Oxidation of Alkylsilane-Based Monolayers on Gold - Langmuir (ACS

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Oxidation of Alkylsilane-Based Monolayers on Gold Thomas M. Owens,† Bonnie J. Ludwig,† Kevin S. Schneider,† Daniel R. Fosnacht,† Bradford G. Orr,‡,§ and Mark M. Banaszak Holl*,†,§ Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055, Department of Physics, University of Michigan, Ann Arbor, Michigan 48109-1120, and The Applied Physics Program, University of Michigan, Ann Arbor, Michigan 48109-1120 Received February 11, 2004. In Final Form: July 30, 2004 The oxidation of alkylsilane monolayers on Au has been studied by X-ray photoelectron spectroscopy, reflection-absorption infrared spectroscopy, contact-angle measurements, and scanning tunneling microscopy. Exposure of the monolayers at 298 K to pure O2 or H2O (>5 × 10-5 Torr and >150 000 L) does not cause oxidation. Ambient atmosphere only causes oxidation if direct sight lines are maintained to the sample. Ozone exposure results in rapid monolayer oxidation. Oxidation initially occurs only at the Si atom, resulting in formation of a cross-linked siloxane monolayer that retains alkyl surface termination. Prolonged ozone exposures result in the oxidation and subsequent loss of the alkyl chain.

Introduction Alkylsilanes (RSiH3) react with gold surfaces to produce chemisorbed layers1-4 analogous to those formed from alkanethiols.5,6 Similar to the alkanethiols, alkylsilane monolayers can be made possessing a variety of alkyl chain lengths. X-ray photoelectron spectroscopy (XPS) indicates that the silicon headgroups bound to the gold are chemically homogeneous on the basis of the narrow full-widthat-half-maximum (fwhm) of 0.4 eV. However, reflection absorption infrared spectroscopy (RAIRS) indicates the alkyl chains are not as ordered within the monolayers relative to the alkanethiols. One of the most interesting comparisons to make between these systems is the oxidative stability. Exposure of alkanethiol self-assembled monolayers (SAMs) on gold to atmosphere and ambient light for 6 h results in substantial formation of sulfonate, sulfonite, sulfate, and/ or sulfite species.7-14 Although some variability has been * Author to whom correspondence should be addressed. E-mail: [email protected]. † Department of Chemistry. ‡ Department of Physics. § The Applied Physics Program. (1) Owens, T. M.; Nicholson, K. T.; Banaszak Holl, M. M.; Su¨zer, S. J. Am. Chem. Soc. 2002, 124, 6800-6801. (2) Owens, T. M.; Su¨zer, S.; Banaszak Holl, M. M. J. Phys. Chem. B 2003, 107, 3177-3182. (3) Schneider, K. S.; Owens, T. M.; Fosnacht, D. R.; Orr, B. G.; Banaszak Holl, M. M. ChemPhysChem 2003, 4,1111-1114. (4) Schneider, K. S.; Lu, W.; Fosnacht, D. R.; Orr, B. G.; Banaszak Holl, M. M. Langmuir 2004, 20, 1258-1268. (5) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (6) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (7) Schoenfisch, M. H.; Pemberton, J. E. J. Am. Chem. Soc. 1998, 120, 4502-4513. (8) Poirier, G. E.; Herne, T. M.; Miller, C. C.; Tarlov, M. J. J. Am. Chem. Soc. 1999, 121, 9703-9711. (9) Lee, M.-T.; Hsueh, C.-C.; Freund, M. S.; Ferguson, G. S. Langmuir 1998, 14, 6419-6423. (10) Zhang, Y.; Terrill, R. H.; Tanzer, T. A.; Bohn, P. W. J. Am. Chem. Soc. 1998, 120, 2654-2655. (11) Zhang, Y.; Terrill, R. H.; Bohn, P. W. Chem. Mater. 1999, 11, 2191-2198. (12) Scott, J. R.; Baker, L. S.; Everett, W. R.; Wilkins, C. L.; Fritsch, I. Anal. Chem. 1997, 69, 2636-2639. (13) Sandhyarani, N.; Pradeep, T. Chem. Phys. Lett. 2001, 338, 3336. (14) Sun, L.; Gardella, J. A., Jr. Langmuir 2002, 18, 9289-9295.

reported in the literature (and several experimental differences cited as the explanation for the discrepancies between studies), XPS studies indicate 24 h of atmospheric exposure is sufficient to oxidize all of the sulfur present within the monolayer.7,8 Alkanethiol SAMs exhibit no evidence of oxidation upon exposure to O2, H2O, and CO2.8 Ozone, however, is generally agreed upon as the primary atmospheric component responsible for monolayer oxidation.7 Oxidized films exhibit decreased order, as exemplified by infrared spectroscopy (IR) and are no longer strongly bound to the underlying gold substrate. Changing the monolayer headgroup from sulfur to silicon was anticipated to dramatically affect the monolayer oxidation. Whereas an RS- fragment will oxidize to form discrete molecular species such as RSO3- or RSO2-, the analogous oxidation products for the RSi3- fragment readily polymerize to generate a well-known class of siloxane polymers (RSiO1.5)n.15 A number of researchers have studied the formation of siloxane polymers on Au surfaces via reaction of trichlorosilanes and adsorbed oxygen species. Allara, Parikh, and Rondelez reported the formation of alkylsiloxane layers via dip coating alkyltrichlorosilanes onto hydrated Au surfaces.16 Sabatani et al. synthesized mixed siloxane-thiol monolayers on oxidized Au from octadecyltrichlorosilane and octadecanethiol.17 Finklea and co-workers investigated the electrochemical properties of alkylsiloxane layers on oxidized Au surfaces.18 Vallant et al. synthesized ultrathin silicon oxide films of controlled thickness on Au via repeated trichlorosilane/UV-ozone exposures.19 In each of these cases, the newly formed polymer net is proposed to be anchored to the Au substrate by adventitious hydroxyl groups on the Au surface. Olson et al. have employed a trimethoxysilane to synthesize siloxaneencapsulated Au nanoparticles suitable for use as surface(15) Baney, R. H.; Itoh, M.; Sakakibara, A.; Suzuki, T. Chem. Rev. 1995, 95, 1409-1430. (16) Allara, D. L.; Parikh, A. N.; Rondelez, F. Langmuir 1995, 11, 2357-2360. (17) Sabatani, E.; Rubinstein, I.; Maoz, R.; Sagiv, J. J. Electroanal. Chem. 1987, 219, 365-371. (18) Finklea, H. O.; Robinson, L. R.; Blackburn, A.; Richter, B.; Allara, D.; Bright, T. Langmuir 1986, 2, 239-244. (19) Vallant, T.; Brunner, H.; Kattner, J.; Mayer, U.; Hoffmann, H.; Leitner, T.; Friedbacher, G.; Schu¨gerl, G.; Svagera, R.; Ebel, M. J. Phys. Chem. B 2000, 104, 5309-5317.

10.1021/la0496385 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/24/2004

Oxidation of Alkylsilane-Based Monolayers on Gold

enhanced Raman spectroscopy substrates.20 Klabunde and co-workers have demonstrated the synthesis of silicon oxide nanowires from RSiH3 and water employing Au nanoparticles as a catalyst.21 Recently, Pallandre et al. demonstrated elegant pattern formation through a combination of electron-beam lithography and silanation of oxidized Si surfaces.22 Of these studies, only Klabunde reported an O/Si ratio (1.5:1) for the material produced. With these results in mind, we initiated an investigation of the oxidative behavior of the alkylsilane monolayers and have characterized the resulting oxidized films by XPS, RAIRS, contact-angle measurements, and scanning tunneling microscopy (STM). Similar to alkanethiol-based systems, alkylsilanes are stable to both water and oxygen exposure but react with ozone present in ambient atmosphere. The products of oxidation by ambient exposure are assigned as polysiloxane polymers of stoichiometry (RSiO1.5)n, with the extent of monolayer oxidation varying with ozone exposure. STM images of the oxidized films are particularly striking as they appear to indicate little to no covalent bonding between the oxidized film and the gold surface. This approach provides a method for forming a polysiloxane polymer net with all of the alkyl chains on a single side.

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Octylsilane (H17C8SiH3) was purchased from Gelest, Inc. (Morrisville, PA) and underwent multiple freeze-pump-thaw cycles prior to use.23 Introduction to ultrahigh vacuum (UHV) was accomplished through a variable sapphire leak valve calibrated prior to dosing. All exposures are reported in Langmuir (1 L ) 1 × 10-6 Torr s) and are uncorrected for the sensitivity of the pressure gauge. Gauges in the dosing chamber were turned off during dosing of the silanes to prevent molecular fragmentation. Three UHV systems were employed in this work. Conventional X-ray photoelectron and reflection-absorption infrared spectroscopies (XPS and RAIRS, respectively) were performed in a previously described UHV system.24 A liquid-N2-cooled MCT detector was used for RAIRS. A Mg KR source (hν ) 1253.6 eV) was employed for conventional XPS. Due to the presence of gold plasmons trailing the Au 4f core level, data for the Si 2p core level could not be obtained with the conventional source. Highresolution soft-XPS was performed at beamline U8B at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory. The experimental endstation and its capabilities have been described elsewhere.25 Si 2p core level and valence band spectra were obtained with an incident photon energy of 160 eV. C 1s core-level spectra were obtained with an incident photon energy of 342 eV. Curvefitting was performed with PHIMAT v4.0A and MatLab v4.2c1. O/Si ratios were calculated by fitting the O 1s and Si 2s core levels (conventional source) and correcting for the atomic sensitivity factor of each element. For the O 1s core level, the published atomic sensitivity factor was employed.26 For the Si 2s core level, spectra of the Si 2p and Si 2s core levels were acquired from a piece of clean Si(100)-2 × 1 with the same instrument and the ratio of the integrated areas determined (Si 2s/Si 2p ) 0.79/1). The published Si 2p core-level atomic sensitivity factor26 multiplied by the ratio of the integrated

Si 2s/2p core-level areas was used for the stoichiometry calculation. A previously described system was employed for STM experiments.27 Advancing H2O contact angles, θa(H2O), were measured with a KSV Instruments CAM 100 contact-angle meter for static drops. Oxygen (either 99.998% or 99.6%) was purchased from Matheson Gas (Montgomeryville, PA) and used without further purification. No difference in reactivity was observed between the two purity levels. H2O was sealed in a UHV-compatible sample container and underwent multiple freeze-pump-thaw cycles prior to use. Introduction to the UHV chamber was accomplished through a variable sapphire leak valve. Ozone was generated using a home-built silent electric discharge ozone generator. The ozone generator converts ∼10% of the O2 to O3. The O3/O2 mixture was admitted directly to the UHV chambers through a variable sapphire leak valve with a direct line of sight to the sample.28 The distance from the leak valve to the sample was ∼20 cm during IR experiments29 and ∼7.5 cm for photoemission experiments performed at the NSLS. Unfortunately, ozone doses do not appear to be directly comparable between the two chambers employed. O2 and H2O were dosed through a leak valve ∼7.5 cm away from the sample in both chambers. All leak valves were calibrated prior to dosing, and the pressure gauge in the chamber was off during dosing.30 Exposures are reported as O3/O2 doses and are uncorrected for the sensitivity of the pressure gauge. Ambient exposures were performed in two ways. In the first case, a sample was dosed by admitting ambient atmosphere to a UHV chamber through a precalibrated variable sapphire leak valve ∼7.5 cm from the sample. The pressure gauge in the chamber was off during dosing. The second procedure for ambient exposure was removal of the sample from the UHV system for controlled lengths of time. Several types of Au substrates were employed in the course of this investigation. For XPS and RAIRS, gold samples were produced by three methods: (1) A Si wafer onto which was evaporated a thin Cr layer followed by ∼200 Å of Au. A fresh layer of gold was evaporated onto the sample in vacuo immediately prior to use. (2) Substrates were purchased from Molecular Imaging, Inc. (Phoenix, AZ) which consisted of ∼1500 Å of Au evaporated onto mica substrates. Samples were annealed in vacuo through resistive heating of a piece of Si in contact with the back of the sample. (3) Au was evaporated onto freshly cleaved ASTM-grade green mica (Asheville-Schoonmaker Mica Co., Newport News, VA) in vacuo. Samples were annealed in vacuo by resistive heating of a piece of Si in contact with the back of the sample. In all cases, XPS was used to confirm the presence of Au and the absence of contamination. No difference was observed between the monolayers formed on different substrate types. All sample dosing and evaporation occurred in separate chambers. Samples for contact-angle measurements were prepared by method 1. Samples for STM were prepared by annealing a commercially fabricated sample of Au deposited on mica (Molecular Imaging). Sample annealing was accomplished by resistively heating a piece of Si(100) situated directly underneath the mica to ∼673-773 K. In a separate chamber, additional gold was evaporated onto the surface for ∼40 min at rate of ∼1 Å/s. Following Au deposition, the sample was further annealed (typically 3 h). Sample cleanliness and quality were assessed by STM prior to octylsilane exposure.

(20) Olson, L. G.; Lo, Y.-S.; Beebe, T. P.; Harris, J. M. Anal. Chem. 2001, 73, 4268-4276. (21) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Zaikovski, V.; Klabunde, K. J. J. Am. Chem. Soc. 2003, 125, 10488-10489. (22) Pallandre, A.; Glinel, K.; Jonas, A. M.; Nysten, B. Nano Lett. 2004, 4, 365-371. (23) CAUTION: Alkylsilanes react with O2 and H2O. All manipulations must be performed in an inert atmosphere. (24) Greeley, J. N.; Meeuwenberg, L. M.; Banaszak Holl, M. M. J. Am. Chem. Soc. 1998, 120, 7776-7782. (25) Lee, S.; Makan, S.; Banaszak Holl, M. M.; McFeely, F. R. J. Am. Chem. Soc. 1994, 116, 11819-11826. (26) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corporation: Eden Prairie, MN, 1992.

(27) Schneider, K. S.; Nicholson, K. T.; Fosnacht, D. R.; Orr, B. G.; Banaszak Holl, M. M. Langmuir 2002, 18, 8116-8122. (28) CAUTION: Ozone is a known health hazard and care must be taken to prevent exposure. Ozone should only be generated and used in a well-ventilated environment, such as a fume hood. In addition, ozone is damaging to electronic equipment. A protocol is available in the literature for trapping ozone on silica gel in a glass vessel at dry-ice temperatures. We chose not to employ this method due to the risk of explosion if the vessel is allowed to warm to near room temperature. (29) Decreasing the valve-sample distance to ∼7.5 cm resulted in no discernible effect. (30) Both a hot filament ion gauge and a cold cathode ionization gauge in operation in the chamber during dosing of O-containing gases were found to generate species capable of oxidizing the Si headgroup of the monolayer.

Experimental Section

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Figure 1. Si 2p core level of octylsilane monolayer on Au (A) as formed in UHV. (B) Octylsilane monolayer on Au after lineof-sight exposure to ambient atmosphere for 15 min. A photon energy of 160 eV was employed, and the spectra are referenced to the Au 4f7/2 core level at -84.0 eV.

Figure 2. Valence-band spectra of octylsilane on Au (A) as formed in UHV and (B) after line-of-sight exposure to ambient atmosphere for 15 min. The incorporation of O into the monolayer is evident from the appearance of photoelectrons at -25 eV emitted from the O 2s orbital. Spectra were acquired with a photon energy of 160 eV.

Results and Discussion Oxidation by Ambient Atmosphere. Exposure of a clean Au surface to >400 L of octylsilane generates a chemisorbed octylsilane monolayer.1 As illustrated in Figure 1A, the Si 2p core level is present at -99.8 eV with a spin-orbit splitting of 0.6 eV. The Si 2p feature has a fwhm of 0.4 eV, indicating the Si atoms of the monolayer are in a chemically homogeneous environment. Previous XPS and RAIRS studies indicate the octyl chains are oriented roughly perpendicular to the Au surface.2 When removed from UHV and exposed to ambient atmosphere, the octylsilane monolayer undergoes oxidation as indicated by a shift in the binding energy of the Si 2p core level to -102.2 eV (Figure 1B), the growth of the O 2s level at ∼ -25 eV (Figure 2), and the growth of the O 1s core level at -532.4 eV (not shown). The magnitude of the binding energy shift of the Si 2p core level is consistent with the formation of RSiO3 groups. When exposed to ambient atmosphere by removal from UHV for 4 h), this ratio increases to ∼4, implying relatively quick formation of a cross-linked siloxane (nominally [RSiO1.5]n) upon exposure, followed by a gradual increase in the extent of

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oxidation of the monolayer. Unfortunately, atmospheric contaminants typically contain oxygen; therefore, a distinction between oxidation and contamination for the longer exposure times cannot be made with certainty when oxidizing the films in this fashion. The Au 4f core level appears unchanged, although slightly diminished in peak height (and therefore area), and is consistent with increased attenuation of the gold photoelectrons. Little useful information can be obtained from the C 1s core level of the monolayers exposed to ambient atmosphere as adventitious carbon contamination has the same binding energy as that of the alkyl chains in the monolayers. The valence band, however, is very informative. The continued presence of a well-defined structure arising from C 2s photoelectrons in the valence band region indicates the octyl chains have remained intact for a monolayer exposed to ambient atmosphere for 15 min (Figure 2). The small changes observed in the structure of the C 2s feature likely arise from oxidation of the terminal methyl on the octyl chain (see below). Changes in the appearance of the valence band spectrum between -4 and -12 eV are also observed and are attributed to the presence of electrons emitted from the O 2p orbital. These electrons have a binding energy of ∼-7 eV and are convoluted into a composite feature arising from the Au 5d, Si 3s, Si 3p, and C 2p electrons. Upon exposure of clean Au (Figure 3A) to a saturating dose of octylsilane, a complex pattern is observed by STM (Figure 3B). The origin of the gray and black sinuous monolayer pattern arises from a strain mediated spinodal decomposition mechanism.4 The STM image is noteworthy for several reasons. Discernible in STM images are (1) the lack of any features indicating the retention of the Au reconstruction, (2) the presence of small islands (white regions) ∼2.5 Å in apparent height, and (3) the formation of irregular step edges (not shown).4 The islands are indistinguishable in height from single-step gold terraces and are appropriately construed as monolayer Au islands with appreciable octylsilane coverage.4 Step edge roughening is indicative of Au migration during monolayer formation. The Au atoms necessary to form these features are available upon relaxation of the Au(111)-23 × x3 reconstruction, which results in ejection of one or two Au atoms per unit cell.31 Ejected Au atoms are mobile on the surface until nucleating as islands within the condensed monolayer or adding to preexisting step edges. Island formation occurs relatively late in the formation of the monolayer.4 The features observed in Figure 3B are stable over time. Further exposure to octylsilane does not produce observable changes to the surface. Following removal from UHV and exposure to ambient atmosphere for 15 min, the STM image of the monolayer is drastically altered. In contrast to the serpentine pattern observed prior to oxidation, the post-oxidation STM image appears to be that of clean Au(111)-23 × x3 (Figure 3C). The image exhibits all the characteristics of the herringbone reconstruction. The Au islands observed in Figure 3 are no longer present, indicating surface migration of Au atoms recommences upon monolayer oxidation. The irregular step edges, however, remain following oxidation and recovery of the herringbone pattern (Figure 4). Additionally, flaws in the 23 × x3 reconstruction are occasionally observed. An interpretation of the STM images that invokes monolayer desorption is refuted by the photoemission data. Recall that, after oxidation, there is substantial signal arising from O 1s, C 1s, and Si 2p core-level photoelectrons present in the core-level XPS (31) Poirier, G. E. Langmuir 1997, 13, 2019-2026.

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Figure 4. UHV STM image of the octylsilane monolayer after oxidation via exposure of the sample to ambient for 15 min. The scale bar is 100 Å. Notice that the step edges are not smooth, but are “ruffled” (as indicated by the arrows) and no gold islands are present. Prior to monolayer formation, the step edges are smooth. During monolayer formation, ejected Au atoms migrate to the step edge and add epitaxially. At a critical monolayer density, the migration of Au atoms in retarded and islands are formed. The migration process appears to recommence upon oxidation, as the Au islands observed in Figure 3 are no longer present. Following oxidation, the 23 × x3 reconstruction is observed, although flaws exist.

Figure 3. UHV-STM images of the same initial Au(111) sample (different sample areas), following successive experimental steps. All images are 35 nm × 35 nm. (A) Clean Au(111) 23 × x3; sample bias (Vs) ) -0.99 V, tunneling current (IT) ) 204 pA. (B) Chemisorbed octylsilane monolayer formed on (a) following exposure to 50 L gaseous octylsilane in UHV; Vs ) -1.00 V, IT ) 262 pA. (C) Oxidized physisorbed alkylsiloxane monolayer formed following exposure of (B) to ambient atmosphere for 15 min; Vs ) 0.51 V, IT ) 172 pA.

data. Additionally, the valence-band region continues to display structures that are highly reminiscent of those observed for the unoxidized monolayer. Furthermore, scanning tunneling spectroscopy confirms the surface in Figure 3C is not clean Au.3 Regeneration of the Au(111)-23 × x3 reconstruction is interesting, considering no evidence of it exists following octylsilane adsorption. Specifically, the presence of Au islands indicate the surface has fully relaxed to Au(111)-1 × 1 during monolayer formation.32 Upon oxidation, the removal of Si-Au bonds allows the Au surface to reconstruct beneath the newly formed siloxane net. Regeneration of the reconstruction indicates the oxidized monolayer does not strongly interact with the underlying substrate. (32) For a case of a chemisorbed species leaving the Au reconstruction in tact, see: Schneider, K. S.; Nicholson, K. T.; Fosnacht, D. R.; Orr, B. G.; Banaszak Holl, M. M. Langmuir 2002, 18, 8116-8122.

To our knowledge, this is the first reported instance of the oxidative conversion of a strongly chemisorbed monolayer to a stable physisorbed monolayer.3 Rinses of hexane, acetone, and water have no detectable effect on the oxidized monolayer, as determined by XPS, consistent with the behavior observed for monolayers synthesized on Au surfaces from trichlorosilanes.16-20 As a fully oxidized alkylsilane monolayer should not be covalently bonded to the Au surface, it is somewhat surprising that it is not readily rinsed off by at least one of the solvents. It is possible that the oxidation is not complete and some Si-Au bonds remain which anchor the siloxane monolayer to the Au surface. However, on the basis of the core-level XPS data, less than 1% of the Si is unoxidized after a 15 min of line-of-sight exposure to ambient atmosphere. Flaws observed by STM in the Au surface reconstruction following oxidation are potential sites where Si-Au covalent bonds remain following oxidation. An H2O advancing contact angle (θa) of 93 ( 4° was obtained for an octylsilane monolayer exposed to ambient atmosphere for 15 min. Freshly evaporated Au removed from the UHV system simultaneously exhibited a contact angle of θa(H2O) ) 80 ( 2°. The above-described oxidation experiments all involved removing the monolayer sample from UHV and placing it in an ambient environment. Additionally, attempts were made to oxidize the alkylsilane monolayers in a more controlled manner by in vacuo exposure to common oxygen containing gases. Upon exposure to O2 (>150 000 L), H2O (>250 000 L), and lab ambient atmosphere (>17 000 L) in vacuo through a precision leak valve, the monolayers exhibit no evidence of oxidation. The O 1s core level is absent from the spectra, and no shift in binding energy is observed for any of the core levels. In addition, no new features or changes are observed in the valence band

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(A) RAIRS (3400-950 cm-1) of octylsilane monolayer

Figure 5. on Au prior to in situ ozone exposure. (B) RAIRS of octylsilane on Au after exposure to ∼3 × 107 L of O3/O2. (C) Difference spectrum showing the changes in the monolayer between (A) and (B). The ν(CH3) modes have changed to a much greater extent than the ν(CH2) modes.

spectrum of the monolayer. As exposure to these oxygencontaining gases is not sufficient to oxidize the monolayers, we have employed ozone, an excited oxygen species known to be present in the atmosphere, to oxidize the monolayers. This approach allows the oxidation to be carried out in vacuo and avoid difficulties caused by ambient contamination in quantifying the XPS data. In Vacuo Ozone Exposure. The data for in vacuo ozone exposure are presented below in two parts. Part 1 includes results for the complete oxidation of the monolayer. Part 2 presents data for partial oxidation of the monolayer. 1. Complete Monolayer Oxidation. Figure 5A is the RAIRS spectrum of a chemisorbed monolayer of octylsilane on Au prior to ozone exposure. The major features observed are attributed to the ν(CHx) modes at 2854, 2879, 2925, and 2967 cm-1. RAIRS data for the oxidation of the monolayer by ozone is shown in Figure 5B, and the difference spectrum is shown in Figure 5C. Following exposure to ∼3 × 107 L of O3/O2, peaks are observed at 1032, 1101, 1378, 1759, 2859, 2929, and 3225 cm-1. In addition, a negative feature exists at 2967 cm-1, the frequency at which the peak assigned as νa(CH3) is observed in the monolayer prior to oxidation. The peaks at 2854 and 2925 cm-1, assigned as ν(CH2) modes, are both shifted higher, indicating the alkyl chains are in a less-crystalline conformation. The new features at 1032 and 1101 cm-1 are attributed to ν(Si-O) vibrational modes and are in agreement with peaks observed for the reaction of octadecylsilane on TiO2.33 The feature at 3225 cm-1 is assigned as ν(OH), likely arising from insertion of an O atom into a C-H bond. The band at 1759 cm-1 is assigned as the ν(CdO) from a carboxylic acid. This position is almost identical to that observed for unassociated carboxyl (33) Fadeev, A. Y.; Helmy, R.; Marcinko, S. Langmuir 2002, 18, 7521-7529.

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Figure 6. Sequential RAIRS data for the oxidation of octylsilane on Au. The top spectrum is the monolayer immediately prior to ozone exposure. Changes in the ν(CHx) features and the appearance of the ν(OH) feature occur prior to the completion of the first set of scans after the introduction of O3/O2. The feature at 1358 cm-1 appears after ∼4 min of ozone exposure, while the feature at 1759 cm-1 appears after ∼9 min. The negative feature observed at 2902 cm-1 results from the loss of carbon contamination from the surface upon the reaction of octylsilane. O3/O2 pressure during ozone exposure was ∼1 × 10-2 Torr. O3/O2 doses are 2 min ) 1.2 × 106 L; 4 min ) 2.4 × 106 L; 5 min ) 3 × 106 L; 7 min ) 4.2 × 106 L; 9 min ) 5.4 × 106 L; 10 min ) 6 × 106 L; 12 min ) 7.2 × 106 L; 13 min ) 7.8 × 106 L; and 15 min ) 9 × 106 L.

groups, indicating that the exposed groups of the monolayer are not involved in hydrogen bonding.34 This assignment is consistent with the binding energy shifts observed in the O 1s and C 1s core levels by XPS (see below). The feature at 1378 cm-1 falls within the range in which ν(C-O) modes are observed and is tentatively assigned as such. Note that peaks still exists for all of the ν(CHx) modes in Figure 5B, indicating the continued presence of methyl groups even after oxidation. The loss of intensity in the peaks arising from the terminal methyl group is indicative of chemical alteration, changes in orientation, loss of material, or a combination of these effects.35 Although a carboxyl group is formed on the alkyl chain, this alteration does not occur concurrently with the changes in the ν(CHx) region. If the loss of intensity arises from a change in the orientation of the dipole with respect to the surface, a feature arising from δ(CH3) is expected in the spectrum due to the orientation of these vibrational modes with respect to one another. New features are observed in the RAIRS spectrum at 1378 and 1759 cm-1 while oxidation is occurring. The appearance of these features does not occur concurrently with the changes observed in the alkyl stretching region (Figure 6, O3/O2 pressure: ∼1 × 10-2 Torr). Although the feature at 1378 cm-1 is in the region in which δ(CH3) has been observed for alkanethiol monolayers,36,37 such an assign(34) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds; 5th ed.; John Wiley and Sons: New York, 1991. (35) Pearce, H. A.; Sheppard, N. Surf. Sci. 1976, 59, 205-217. (36) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569.

Oxidation of Alkylsilane-Based Monolayers on Gold

ment would be contrary to the distinct temporal separation of the effects. Growth of the ν(O-H) feature occurs concurrently with the observable changes in the alkyl stretching modes. Thus, one could hypothesize the first step in oxidation of the alkyl chain is insertion of an O atom into the C-H bond to form an alcohol. This transformation may also reorient the alkyl chain with respect to the surface to minimize energy. Such a change in orientation would account for the changes seen in the ν(CH2) features which occur simultaneously with the growth of the ν(O-H) feature. The negative feature observed at 2902 cm-1 (most apparent in the initial spectrum) arises from loss of carbon contamination from the surface upon reaction of the octylsilane. The alkylsiloxane polymer net formed by the oxidation of the octylsilane monolayer is expected to be more stable than the oxidized sulfonate and sulfinite species observed following alkanethiol-SAM oxidation. Oxidized alkanethiols are known to desorb in ambient atmosphere,7 solution,38-40 and vacuum upon X-ray exposure.41,42 Spectroscopic data investigating alkylsilane monolayer oxidation indicate no loss of material in a vacuum. Alkylsilanebased monolayers monitored for 15 min following ozone exposure to investigate the possibility of material loss showed no change in the RAIRS spectra. Additionally, no loss of material is observed upon prolonged X-ray irradiation (>24 h), a condition known to result in desorption of oxidized alkanethiols. The presence of the O 1s core level (Mg KR source) in the XPS spectrum clearly indicates incorporation of oxygen into the monolayer.43 Interestingly, the data exhibits a dependence between the rate at which ozone is dosed and the final distribution of species present in the monolayer. When exposed to a high O3/O2 flux (pressure of ∼1 × 10-2 Torr, referred to as “high flux”) the O 1s core level exhibits a distinct asymmetry toward lower binding energy. This asymmetry is absent for an equivalent ozone exposure when a lower O3/O2 flux (1 × 10-4 Torr, referred to as “low flux”) is employed. Although the O 1s core-level line-shape changes, the Si line shape is static. This suggests formation of stable, flux-independent silicon species within the monolayer. The C 1s core level (Mg KR source) exhibits a decrease in intensity at the initial peak maxima of -284.2 eV and an increase at ∼-287 eV (resulting from at least two new peaks growing in) for both high and low O3/O2 fluxes.44 Greater ozone exposures result in loss of total integrated area from these C 1s core levels. By XPS, the monolayer sample in Figure 5B (RAIRS of the monolayer after 3 × 107 L O3/O2 at high flux) still retains ∼75% of the C 1s integrated area observed for an unoxidized monolayer. This suggests that, on average, two C atoms have been lost from each alkyl chain in the monolayer. The binding energy shifts observed in the O 1s and C 1s core levels are (37) Dunbar, T. D.; Cygan, M. T.; Bumm, L. A.; McCarty, 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. (38) Garrell, R. L.; Chadwick, J. E.; Severance, D. L.; McDonald, N. A.; Myles, D. C. J. Am. Chem. Soc. 1995, 117, 11563-11571. (39) Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305-5306. (40) Tarlov, M. J.; Newman, J. G. Langmuir 1992, 8, 1398-1405. (41) Frydman, E.; Cohen, H.; Maoz, R.; Sagiv, J. Langmuir 1997, 13, 5089-5106. (42) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O.; Ulman, A. Langmuir 2001, 17, 8-11. (43) See Supporting Information for XPS wide scans before and after ozone-induced oxidation. (44) The Si 2s core level exhibits a shift to higher binding energy but is difficult to quantify due to its low photoelectron cross section and broad natural line width.

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consistent with formation of carboxyl moieties within the monolayer.45,46 Si 2p core-level photoemission data acquired with a photon energy of 160 eV exhibit a final binding energy shift of -102.2 eV following exposure to ozone in vacuo. This binding-energy shift is consistent with formation of RSiO3 species. Further exposure to ozone yields no significant changes in the Si 2p core level. Incorporation of oxygen into the monolayer is evident from the growth of the O 2s feature in the valence band spectrum at ∼-25 eV. An increase in intensity is observed at ∼-5 eV, likely arising from electrons emitted from the O 2p orbital (generally observed at ∼-7 eV), thus changing the structure of the leading edge of the major spectral feature. Further ozone exposure results in a single feature in the Si 2p spectrum at the same binding energy as the furthest shifted feature in Figure 1B (oxidation by ambient exposure). A monolayer of octylsilane on Au exposed to the high flux of O3/O2 for 1 h exhibits θa(H2O) ) 71 ( 2°. This value is lower than that observed for the octylsilane monolayer exposed to ambient for 15 min (θa(H2O) ) 93 ( 4°), indicating the presence of a more-hydrophilic surface. The contact-angle measurements for the oxidized alkylsilane monolayer are consistent with recent reports by Pallandre and co-workers (θa(H2O) ) 70°) for oxidized silane-based monolayers with carboxylic termination on SiO2.22 However, other reports for carboxylic surfaces have differed, and values of 40° (oxidized 16-heptadecenyltrichlorosilane on SiO2) and ∼0° (16-mercaptohexadecanoic acid on Au) have also been observed.47,48 Given the disparate reports in the literature, the contact-angle data do not allow definitive conclusions to be drawn. 2. Partial Monolayer Oxidation. To further examine the oxidation reaction, a monolayer of octylsilane on Au was exposed to ozone in doses insufficient to achieve full oxidation. XPS data were collected for the O1s (conventional source, Figure 7A), C 1s (conventional (Figure 7B) and synchrotron sources (Figure 8, Panel II, A-E)), Si 2s (conventional source) and Si 2p (synchrotron source, Figure 8, Panel I, A-E) core levels, and the valence band region (synchrotron source, Figure 9). Care must be taken in interpretation and comparison of the data acquired with the conventional source and the data acquired with the synchrotron source. Although the system under study is nominally identical, the characteristics of the source influence the information obtained. Due to the surface sensitivity achieved by employing a photon with an energy ∼60 eV above the binding energy of the core level, quantitative measurements employing the synchrotron data are complicated by the changing thickness of the monolayer, which affects the absolute depth from which photoelectrons will be detected. The conventional source allows a more facile determination of changes in the integrated area of peaks, at the expense of surface sensitivity and the ability to measure the Si 2p core level. The O 1s and C 1s core levels are presented in Figure 7 for the partial oxidation of the octylsilane monolayer at the low flux rate. Each spectrum (except for the pre-ozone exposure; I, blue) covers an exposure of ∼1.08 × 106 L of O3/O2. For reference, the O:Si ratio is 2.85:1 for the first data set (II, pink). A decrease in the integrated area of the C 1s peak is observed, though there is a slight amount of (45) Briggs, D.; Beamson, G. Anal. Chem. 1992, 64, 1729-1736. (46) Briggs, D.; Beamson, G. Anal. Chem. 1993, 65, 1517-1523. (47) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. Soc. 1989, 111, 5852-5861. (48) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335.

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Figure 7. (A) O 1s and (B) C 1s core levels (conventional Mg KR source, hν ) 1253.6 eV) of an octylsilane monolayer on gold exposed to successive dose of O3/O2. After monolayer formation (I) exposure to ozone results in the oxidation of the alkyl chain and the eventual loss of C from the monolayer, determined from the integrated area of the C 1s core level.

Figure 8. (Panel I, A-E) Si 2p and Au 4f (hν ) 160 eV), and (Panel II, A-E) C 1s (hν ) 342 eV) core levels of an octylsilane monolayer exposed to successive dose of O3/O2. (A)No O3/O2, (B) first exposure, (C) second exposure, (D) third exposure, and (E) fourth exposure. Each ozone dose was ∼72 000 L of O3/O2. Peak intensity has been normalized to the Au 4f7/2 core level at - 84 eV (Panel I) and the Au Auger feature at -275 eV (Panel II).

tailing. Further ozone exposures (III, light blue, and IV, brown) result in a decrease in intensity of the peak maxima of the unoxidized peak at -284.2 eV and the continued growth of a shoulder at ∼-287 eV. Following ∼1.2 × 107 L O3/O2 exposure (V, red), no further decrease is observed in the C 1s intensity at the peak maximum and no further increase is observed in the O 1s peak area. Photoemission data obtained with a synchrotron source are informative with regards to the oxidation of the monolayer (Figure 8). Panel I highlights the changes in

the Si 2p core level, and panel II features the changes in the C 1s core level. The peak intensity has been normalized to the Au 4f7/2 core level at -84 eV (Panel I) and the Au Auger feature at -275 eV (Panel II). After the first exposure, the Si 2p core level is partially oxidized, exhibiting a decrease in intensity at the original binding energy and substantial new signal at higher binding energy (Figure 8-I). At least three distinct features are observable in the Si 2p core level in Figure 8-I-B. Oxidation of the Si atoms in the monolayer clearly does not proceed

Oxidation of Alkylsilane-Based Monolayers on Gold

Figure 9. Valence-band region (hν ) 160 eV) of the octylsilane monolayer exposed to successive dose of O3/O2. (A) No O3/O2, (B) first exposure, (C) second exposure (D) third exposure, and (E) fourth exposure. Each ozone dose was ∼72 000 L of O3/O2.

in a uniform fashion. Some of the Si atoms have reached their final oxidation state while other Si atoms remain unoxidized. Following the second exposure to ozone (8I-C), all of the Si atoms in the monolayer are oxidized and the Si 2p core level exhibits no further change with subsequent ozone exposures. However, the C 1s core level (Figure 8-II-C) begins to undergo changes at this point. A slight tailing is observed to the high-binding-energy side of the C 1s core level, indicating some oxidation of the alkyl chain has occurred. Further O3 exposure results in the continued growth of the C tail feature and a decrease in the overall height and integrated area of the unoxidized peak (relative to the Au Auger feature at ∼-276 eV) (Figure 8-II-D). A fourth ozone exposure resulted in a further loss of C (Figure 8-II-E).49 Spectra of the valence band region (Figure 9) corroborate the loss of C from the alkyl chains during the third ozone exposure. No significant change is observed in the C 2s region between parts A and B of Figure 9, in accord with the C 1s core level. The O 2s level (∼-25 eV) is observable after the first ozone exposure. The second ozone exposure (Figure 9C) results in a substantial increase in the intensity of the O 2s level, as well as an apparent decrease in the C 2s region. The shape of the feature between -2.5 and -12.5 eV changes concurrent with the intensity loss in the C 2s region,. This is understandable as electrons emitted from the C 2p orbitals of the alkyl chains contribute to this feature. In particular, the change in the high-binding-energy side of the feature likely arises from the loss and/or oxidation of C atoms. Changes in the structure of the leading edge (low-binding-energy side) of (49) The difference in the amount of C remaining for the experiments performed in the two chambers may arise from differences in the ozone flux that reaches the sample.

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the feature can be assigned to the lone pair electrons of the O 2p orbitals, which typically have a binding energy of ∼-7 eV. Further changes in the feature arise from the shift to higher binding energy that will occur for the Si 3s and 3p orbitals upon oxidation of the Si atoms. The final two ozone exposures (Figure 9D and E) continue the trend observed in Figure 9C, namely the loss of intensity for C-related features and an increase in intensity of the O-related features. While C remains on the surface following the fourth ozone exposure, the C 2s region no longer exhibits the structure observed for intact alkyl chains. New in Figure 9D is the loss of intensity in the Au 6s feature (∼0 eV). This intensity loss was not observed for oxidation of the monolayer by ambient exposure and may be attributed to oxidation of the surface Au atoms, though no change is apparent in the Au 4f core level (conventional or synchrotron). A second possibility is the interaction of the remaining SiOx layer with the Au 6s orbital to a greater extent than the alkylsilane or alkylsiloxane monolayers result in a change in the binding energy or shape. Stoichiometry of the Oxidized Monolayer. An interesting comparison can be made between the O/Si ratios for the ambient-exposed and ozone-exposed cases. Recall the O/Si ratio for the ambient-exposed monolayer is 1.6 ( 0.3. Prior to ozone exposure, no O is detectable in the monolayer. Following exposure to 360 000 L of O3/ O2, the O/Si ratio is found to be 1.6:1 and a small decrease in intensity at the peak maxima of the C 1s core-level feature is observed. However, the decrease in intensity is offset by the growth of a tailing feature on the high bindingenergy-side of the core level, and the overall integrated area of the C 1s core level remains constant throughout the oxidation of the Si headgroup. The O/Si ratio is roughly consistent with a chemical formula of (RSiO1.5)n, the same stoichiometry observed for monolayers oxidized by short (4 h) exposures of the octylsilane monolayer to ambient atmosphere. The formation of carboxylic acid groups upon oxidation of the alkylsilane monolayer is consistent with all of the spectroscopic data available. Recall that peaks exist in the post-oxidation RAIRS spectrum, indicating the continued presence of methyl groups and, thus, the presence (50) Uosaki, K.; Quayum, M. E.; Nihonyanagi, S.; Kondo, T. Langmuir 2004, 20, 1207-1212.

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Table 1. possible species

predicted O/Si ratio

(RSiO1.5)n (R-OSiO1.5) n (HO-R-SiO1.5) n RSiO3 (HOOC-R-SiO1.5) n (HO-R-OSiO1.5) n (HOOC-R-OSiO1.5)n HOOC-R-OSiO3

1.5 2.5 2.5 3 3.5 3.5 4.5 6

of at least two functional groups at the monolayer surface. As O3/O2 exposure does not oxidize the surface evenly, it is likely that multiple chemical species exist after alkylchain oxidation commences. The contact-angle data indicate the possibility that other chemical species may be present on the surface. We caution that interpretation of contact-angle data with respect to species present can be fraught with complications. Factors known to influence the contact angle of similar systems include the pH of the H2O employed, degree of order in the alkylchains, exposed methylene groups, and physisorbed layers (such as H2O or contamination).51 Nevertheless, we would be remiss not to enumerate some of the possible combinations of species which could also produce the observed ozoneproduced O/Si ratio of 3.2:1. Plausible species and their expected O/Si ratio are listed in Table 1. If present, these species are either below our level of detection or we are unable to differentiate them from carboxylic-acid-terminated siloxane or from one another by either RAIRS or XPS. Comparison of the oxidation results by ambient exposure with those obtained by ozone exposure is informative. The Si atoms of the monolayer can be oxidized to the same degree by exposure to ambient atmosphere or ozone. The ozone also reacts directly with the alkyl chains. Surprisingly, reaction with the alkyl chains does not appear to occur to any appreciable extent until the Si atoms have been fully oxidized. Penetration of ozone into the monolayer may be a result of the degree of disorder present in the alkyl chains, as implied by the position of the νa(CH2) feature in the RAIRS spectrum. Atmospheric oxidation, presumably by ozone, rapidly oxidizes the silicon headgroup without causing any alkyl chain damage, as indicated by XPS. Further analysis, either of the total carbon content or of the degree of silicon and alkyl chain oxidation, is difficult owing to other contaminants present in the atmosphere. We caution the reader once again that the ozone exposures do not appear to be directly comparable between the different chambers employed and that a dependence on flux exists. The 10% O3/O2 mixture fully oxidizes the silicon headgroup following 144 000 L exposure without causing alkyl chain damage, as judged by XPS. Assuming a typical 100 ppb ozone concentration in the atmosphere, this corresponds to a 1.44 × 1011 L dose of ambient atmosphere or an exposure time of ∼3.2 min. Thus, complete destruction of the carbon chains observed in Figure 8-II-E should correspond to an ∼15 min atmospheric exposure. On the basis of the valence-band data for the ambient-exposed monolayer (Figure 2), oxidation of the alkyl chains is clearly retarded during atmospheric exposure. The O3/O2 exposure in the RAIRS data presented in Figures 5 and 6 are greater than that for the photoemission data, yet they continue to display clear evidence of alkyl chain retention. Several groups have reported on alkanethiol SAM oxidation by ozone. Poirier and co-workers report a loss (51) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370-1378.

in C 1s intensity after 24 h ambient exposure or 120 L of O3 in vacuo for decanethiol on Au.8 For undecanethiol on Au, Schoenfisch and Pemberton report the loss of approximately half of the C 1s core-level intensity after 24 h of ambient atmosphere exposure (∼41 ppb O3).7 The loss in C 1s intensity presented by Schoenfisch and Pemberton is similar to those reported by Poirier and coworkers. Ferguson and co-workers report a decrease (∼63%) in the C/Au ratio after 6 days of ambient exposure in the dark for a monolayer of dodecanthiol on Au substrates with small grain sizes.9 Samples produced on Au films with larger grain sizes exhibited a substantially smaller (∼16%) decrease in this ratio over the same time period. The loss of C 1s intensity is attributed to the desorption of oxidized thiol molecules.9 All of these groups focus on oxidation of the sulfur headgroup and note loss of carbon intensity; however, none comment on oxidation of the carbon chain. It is possible that the short XPS acquisition times employed to limit monolayer damage for the alkanethiol studies may reduce the signal-to-noise level such that the features arising from the oxidized C are below the levels of detection. Bohn and co-workers observed oxidation of the S headgroup but no changes in the ν(CHx) region of the IR spectrum of a monolayer of hexadecanethiol on Au (550 ppb O3 for 30 min.).11 This study stands out as the only case in which carbon loss is not observed. Sun and Gardella observed a decrease in the intensity of the C 1s core level and a chemical shift indicative of partial oxidation of the alkyl chain after exposing a decanethiol monolayer on Au to 10 min of ∼80 ppm ozone at ambient pressure.14 Hutt and Leggett have reported the appearance of C 1s corelevel features at higher binding energy for the UVphotooxidation of alkanethiols on Au, which are consistent with oxidation of the alkyl chain, though the authors do not draw this conclusion.52 Our work clearly indicates the occurrence of both oxidation of the alkyl chain and net carbon loss for alkylsilane monolayers exposed to ozone. The octylsilane monolayer serves as an interesting control for the alkanethiol system since the silicon headgroup is less sensitive to prolonged X-ray exposure. Conclusions The oxidation of alkylsilane monolayers on Au has been investigated by XPS, RAIRS, contact-angle measurements, and STM. The monolayers oxidize readily in ambient atmosphere when a line of sight exists between the sample and the flow of ambient gases, with full oxidation occurring over a period of several hours. The extent of oxidation varies slightly among the samples, indicating reaction with an atmospheric component which may vary with time. Ozone has been demonstrated to be capable of oxidizing the Si atoms in alkylsilane monolayers. In contrast to analogous alkanethiol SAMs, the alkylsilane monolayers do not desorb following oxidation. The silane monolayer cross-links to form an RSiO3/2 network on the gold surface. Prolonged exposure to ozone results in the oxidation and eventual loss of C from the alkyl chains. The concentration of ozone in ambient atmosphere is not sufficiently high enough to cause the oxidative loss of alkyl chains on a time scale of several hours, though it is likely oxidation of the alkyl chains occurs to some degree. Long-term exposure, however, likely leads to oxidation and ultimately loss of the alkyl chains. This is an important consideration when designing molecular devices incorporating similar molecules. (52) Hutt, D. A.; Leggett, G. J. J. Phys. Chem. 1996, 100, 66576662.

Oxidation of Alkylsilane-Based Monolayers on Gold

As in the case of alkanethiol SAMs, the alkylsilanebased monolayers exhibit reactivity toward ozone, though the results of the reaction are dramatically different in the two systems. For the thiol-based monolayers, reaction with ozone results in formation of a sulfonate species, which eventually desorbs. For the silanes, a stable alkylsiloxane layer is formed. Further ozone exposure results in the loss of the alkyl chains, leaving a siloxane monolayer on the Au surface. STM images suggest the Au23 × x3 reconstruction is lifted during formation of the octylsilane monolayer, expected for a chemisorption process. Au atoms ejected during relaxation of the surface form islands or add to step edges, both of which are also covered by the silane monolayer. Surprisingly, the Au surface re-adopts the reconstruction as the monolayer is oxidized from discrete silyl surface groups to a cross-linked siloxane network. The STM data presented in this paper for the octylsilane monolayer exposed to ambient atmosphere still have intact alkyl chains present, though they are not observed by STM. The implications of this statement are that the alkyl chains, present in both the oxidized and unoxidized monolayer, are always transparent to STM imaging. This implies that the pattern observed in Figure 3B is a result of tunneling into Au-Si-related states and not electronic states associated with the alkyl chain.4

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Acknowledgment. Dow Corning, RHK Technology, Inc., and the NSF (DMR-0093641) are thanked for their support. This 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. K.S.S., B.J.L., and D.R.F. thank the NSF for IGERT fellowships (DGE9972776). Prof. J. R. Barker is thanked for the use of the ozone generator and insightful discussions. M. L. Clarke and Prof. Z. Chen are thanked for assistance with the contact-angle measurements. Dr. K. A. Miller is gratefully acknowledged for assistance in sample preparation. Dr. S. Hulbert of the NSLS is thanked for assistance. D. Khan and Y. Q. Chen are acknowledged for assistance. J. Kulman is thanked for the initial evaporation of the Au onto the Si. Supporting Information Available: XPS wide scans of the oxidized and unoxidized octylsilane monolayer obtained with a conventional Mg KR source. This material is available free of charge via the Internet at http://pubs.acs.org. LA0496385