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Friction Force Microscopy of Self-Assembled Monolayers: Influence of Adsorbate Alkyl Chain Length, Terminal Group Chemistry, and Scan Velocity Nicholas J. Brewer, Ben D. Beake, and Graham J. Leggett* Department of Chemistry, The University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester, United Kingdom M60 1QD Received November 10, 2000 Friction force measurements have been made using friction force microscopy for a series of self-assembled monolayers (SAMs) of alkanethiols of varying chain lengths and terminal groups adsorbed on gold. The chemistry of the tip was controlled by the deposition of a SAM. When carboxylic acid terminated tips were employed, the friction coefficients of hydroxyl and carboxylic acid terminated SAMs were found to be greater than those of methyl terminated SAMs. However, although the friction coefficients of short-chain methyl terminated SAMs were significantly greater than those of long-chain methyl terminated SAMs, there was not a significant difference between the values determined for short- and long-chain SAMs with polar terminal groups. When methyl terminated tips were employed, there was no difference between the behavior observed for hydroxyl and carboxylic acid terminated SAMs of equal chain length, but methyl terminated SAMs exhibited increased friction coefficients which were larger for short-chain adsorbates. Friction-velocity plots exhibited markedly different behavior for polar and nonpolar SAMs when carboxylic acid terminated tips were employed. These observations are explained in terms of the stabilization of the adsorbates by intermolecular hydrogen bonding in SAMs with polar terminal groups.
Introduction There has recently been considerable interest in probing the frictional1,2 and adhesive3,4 properties of organic materials using atomic force microscopy (AFM). The development of chemical force microscopy4,5 has additionally enabled control of the chemistry of the tip-sample interaction, and a number of groups have successfully attached monolayers of alkylthiols and alkylsilanes to AFM probe tips.5-9 Because of the capability that exists to tailor their structures and properties through control of the adsorbate terminal group, self-assembled monolayers (SAMs) are ideal systems for investigations into the molecular origins of phenomena such as friction, and a number of authors have investigated the relationship between SAM structure and frictional behavior. Xiao et al.10 and Lio et al.11 investigated the frictional properties of alkanethiols and alkylsilanes as a function of adsorbate alkyl chain length, concluding that short-chain SAMs exhibited higher friction coefficients than long-chain monolayers because of their greater disorder. These * Corresponding author. E-mail:
[email protected]. (1) Overney, R. M.; Meyer, E.; Frommer, J.; Brodbeck, D.; Luthi, R.; Howald, L.; Guntherodt, H.-J.; Fujihira, M.; Takano, H.; Gotoh, Y. Nature 1992, 359, 133. (2) Meyer, E.; Overney, R. M.; Brodbeck, D.; Howald, L.; Luthi, R.; Frommer, J.; Guntherodt, H.-J. Phys. Rev. Lett. 1993, 69, 1777. (3) van der Werf, K. O.; Putman, C. A. J.; de Grooth, B. G.; Greve, J. Appl. Phys. Lett. 1994, 65, 1195. (4) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. (5) Noy, A.; Frisbie, C. D.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1995, 117, 7943. (6) Green, J.-B. D.; McDermott, M. T.; Porter, M. D. J. Phys. Chem. 1995, 99, 10960. (7) Sinniah, S. K.; Steel, A. B.; Miller, C. J.; Reutt-Robey, J. E. J. Am. Chem. Soc. 1996, 118, 8925. (8) Tsukruk, V. V.; Bliznyuk, V. N. Langmuir 1998, 14, 446. (9) van der Vegte, E. W.; Hadziioannou, G. Langmuir 1997, 13, 4357. (10) Xiao, X.; Hu, J.; Charych, D. H.; Salmeron, M. Langmuir 1996, 12, 235. (11) Lio, A.; Charych, D. H.; Salmeron, M. J. Phys. Chem. B 1997, 101, 3800.
observations have been supported by other experimental studies12,13 and have recently been modeled by Harrison and co-workers.14 Beake and Leggett studied mixed SAM systems composed of short- and long-chain alkanethiols. They found that the friction coefficient varied substantially with monolayer composition and that these variations could be explained in terms of changes in alkyl chain structure.15 Kiely and Houston measured increased coefficients of friction for aged SAMs, which they attributed to a collapse in molecular order following oxidation of the gold-thiolate bond.16 The nature of the terminal functionality of the SAM also has an influence on the tipsample frictional interaction,4,5,9,17 and there is recent evidence that its size may be important.18,19 Although the effect of adsorbate alkyl chain length on frictional interactions with the probe tip has been explored, the studies published to date have dealt exclusively with methyl terminated SAMs and the relative effect of the alkyl chain length on frictional behavior has not been systematically compared for molecules with both polar and nonpolar terminal groups. In addition, although there have been some studies of the relationship between frictional behavior and the scan speed for SAMs16,20 there has not, to date, been a systematic study encompassing a range of terminal group chemistries and alkyl chain lengths. In this study, we investigated the frictional (12) McDermott, M. T.; Green, J.-B. D.; Porter, M. D. Langmuir 1997, 13, 2504. (13) Li, L.; Yu, Q.; Jiang, S. J. Phys. Chem. B 1999, 103, 8290. (14) Tutein, A. B.; Stuart, S. J.; Harrison, J. A. Langmuir 2000, 16, 291. (15) Beake, B. D.; Leggett, G. J. Langmuir 2000, 16, 735. (16) Kiely, J. D.; Houston, J. E. Langmuir 1999, 15, 4513. (17) Tsukruk, V. V.; Everson, M. P.; Lander, L. M.; Brittain, W. J. Langmuir 1996, 12, 3905. (18) Kim, H. I.; Koini, T.; Lee, T. R.; Perry, S. S. Langmuir 1997, 13, 7192. (19) Kim, H. I.; Graupe, M.; Oloba, O.; Koini, T.; Imaduddin, S.; Lee, T. R.; Perry, S. S. Langmuir 1999, 15, 3179. (20) Bliznyuk, V. N.; Everson, M. P.; Tsukruk, V. V. J. Tribol. 1998, 120, 489.
10.1021/la001568o CCC: $20.00 © 2001 American Chemical Society Published on Web 02/20/2001
Friction Force Microscopy of SAMs
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Table 1. Thiols Used in This Study thiol
molecular formula
abbreviation
butanethiol mercaptopropanol mercaptopropanoic acid dodecanethiol mercaptoundecanol mercaptoundecanoic acid
HS(CH2)3CH3 HS(CH2)3OH HS(CH2)2COOH HS(CH2)11CH3 HS(CH2)11OH HS(CH2)10COOH
C3CH3 C3OH C2COOH C11CH3 C11OH C10COOH
behavior of SAMs with two different chain lengths and methyl, hydroxyl, and carboxylic acid terminal groups (see Table 1). We have investigated the dependence of the friction force on the scan velocity for each SAM. To identify the contribution of adhesive interactions to the friction force, we have controlled the tip chemistry throughout. The resulting data reveal very different chain-length dependences and friction-velocity correlations for SAMs with polar and nonpolar terminal groups. Experimental Section Contact mode imaging was performed in ethanol and in air with chemically modified silicon nitride Nanoprobe cantilevers using a Digital Instruments Nanoscope IIIa Multimode atomic force microscope. The nominal force constant of the cantilevers was 0.12 N m-1. The spring constants of individual cantilevers were determined using a resonance method implemented in the microscope software and were found to be in the range of 0.100.14 N m-1. The approach speeds were in the region of 0.35 µm s-1. Friction force measurements were made from friction loops recorded in scope mode at 10 separate points on the monolayer surfaces with the scan velocity fixed at 3 µm s-1. For measurements of friction-velocity relationships, the cantilever was placed under a constant load, and the scan velocity was increased incrementally while the friction force was recorded. SAMs were prepared by immersion of gold-coated microscope slides or AFM cantilevers in 1 mM solutions of the appropriate thiol in degassed ethanol. Butanethiol (C3CH3), mercaptopropanol (C3OH), mercaptopropanoic acid (C2COOH), and dodecanethiol (C11CH3) were obtained from Fluka Chemicals Ltd and used as received. Mercaptoundecanol (C11OH) and mercaptoundecanoic acid (C10COOH) were purchased from Aldrich Chemicals Ltd and also used as received. All glassware used was cleaned with piranha solution (a 3:7 mixture of 30% hydrogen peroxide and concentrated sulfuric acid). This mixture is a very strong oxidizing agent and has been known to detonate spontaneously with organic material, so great care should be employed when using it. Glass microscope slides (22 mm × 64 mm and no. 2 thickness) were obtained from Chance Proper Ltd. The glass slides and Nanoprobe AFM tips were placed into a stainless steel slide holder mounted in an Edwards bell-jar thermal evaporation system. Chromium (10 Å) was deposited as an adhesion promoter, followed by polycrystalline gold to a thickness of approximately 250 Å. A quartz crystal film thickness monitor was used to determine the evaporation rate. The evaporation rate was always kept below 3 Å s-1 to ensure that the cantilevers did not bend during deposition.21 After deposition of the metals, the slides and cantilevers were left to cool. The gold slides were then immersed in a 1 mM solution of the respective thiol for 18 h. Once formed, the SAM slides and tips were removed from solution, gently rinsed in copious amounts of absolute ethanol, and blown dry in a light stream of oxygen-free nitrogen gas. The SAMs were then ready for experimentation. Contact mode images of all our gold surfaces exhibited low roughness (root-mean-squared (rms) roughness ) 0.6-1.4 nm at 1 µm length scale) and were polycrystalline with the crystallites approximately 50-70 nm across. Average roughness measurements were very similar for all the samples used in the study, and no significant variation was observed with the composition of the self-assembled monolayers. (21) Beake, B. D.; Leggett, G. J. Phys. Chem. Chem. Phys. 1999, 1, 3345.
Figure 1. Variation in friction force with load for C10COOH SAMs in contact with a C10COOH functionalized tip in air (0) and under ethanol (9) and for C11CH3 SAMs in contact with a C11CH3 functionalized tip in air (4) and under ethanol (2).
Results Effect of SAM Chain Length and Terminal Group. The friction force was measured as a function of the applied load for tips modified with carboxylic acid (C10COOH) and methyl (C11CH3) terminated thiols, in contact with carboxylic acid, hydroxyl, and methyl terminated alkanethiol monolayer surfaces in ethanol and air. For all of the tip/ sample combinations studied, the friction forces increased linearly with the normal load applied through the cantilever. The friction forces measured in air were consistently greater than those measured under ethanol, in accordance with expectations, because of the presence of the capillary. Figure 1 shows, as illustrative examples, the data for the acid-acid combination (C10COOH functionalized tip in contact with a C10COOH monolayer) and for a methyl-methyl (C11CH3/C11CH3) combination. The frictional behavior of surfaces can be modeled by a modified form of Amonton’s Law where the lateral force (FL) is given by
FL ) µFN + F0 where µ is the coefficient of friction, FN is the normal force (applied load), and F0 is the friction force when the normal force is zero. µ may therefore be obtained by linear regression analysis of the slopes of friction-load plots. Friction forces were measured as a function of the load for a series of polar and nonpolar SAMs with short and long alkyl chains, and the coefficients of friction were calculated using this method. Both carboxylic acid and methyl terminated tips were used, and the experiments were performed in air and under ethanol for each combination. Each experiment was performed at least seven times. In each case, all six surfaces were examined using the same tip. The coefficients of friction were determined and then normalized to the value for the C2COOH surface (in the case of an acid terminated tip) or the C3CH3 surface (in the case of the methyl terminated tip). This enabled separate sets of data, recorded using different cantilevers, to be compared realistically without the problem of errors introduced because of difficulties associated with the calibration of cantilever lateral force constants. The agreement between different sets of data, even when collected several months apart, was very good. Figure 2 shows the data for a carboxylic acid terminated tip. It may be seen that for each of the six tip/sample combinations, the coefficients of friction are very similar
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Figure 2. Friction coefficients for six different SAMs measured using a C10COOH functionalized tip in ethanol (white bars) and in air (hatched bars).
Brewer et al.
Figure 4. Variation in friction force with scan velocity for a C10COOH functionalized tip in contact with C10COOH and C11CH3 SAMs in air and under ethanol.
Figure 3. Friction coefficients for six different SAMs measured using a C11CH3 functionalized tip in ethanol (white bars) and in air (hatched bars).
for measurements performed in air and under ethanol. Within the limits of experimental error, the results for a given combination are indistinguishable. As can be seen, in both air and ethanol the acid terminated SAMs give the highest friction coefficient values, with the short-chain acid SAM (C2COOH) giving the highest value of all. The nonpolar methyl terminated SAMs gave the lowest values of µ, with the long-chain methyl SAM (C11CH3) giving the lowest of all. Hydroxyl terminated SAMs exhibited slightly smaller values of µ than acid terminated SAMs with the same chain length. The friction coefficient measured for the C3CH3 SAMs was significantly greater than (approximately double) that measured for the C11CH3 SAMs, in agreement with previous findings. However, within the limits of experimental error there was not a significant difference between the coefficients of friction measured for the short- and long-chain acid terminated SAMs (C2COOH and C10COOH) or the short- and long-chain hydroxyl terminated SAMs (C3OH and C11OH), although in each case the shorter adsorbate of each pair exhibited a slightly higher value of µ. Figure 3 shows the data for the methyl terminated tips. Although the friction forces measured were higher in air than under ethanol, it is clear from Figure 3 that the values of µ determined using the methyl terminated tip are also not significantly different for the two environments. The highest value of µ was determined for C3CH3 monolayers. Again, this value was different from that measured for C11CH3 SAMs, although the magnitude of the difference was less than that determined using a carboxylic acid terminated tip. The carboxylic acid and hydroxyl terminated SAMs yielded the smallest coefficients of friction for a given alkyl chain length. However, these were only slightly less than the values recorded for
Figure 5. Variation in friction force with scan speed for a carboxylic acid (C10COOH) functionalized tip in contact with six different SAMs.
the C11CH3 SAMs. It was again observed that the coefficients of friction for short- and long-chain acid terminated and hydroxyl terminated SAMs were little different. The difference was slightly greater than was the case for the carboxylic acid tips, but given the limits of experimental error, it was not a significant difference. For a given chain length, there was no difference in the values of µ determined for hydroxyl and carboxylic acid terminated SAMs. Effect of Sliding Velocity. Figure 4 shows the variation in the friction force with scan velocity at a fixed load of 80 nN for C10COOH and C11CH3 SAMs with a carboxylic acid terminated tip. For C11CH3 SAMs, in both air and ethanol the friction force rises with scan velocity. For C10COOH SAMs, however, the initial rise is much more rapid, before a plateau is reached at a velocity of only 1-3 µm s-1. There is again little difference between the general behavior observed in air and under ethanol and little difference in the point at which the gradient of the friction-velocity plot changes suddenly. In light of these findings, subsequent data were recorded only under ethanol. Figure 5 shows the friction-velocity behavior of a range of short- and long-chain SAMs with polar and nonpolar terminal groups using a carboxylic acid modified (C10COOH) tip in ethanol. It can be seen that for all of the SAMs with polar terminal groups a plateau in the frictionvelocity plot is reached at small velocities. In the course of a large number of experiments, we did not observe a
Friction Force Microscopy of SAMs
Figure 6. Variation in friction force with scan speed for a methyl (C11CH3) functionalized tip in contact with six different SAMs.
significant difference between hydroxyl and carboxylic acid terminated SAMs. Moreover, there did not appear to be a substantive difference between the behavior of SAMs with the same terminal group and different alkyl chain lengths. In contrast, for both of the methyl terminated SAMs the friction force increases steadily with the scan velocity. Different behavior was observed when C11CH3 coated tips were employed (Figure 6). It was found that the friction force now increased much more slowly for the polar SAMs, which exhibited behavior similar to that of the methyl terminated monolayers. Discussion Friction Force Measurements: Friction versus Load. As expected, based on the findings of previous studies,4-6,12 the largest coefficients of friction were measured when similar surfaces interacted (acid/acid and methyl/methyl), and the smallest were observed when dissimilar surfaces were in contact (methyl/acid and methyl/hydroxyl combinations). Adhesion between the tip and the sample may make a substantial contribution to the coefficient of friction,4,5,21 because of the energy that must be dissipated in the shearing of intermolecular interactions, and the adhesive component of the interaction is greatest where strong hydrogen bonds occur (acid/ acid) and weakest where dissimilar groups are involved that interact weakly (acid/methyl).4,5,21 Very different behavior was exhibited when short- and long-chain SAMs with polar and nonpolar terminal groups were compared. A number of studies have reported that µ increases with decreasing alkyl chain length for methyl terminated SAMs.10-12 The increased values of µ determined for short-chain methyl terminated SAMs reflect the greater energy dissipation in these disordered systems. Surprisingly, however, there have been no studies in which µ has been determined as a function of chain length for SAMs with polar terminal groups. Although for the methyl terminated SAMs our data show the expected increase in the value of µ on going from C3CH3 to C11CH3, there was, within the limits of experimental error, no significant difference between the values determined for short- and long-chain SAMs with polar terminal groups. These findings suggest that there is a fundamental difference in the deformation of methyl and carboxylic acid terminated SAMs under the AFM tip. The most likely explanation for the similarity in the behavior of short- and longchain polar SAMs is that they do not exhibit different modes of alkyl chain deformation during scanning in the
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way that methyl terminated SAMs do. There are two possible explanations for this. First, it is possible that the packing densities of shortand long-chain SAMs formed from thiols with polar terminal groups are similar to and also less than those of the ordered methyl terminated SAMs. There is surprisingly little detailed structural information on SAMs formed from adsorbates with polar terminal groups. However, determinations of the area per adsorbate for polar SAMs yield values similar to those determined for methyl terminated SAMs, and the structure of these monolayers is expected to be similar to that of a methyl terminated SAM, unless the presence of the polar terminal group causes a structural rearrangement in order to minimize the interfacial free energy.22 Second, it is possible that intermolecular hydrogen bonding between adsorbate terminal groups has a determining effect on their stabilities and their modes of deformation during friction force measurement. Work in the authors’ laboratory has examined the effect of terminal group hydrogen bonding on the stabilities of patterned SAMs towards displacement by solution-phase thiols.23 Our data suggest that a carboxylic acid group may contribute as much as 33 kJ mol-1 to the enthalpy of adsorption of a SAM. Significantly, in these studies little difference was observed in the ability of C2COOH, C10COOH, C3OH, and C11OH to displace methyl terminated thiols. It was not clear what the mechanism of stabilization due to the polar terminal group was, however. One possibility is that intermolecular hydrogen bonds order the adsorbates so that even short-chain thiols with polar terminal groups form well-ordered layers. Alternatively, the hydrogen bonds may freeze the adsorbates into a twodimensional glass. On balance, we favor the second hypothesis. The data in Figure 3 correspond closely with expectations based on the work of Cooper and Leggett.23 With the methyl terminated tip, when little adhesion is expected between the tip and the polar SAMs,4,5 they exhibit smaller values of µ than C11CH3. When there is little tip-sample adhesion, the bulk of energy dissipation is expected to be in the form of adsorbate deformation. If the polar SAMs were less closely packed than their methyl terminated analogues, the value of µ would be expected to be higher. However, for adsorbates with greater intermolecular interaction energies than that of a C11CH3 SAM (as predicted by Cooper and Leggett23) the values of µ for the polar SAMs would in these circumstances be expected to be smaller than µ(C11CH3), as, indeed, they are. Friction Force Measurements: Friction versus Sliding Velocity. A number of other workers have studied the dependence of friction on tip velocity in related systems.16,17,20,24,25 Liu et al.25 and van der Vegte et al.24 reported variations in friction for monolayers of (respectively) dialkylammonium salts and unsymmetrical ndialkyl sulfides that were correlated with chain melting temperatures. However, the form of the friction-velocity relationships that they observed is quite different from the behavior exhibited in Figures 4-6. It seems unlikely that such explanations apply in the present circumstances, therefore. More relevant is the work of Tsukruk and coworkers on stearic acid monolayers.20,26 They noted that (22) Sprik, M.; Delamarche, E.; Michel, B.; Rothlisberger, U.; Klein, M. L.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 4116. (23) Cooper, E.; Leggett, G. J. Langmuir 1999, 15, 1024. (24) van der Vegte, E. W.; Subbotin, A.; Hadziioannou, G.; Ashton, P. R.; Preece, J. A. Langmuir 2000, 16, 3249. (25) Liu, Y.; Evans, D. F.; Song, Q.; Grainger, D. W. Langmuir 1996, 12, 1235.
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for solid surfaces the relationship between the shear force Fx and the shear velocity vx is logarithmic and reported observing the expected logarithmic behavior for several solid materials. On this basis, a monotonic increase in the friction force should be expected in Figures 4-6 as the scan velocity is increased. This behavior is indeed observed for the methyl terminated SAMs, and, although the friction data in Figure 6 do not exhibit an exactly linear relationship to the scan velocity, there is nevertheless a steady increase. However, the data for the polar terminated SAMs in Figures 4 and 5 are clearly anomalous, with their distinctive pattern of rapidly increasing friction at low speeds, followed by the attainment of a plateau. To summarize our data, the expected friction-velocity behavior is observed for all tip/sample combinations, except when a polar tip contacts a polar SAM. The chemistry of the tip is clearly important, because these same monolayers exhibit behavior similar to that observed for the methyl terminated SAMs when a methyl terminated tip is employed. However, the explanation is not simply that for polar-polar contacts there is increased adhesion. Stronger adhesion is expected between methyl terminated tips and methyl terminated SAMs than between methyl terminated tips and polar SAMs.4,5,21 On this basis, one would expect that the friction-velocity relationships for methyl terminated SAMs would be different from those exhibited by the polar SAMs when a methyl terminated tip is used and might in fact be more like the friction-velocity behavior exhibited by the polar SAMs when a polar tip is used. This is clearly not the case, indicating that a more complex explanation is required. Instead, we suggest that these data are best explained in terms of the mechanism of deformation of the adsorbates during scanning. If intermolecular hydrogen bonding occurs in carboxylic acid and hydroxyl terminated SAMs, then deformation of the alkyl chains is not the only mode of deformation during shearing. The relaxation behavior of the alkyl chains will certainly influence the velocity dependence of the friction force, but there may also be additional relaxation phenomena associated with the intermolecular hydrogen bonding interactions between terminal groups in the SAM. It is possible that when the tip is polar and is capable of interacting strongly with polar terminal groups in the SAM, additional energy is dissipated in breaking and reforming the intermolecular hydrogen bonds. This is effectively additional work against cohesive forces within the SAM rather than work done against tip-sample adhesive interactions. If the hypothesis of Cooper and Leggett is correct,23 the energies of these cohesive interactions may be substantial; for the SAMs studied here, they may be greater than the chainchain van der Waals interactions in the long-chain SAMs. Consequently, these deformations may make a significant contribution to the friction-velocity relationships for polar SAMs, explaining the behavior in Figures 4 and 5. This (26) Tsukruk, V. V.; Bliznyuk, V. N.; Hazel, J.; Visser, D.; Everson, M. P. Langmuir 1996, 12, 4840.
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is admittedly a speculative hypothesis, but at present it seems the best one capable of explaining the experimental data. It is also evident from Figures 5 and 6 that there is little difference in the friction-velocity behavior of SAMs with similar terminal groups but different alkyl chain lengths. Kiely and Houston have explained the friction-velocity relationships observed in aged SAMs in terms of viscoelastic effects.16 Their analysis is helpful in attempting to understand the present data. Treating the SAM as a standard linear solid, they have derived an expression for the friction force Fµ as a function of the normal force FN.16 According to their analysis, Fµ/FN peaks at vx ) 12hτ0 (where h is the film thickness and τ0 is the relaxation time constant) and the friction force is thus expected to rise more rapidly with velocity for a thick film than for a thin film of the same material. Long-chain SAMs would therefore be expected to exhibit friction forces that increase more rapidly with velocity than short-chain analogues. However, methylene chain order is also known to increase with alkyl chain length. This would tend to make the behavior of the SAMs less viscous and more elastic, decreasing the time constant for relaxation. It is therefore possible that the increase in h on going from short- to long-chain SAMs is counterbalanced by the decrease in τ0. Conclusions Friction force microscopy of a range of SAMs with different alkyl chain lengths and terminal group chemistries has revealed unexpected differences in behavior for monolayers with polar and nonpolar terminal groups. Our data support previous reports that methyl terminated SAMs exhibit coefficients of friction that increase with decreasing alkyl chain length but show that there is, in contrast, only a small difference between the coefficients of friction exhibited by hydroxyl and carboxylic acid terminated SAMs with short and long alkyl chains. This may be understood best in terms of the intermolecular hydrogen bonding that some authors have suggested occurs in these systems. Friction-velocity plots for SAMs exhibit markedly different behavior when polar and nonpolar tips are used. Little difference in behavior was observed for SAMs with polar and nonpolar terminal groups when methyl terminated tips were used. However, for carboxylic acid terminated tips the polar SAMs exhibited a much sharper increase in friction with velocity than the methyl terminated SAMs. It is proposed that cohesive forces between polar terminal groups may explain this behavior, with the breaking and remaking of hydrogen bonds forming an important pathway for deformation of polar SAMs when acid terminated tips are used. Acknowledgment. The authors are grateful to the Engineering and Physical Sciences Research Council (EPSRC) for financial support (Grant GR/K88071). N.J.B. thanks EPSRC for a research studentship. LA001568O
