spudich, J. A.; Volkmuth, W. D.; Chu, S.; Cicero, R. L.; Wade, C. P.; Linford, M. R.; Chidsey, C. E. D. J. Struct. Biol. 1997, 119, 189. (6) Bansal, A.; Li, X.; Lauerman, I.; Lewis, N. S. J. Am. Chem. Soc. 1996, 118, 7225. (7) Bansal, A.; Lewis, N. S. J. Phys. Chem. B 1998, 102, 1067. (8) Bansal, A.; Lewis, N. S. J. Phys. Chem. B 1998, 102, 4058.
with Si(111)-H.9 While it is too early to generalize, it appears there are a number of parallels between silicon surface chemistry and organosilicon molecular chemistry. For example, Effenberger et al.10 reported the photochemical hydrosilylation of aldehydes (RCHO) in a reaction with Si(111)-H to form Si(111)-OCH2R surfaces, a reaction that is known to occur with tris-trimethylsilylsilane,11 and demonstrated, for the first time, that it is possible to photopattern surfaces with reasonable spatial precision, by irradiation through a mask. On the other hand, there are examples of surface reactivity that does not have a parallel in the molecular organosilane literature. For example we showed that there is a direct reaction between Grignard reagents (RMgBr) and Si(111)-H to form what appears to be Si(111)-R monolayers.9 The mechanism of this reaction is not understood. In another example, Cleland et al. reported the direct functionalization of Si(111)-H by the thermal reaction with alcohols to give Si(111)-OR monolayers.12 A mechanism of formation of these monolayers was not suggested but must involve the oxidative addition of the alcohol and formation of H2. We now report the thermal reaction of an aldehyde to form Si(111)-OR surfaces which complements the (9) Boukherroub, R.; Bensebaa, F.; Morin, S.; Wayner, D. D. M. Langmuir 1999, 15, 3831. (10) Effenberger, F.; Gotz, G.; Bidlingmaier, B.; Wezstein, M. Angew. Chem., Int. Ed. 1998, 37, 2462. (11) Kopping, B.; Chatgilialoglu, C.; Zehnder, M.; Geise, B. J. Org. Chem. 1992, 57, 3994. (12) Cleland, G.; Horrocks, B. R.; Houlton, A. J. Chem. Soc., Faraday Trans. 1995, 91, 4001.
10.1021/la991678z CCC: $19.00 © 2000 American Chemical Society Published on Web 08/16/2000
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Figure 2. XP survey spectra of Si(111)-OC10H21 prepared by thermal reaction of Si(111)-H with (a) decanol and (b) decanal and (c) the survey spectrum of Si(111)-H.
Figure 1. ATR-FTIR of the C-H stretch region of Si(111)OC10H21 prepared by the thermal reaction of Si(111)-H with (a) decanol and (b) decanal.
photochemical approach.10 This reaction is compared to the thermal reaction of an alcohol using attenuated total internal reflection (ATR) FTIR, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). While the alcohol- and aldehyde-modified surfaces have similar spectroscopic characteristics, AFM and chemical stability studies provide insights into mechanistic differences between the two processes. Results and Discussion The Si(111)-H surfaces were prepared as previously reported.13,14 The asymmetric stretching bands (νa) in the FTIR spectra (Figure 1) obtained from surfaces reacting with decanol and decanal (2922 and 2920 cm-1, respectively) have integrated areas that are within 5% of each other. In our experience, this is similar to the variation in the infrared absorbance from the Si(111)-H stretch (2083.7 cm-1, Figure 1) from one sample to the next. XP survey spectra show an increase in the oxygen and carbon content of the reacted surfaces (Figure 2a,b) compared to the hydrogen-terminated surface (Figure 2c). The oxygen: carbon ratio appears to be higher for the surface prepared from the alcohol compared to that from the aldehyde. Highresolution XP spectra of the Si2p and C1s regions are similar (Figure 3). In addition to the main peaks for C1s (285 eV) and Si2p (99.2, 99.7 eV) these spectra are characterized by chemically shifted C1s (286 eV) and Si2p (100.3 eV) peaks as expected for the respective C-O and Si-O linkages. This assignment cannot be considered unequivocal as the instrument does not allow resolution of each of the peaks. However, we note that the decanol- and decanal-modified surfaces are both broadened on the high-energy side of the Si2p peak compared to a hydrogen-terminated sample. For both samples the amount of SiO2, which would appear near 103 eV, is below the detection limit for the experimental conditions used. When the two sets of data are overlayed (Figure 3e,f), the differences between the aldehyde- and alcohol-modified surfaces are not obvious; however, a contribution from a species such as Si-OH in (13) Higashi, G. S.; Chabal, Y. J.; Trucks, G. W.; Raghavachari, K. Appl. Phys. Lett. 1990, 12, 656. (14) Wade, C. P.; Chidsey, C. E. D. Appl. Phys. Lett. 1998, 71, 1679.
Figure 3. High-resolution XP spectra of the Si2p (left) and C1s (right) regions of Si(111)-OC10H21 surfaces prepared by the thermal reaction of Si(111)-H with (a), (b) decanol, and (c), (d) decanal. Overlays of the data for the alcohol (open squares) and aldehyde (lines) are shown in (e) and (f).
the reaction of decanol (which could account for the higher O:C ratio) cannot be ruled out. AFM images of the modified surfaces (Figure 4) show a striking difference in the topography of the two surfaces. The aldehyde reaction results in relatively clean terraces and steps (Figure 4b) that resemble the starting hydrogenterminated surface. On the other hand, the surface from the alcohol reaction has irregular step edges and pitted terraces, indicative of a reaction in which silicon is removed competitively both from the step edges and the terraces. A particularly extreme example of this etching is shown
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Figure 4. Contact AFM images (1 × 1 µm2) of Si(111)-OC10H21 surfaces prepared by the thermal reaction of Si(111)-H with (a) decanol and (b) decanal. The images were acquired at 1 Hz using sharpened silicon nitride cantilevers at a constant force of 1-2 nN.
in Figure 4a. This etching morphology is in contrast to the steady-state morphology of ammonium fluoride etched Si(111), which proceeds by a step flow mechanism.15-17 Chemical Stability of the Surfaces. The observation of alkyl C-H stretch in the IR spectra is consistent with the presence of an alkyl layer on the Si(111)-H surface. In the present case, evidence for a covalent Si-O-C link at the surface has been inferred from the chemically shifted Si2p and C1s peaks from high-resolution XPS. In the absence of other direct spectroscopic evidence chemical robustness has been taken as further indirect evidence of a covalent link.4 To test the robustness of the films, the chemically modified ATR crystals were subjected to the following sequential treatments: (a) rinsed with trichloroethane, (b) sonication in dichloromethane for 5 min., (c) boiling in chloroform for 1 h, (d) boiling in water for 1 h, (e) immersion in 1.2 N HCl for 1 h at 25 °C, (f) immersion in MilliQ water for 16 h, (g) immersion in 2% HF for 2 min, and (h) immersion in 2% HF for 10 min. Spectra of the alcoholand aldehyde-modified surfaces are shown in Figure 5. Although these surfaces appear to be very similar from spectroscopic analysis (IR, XPS), the clear differences in chemical stability and topography suggest that the two reactions do not result in structurally identical surfaces. Both surfaces are essentially unaffected by washing with halocarbons (Figure 5, spectra a-c), which we have found to be effective for the removal of physisorbed hydrocarbons. Boiling in water, which will lead to etching of any exposed silicon, results in the loss of about 50% of the hydrocarbon signal from the alcohol-modified surface (Figure 5A) but only about 8% of the aldehyde modified surface (Figure 5B). These surfaces are chemically stable toward further washing with trichloroethane or immersion in HCl or water for extended periods (Figure 5, spectra d-f). Finally, as expected both surfaces are unstable toward HF with the alcohol-modified surface being considerably less stable than the aldehyde-modified surface. The instability of both Si-OCnH2n+1 surfaces toward water and fluoride compared to Si-CnH2n+1 surfaces9 can be understood from known organosilicon chemistry; the former is more susceptible (15) Luo, H.; Chidsey, C. E. D. Appl. Phys. Lett. 1998, 72, 477. (16) Flidr, J.; Huang, Y. C.; Newton, T. A.; Hines, M. A. Chem. Phys. Lett. 1999, 302, 85. (17) Huang, Y. C.; Flidr, J.; Newton, T. A.; Hines, M. A. J. Chem. Phys. 1998, 109, 5025.
Figure 5. Infrared spectra of the C-H stretch region of the alcohol- (A) and aldehyde-modified (B) surfaces. The surfaces were subjected to the following treatments: (a) rinsed with trichloroethane; (b) sonicated in dichloromethane for 5 min; (c) immersed in boiling chloroform for 1 h; (d) immersed in boiling water for 1 h; (e) immersed in 1.2 N HCl at 25 °C for 1 h; (f) immersed in water at 25 °C for 16 h; (g) immersed in 2% HF for 2 min; (h) immersed in 2% HF for 10 min.
to hydrolysis or reaction with fluoride ion.18 Assuming that both the decanol- and decanal-modified surfaces are covalently linked by an Si-O-C bond to the surface, the greater stability of the aldehyde-modified surface suggests better packing of molecules, with more difficult access to the surface by chemical etchants. (18) Ojima, I. The Chemistry of Organic Silicon Compounds; John Wiley & Sons: New York, 1989.
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Grafting Mechanisms and Surface Linkage. In the case of decanol, it is reasonable to suggest that the reaction is similar to one for the reaction of water19 and alcohols20 (Scheme 1). In this case nucleophilic attack of the Si-H bond by an oxygen lone pair followed by loss of dihydrogen leads to an overall oxidative addition and the formation of the Si(111)-OR link. This is in contrast to the mechanism suggested for reactions of alcohols with porous silicon surfaces in which nucleophilic attack leads to the cleavage of Si-Si back-bonds.21 We have considered two possible mechanisms for the reaction of the aldehyde (Scheme 2). The first mechanism is initiated by a nucleophilic attack of the Si(111)-H surface by the aldehyde forming a pentavalent silicon intermediate or transition state followed by a {1,3}-hydride shift. The second mechanism is a surface radical chain reaction that is analogous to the Chidsey mechanism for the addition of alkenes.4 While it is difficult to rule out the possibility of adventitious radical initiation, two points argue against the radical process: (1) the rate constants for addition of silicon-centered radicals to carbonyls is about the same as for alkenes22 even though the direct thermal reaction of alkenes occurs at much higher temperatures (>200 °C) and does not result in the formation of uniform films;4 (2) the reaction of the aldehyde occurs even at 55 °C where the uncatalyzed thermal decomposition of the most likely radical precursors in the neat aldehyde (peracids or diacyl peroxides) is too slow to be kinetically viable.23 On the other hand, the better packing of the decanal-modified surface is more consistent with the radical chain mechanism. Furthermore, it is very difficult to remove all traces of peracid from neat aldehydes and it is possible that the decomposition may be catalyzed at the Si(111)-H surface. Therefore, adventitious radical initiation cannot be unequivocally ruled out, and it is not possible to distinguish between these alternatives at this time. It is somewhat puzzling that the reaction of the singlecrystal surface leads to the apparent disappearance of the of Si(111)-H stretch without the appearance of new absorptions (especially since the alkyl chain diameter, 4.2 Å, is larger than the Si-Si distance, 3.85 Å, so it is not possible to achieve a 1 × 1 packing density on the silicon surface).4 One explanation lies in the possibility that reaction with the surface can lead to heterogeneous broadening of Si(111)-H absorption. This has been observed in the electrochemical modification of Si(111)-H by aryldiazonium ions24 which, after subtraction of the original Si(111)-H FTIR spectrum, shows a broad peak centered near 2100 cm-1. Similarly, it has been found that simple physisorption of a hydrocarbon on the Si(111)-H surface results in an increase in the peak width from 1 cm-1 to about 30 cm-1.25,26 When the surface was (19) Allongue, P.; Costa-Kieling, V.; Gerischer, H. J. Electrochem. Soc. 1993, 140, 1018. (20) Newton, T. A.; Huang, Y.-C.; Lepak, L. A.; Hines, M. A. J. Chem. Phys. 1999, 111, 9125. (21) Kim, N. Y.; Laibinis, P. E. J. Am. Chem. Soc. 1997, 119, 2297. (22) Chatgilialoglu, C. Chem. Rev. 1995, 95, 1229. (23) Fujimori, K. In Organic Peroxides; Ando, W., Ed.; John Wiley & Sons: New York, 1992; p 318. (24) Allongue, P.; de Villeneuve, C. H.; Pinson, J.; Ozanam, F.; Chazaviel, J. N.; Wallart, X. Electrochim. Acta 1998, 43, 2791.
Boukherroub et al. Scheme 2
rinsed with a low molecular weight hydrocarbon, the absorption recovered to its original width and intensity. Given that this apparently weakly interacting system has such a dramatic effect, perhaps it is not surprising that the Si-H peak is broadened beyond detection in the alcohol- or aldehyde-modified surface. Origin of Etching at Decanol-Modified Surfaces. In NaOH solution at open circuit there is a competition between an electrochemical process (reaction 1) leading to the formation of H2 and a hydrolytic process (reaction 2) leading the breaking of Si-Si back-bonds.16,17,19,27 Both ultimately lead to the dissolution of silicon by hydrolysis of the hydroxylated surface through the formation of (HO)4Si (or F3SiOH in the case of fluoride etching). For short-chain alkoxides, etching of the silicon surface also has been observed. For example, the reaction of methoxide with Si(111)-H appears to be very similar to etching by hydroxide, although some steady-state grafting occurs.28 Hines and co-workers20 showed that the addition of only 2% 2-propanol to ammonium fluoride solution is sufficient to completely suppress the step-flow etching due to preferential grafting at steps. It was surprising, therefore, to see evidence of continued step-flow etching in the reactions of the alcohol. For steric reasons, one might expect the steady-state density of molecules attached at steps to increase as the length of the alkyl chain increases which should further limit step-flow etching. This should then lead (contrary to the observation) to predominant terrace modification (Scheme 1) without significant pitting.
tSi-H + OH- + H2O f tSiOH + H2 + OH-
(1)
(tSi)3SiOH + 3HX (X ) F, OH) f 3(tSiH) + SiX3OH (2) There are other discrepancies that suggest the possibility of competitive processes. The extent of etch pit formation is variable from one decanol-modified sample to the next, even for surfaces prepared from the same wafer on the same day. We found that the extent of pit formation depended on length of time that the alcohol was purged with argon before heating. Figure 4 represents one extreme in which the shard of silicon was removed from the etchant, blown “dry” in a stream of filtered nitrogen, placed in the previously deoxygenated neat (25) Lopinski, G. L.; DeJong, K.; Wolkow, R. A. To be submitted. (26) Ye, S.; Ichihara, T.; Uosaki, K. Appl. Phys. Lett. 1999, 75, 1562. (27) Allongue, P.; Kieling, V.; Gerischer, H. Electrochim. Acta 1995, 40, 1353. (28) Warntjes, M.; Viellard, C.; Ozanam, F.; Chazaviel, J.-N. J. Electrochem. Soc. 1995, 142, 4138.
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Figure 6. Contact AFM images (0.75 × 0.75 µm2) of Si(111)-OC10H21 surfaces prepared by the thermal reaction of Si(111)-H with decanol. The silicon shard was added to a Schlenk tube with previously deoxygenated decanol and purged with argon for an additional (a) 60 min or (b) 30 min before heating at 85 °C.
alcohol, and then heated at 85 °C. However, if the sample was further purged with argon for 1 h in the neat alcohol before being heated, it was not possible to distinguish between the surface derived from the alcohol and that from the aldehyde by AFM (compare Figures 6a and 4b). Purging for intermediate times led to surfaces which are etched to varying extents. After only 30 min of purging before heating, the step edges are jagged and there are a significant number of monatomic etch pits on the terraces which appear to be localized near the step edge (Figure 6b). Finally, if the silicon is dipped in 40% HF, rinsed, and then reacted with decanol, there is no evidence for terrace formation although the Si(111)OC10H21 surface forms. Given the discussion above and the experimental evidence that longer Ar purging suppress etching, it is clear that the etching of the surface (Figures 4a and 6) does not arise from decanol but most likely from residual water and/or dioxygen. While oxygen, in analogy with NH4F etching, is most probably responsible for pit initiation,29 water will etch the surface mainly from steps. This will widen the etch pits and lead to triangles due to a step-flow mechanism. Both processes compete with the decanol modification reaction, and the resulting morphology depends on the relative kinetics.20 One may consider two extreme cases. In the first, the decanol modification process is uniform and faster than etching, one would expect etch pits randomly distributed over terraces. This the basis of assumption the dissolution would indeed occur at defects of the layer under formation. This does not appear to be the case as the etch pits in Figure 6b tend to have a preference to be near a step edge. Alternatively, if etching is much faster, the surface will first etch and reach a morphology similar to that of a Si(111)-H surface NH4F etched in the presence of a trace amount of oxygen (based on a mechanism in which step-flow etching competes with pitting). Slow alkoxylation will then occur more or less uniformly. The images in Figures 4 and 6 are consistent with the latter case in which etching is faster. It appears that the reaction of decanol at kink sites on the step edge either is slower than that observed in the reaction of 2-propanol, which completely inhibits step-flow etching (29) Allongue, P.; Henry de Villeneuve, C.; Morin, S.; Boukherroub, R.; Wayner, D. D. M. Electrochim. Acta, in press.
at a concentration of only 2% in aqueous ammonium fluoride20 or is more susceptible to attack by water. To improve the surface morphology, Figure 6 clearly shows that Ar purging is one obvious possibility. To further prove that water/NH4F is indeed involved, we carried out a reaction with alcohol using a chemical scavenger but without purging with argon prior to heating. The scavenger, chlorotrimethylsilane (5% by volume), reacts very quickly with traces of water to form bis-trimethylsilyl ether and with traces of fluoride to form fluorotrimethylsilane. As a control, a sample from the same wafer was subjected to the same treatment but reacted in the absence of the drying agent (Figure 7a). It is clear from Figure 7b that the reaction in the presence of chlorotrimethylsilane produces a surface with little or no etch pit formation. Therefore, we conclude that etching during alcohol modification must arise from water and traces of oxygen. Surprisingly, the chemical stability of the improved alcohol-modified surface is unchanged (Figure 5A) even though it is now topographically indistinguishable from the aldehyde-modified surface. The simple mechanisms proposed in Schemes 1 and 2 require, however, that the chemical stability these surfaces should be the same. This result is consistent with a reaction of decanol leading to an organic film that provides better access of solvent molecules to the surface and/or a surface with more easily hydrolyzable structures. Conclusions The direct thermal reaction of decanol with Si(111)-H leads to incorporation of the alkoxyl function onto the surface by an oxidative addition to the Si-H with the concomitant loss of dihydrogen. The reaction of aldehydes with the Si(111)-H leads to surfaces that appear very similar both spectroscopically and topographically. The proposed mechanisms for reaction of decanal are analogous to the hydrosilylation of an aldehyde either by nucleophilic attack/hydride transfer or a surface radical chain process. The higher chemical stability of the decanal-modified surface compared to the decanol-modified surface may point to a radical chain reaction similar to the mechanism proposed for the reaction of alkenes,4 although the origin of the radical initiation under these conditions is not clear.
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Figure 7. Contact AFM images (3 × 3 µm2) of Si(111)-OC10H21 surfaces prepared by the thermal reaction of Si(111)-H with decanol (a) in the absence and (b) in the presence of 5% (v/v) of chlorotrimethylsilane. The silicon shard was added to a Schlenk tube with the deoxygenated alcohol and immediately immersed in the heating bath at 85 °C.
Surfaces prepared by the slower alcohol reactions are susceptible to etching by traces of water and/or fluoride. These traces of water and fluoride can be sequestered using a chemical scavenger of water and fluoride, chlorotrimethylsilane. This reagent appears to be otherwise unreactive and may turn out to be a useful reagent in thermal and photochemical reactions of Si(111)-H. Experimental Section Silicon wafers were purchased from Virginia semiconductor. ATR elements (25 × 5 × 1 mm3) were purchased from Harrick. All cleaning and etching reagents were clean room grade and were supplied by Amplex. All other reagents were obtained from Aldrich and were the highest purity available. Hydrogen-terminated Si(111) was prepared from shards of silicon (0.5-5.0 Ω cm, n-type) by cleaning in 3:1 concentrated H2SO4/30% H2O2 at 100 °C for 20 min, followed by copious rinsing with MilliQ water. The surfaces were etched with clean room grade 40% aqueous deoxygenated NH4F (15 min)14 and transferred, without rinsing, into the reaction vessel. The Si(111) attenuated total internal reflectance (ATR) crystals were cleaned by the standard RCA procedure prior to etching. We found that the RCA clean was more effective when the surfaces are reused many times, as judged by the reproducibility of the Si-H absorption intensity at 2083.7 cm-1.
Film Formation. A shard or ATR crystal of freshly hydrogen terminated Si(111) was placed in the previously deoxygenated neat decanol or decanal and allowed to react for 16 h at 85 °C. The monolayers were characterized by ATR-FTIR, XPS, and AFM. ATR-FTIR spectra were recorded using a Nicolet MAGNA-IR 860 spectrometer at 2 cm-1 resolution. The ATR crystals were mounted in a purged sample chamber with the light focused normal to one of the 45° bevels. Background spectra were obtained using a freshly hydrogenated surface. Atomic force microscopy was carried out using a Molecular Imaging PicoSPM equipped with an environmental chamber and a Nanoscope IIIa controller (Digital Instruments). The sample was kept in an argon environment for all measurements. AFM images were acquired at 1 Hz in contact mode using silicon nitride sharpened tips (Digital Instruments. 0.12 N m-1) at a constant force of 1-2 nN. All images are leveled but otherwise unfiltered. X-ray photoelectron spectra (XPS) were recorded on a Kratos Axis Instrument, using monochromated Al KR (1486 eV) radiation with detection on the surface normal. The pressure during analysis was about 5 × 10-8 Torr.
Acknowledgment. We gratefully acknowledge Dr. Robert Wolkow for helpful comments. LA991678Z
