Thermal Treatment of n-Alkanethiolate Monolayers on Gold, As

F. Bensebaa and T. H. Ellis*. De´partement de .... We have also found that the shorter chain thiols (C12 ... been performed on the short chain system...
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Langmuir 1998, 14, 2361-2367

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Thermal Treatment of n-Alkanethiolate Monolayers on Gold, As Observed by Infrared Spectroscopy F. Bensebaa and T. H. Ellis* De´ partement de chimie and GCM, Universite´ de Montre´ al, C.P. 6128, Succ. Centre-ville, Montre´ al, Que´ bec, Canada H3C 3J7

A. Badia and R. B. Lennox Department of Chemistry, McGill University, Montre´ al, Que´ bec, Canada H3A 2K6 Received October 27, 1997. In Final Form: February 12, 1998 In the present work, we have used reflection-absorption infrared spectroscopy (RAIRS) to investigate the temperature behavior of n-alkanethiols adsorbed on gold substrates. A careful examination of the spectra (frequencies, integrated intensities, and line widths) reveals at least two different types of thermally induced disorder as the temperature is increased. These observations are discussed in light of other recent studies using complementary techniques. The overall picture that emerges is that an initial stage is characterized by a gradual untilting of the alkyl chain and the appearance of gauche conformational defects near the surface, and both of these effects are largely reversible. The subsequent nonreversible disordering of the monolayer is characterized by a large number of defects located below the surface. Infrared spectroscopy is particularly well suited for detecting these changes.

1. Introduction The properties of n-alkanethiolate self-assembled monolayers (SAMs) on gold have been extensively studied in recent years (see refs 1 and 2 for thorough recent bibliographies). This interest is largely motivated by the numerous applications of these easily prepared monolayers in many areas of fundamental and applied research. For some of these applications a limiting factor will be thermal stability and the presence of defects. Only a limited number of studies have addressed this issue, and an overall picture has not yet emerged. Dubois and coworkers3,4 used infrared reflection-absorption infrared spectroscopy (RAIRS) to follow changes upon heating in ultrahigh vacuum (UHV) and concluded that the degree of disorder gradually increased with temperature. This was in agreement with RAIRS studies of LangmuirBlodgett (LB) films which concluded that disorder always increases with temperature.5 Importantly, it was found that disorder was reversible up to a certain point, after which the disorder became nonreversible. In no case did thermal treatments lead to improved order. In recent years, a large number of studies have addressed the issue of order in n-alkanethiolate SAMs on gold using diffraction and scanning probe techniques. Many of these studies were performed under UHV conditions and, as such, used principles developed in the study of chemisorbed molecules on surfaces. Attempts were made to increase the order of as deposited monolayers by annealing them in a vacuum. In several studies it was in fact determined that this is the procedure which * To whom correspondence should be addressed. Telephone: (514) 343-6910. FAX: (514) 343-7586. E-mail: [email protected]. (1) Poirier, G. E. J. Vac. Sci. Technol. B 1996, 14, 1453-1460. (2) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (3) Nuzzo, R. G.; Korenic E. M.; Dubois, L. H. J. Chem. Phys. 1990, 93, 767-773. (4) Dubois, L. H.; Zegarski B. R.; Nuzzo, R. G. J. Electron Spectrosc. Relat. Phenom. 1990 54/55, 1143-1152. (5) Naseli, C.; Rabolt J. F.; Swalen J. D. J. Chem. Phys. 1985, 82, 2136-2140.

produces the most highly ordered SAMs.6-8 (Annealing in solution was also found to produce more order.9) The thermal ordering and disordering of SAMs is therefore an issue which requires some clarification. Overall, in any study of surface order, there are a number of factors which need to be considered. Most importantly, the type of order which is being probed is a function of the technique being used. With RAIRS, it is most common to monitor the degree of order based on the stretching frequency of the methylene bands. This approach has evolved from many years of experience in other long alkyl chain systems. From those studies, it is known that such a measurement is in fact most sensitive to the conformational order of the alkyl chain.10 On the other hand, diffraction and scanning probe imaging measurements are most sensitive to the spatial positions of the molecules. Therefore, it will be important to understand the exact details of each experiment when comparing results obtained with different techniques. Even taking this into account, one would still expect that the increasing disorder which has been detected in monolayers using RAIRS should also be detected with diffraction and imaging techniques. Another critical consideration is the wide variability in sample preparation conditions. It is becoming clear that despite the fact that alkanethiolate SAMs on gold are easily prepared, there are many factors which greatly affect the final structure of the SAM. These factors include substrate quality (degree of order and cleaning procedure), (6) Fenter, P. Eisenberger, P. and Liang, K. S. Phys. Rev. Lett. 1993, 70, 2447-2450. (7) Camillone, N., III; Eisenberger, P.; Leung, T. Y. B.; Schwartz, P.; Scoles, G.; Poirier G. E.; Tarlov, M. J. J. Chem. Phys. 1994, 101, 1103111036. (8) Bucher, J.-P.; Santesson, L.; Kern, K. Langmuir 1994, 10, 979983. (9) Delamarche, E.; Michel, B.; Gerber, Ch.; Anselmetti, D.; Gu¨ntherodt, H.-J.; Wolf H.; Ringsdorf, H. Langmuir 1994, 10, 28692871. (10) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145-5150.

S0743-7463(97)01158-X CCC: $15.00 © 1998 American Chemical Society Published on Web 03/31/1998

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the nature of the solvent, the time of incubation, and the thiol concentration. Once again, great care must be taken in making comparisons between published results since there is generally at least one of the above variables which will be different. One further factor which will certainly play an important role in determining the properties of any SAM is the length of the alkyl chain. Fortunately, this is a parameter that can be varied systematically. The goal of the present study was therefore to use RAIRS to follow the thermal stability of n-alkanethiolate SAMs (Au-S-(CH2)n-1-CH3, abbreviated as Cn) on gold as a function of chain length. The long chain thiols (C15 and above) form well-ordered monolayers easily. We have found that the conformational disorder of SAMs formed from long chain thiols increases continuously as a function of temperature, and as such, an annealing procedure to improve order has not been found. Further, there are clearly two stages in the disordering process. An initial, largely reversible stage is characterized by chain untilting and relatively few conformational defects. This is followed by a mostly nonreversible stage characterized by a large number of conformational defects. These obsevations are in excellent agreement with recent theoretical studies. We have also found that the shorter chain thiols (C12 and below) tend to be more disordered “as deposited”, when using conventional preparation techniques. However, highly disordered short chain SAMs can be somewhat improved though vacuum annealing. It is worth noting that the majority of scanning probe measurements have been performed on the short chain systems, for experimental reasons.9 This helps to explain why many of those studies found that annealing produces the most highly ordered SAMs. 2. Experimental Procedures Sample preparation of self-assembled monolayers can have a great impact on the results obtained. The substrates used in the present study are 1000 and 2000 Å thick evaporated gold films on glass (precovered with 50 Å of titanium), purchased from Evaporated Metal Films, Inc. (Ithaca, NY). These substrates are expected to be relatively poorly ordered (they have not been annealed), with small domains of (111) crystallites. They were chosen partly so that a large number of reproducible samples could be easily prepared. No effect of gold film thickness was observed, as expected. The substrates were first cleaned in a boiling 50/50 ethanol and chloroform solution and sonicated for 5 min. The surfaces were then exposed for 1 min to drops of a piranha solution (1:4 H2O2/H2SO4), then rinsed with peroxide and ethanol. After drying in a jet of N2, they were then immersed in 1 mM thiol solutions in ethanol, at room temperature, for periods ranging from at least 24 h to several days. After removal from the solution, they were extensively rinsed with ethanol, and mounted in the vacuum chamber in less than 1 h. The chamber was quickly pumped to about 10-6 Torr, and the sample was left in the chamber for up to a few days while the pressure decreased to about 5 × 10-9 Torr. All spectra were obtained on a Mattson RS-1 FTIR spectrometer equipped with a water cooled source and a liquid nitrogen cooled midrange MCT (mercury-cadmium-tellurium) detector. The nonpolarized infrared beam was reflected from the surface at an angle of about 85° from the surface normal. Spectra were recorded using 2000 co-additions with a resolution of 4 cm-1, and triangular apodization was used. All spectra are referenced to a clean gold substrate. The references were obtained for each sample at the end of the complete series of measurements. These clean surfaces were obtained by sputtering away the monolayer film using a flux of 600 eV argon ions with the sample at around 700 K. Using this procedure, it was possible to obtain references in the identical position as the sample, and this resulted in high quality spectra. Reference spectra obtained simply with an

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Figure 1. Representative RAIRS spectra of a C20 monolayer measured at 145, 296, and 435 K. uncoated gold surface did not give reliable intensities or frequencies, probably because of the presence of C-H groups on the reference surface and/or small changes in the optical path. Spectral analysis was performed using GRAMS software (Galactic Industries Corp., Salem, NH). Peak positions were obtained using a center of mass analysis, which is always within 0.5 cm-1 of the peak maximum. Peak intensities were obtained by integrating over the entire range between peaks using a standard trapezoidal routine. The reported line widths represent the full width at half-maximum (fwhm). Sample temperature was measured using a chromel-alumel thermocouple clamped onto the sample surface. The 10 mm × 30 mm × 1.5 mm thick sample was mounted on a 1 mm thick molybdenum plate, which was heated from the back by a 25 mm diameter boron nitride heating element. The sample plate was supported by two copper blocks which could be cooled to liquid nitrogen temperatures. Under these conditions, the sample was heated and cooled uniformly, but we cannot rule out absolute errors in the temperature measurement of the monolayer film of (5 K.

3. Results from Infrared Spectroscopy A series of spectra as a function of temperature have been obtained with C15, C18, C20, and C22 monolayers. At each temperature, peak frequencies, integrated intensities, and line widths have been determined. The basic trends in all of these parameters are similar for all of these chain lengths. Figure 1 presents representative infrared spectra for the same C20 monolayer recorded at three different temperatures, 145, 296, and 435 K, chosen from among the roughly 20 temperatures that were measured. All spectra were measured at the temperatures indicated, with the temperature held constant to (1 K while the spectra were being recorded. Overall, these spectra are similar to previous spectra obtained for these types of systems.3,4,11-13 Detailed information about the (11) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568.

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structural properties can be obtained from a careful analysis of the spectral features. The temperature dependence of the peak frequency of the following modes for a C20 SAM are plotted in Figure 2: (a) methylene antisymmetric C-H stretch (d -), (b) methylene symmetric C-H stretch (d +), and (c) methyl symmetric C-H stretch (r +). (The methyl antisymmetric C-H stretch (r -) is composed of two modes, in-plane and out-of-plane, which change in relative intensity as a function of temperature, so it is not possible to accurately follow the peak position.) Each data point was obtained from a measurement at the indicated temperature, with the temperature being increased for each subsequent point. The significant increases in the CH2 stretching frequencies as a function of temperature are well-known from previous studies of alkyl chain systems, including bulk solids,10 lipid bilayers,14 and Langmuir-Blodgett films.5 Qualitatively, it has been suggested that when the frequency of the d - peak is below 2918 cm-1, the alkyl chains are considered to be “ordered”, and when the frequency is above this value, it is considered that a large degree of disorder, in the form of gauche defects, exists.10,14 Our observed onset of disorder (i.e., increasing frequency) is more gradual than the behavior of bulk alkanes and phospholipid bilayers (such as DPPC), where a clear firstorder phase transition is observed, resulting in a sharp increase in frequency at the melting temperature.14 Instead, alkanethiolate monolayers undergo a gradual increase in the frequency up to about 350 K, followed by a steeper rise. A similar behavior is seen for the methylene d + peak, whereas the methyl peaks shift less in frequency. In certain cases heating cycles have been performed15 where the temperature was increased to only 320 K, following which the sample was cooled back down to 150 K. Under these conditions the measurements are reversible. In contrast, when the monolayer is heated above 350 K, the frequencies do not return to their original values upon cooling to 150 K, even after several hours, although there is some decrease in frequency upon cooling. The nonreversible (or perhaps kinetically frozen) disorder created above 350 K contrasts to the melting transition in bulk alkanes10 and the gel to liquid crystal transition in lipid bilayers,14 which are completely reversible. The integrated intensities of each of the above three peaks are plotted in Figure 3, again for the C20 monolayer. The intensities were determined by integrating over a region of about 50 cm-1 for each peak. Note that by using this procedure, the integrated intensity of the d - peak includes the shoulder which has been assigned to a Fermi resonance. The most important result from these measurements is that the integrated intensities of the methylene peaks show a large initial decrease in intensity up to about 350 K, followed by a significant increase at higher temperature. This increase in integrated intensity is not observed for the methyl peaks. The methylene peaks of other long chain thiols show this same behavior, as shown in Figure 4. In particular, the minimum intensity is always near 350 K. It is clear that the different form of this curve as compared to the behavior of the CH2 frequencies shown in Figure 2 indicates that these measured quantities are sensitive to different properties of the monolayer. (12) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167. (13) Bertilsson, L.; Liedberg, B. Langmuir 1993, 9, 141-149. (14) Cameron, D. G.; Casal, H. L.; Mantsch, H. H. Biochemistry 1980, 19, 3665-3672. (15) Bensebaa, F.; Ellis, T. H.; Badia, A.; Lennox, R. B. J. Vac. Sci. Technol. A 1995, 13, 1331-1336.

Figure 2. Temperature dependence of the peak frequency of the following C20 vibrational peaks: (a) antisymmetric methylene stretch, d -; (b) symmetric methylene stretch, d +; (c) symmetric methyl stretch, r +.

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Figure 4. Temperature dependence of the intensity of the antisymmetric methylene stretching peak, d -; for C15, C18, and C22 monolayers. Note that the curve for the C15 monolayer has been displaced downward by 0.015 units for clarity.

Figure 5. Temperature dependence of the full width at halfmaximun (fwhm) of the C20 antisymmetric methylene stretching peak, d -.

Figure 3. Temperature dependence of the integrated intensity of the following C20 vibrational peaks: (a) antisymmetric methylene stretch, d -; (b) symmetric methylene stretch, d +; (c) symmetric methyl stretch, r +.

The temperature dependence of the line width of the d- antisymmetric CH2 stretching peak is plotted in Figure 5. Given that the form of this peak changes as a function of temperature, it is not possible to assign a precise physical significance to the line width measurement. However, its measurement provides another way of monitoring spectral changes. A large increase is observed above 350 K, and this increase in the line width in this region contributes to the increase in integrated intensity discussed above. A different behavior was observed for shorter chain thiols (C12 and below). These films were routinely less ordered as deposited. For example, the d - antisymmetric

Thermal Treatment of n-Alkanethiolate Monolayers

CH2 stretching peak was always 2919 cm-1 or higher as deposited, and in some cases it was as high as 2924 cm-1. These latter, highly disordered films could be ordered by annealing to between 350 and 380 K, although the d peak never went below 2919 cm-1. We therefore chose to study the thermal disordering process only on those films which were initially highly ordered (with a d - peak at or below 2918 cm-1 as deposited), which in our case was for longer chain films (C15 and above). It has recently been reported that short chain SAMs are distincly more strained than long chain ones, with the dividing line being around C14.16 4. Interpretation of the Infrared Spectra By analogy to other long alkyl chain systems, one can anticipate a large number of possible structural changes as well-ordered, low-temperature monlayers are heated. These effects include trans-gauche isomerization within the chains, chain untilting, rotator phases, chain diffusion, and eventually desorption. In addition, there are several types of gauche defects, both single and double, and they may be located near the chain ends or near the chain center. A detailed analysis of the infrared spectra provides a wealth of information. The integrated intensity changes of the methylene C-H stretching peaks show a considerable decrease in intensity upon heating, followed by a rapid increase in intensity above about 350 K. This can be explained to be the result of two different temperature-induced changes in the film structure which have opposite effects on the intensities of these peaks. The initial decrease is believed to be related to a change in the average tilt angle of the alkyl chain. The subsequent increase in integrated intensity reflects the onset of strong conformational disorder in the alkyl chains. Each of these effects will initially be discussed separately. 4.1. Evidence for Chain Untilting. There is supporting evidence for chain untilting upon heating. Molecular dynamics calculations by Mar and Klein17 have shown that the average tilt angle of a C15 chain decreases continuously from 32° at 200 K to 21° at 350 K. The chains are initially tilted because the packing of the sulfur atoms on the gold substrate is less dense than the packing of the chains that would optimize interchain van der Waals attractive forces (5 vs 4.4 Å nearest neighbor distances). Tilting is a means to increase the three-dimensional density of the monolayer. As the monolayer is heated, vibrational motion must be accommodated through a decrease in density, and untilting is a way to achieve this. The important question is whether this untilting is sufficient to explain the large intensity decreases seen in Figures 3 and 4. First, it is important to point out that the sensitivity of RAIRS to chain tilting arises from the fact that the metal surface renders grazing incidence RAIRS sensitive only to vibrational modes with components perpendicular to the surface. In fact, for tilt angles close to zero, the methylene C-H stretching peaks would be very weak (i.e., the methylene stretching vibrations would be parallel to the surface). The intensities vary as (cos θ)2, where θ is the angle between the dipole and the surface normal, so that changing the tilt from 32 to 21° will cause the intensity to decrease by a factor of 2.2 (assuming a twist angle of 0°). A more extensive simulation of RAIRS intensities by Parikh and Allara18 confirms the sensitivity of the absolute intensity of both the d+ and (16) Fenter, P.; Eisenberger, P.; Burrows, P.; Forrest, S. R.; Liang, K. S. Physica B 1996, 221, 145-. (17) Mar W.; Klein, M. L. Langmuir 1994, 10, 188-196.

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d- bands to small changes in tilt angle, even when the twist angle is not zero. Changing the tilt angle from 31 to 23° results in a calculated intensity decrease by about a factor of 2 (see part c and d of Figure 8 of ref 18). This was for a two-chain model having twist angles of 50° and -48°. At the same time, the relative intensities of the symmetric and antisymmetric bands should also change, but this effect is somewhat smaller, as can be seen in the same figure in ref 18. In fact, even though the two bands show the same overall trend in Figure 2, the relative intensities do show a gradual change with temperature. However, the relative intensity is sensitive to both the tilt angle and also the degree of conformational disorder,19 so the best evidence for untilting comes from an analysis of the absolute peak intensities. There is a second factor which contributes to the changes in the integrated intensity seen in Figures 3 and 4. As reported by Snyder et al.,20 the intensities of infrared bands in both crystalline n-alkanes and polyethylene decrease significantly with increasing temperature. Such an effect is not related to the orientation of the chains relative to the metal surface. However, the above study found that the C-H stretching bands decrease much less than the bending bands. For a C21 n-alkane, the intensity of the C-H stretching region decreases by less than 5% between 50 and 300 K, compared to the greater than 25% decrease observed over the same temperature range as seen for SAMs (Figures 3 and 4). Further, given the mechanism involving low-frequency torsional modes that was given to explain this effect for bulk alkanes, one might expect an even weaker effect in monolayers tightly bound to surfaces. As a result of these considerations, we conclude that the reorientation of the alkyl chains toward the surface normal is the major contributing factor for the initial intensity decreases seen in the methylene C-H stretching peaks. A very recent molecular dynamics simulation21 for a C13 thiolate SAM on gold confirms that the chains gradually untilt as the temperature is raised from 40 to 375 K, with the average tilt angle going from 35 to 24°. A rough estimate of the resulting changes in infrared intensities is shown in Figure 6, where the tilt angles are converted to a projection in the direction of the surface normal, assuming a twist angle of zero. (The calculations show a broad distribution of twist angles that vary with temperature.) This confirms the high sensitivity of the IR measurements to changes in tilt angle. The present IR results and both molecular dynamics calculations contradict a recent NEXAFS experiment reported for a C22 thiolate SAM on gold.22 In that case the tilt angle was determined from the R* resonance dichroism, and was found to increase continuously between 310 and 400 K. A possible explanation for this discrepancy is the different depth sensitivity of NEXAFS and RAIRS. The former is highly surface sensitive, and may not be probing the full length of the chain. 4.2. Conformational Disorder. Above about 350 K, there is a significant increase in the integrated intensity of both methylene C-H stretching bands. In this case, by analogy with the melting of crystalline n-alkanes and lipid bilayers, one might have expected a further decrease (18) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927-945. (19) Snyder, R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta 1978, 34A, 395-406. (20) Snyder, R. G.; Maroncelli, M.; Strauss H. L.; Hallmark, V. M. J. Phys. Chem. 1986, 90, 5623-5630. (21) Bhatia, R.; Garrison, B. J. Langmuir 1997, 13, 765-769. (22) Schertel, A.; Ha¨hner, G.; Grunze, M.; Wo¨ll, Ch. J. Vac. Sci. Technol. A 1996, 14, 1801-1806.

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Figure 6. Estimated changes in the CH2 stretching intensities based on the polar tilt angles determined by Bhatia and Garrison.21

in intensity. Once again, however, the dominant effect arises because RAIRS on metal surfaces is sensitive mostly to vibrations perpendicular to the surface. Below 350 K, the chains are mostly linear, and the proposed untilting observed to this point results in a low intensity. The large intensity increase is the result of the onset of considerable disorder in the chains, leading eventually to a random orientation of the methylene groups relative to the surface, which increases the number of methylene groups with large components perpendicular to the surface. Large increases in the intensities of these same two peaks are also observed with RAIRS upon heating LangmuirBlodgett layers adsorbed on metal substrates, which has also been explained in terms of disorder in the alkyl chain.23 Although not shown in Figures 3 and 4, the intensities of the methylene peaks decrease sharply near 450 K, which is probably a result of desorption and/or decomposition. We do not know exactly which bond is being broken (AuAu, Au-S, or S-C), but most evidence points to Au-S bond scission. It has recently been reported that butanethiol desorbs intact from Ag(100) with a desorption maximum at 550 K, although desorption begins at 370 K.1 This is in agreement with He diffraction and STM studies showing that the gold surface which remains after desorption is clean and well-ordered.7 Very recently it has been reported that C18 desorbs as dimers from Au(111), with the desorption maximum being at about 450 K.24 We address now the correlation between the intensities and the frequencies of the methylene C-H stretching bands. It has been well documented that increases in both the d+ and d- frequencies are correlated to the degree of local disorder in long chain n-alkyl systems and are closely related to the number of gauche defects.25 The low frequencies of both peaks at low temperature is a confirmation of the high degree of conformational order of our SAMs, similar to that in crystalline n-alkanes. This is interesting since the gold substrates used in the present (23) Ulman, A. B. Adv. Mater. 1991, 3, 298-303. (24) Nishida, N.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys. 1996, 35, L799-802. (25) Dubois L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463.

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experiments are not well-ordered. However, AFM measurements have confirmed that monolayers of long chain thiols can be well-ordered even on relatively poorly ordered substrates.26 It is even possible that monolayer formation increases the order of the underlying substrate, since it is known that gold can be removed from the surface during monolayer formation.27,28 Starting from this ordered, crystalline state, the data shown in Figure 2 indicate increasing disorder at higher temperatures. The data also show a larger rate of increase in disorder above 350 K. Between 150 and 350 K, the d- frequency increases by less than 2 cm-1. Between 350 and 440 K, it increases by more than 4 cm-1. This change in behavior above 350 K is consistent with the measurements of the integrated intensities and line widths. The small increase in conformational disorder below 350 K occurs at the same time as the chains untilt and the packing density decreases. It has been proposed that in this temperature range the defects are concentrated near the top surface.25 Above 350 K, we propose a much greater increase in disorder, as a result of disorder throughout the chain, which is reflected in the increase in frequency and line width. This latter type of disorder was observed to remain upon cooling. Recent molecular dynamics calculation of Bhatia and Garrison observed a sudden increase in the fraction of gauche defects near 400 K. Some of these defects remained trapped in the system upon cooling, at least within the short time scale of the simulations.21 There has been considerable research into the nature of chain disorder in similar systems. This type of disorder results from defects, of which there are many different types such as kinks, double gauche, and end gauche. Defects near the top surface should be the lowest energy in monolayers, since they do not change the orientation of the rest of the chain. These are the types of defects that are proposed to first appear at lower temperature.21 Near melting, the creation of gauche defects below the surface will create a larger degree of disorder.21 A single defect would change the orientation of a large section of the chain which favors the formation of double defects. The appearance of defects toward the bottom half of the chain would therefore create large increases in the intensity of the d + and d - peaks. The above discussion has concentrated on the methylene peaks. However, there is important information in the methyl peaks as well. As discussed by Dubois and coworkers,4 the appearance of the out of plane antisymmetric stretch at low temperature provides evidence for surface ordering of the methyl groups at 80 K. The intensity of the antisymmetric methyl C-H stretching peak (r -) has also been used to provide information on surface ordering. Precise simulations of the RAIRS spectra reveal that for well-ordered monolayers, the intensity of this peak, compared to those of the methylene peaks, is larger than predicted by the calculations. Parikh and Allara18 showed that the spectra of a C18 alkanethiolate could be better reproduced if they added in the simulations 10-45% of single gauche defects in the terminal methylene group, which results from freely rotating methyl groups. However, if this was the case, then one would expect to see a significant decrease in the intensity of this peak as the system is cooled and the concentration of gauche defects at the surface diminishes. In fact, the opposite is observed. (26) Butt, H.-J.; Seifert, K.; Bamber, E. J. Phys. Chem. 1993, 97, 7316-7320. (27) Edinger, K.; Go¨lzha¨user, A.; Demota, K.; Wo¨ll Ch.; Grunze, M. Langmuir 1993, 9, 4-8.151. (28) Sondag-Huethorst, J. A. M.; Scho¨nenberger, C.; Fokkink, L. G. J. J. Phys. Chem. 1994, 98, 6826-6834.

Thermal Treatment of n-Alkanethiolate Monolayers

The data shown in Figure 1 and also the data of Dubois and co-workers3,4 show that the intensity of this peak actually increases as the temperature is lowered. The discrepancy with the calculations of Parikh and Allara18 may arise from the fact that the reference spectrum used as an input to these simulations had a relatively low intensity for the r + peak, compared, for example, to a low temperature n-alkane. The temperature dependence of the intensity of this peak is the result of the changing tilt angle. Finally, it is intriguing that nonreversible transformations in alkanethiolate SAMs on gold usually occur in the temperature range of 350-380 K.6-8 In the present study, this is the temperature where the onset of nonreversible disorder was observed with long chain SAMs. This is also the annealing temperature which produces the most ordered short chain SAMs, both in our own laboratory and in a number of other studies.6-8 These effects can be explained in terms of a decrease in the two-dimensional density of the monolayer upon heating to these temperatures. It is often the case that annealing occurs near the onset of desorption. In this particular case, these transformations may result from desorption of intact thiol molecules, the annealing away of defects, desorption of molecules trapped in the monolayer, or partial molecular decomposition. In the case of long chain thiols, this loss of density is required for the monolayer to disorder. The strong bond to the surface constrains the monolayer, and once the chains have untilted there is no other mechanism to reduce

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the density further without loss of material. Once this material is lost, the monolayer disorders and remains disordered. When the SAMs are cooled, the disorder which remains is similar to the disorder seen when SAMs are prepared with less than full coverage, indicating that a high packing density is required to order long chain monolayers. Conclusions Ordered monolayers of long chain n-alkanethiols (C15 and above) on gold substrates undergo a series of changes as they are heated in a vacuum. The average tilt angle is a strong function of temperature, and the decrease in the intensities of methylene C-H stretching peaks provides evidence for a gradual chain untilting up to 350 K. In this same temperature range, there is a small increase in the number of gauche defects located near the surface, but the chains remain largely all-trans. At about 350 K, there is an onset of an nonreversible disordering, characterized by a large number of conformational defects. This transition is thought to be correlated with a decrease in monolayer density. Acknowledgment. This work has been funded by the Natural Sciences and Engineering Research Council of Canada. F.B. thanks the Groupe de Recherche en Physique et Technongie des Couches Minces for a fellowship. LA9711589