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J. Phys. Chem. C 2007, 111, 374-382
Modification of Alkanethiolate Self-Assembled Monolayers by Atomic Hydrogen: Influence of Alkyl Chain Length Justin Gorham,† Billy Smith,† and D. Howard Fairbrother*,†,‡ Department of Chemistry, and Department of Materials Science and Engineering, Johns Hopkins UniVersity, 3400 North Charles Street, Baltimore, Maryland 21218 ReceiVed: July 20, 2006; In Final Form: October 4, 2006
The interaction of atomic hydrogen with four alkanethiolate self-assembled monolayers (SAMs), octadecanethiolate, hexadecanethiolate, dodecanethiolate, and nonanethiolate, has been studied in situ, using X-ray photoelectron spectroscopy. Atomic hydrogen reactions with gold-sulfur bonds result in sulfur desorption and the formation of new sulfur species within the film. The loss of sulfur from each SAM exhibits firstorder kinetics with a rate constant that decreases with increasing alkyl chain length. This supports the idea that sulfur desorption is controlled by the diffusion of atomic hydrogen through the alkyl chains to the film/ substrate interface. In the two shorter chain SAMs, sulfur and carbon desorption is rapid and concerted. This indicates that the modification of dodecanethiolate and nonanethiolate SAMs is dominated by desorption of intact adsorbate chains and alkyl sulfur fragments formed by atomic hydrogen reactions at the film/substrate interface. For octadecanethiolate and hexadecanethiolate SAMs, however, the increased residence time of atomic hydrogen within the hydrocarbon film is responsible for the formation of a disordered hydrocarbon overlayer that prevents desorption of intact adsorbate chains or alkyl sulfide species. As a result, the rate of carbon desorption from these two SAMs is dramatically reduced and the loss of carbon and sulfur is not concerted. In the longer alkyl chain SAMs, sulfur is lost as small volatile fragments (e.g., H2S), while carbon desorption proceeds through chemical erosion of the hydrocarbon film by atomic hydrogen. For alkanethiolate SAMs exposed to atomic hydrogen, reactions between carbon-centered radicals and atmospheric oxygen containing species increase the surface hydrophilicity. Results from this investigation highlight the determinant role that film thickness can play in moderating the reactivity of gas-phase species with organized thin film assemblies.
I. Introduction Self-assembled monolayers (SAMs) are nanometer-scaled organic films composed of well-ordered and densely packed two-dimensional assemblies of long-chain molecules chemisorbed onto a substrate by a suitable headgroup.1 One of the most widely studied classes of SAMs are alkanethiols (CH3(CH2)n-1SH) anchored to a Au substrate by the formation of a thiolate (R-S-Au) bond.2,3 The structural and chemical properties of SAMs can be conveniently tuned by varying the chemical composition of the adsorbate.1 As a result, SAMs have been employed effectively as model systems to understand the structural and chemical properties of organic interfaces and thin films.4-6 SAMs also possess a number of important advantages over traditional polymers in studies designed to probe the mechanisms of vacuum-based organic surface modification strategies.7-17 For example, SAMs can be used to generate organic surfaces routinely and reproducibly without the unwanted presence of surface contaminants (e.g., adventitious carbon). Electron-based surface spectroscopies can also accurately quantify changes in the film’s chemical composition without an overwhelming contribution from bulk species.10,16,17 Information on the effect of atomic hydrogen (AH) on alkanethiolate SAMs can therefore * Corresponding author. E-mail:
[email protected]. Phone: (410) 5164328. Fax: (410) 516-8420. † Department of Chemistry. ‡ Department of Materials Science and Engineering.
provide a more detailed understanding of gas-surface interactions with hydrocarbon films and polymers. Furthermore, by appropriately varying the adsorbate, the effect of specific film characteristics on the overall reaction dynamics can be investigated. AH is a ubiquitous component of all hydrocarbon-based plasmas.18 As a result, AH reactions with hydrocarbon surfaces play an important role in numerous plasma processes, including the growth and etching of carbonaceous thin films19-22 and the surface modification of polymers.23 In these materials processes, however, the detailed role of an individual species is often masked by the variety of reactive species present in the gasphase environment.24 The complexity of the situation is often further complicated by synergistic effects between different gasphase species at the growing surface.25-30 Previous studies have established that AH reacts with hydrocarbons adsorbed on solid substrates to form an alkyl radical by an Eley-Rideal mechanism.31,32 In this process, a bimolecular reaction occurs between a surface-bound species and an incident gas-phase species in the absence of thermal equilibration. Once formed, numerous reaction pathways are available to the alkyl radical, including carbon-carbon bondforming reactions with other alkyl radicals, dehydrogenation, and secondary reactions with hydrogen atoms. Information obtained from these studies has also formed a useful scientific framework to understand how AH reacts with well-defined thin films. For example, Kluth et al. have used HREELS to study
10.1021/jp0646224 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/02/2006
Modification of Alkanethiolate SAMs by Atomic H the reactions of deuterium atoms with octadecylsiloxane SAMs.33 Results from these studies revealed that H/D exchange and cross-linking reactions occur within the hydrocarbon film as well as chemical erosion. In the present study, the reactions of AH with n-alkanethiolate SAMs have been investigated using a combination of contact angle measurements and in situ surface spectroscopy. Changes observed in the chemical and physical properties of the SAMs are indicative of AH-mediated reactions both within the hydrocarbon film and at the film/substrate interface. The key finding of this study is that film thickness plays a determinant role in moderating the interactions of atomic hydrogen with organized nanometer-scaled film assemblies. The effect of alkyl chain length on the modification process is correlated with changes in the importance of AH reactions within the hydrocarbon film compared to those at the film/substrate interface. II. Experimental Section II.A. Preparation of n-Alkanethiolate Self-Assembled Monolayers (SAMs). SAMs were prepared by immersing Ar+ sputter-cleaned polycrystalline gold substrates in an ethanol solution of 5 mM (i) octadecanethiol (CH3(CH2)17SH), C18 SAMs; (ii) hexadecanethiol (CH3(CH2)15SH), C16 SAMs; (iii) dodecanethiol (CH3(CH2)11SH), C12 SAMs; and (iv) nonanethiol (CH3(CH2)8SH), C9 SAMs for at least 12 h. Gold substrates were removed from the thiol solution and rinsed with ethanol, hexanes, and water. Samples were then introduced into the vacuum chamber and exposed to atomic hydrogen. Low-energy secondary electrons produced by X-ray irradiation initiate electron-stimulated C-H, C-C, and S-Au bond cleavage in SAMs, modifying the chemical composition and compromising the film’s structural integrity.34-36 To minimize the effects of X-ray irradiation in the present investigation, XPS analysis was carried out on the SAMs only after AH exposure. Furthermore, each SAM was exposed to AH once. As a result, systematic variations in the chemical composition of each alkanethiolate SAM as a function of AH exposure (discussed in section III) reflect the reproducibility of (i) the SAM’s chemical/structural properties and (ii) the AH flux. II.B. Hydrogen Radical/Atom Source. Atomic hydrogen (AH) was generated using a thermal gas cracker TC-50 (Oxford Applied Research) positioned in a line of sight to the sample (target-to-sample distance of ∼5 cm). The thermal gas cracker operates by passing molecular hydrogen through an Ir capillary (biased to 2 kV), which is heated to ≈2000 °C by electron bombardment. The heated capillary causes molecular hydrogen to dissociate into a stream of atomic species. Assuming thermal equilibration with the Ir tube, the AH that emerges from the gas cracker will possess an average kinetic energy of 3/2kTcapillary (≈0.3 eV). All AH exposures were carried out at PH2 ≈ 2 × 10-5 torr with the AH source operating at 60 W, ensuring a constant flux of AH at the sample surface. II.B.1. Atomic Hydrogen Flux. We have recently developed an in situ method to calibrate the AH flux. This involves utilizing atomic force microscopy to measure the etch rate of highly ordered pyrolytic graphite (HOPG) exposed to AH.37 With the use of this methodology, the AH flux at the surface was determined to be 5 × 1014 AH molecules cm-2 s-1 when PH2 ) 2 × 10-5 torr. We estimate that this value is accurate to within a factor of 5. II.C. Sample Analysis. To prevent the unwanted effects of atmospheric contamination and surface oxidation, XPS was carried out using a physical electronics 5400 system in the same UHV chamber (Pbase ≈ 5 × 10-8 torr) that housed the AH
J. Phys. Chem. C, Vol. 111, No. 1, 2007 375 source. In all experiments, Mg KR (1253.6 eV) X-ray radiation was generated from a physical electronics 04-500 X-ray source. Ejected photoelectrons were analyzed using a hemispherical electron energy analyzer operating at a pass energy of 44.75 eV and a step size of 0.125 eV/step with a takeoff angle of 45° from the sample normal. For all experiments, binding energy scales were referenced to the Au(4f7/2) peak at 83.8 eV.38 XPS data analysis was performed using commercially available software; the S(2p) region was fit with 100% Gaussian peaks and a linear baseline: a Shirley baseline was used for the Au(4f) and C(1s) regions. To verify that the changes observed in the SAM’s chemical composition were due only to the effects of atomic hydrogen, control experiments were carried out that involved exposing alkanethiolate SAM to the heated thermal cracker in the absence of hydrogen gas. XPS analysis revealed that these experiments produced little or no change in the chemical composition of the SAMs, in contrast to the effect of AH. To quantify the relationship between the C(1s) and Au(4f) XPS areas and the alkyl chain length, five different alkanethiolates as well as a sputter-cleaned Au substrate were analyzed. Results from these experiments, shown in the Supporting Information (Figure 1), illustrate that for these nanometer-thick films the C(1s) and Au(4f) XPS intensities vary approximately linearly as a function of the total number of carbon atoms in alkanethiolate SAM. As a result, the effect of AH on the integrated C(1s) and Au(4f) XPS signals can provide information on changes in the film’s carbon content and thickness. Contact angle measurements on C18 SAMs modified by AH were performed using a KSV Instruments CAM 100. In these experiments, C18 SAMs were exposed to AH for a prescribed time period and subsequently removed from the vacuum chamber. The water contact angle was then measured under ambient conditions. Preliminary experiments revealed that in order to obtain reproducible contact angle values, samples should sit in air for at least 30 min. The values reported in Figure 4 represent the average of 4-6 measurements recorded at different points on the sample surface. Once contact angle measurements were completed, samples were reintroduced into the vacuum chamber, and the film’s chemical composition was determined using XPS. III. Results The effect of atomic hydrogen (AH) on the C(1s) and S(2p) XPS regions of (a) C9 and (b) C16 SAMs is shown in Figure 1. Prior to AH exposure, the C(1s) region of both SAMs consisted of a single peak centered at 284.6 eV; a well-resolved Au 4f(7/2,5/2) doublet at 83.8 and 87.4 eV is also observed (Au XPS region not shown). The S(2p) region of both SAMs was well fit by the characteristic S(2p3/2,1/2) doublet associated with the native thiolate species (S(2p3/2) peak at 161.5 eV),39 although it was necessary to include a small contribution from a radiationinduced sulfur species (S(2p3/2) peak at 163.2 eV) formed by X-ray irradiation.34,40 For both the C16 and C9 SAMs, the same ordinate scale was used for the C(1s) and S(2p) regions. Differences in the peak intensities between these two regions can therefore be directly compared. For example, the fact that the S(2p) XPS peak is larger in the native C9 SAM (Figure 1, bottom spectra) is a reflection of the smaller number of carbon atoms in the alkyl chain (9 vs 16). Figure 1 shows that after prolonged AH exposures (240 min, top spectra), the C(1s) and S(2p) XPS regions of the C9 and C16 SAMs are virtually identical with a small residual carbon signal and no detectable sulfur. However, although the long-
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Figure 1. Evolution of the C(1s) and S(2p) regions of (a) nonanethiolate (C9) and (b) hexadecanethiolate (C16) self-assembled monolayers (SAMs) as a function of atomic hydrogen (AH) exposure. For each SAM, the same vertical scale has been used for the C(1s) and S(2p) regions.
term effect of AH on the chemical composition of both SAMs is similar, the time scales for carbon and sulfur desorption as well as the relationship between the sulfur and carbon content in the two hydrocarbon films are very different. For the C9 SAM, all of the sulfur has desorbed after 2 min of AH exposure. Indeed, Figure 1 demonstrates that the C(1s) and S(2p) XPS regions of the C9 SAM remain unchanged for AH exposures in excess of 2 min. In contrast, sulfur is still clearly visible in the C16 SAM after 8 min of AH exposure. The most dramatic difference in the behavior of the C9 and C16 SAMs, however, is the relationship between the carbon and sulfur content in the films as a function of increasing AH exposure. For example, in the C9 SAM, sulfur desorption from the film is accompanied by a correspondingly sharp decrease in intensity within the C(1s) region. In contrast, sulfur desorption from the C16 SAM is complete before any significant change in the carbon content is observed. Figure 1 also demonstrates that the S(2p) region broadens to higher binding energies as a result of AH exposure. The resultant spectral envelope was well fit by a combination of the native thiolate species and new reaction-induced sulfur species (S(2p3/2) peak at 163.2 eV). The chemical identity of the new sulfur species produced by AH cannot be determined unambiguously from XPS analysis of the S(2p) region and may correspond to unbound thiol species that remain trapped within the hydrocarbon film, alkyl sulfur (C-S-C) species analogous to those produced by X-ray irradiation, or disulfides (C-S-S-C). However, in contrast to the radiation-induced sulfur species, the new sulfur species produced by AH reactions exhibit a
systematic dependence upon AH exposure and alkyl chain length and are also produced in much higher concentrations. In Figure 2 the variations in the C(1s) and Au(4f) XPS peak areas for all four alkanethiolates are plotted as a function of AH exposure. This demonstrates the influence of alkyl chain length on the carbon content and corresponding change in the film’s thickness. Since each SAM was only used once, the data shown in Figure 2 corresponds to ≈80 individual experiments. To facilitate a direct comparison of the changing chemical composition, the carbon and gold peak areas were plotted on the same ordinate scale for each SAM. Figure 2 reinforces the idea presented in Figure 1 that the alkyl chain length plays a key role in moderating the effect of AH on alkanethiolates SAMs. Specifically, Figure 2 demonstrates that the effect of AH on the two shorter chain alkanethiolate SAMs (C9 and C12) differs markedly from that of the C16 and C18 SAMs. Thus, for the C9 and C12 SAMs, the carbon content decreases sharply during short AH exposure times (10 min), however, the C(1s) and Au(4f) XPS areas remain essentially constant. In contrast, for the C16 and C18 SAMs, the film’s carbon content decreases monotonically over the entire range of AH exposures studies (0-250 min), with a correspondingly steady increase in the Au(4f) substrate signal. Figure 3 describes the influence of AH on the sulfur species in each of the four alkanethiolate SAMs. In Figure 3a the variation in the S(2p) XPS regions after 2 min of AH exposure is shown. In the C9 SAMs, no sulfur remains in the film. In contrast, for the same AH exposure sulfur atoms are still present
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Figure 2. Variation in the (left) C(1s) and (right) Au(4f) peak areas of octadecanethiolate (C18), hexadecanethiolate (C16), dodecanethiolate (C12), and nonanethiolate (C9) self-assembled monolayers (SAMs) as a function of atomic hydrogen (AH) exposure. For each SAM, the same ordinate scale has been used. The solid lines represent best fits to the data based on a multicomponent polynomial fit and are merely meant to guide the eye.
in the C12, C16, and C18 SAMs. Changes in the S(2p) spectral envelope are strongly dependent upon the alkyl chain length. For the C12 SAM, the new AH-induced sulfur species are formed in greater concentration than the native thiolate species, while spectral deconvolution of the S(2p) region of C16 SAMs reveals a roughly equal concentration of native thiolate species and AHinduced sulfur species. In contrast, for the C18 SAM, the S(2p) region is dominated by the native thiolate species. Indeed, XPS analysis of the S(2p) region obtained for the C18 SAM after 2 min of AH exposure is indistinguishable from the spectra obtained from the native SAM prior to any AH exposure. This indicates that the well-ordered hydrocarbon films generated by alkanethiolate SAMs represent a barrier toward AH diffusion to the film/substrate interface. In Figure 3b the influence of AH on the integrated S(2p) XPS intensity is shown explicitly. For all four alkanethiolate SAMs, the S(2p) signal intensity decreases upon exposure to AH. The rate of sulfur loss, however, decreases as the chain
length increases. Thus, for C9 and C18 SAMs complete removal of the sulfur atoms requires AH exposures in excess of 2 and 45 min, respectively. Despite these different time scales, the kinetics of sulfur desorption from each one of the four SAMs can be well fit by a first-order loss process (best fit values are shown as solid lines in Figure 3b). In Figure 3c, the corresponding rate constants for sulfur desorption (kS-des) are plotted as a function of the number of carbon atoms in the alkyl chain. This analysis reveals that the rate constant for sulfur desorption decreases linearly with increasing chain length for the C12, C16, and C18 SAMs. The rate constant for sulfur desorption from the C9 SAM is, however, significantly greater than those values measured for the three longer chain alkanethiolate SAMs. The effect of AH on the static water contact angle of C18 SAMs is shown in Figure 4. The water contact angle of the native C18 SAM is ≈110°, consistent with typical values for alkanethiolate SAMs adsorbed on coinage metals.41 As a result of reactions with AH, the surface becomes more hydrophilic
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Figure 3. (a) S(2p) XPS region of octadecanethiolate (C18), hexadecanethiolate (C16), dodecanethiolate (C12), and nonanethiolate (C9) self-assembled monolayers (SAMs) after 2 min of atomic hydrogen (AH) exposure. (b) Variation in the integrated S(2p) area for each alkanethiolate SAM as a function of AH exposure. The solid lines represent the result of a best fit analysis based on a first-order decay process. (c) Variation in the firstorder rate constants for sulfur desorption (kS-des) calculated from (b) are plotted in (c) as a function of the number of carbon atoms in the SAM.
Figure 4. Static water contact angle of octadecanethiolate (C18) self-assembled monolayers (SAMs) as a function of atomic hydrogen (AH) exposure. Inset: variation in the O(1s) signal intensity of C18 SAMs following AH modification and subsequent exposure to atmospheric conditions.
following air exposure. For AH exposures in excess of 5 min the static water contact angle reaches a steady-state value of ≈70°. The inset to Figure 4 shows that the change in wettability is mirrored by an increase in the film’s oxygen content following AH exposure.
IV. Discussion IV.A. C9 and C12 Alkanethiolate SAMs. When AH interacts with the shorter alkyl chain SAMs, carbon and sulfur atoms desorb rapidly from the film. This produces a correspondingly rapid decrease in the film thickness, the latter evidenced by the
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Figure 5. Schematic representation illustrating the effect of alkyl chain length on the reactions of AH with alkanethiolate self-assembled monolayers (SAMs): (top) shorter chain alkanethiolates (dodecanethiolate and nonanethiolate) where film modification by atomic hydrogen is rapid and dominated by desorption of intact adsorbate chains and alkyl sulfide species; (bottom) longer chain alkanethiolates (octadecanethiolate and hexadecanethiolate) where carbon and sulfur desorption are not concerted and carbon desorption proceeds via chemical erosion.
change in Au(4f) substrate signal intensity (Figure 2). For the C12 SAMs, Figure 2 illustrates that essentially all of the changes in film thickness occur during the first 10 min of AH exposure. Similarly, for C9 SAMs exposed to AH, Figures 1-3 demonstrate that all of the changes in the C(1s), S(2p), and Au(4f) regions occur within the first 2 min of AH exposure. Indeed, for AH exposures >2 min, the C(1s), S(2p), or Au(4f) regions of the C9 SAM remain essentially unchanged. The residual peak in the C(1s) region, whose intensity remains constant for prolonged AH exposures, is attributed to the presence of carbonaceous surface impurities that were either not removed during Ar+ sputtering or accumulated on the surface during film preparation. The concerted loss of carbon and sulfur from the C9 and C12 SAMs suggests that in the shorter chain alkanethiolate SAMs reactions are initiated by AH reactions at the film/substrate interface. We postulate that one of the dominant processes responsible for carbon and sulfur desorption in the C9 and C12 SAMs involves AH reduction of the native thiolate species at the film/substrate interface:
AH + RS-Au f RSHv + Au
(∆H ) -197 kJ mol-1) (1)
The strongly exothermic nature of this reaction reflects the relative strength of the S-H and Au-S bonds (364 vs 167 kJ mol-1).1 Since the thiolate (Au-S-R) bond anchors the alkyl chains to the Au substrate, cleavage of the Au-S bond by AH will produce intact adsorbate chains that can then desorb, as shown in Figure 5. Desorption of intact adsorbate chains has also been reported in previous studies of alkanethiolate and semifluorinated SAMs modified/damaged by electrons and ionizing radiation.42 In addition to carbon and sulfur desorption, new sulfur species (Figures 1 and 3) are also formed during AH exposure. As mentioned in section III, these new species are a consequence of AH-mediated Au-S bond cleavage and may be either unbound thiol species that remain trapped within the hydrocarbon film, alkyl sulfur (C-S-C) species analogous to those produced by X-ray irradiation, or disulfides (C-S-S-C). During the initial stages of AH exposure the balance between adsorbate chain desorption and the formation of these new AH-
mediated sulfur species can be estimated by analyzing the S(2p) region. In the case of the C9 SAM, deconvolution of the S(2p) region after 10 s of AH exposure (Figure 1a) reveals that 22% of the native thiolate species have desorbed while 53% have been converted to the new AH-mediated S species. The evolution of the S(2p) region indicates that these new sulfur species also react in secondary reactions with AH and contribute to the concerted loss of carbon and sulfur atom from the film, presumably due to formation of volatile alkyl sulfide species. A schematic representation of the reactions of AH with the C9 and C12 SAMs is shown in Figure 5 (top). IV.B. C16 and C18 Alkanethiolate SAMs. Figures 1 and 2 demonstrate that AH modifies the longer alkyl chain hexadecanethiolate and octadecanethiolate SAMs differently than the shorter alkyl chain alkanethiolate SAMs. In particular, the rate of carbon desorption slows dramatically and occurs independently from the loss of sulfur. For example, if we compare the C(1s) and S(2p) regions in Figure 1b it is apparent that after 20 min of AH exposure all of the sulfur has desorbed from the C16 SAM while the carbon content, measured by the C(1s) peak intensity is, within experimental uncertainty, unchanged compared to that of the native SAM. The carbon content of C16 and C18 SAMs exposed to AH does decrease (Figures 1 and 2). However, this occurs at a much slower rate than sulfur desorption and continues over the entire range of AH exposures studied (0-240 min). The markedly different behavior of the C16 and C18 SAMs compared to that of the C9 and C12 SAMs indicates that the hydrocarbon film thickness plays a crucial role in determining how AH modifies alkanethiolate SAMs. In the shorter alkyl chain SAMs, the rapid and concerted loss of sulfur and carbon occurs because AH can diffuse relatively easily through the hydrocarbon film and initiates desorption from reactions at the film/substrate interface. In contrast, for the longer chain alkyl SAMs, AH reactions at the film/substrate interface become increasing hindered by the thick, well-ordered, hydrocarbon overlayer. This phenomenon is well illustrated in Figure 3 by the variation in the S(2p) region of the various alkanethiolates after 2 min of AH exposure. Thus, for the C9 SAM all of the
380 J. Phys. Chem. C, Vol. 111, No. 1, 2007 sulfur has desorbed, while the S(2p) region of the C18 SAM is virtually identical to the spectrum obtained before any AH exposure. As the alkyl chain length increases, the residence time of AH within the hydrocarbon film will also increase. Consequently, AH reactions within the hydrocarbon film itself become increasingly important. Previous studies on the interaction of AH with adsorbed hydrocarbons and hydrocarbon thin films have shown that reactions between AH and the alkyl chains are initiated by hydrogen abstraction from C-H bonds to form a carbon-centered radical:31,32
In the longer chain SAMs, we postulate that carbon-carbon coupling reactions between proximate carbon-centered radicals create a cross-linked hydrocarbon overlayer, as shown in Figure 5. A similar process leading to the formation of a polymeric film has been shown to occur during the interaction of hydrogen atoms with octadecylsiloxane SAMs and as a component of the electron-induced modification/damage of alkanethiolate SAMs.33,34,36 Once AH reaches the film/substrate interface in either the C16 or C18 SAMs, this carbon-carbon cross-linking process significantly reduces the probability that intact adsorbate chains or new AH-mediated sulfur species will form as a result of AH-mediated Au-S cleavage (see Figure 5).34,36 Conversely, in the modification of the shorter C9 and C12 SAMs, the rapid permeation of AH through the hydrocarbon film to the film/ substrate interface means that AH reactions with the alkyl chains do not play a major role. Changes in the chemical composition of the C12 SAM during AH exposure (Figure 2) do, however, reveal that a small amount of carbon desorption continues after all of the sulfur has been removed from the film. This suggests that cross-linking reactions within the hydrocarbon film contribute to the AH modification of the C12 SAM. The hydrocarbon matrix that forms during the initial reactions between AH and the C16 and C18 SAMs will also trap any new sulfur species produced by AH reactions at the film/substrate interface. This effect contributes to the absence of concerted loss of carbon and sulfur from the longer chain SAMs. New sulfur species formed by AH-mediated Au-S bond cleavage are likely to become trapped with the polymeric film and are responsible for the new S(2p) peak observed in Figure 3. In contrast to the concerted loss of sulfur and carbon from the shorter alkyl chain SAMs, all of the sulfur desorbs from the C16 and C18 SAMs in the absence of any significant carbon loss from the film (for example, compare the evolution in the S(2p) and C(1s) regions of the C16 SAM in Figure 1). This suggests that in the longer alkyl chain SAMs sulfur desorbs as volatile, low-weight fragments such as HS‚ and/or H2S, presumably as a result of reactions between AH and sulfur-containing bonds within the hydrocarbon film (Figure 5). The ability of AH to remove adsorbed sulfur species as H2S has previously been observed in studies involving the reactions of AH with sulfided Ni(100) surfaces.43 Since desorption of adsorbate chains or alkyl sulfide species does not play a role in the modification of the longer chain SAMs by AH, the steady decrease in the carbon content of both the C16 and C18 SAMs as well as the corresponding increase in the Au(4f) signal (Figure 2) provides evidence for the presence of another, slower desorption mechanism capable of removing carbon. This process is AH-mediated chemical erosion of the hydrocarbon film.33 In this mechanism, the initial step involves AH reactions with carbon-centered
Gorham et al. radicals in the film (R‚ + AH f R-H). The energy released during this C-H bond formation (>400 kJ mol-1) process is sufficient to cleave a C-C bond (bond strength
