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Dynamical Exchange Behavior in Organic Monolayers Studied by STM Analysis of Labeled Mixtures F. Stevens and T. P. Beebe, Jr.* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 Received April 10, 1998. In Final Form: June 2, 1999 By using mixtures of molecules which are sterically similar but contain functional groups with differing electronic properties, we have obtained scanning tunneling microscopy (STM) images of domains of randomly mixed monolayers of organic molecules on graphite, and the different molecules could be distinguished by STM. It was determined that molecules in the centers of the domains could still exchange readily with the solution, something which could not be determined using monolayers with single-component domains. The typical residence time of a molecule in the monolayers appeared to be between 0.2 and 20 s, and some variation was observed for different mixtures. We have also shown one case in which the composition of the adsorbed monolayer appears to be nearly independent of the composition of the bulk solution over almost 3 orders of magnitude. Although the mixture components could often be distinguished by STM, other times little or no difference was observed, even when image resolution appeared to be quite good in general. This suggests that the factors leading to STM contrast in organic molecules are complex, and the quest for reliable functional group identification by STM may be more difficult than expected.
Introduction Scanning tunneling microscopy (STM) has been widely used to study monolayers of pure organic compounds. Mixed monolayers containing more than one organic compound are also of interest, but they greatly complicate the interpretation of STM images. However, several mixtures have been studied, including alkane/alcohol mixtures,1 alcohols2 or acids3 of different chain lengths, n-alkylcyanobiphenyls,4-6 unsaturated liquid crystals,7 and others.8-10 In most cases, the mixtures were observed to form separate domains, each composed of a pure component. The cyanobiphenyls form stoichiometric mixedcrystal domains, with the monolayer containing the two components in a fixed ratio and with a specific ordered structure in each unit cell. One question of interest regarding organic monolayers is whether molecules in the monolayer are fixed or can exchange with molecules in the solution or neat liquid covering the monolayer. STM analysis of a mixed monolayer could address this question. To determine if exchange occurs, it is necessary to use a mixture in which the two components are similar enough in size and shape to be interchangeable in the monolayer but different enough * Corresponding author. (1) Venkataraman, B.; Breen, J. J.; Flynn, G. W. J. Phys. Chem. 1995, 99, 6608-6619. (2) Poulin, J.-C. Microsc. Microanal. Microstruct. 1994, 5, 351-358. (3) Hibino, M.; Sumi, A.; Hatta, I. Thin Solid Films 1996, 281-282, 594-597. (4) Kadotani, T.; Taki, S.; Kai, S. Jpn. J. Appl. Phys. 1997, 36, 44404445. (5) Kadotani, T.; Taki, S.; Okabe, H.; Kai, S. Jpn. J. Appl. Phys. 1996, 35, L1345-L1347. (6) Iwakabe, Y.; Hara, M.; Kondo, K.; Oh-hara, S.; Mukoh, A.; Sasabe, H. Jpn. J. Appl. Phys. 1992, 31, L1771-L1774. (7) Stevens, F.; Dyer, D. J.; Walba, D. M. Langmuir 1996, 12, 436440. (8) Vanoppen, P.; Grim, P. C. M.; Ru¨cker, M.; De Feyter, S.; Moessner, G.; Valiyaveettil, S.; Mu¨llen, K.; De Schryver, F. C. J. Phys. Chem. 1996, 100, 19636-19641. (9) Eichhorst-Gerner, K.; Stabel, A.; Moessner, G.; Declerq, D.; Valiyaveettil, S.; Enkelmann, V.; Mu¨llen, K.; Rabe, J. P. Angew. Chem., Int. Ed. Eng. 1996, 35, 1492-1495. (10) Grim, P. C. M.; Vanoppen, P.; Ru¨cker, M.; De Feyter, S.; Valiyaveettil, S.; Moessner, G.; Mu¨llen, K.; De Schryver, F. C. J. Vac. Sci. Technol., B 1997, 15, 1419-1424.
electronically that they will appear different by STM. So, if exchange occurs, a molecule of component A might be replaced by a molecule of component B, and this change would be observable in a sequence of STM images. However, in nearly all mixtures reported to date, the two components were different enough that component A could only be replaced by another molecule of A, so it was impossible to tell if molecules in the monolayer exchanged with molecules in the solution. A few randomly mixed monolayers have been reported previously, but no dynamic processes were observed. In layers of mixed phthalocyanines, STM could be used to differentiate between phthalocyanines containing Co and those containing Cu.11,12 Lead phthalocyanine produces monolayers in which some molecules show a bright center by STM, whereas others show a dark center.13 These molecules have been observed to change between bright and dark centers, but this is believed to be due to conformational changes rather than molecular exchange. A mixed-monolayer liquid crystal was tentatively identified based on unit-cell dimensions, but the two different molecules could not be distinguished.7 In a final case, protoporphyrin containing iron appeared bright by STM, whereas protoporphyrin without iron appeared relatively dark. Solutions of protoporphyrin with and without iron form mixed monolayers on graphite, and the two different molecules could be differentiated.14 The monolayer ratios were found to be approximately the same as the solution ratios, but no dynamical exchange processes were reported. Here, we report data from two mixtures which form random mixed monolayers on graphite and which also show dynamical processes observable on the STM time scale. The two mixtures studied were (1) a mixture of a saturated and an unsaturated fatty acid; and (2) a mixture of a long-chain alcohol and a long-chain thiol. The results clearly show that molecules in the monolayers can (11) Hipps, K. W.; Lu, X.; Wang, X. D.; Mazur, U. J. Phys. Chem. 1996, 100, 11207-11210. (12) Lu, X.; Hipps, K. W.; Wang, X. D.; Mazur, U. J. Am. Chem. Soc. 1996, 118, 7197-7202. (13) Strohmaier, R.; Ludwig, C.; Petersen, J.; Gompf, B.; Eisenmenger, W. J. Vac. Sci. Technol. B 1996, 14, 1079-1082. (14) Tao, N. J. Phys. Rev. Lett. 1996, 76, 4066-4069.
10.1021/la980416e CCC: $15.00 © 1999 American Chemical Society Published on Web 07/23/1999
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Figure 1. Comparison of constant-height STM images of a saturated acid, an unsaturated acid, and a mixture: (A) Stearic acid [saturated, CH3(CH2)16CO2H]; 0.53 V, 141 pA, 90 Å × 100 Å; (B) Elaidic acid [unsaturated, CH3(CH2)7CHdCH(CH2)7CO2H]; 0.85 V, 162 pA, 90 Å × 100 Å; (C) Mixture of stearic and elaidic acids; 0.30 V, 150 pA, 150 Å × 180 Å. Insets in A and B show the monolayer structure; COOH groups are denoted by round dots. Note that the contrast between the COOH groups and the alkyl chains varies somewhat between images. This may be due to some type of tip variation.
exchange with the solution and that the time scale for this exchange can be estimated by analysis of sequential STM images. Experimental Section The STM used in these studies is custom built and has been described elsewhere.15 All images were acquired under a liquid droplet of the compounds in phenyloctane solution at room temperature, using mechanically cut tips of Pt/Ir (80:20). Stearic acid (CH3(CH2)16CO2H) and elaidic acid (CH3(CH2)7CHdCH(CH2)7CO2H) were obtained from Sigma. Octadecanethiol (CH3(CH2)17SH) was obtained from Aldrich. 1-phenyloctane and octadecanol (CH3(CH2)17OH) were obtained from Fluka. All chemicals were used as received. HOPG was supplied by Dr. Arthur W. Moore of Union Carbide. Samples were prepared by cleaving the HOPG surface with tape and applying a drop of the organic material as a saturated solution in phenyloctane. Typical constant-height scanning conditions were 0.6 V (tip positive) and 160 pA. Images were not filtered. STM images could not always be obtained immediately, and voltage pulses were applied until the monolayer could be imaged. Voltage pulses were produced by briefly increasing the bias voltage by a factor of 10. Component ratios in solution were analyzed by gas chromatography (GC) using a Shimadzu GC-14A with FID detector and a DB-5 column heated to a constant temperature of 220 °C. The as-received octadecanethiol and octadecanol showed only single peaks by GC, from which we ruled out the possibility of any significant amount of disulfide formation.
Results Saturated/Unsaturated Acids. Both saturated and unsaturated fatty acids have been previously observed by STM,16-18 and the double bonds in the unsaturated acids were reported to image “bright” (enhanced electron tunneling) relative to the alkyl chains. The 18-carbon acids stearic acid (saturated) and elaidic acid (unsaturated between carbons 9 and 10) have also been found to form identical unit cells16 and so appeared to be good candidates (15) Zeglinski, D. M.; Ogletree, D. F.; Beebe Jr., T. P.; Hwang, R. Q.; Somorjai, G. A.; Salmeron, M. B. Rev. Sci. Instrum. 1990, 61, 37693774. (16) Hibino, M.; Sumi, A.; Hatta, I. Jpn. J. Appl. Phys. 1995, 34, 610-614. (17) Canet, D.; Guillain, F.; Sanson, A. J. Trace and Microprobe Techniques 1995, 13, 361-362. (18) Hatta, I.; Nishino, J.; Sumi, A.; Hibino, M. Jpn. J. Appl. Phys. 1995, 34, 3930-3936.
as a mixture for forming a random cocrystal. It was expected that the saturated and unsaturated molecules should be the same size and should be distinguishable by STM due to the presence or absence of a “bright spot” in the STM image, signifying the double bond. We obtained numerous images of both the pure acids and the mixture. While we found strong evidence that a mixed monolayer did indeed form, the results were less clear-cut than anticipated. Figure 1A shows an STM image of the pure saturated acid showing only alkyl chains with low contrast, as expected. STM images of the pure unsaturated acid sometimes showed a bright spot in the center of each alkyl chain, as has been reported before and as shown in Figure 1B, but other times were indistinguishable from the saturated acid images. Images of the monolayer formed by a mixture of the two acids sometimes showed a mixed monolayer, with bright spots on only some of the molecules, as shown in Figure 1C; other times the mixture produced STM images which were similar to images of the pure saturated acid or the pure unsaturated acid. In summary, pure saturated acid only produced images similar to Figure 1A; pure unsaturated acid produced images similar to Figure 1A or Figure 1B; and the mixture produced images similar to Figure 1A, Figure 1B and Figure 1C. These results are consistent with the double bond sometimes imaging brighter than the alkyl chains and sometimes imaging with the same brightness as the alkyl chains. Despite this unexpected variability, we can state with some confidence that Figure 1C represents a mixed crystal because images such as Figure 1C were never obtained from either of the pure acids. The fact that the mixture sometimes produced images identical to the pure acids could be explained if the mixture formed domains of pure saturated or pure unsaturated acid as well as mixed domains. However, the fact that the pure unsaturated acid sometimes produced images indistinguishable from the pure saturated acid appears to demonstrate that the double bonds are not always observed by STM. This is consistent with an earlier report that the double bonds in an unsaturated liquid crystal could not be reliably observed by STM.7 The reason for this variability is not clear, although it could be tip-related. Because the double bond could not be consistently
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observed, no attempts were made to analyze the saturated/ unsaturated monolayer ratios quantitatively. A crude estimate of the rate of exchange between the monolayer and the solution can be found by noting the following observations. In Figure 1C, because the “bright spots” are 5 to 7 scan lines across and each scan line took 0.032 s to acquire, the molecules appear to be stable on a time scale of at least 0.16 s. Images taken immediately after Figure 1C show different patterns of bright spots. Although there is no way to reference the image position absolutely, no correlation between images could be found, even if the two images were offset. Unless the drift rate was extremely high (much higher than the drift rates we typically observed), this shows that a significant amount of molecular exchange had occurred between images, which took 21 s to acquire. Thus, the average time a molecule remains fixed in the monolayer without exchanging must be between approximately 0.2 and 20 s. It is possible that the presence of the STM tip is affecting the exchange rates, because scanning has been shown to realign adsorbed monolayers in some cases.19 Alcohol/Thiol Mixtures. In hopes of finding a mixture with more consistent behavior that could be studied more quantitatively, monolayers composed of octadecanol mixed with octadecanethiol were studied. Alcohols form ordered monolayers readily and have been studied by many groups. The alcohol functional group typically appears with a similar brightness as the alkyl chain.20 Thiols have been observed less frequently, but STM images of thiols,21,22 thioethers,23,24 and disulfides21,23,25 have been reported. In all cases, the sulfur atom appears considerably brighter than the alkyl chain. A series of six mixtures of octadecanethiol and octadecanol having a range of [thiol]/[alcohol] molar ratios were made, and the solution ratios were measured precisely by gas chromatography. STM images of the mixtures showed qualitative differences correlated to the varying [thiol]/[alcohol] ratios.26 When the solution had a [thiol]/[alcohol] ratio of ∼0.6 or less, a herringbone pattern of molecules oriented at 60° to the rows was observed, as seen in Figure 2, identical to the structure seen in monolayers of pure alcohol. Solutions having [thiol]/[alcohol] ratios of ∼1 or greater showed a perpendicular pattern of molecules oriented at 90° to the rows, as seen in Figure 3, identical to the structure seen in monolayers of pure thiol. In both cases, some bright spots (identified as thiol groups) were observed at the edges of the lamellae. The change in structure thus occurs at between ∼0.6 and ∼1 for the [thiol]/[alcohol] ratio. Monolayers from solutions with [thiol]/[alcohol] ratios below ∼1 showed only a few isolated bright spots (Figure 2), which is consistent with a monolayer composed primarily of alcohol but containing a few thiol molecules. (19) Stevens, F.; Buehner, D.; Beebe Jr., T. P. J. Phys. Chem. B 1997, 101, 6491-6496. (20) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. Chem. Mater. 1996, 8, 1600-1615. (21) Venkataraman, B.; Flynn, G. W.; Wilbur, J. L.; Folkers, J. P.; Whitesides, G. M. J. Phys. Chem. 1995, 99, 8684-8689. (22) Gunning, A. P.; Kirby, A. R.; Mallard, X.; Morris, V. J. J. Chem. Soc.. Faraday Trans. 1994, 90, 2551-2554. (23) Claypool, C. L.; Faglioni, F.; Goddard III, W. A.; Gray, H. B.; Lewis, N. S.; Marcus, R. A. J. Phys. Chem. B 1997, 101, 5978-5995. (24) Giancarlo, L.; Cyr, D.; Muyskens, K.; Flynn, G. W. Langmuir 1998, 14, 1465-1471. (25) Rabe, J. P.; Buchholz, S.; Askadskaya, L. Synth. Met. 1993, 54, 339-349. (26) It is possible that disulfides are also present in the monolayer. However, disulfides would cause additional “bright spots” and would increase the observed [thiol]/[alcohol] ratio. Since this ratio is low, there must be few, if any, disulfides in the monolayer. GC analysis did not indicate the presence of this impurity.
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These bright spots were observed only at every other row edge, as seen in Figure 4. Alcohols form monolayers with lamella presenting alternating head-to-head and tail-totail contacts, so that the hydrophilic alcohol groups become segregated away from the hydrophobic alkyl tails, similar to a lipid bilayer. If one alcohol molecule leaves the monolayer and is replaced by a thiol, the new molecule could orient with the thiol group contacting the hydroxyls of one adjacent row (position A in Figure 4) or with the thiol contacting the methyls of the other adjacent row (position B in Figure 4). It was observed that the bright spots in the STM images always appeared with a twomolecule spacing, as seen in Figures 2-4. This is consistent with the thiols showing a preference for thiol-hydroxyl and methyl-methyl contacts, as opposed to methyl-thiol and methyl-hydroxyl contacts. Monolayers from solutions having [thiol]/[alcohol] ratios of ∼1 or greater appeared to show more bright spots than monolayers from solutions dominated by alcohol; however, the number of spots that were clearly bright was still low, and the contrast in these monolayers more closely resembled that seen in monolayers of pure alcohol rather than that seen in monolayers of pure thiol. Bright spots seen in monolayers from solutions dominated by thiol also showed a continuous variation: from no brightness, to faintly bright, to strongly bright, as seen in Figure 3. This variation made accurate counting problematic. The bright spots were still observed to occur only at every other row edge, which suggests that the monolayers contain enough alcohol to preserve a predominant pattern of alternating hydroxyl-hydroxyl and methyl-methyl contacts. In monolayers of pure thiol, the thiols display only a small preference for head-to-head over head-to-tail interaction21 because thiol-thiol hydrogen bonding is very weak to nonexistent.27,28 As seen in the time series of images in Figure 2, the number and pattern of bright spots (thiol molecules) varied from scan to scan, showing that molecules in the monolayer were exchanging with molecules in the solution between scans. The residence time of a molecule within these alcohol/thiol monolayers can be estimated in the same manner as was done for the acid mixtures. For mixtures from solutions with [thiol]/[alcohol] ratios below ∼1, the results are comparable to the results from the acid mixtures: namely, that molecules appear stable over a period of several scan lines but change between image frames, giving a time scale for exchange of between 0.2 and 20 s. However, for mixtures having [thiol]/[alcohol] ratios above ∼1, the alcohol/thiol exchange time scale appeared to be comparable to the scan line time (0.032 s). This can be seen in Figure 3, which shows faint spots of variable brightness that are consistent with molecules that sometimes remained in place over several scan lines (“a” arrows in Figure 3) but often exchanged after being imaged over only one or two scan lines (“b” arrows in Figure 3). Thus, for these monolayers, the exchange time scale must be on the order of 0.03 to 0.3 s. Despite the difficulties in counting the thiol groups, an attempt was made to measure the steady-state [thiol]/ [alcohol] ratio in monolayers from each solution. The number of thiol molecules was directly measured by counting the number of spots in each image that were clearly “bright”, and the number of alcohol molecules was obtained by dividing the area of the image by the area occupied by one alcohol molecule (from the known alcohol (27) Joesten, M. D.; Schaad, L. J. Hydrogen Bonding; Marcel Dekker: New York, 1974; pp 278, 283. (28) Wenzler, L. A.; Moyes, G. L.; Raiker, G. N.; Hansen, R. L.; Harris, J. M.; Beebe Jr., T. P. Langmuir 1997, 13, 3761-3768.
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Figure 2. A sequence of four STM images of a monolayer from a [thiol]/[alcohol] mixture. The herringbone lamellae pattern characteristic of alcohol monolayers is visible (also see Figure 4). Arrows point to isolated bright spots, which we identify as SH groups on thiol molecules. Note that the number, pattern, and position of bright spots varies, and that the domain boundary at the bottom of each image serves as an approximate reference location. Each image was obtained in constant-height mode, 0.56 V, 176 pA, 350 Å × 400 Å. Elapsed time is given in minutes:seconds.
unit cell) and subtracting the number of thiol molecules. Although assignment of “bright” spots was often problematic, it was thought that reasonable statistics could be obtained by averaging over a large number of images, and any trends should be observable, even if absolute measurements contained a systematic undercounting of the thiols. A total of 430 images were obtained, spread fairly evenly over the six solutions having different [thiol]/ [alcohol] ratios. The thiol/alcohol ratios found in the monolayers by STM could now be compared to the thiol/ alcohol ratios in the solution, as determined by GC. The results, given in Figure 5, show that the monolayer ratio
remains essentially independent of the solution ratio and that the monolayer was dominated by alcohol at all solution ratios studied. The only change in monolayer ratio appears as a modest difference between solution ratios above and below ∼1; this difference is associated with a change in monolayer structure, as previously discussed. Discussion Although functional groups are generally assumed to have a constant “STM brightness,” one other case has appeared in the literature,29 in which a functional group was observed with widely varying contrast. In that case,
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Figure 3. STM image of a monolayer from a [thiol]/[alcohol] solution with [thiol]/[alcohol] ratio of ∼1. The pattern of molecules oriented perpendicularly to the rows is visible. Labeled arrows show examples of: (a) strongly bright spots, (b) faintly bright spots, and (c) locations without bright spots. Arrows at top show the two types of row contacts: one dominated by bright spots (right arrow), and one showing no bright spots but displaying a gap slightly darker than the alkyl chains (left arrow). Inset shows schematic of monolayer structure. Constant height, 0.59 V, 165 pA, 320 Å × 360 Å.
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Figure 4. Expanded section of Figure 2D (124 Å × 498 Å), along with a schematic model of the structure. A gap has been left in the structure, and an incoming exchanging molecule could be placed in the gap with its functional group directed toward A or toward B. It was observed that thiol molecules always oriented with the SH group at the A position, as can be seen by the two bright spots (SH groups) in the STM image.
the Br in a primary bromoalkane sometimes appeared brighter than the alkyl chain and other times showed no contrast relative to the alkyl chain. However, that case was explained by a conformational shift, and theoretical studies supported the idea that the Br could be “bright” when the C-Br bond was angled off the HOPG surface and “dark” when it was parallel to the surface.30 Similar explanations do not appear to be valid for the current study. At present, we have no explanation for why the brightness of the double bond should be variable, although it could be due to subtle changes in the tip structure or tip wavefunction. In general, the “STM brightness” of functional groups has been correlated to the presence of orbitals relatively close to the substrate Fermi level, which tend to image bright by STM due to increased electronic coupling.23,29,30 However, this correlation is not perfect,31 and the reasons for functional group brightness by STM are not completely understood. STM identification of functional groups is of great interest, both for a better understanding of how image contrast is developed in molecular STM images and for identification of molecules by STM. Our findings that double bonds cannot be reliably observed and identified by STM implies that identification of functional groups by STM may be even more problematic than has been thought. The Langmuir model of competitive adsorption32 predicts that the equilibrium ratio of two components
where C is a proportionality constant. Figure 5 clearly shows that this was not observed. Instead, the adsorbate ratio [A]ad/[B]ad was nearly constant and totally dominated by the alcohol, whereas the solution ratio changed by nearly 3 orders of magnitude. We propose that, in this system, one or more of the assumptions made by the Langmuir model are not applicable. Specifically, the Langmuir model assumes that there are no interactions between adsorbed molecules, so that the equilibrium constant for adsorption of each species is independent of the composition of the monolayer. That this is not true in this case can be seen from the change in monolayer structure, from herringbone lamellae when the solution is dominated by alcohol, to perpendicular lamellae when the solution is dominated by thiol. The relatively constant monolayer composition observed experimentally could be explained if a certain monolayer composition (or range of compositions) is favored, regardless of solution composition. The two different monolayer structures evidently have slightly different preferred ratios and ranges, as seen in Figure 5. If this is the case, then the Langmuir model cannot be applied because it is based on an equilibrium
(29) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. J. Phys. Chem. 1996, 100, 13747-13759. (30) Faglioni, F.; Claypool, C. L.; Lewis, N. S.; Goddard III, W. A. J. Phys. Chem. B 1997, 101, 5996-6020.
(31) Stevens, F.; Dyer, D. J.; Mu¨ller, U.; Walba, D. M. Langmuir 1996, 12, 2, 5625-5629. (32) Masel, R. I. Principles of Adsorption and Reaction on Solid Surfaces; Wiley: New York, 1996; p 804.
adsorbed on a surface, [A]ad/[B]ad, should follow a similar trend as the solution ratio [A]soln/[B]soln. That is,
[A]ad [B]ad
)C
[A]soln [B]soln
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Figure 5. A plot of [thiol]/[alcohol] ratios in monolayers (measured from STM images) versus [thiol]/[alcohol] ratios in the solutions covering the monolayers (measured by gas chromatography). Error bars represent one standard deviation within each set of measurements. Note the use of a log scale for solution ratios.
between molecules adsorbed and in solution. If a certain monolayer composition is favored, independent of solution composition, then the equilibrium theory does not apply. Monolayer ratios found by counting “bright spots” in the STM images show only a small amount of thiol in all monolayers (25 to 1). For monolayers formed from solutions having [thiol]/[alcohol] ratios of less than ∼1, the monolayers show the same herringbone lamella structure as monolayers of pure alcohol, consistent with the low thiol content measured. Monolayers formed from solutions having [thiol]/[alcohol] ratios above ∼1 show two changes consistent with an increased amount of thiol. First, these monolayers show the same perpendicular lamella structure as pure thiols. Second, the exchange rate increases, consistent with weaker molecule-molecule interactions (decreased hydrogen bonding). Monolayer ratios determined by STM did show a factor of ∼5 to 10 increase in thiol coverage for monolayers derived from solutions of [thiol]/[alcohol] ratios above ∼1, but the absolute amount of thiol measured was still quite low. From the change in monolayer structure, one might have expected a larger change in monolayer composition; however, the orientation of thiols in the monolayer (remaining head-to-head, rather than random) suggests that even monolayers derived from solutions dominated by thiol still contain significant amounts of alcohol. The more rapid exchange seen in the monolayers from solutions of [thiol]/[alcohol] ratios above ∼1 makes accurate counting much more difficult because spots with a range of brightness were observed, rather than simply a few spots much brighter than the background (compare Figure 2 and Figure 3), and the images generally showed poorer resolution. The method of counting only the spots which show up clearly above the background probably
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undercounts the amount of thiol in the monolayer but should still accurately reflect any trends in monolayer composition. Both the variation in brightness and the loss of resolution are probably due to the increased rate of exchange between the monolayer and the solution. Our finding that the residence time for a molecule33 in a mixed alcohol/thiol monolayer decreased at a higher solution thiol concentration agrees with expectations based on hydrogen bonding. An alcohol monolayer is stabilized by hydrogen bonding between alternate rows of molecules. Because thiols form much weaker hydrogen bonds than alcohols do, adding thiol to the monolayer disrupts the hydrogen bonding, thus weakening the forces that hold molecules within the monolayer and allowing more rapid exchange with solution. This result suggests that the residence time of molecules within the monolayer can be controlled to some degree by changing the functionality of the molecules or by changing the composition of the monolayer. Conclusions The strategy of mixing sterically similar but electronically different molecules was used to form two different randomly mixed organic monolayers. Although reliable differentiation of the molecules by STM was more difficult than anticipated, images were obtained in which it was possible to identify the molecules based on their different appearance in the STM images. In addition, sequences of images showed molecules in the centers of domains exchanging with solution. Analysis of sequences of images permitted estimates of residence times for exchange between the monolayer and the solution, which were generally between 0.2 and 20 s, except for the monolayers from thiol/alcohol solution, when the [thiol]/[alcohol] ratio was above ∼1. In those cases, the time-scale for exchange appeared to be faster, 0.03 to 0.3 s. Quantitative studies of steady-state molecular coverage ratios showed no correlation between monolayer composition and solution composition over a range of solution concentration ratios of almost 3 orders of magnitude. Analysis of mixed monolayers by STM presents challenges in design of mixtures and in interpretation of images. For quantitative measurement of mixed monolayers, it is important not only that a mixed monolayer form and that the components be distinguishable by STM, but also that the exchange rate be slow enough that the different molecules can be observed clearly. We have shown that these challenges can be overcome, and mixed monolayers can be used to gain information about dynamic processes that is not available from studies of pure monolayers. Acknowledgment. This work was supported by the National Science Foundation (CHE-9357188), the Camille and Henry Dreyfus Foundation, and the Alfred P. Sloan Foundation. LA980416E (33) Because both a molecule of alcohol and a molecule of thiol are involved in any single measurable exchange event, the determined rate is a composite of both molecules. Individual exchange rates could not be calculated.
