Scanning Force Microscopic Exploration of the Lubrication

This paper explores the ability of n-alkanethiolates chemisorbed at Au(111) to function as boundary lubricants at microscopic length scales as probed ...
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Langmuir 1997, 13, 2504-2510

Scanning Force Microscopic Exploration of the Lubrication Capabilities of n-Alkanethiolate Monolayers Chemisorbed at Gold: Structural Basis of Microscopic Friction and Wear Mark T. McDermott,† John-Bruce D. Green,‡ and Marc D. Porter* Ames Laboratory-U.S. Department of Energy, Microanalytical Instrumentation Center, and Department of Chemistry, Iowa State University, Ames, Iowa 50011 Received December 11, 1996. In Final Form: February 21, 1997X This paper explores the ability of n-alkanethiolates chemisorbed at Au(111) to function as boundary lubricants at microscopic length scales as probed by scanning force microscopy (SFM). Through an examination of the influence of alkyl chain length, we show that the macroscopic structure of this system, as developed from insights into the chain-packing density via infrared reflection spectroscopy, greatly influences the observed friction and wear. That is, the longer chain monolayers exhibit a markedly lower friction and a reduced propensity to wear than the shorter chain monolayers, a situation that reflects the more extensive cohesive interactions between chains. From the combined weight of these findings, we examine the frictional process within the context of an activation mechanism that involves pressure and shear activation volumes. The ability of longer chain alkanethiolate monolayers to lubricate features that arise from changes in substrate topography is also presented, and the resulting mechanistic issues are discussed.

Introduction The spontaneous adsorption of n-alkanethiolates and their ω-substituted analogues at gold yields well-ordered monolayers of value as models of organic interfaces.1 This system has been used as barriers in electrochemical charge- transfer studies,2 as templates for the adsorption of proteins and related materials,3 coatings for molecular recognition sensors,4 and in a variety of additional applications. We note that these monolayers exhibit many of the characteristics requisite for a model boundary lubricant, viz., a strong headgroup-substrate binding and a densely packed chain structure. Thus, a detailed exploration of the tribological properties of this system is of fundamental interest. We report herein the results of an in-depth study of the friction and wear of this adsorbate-substrate combination at a microscopic length scale. Several reports have described the frictional properties of monolayers at large (e.g., >100 µm2) contact areas.5-8 In general, these studies were conducted at films that * To whom correspondence should be addressed. † Present address: Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2. ‡ Present address: Naval Research Laboratory, Code 6170, 4555 Overlook Ave, Washington, DC 20375-5342. X Abstract published in Advance ACS Abstracts, April, 1; 1997. (1) (a) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic: San Diego, CA, 1991. (b) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (2) Examples include: (a) Chidsey, C. E. D. Science 1991, 251, 919. (b) Rowe, G. K.; Creager, S. E. Langmuir 1994, 10, 1186. (c) Miller, C.; Cuendet, P.; Gra¨tzel, M. J. Phys. Chem. 1991, 95, 877. (3) (a) Lo´pez, G. P.; Biebuyck, H. A.; Ha¨rter, R.; Kumar, A.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 10774. (b) Singhvi, R.; Kumar, A.; Lo´pez, G.; Stephanpooulos, G. N.; Wang, D. I. C.; Whitesides, G. M.; Ingber, D. E. Science 1994, 264, 696. (4) Spinke, J.; Liley, M.; Guder, H.-J.; Angermaier, L.; Knoll, W. Langmuir 1993, 9, 1821. (5) (a) Bailey, A. I.; Courtney-Pratt, J. S. Proc. R. Soc. London, Ser. A, 1954, 227, 501. (b) Levine, O.; Zisman, W. A. J. Phys. Chem. 1957, 61, 1068. (6) Briscoe, B. J.; Evans, D. C. B. Proc. R. Soc. London, Ser. A 1982, 380, 389. (7) (a) Yoshizawa, H.; Chen, Y.-L.; Israelachvili, J. J. Phys. Chem. 1993, 97, 4128. (b) Yoshizawa, H.; McGuiggan, P.; Israelachvili, J. Science 1993, 259, 1305.

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were deposited on smooth substrates (e.g., mica) to minimize contributions to the measurement from surface topography. n-Alkanethiolate monolayers, however, are usually chemisorbed at sputtered or vapor-deposited gold films. While these gold films exhibit a strong (111) surface crystallinity, their topography generally consists of atomically flat terraces that are, at most, 200-300 nm in size.9 Thus, an examination of the frictional properties at this system without contributions from substrate topography requires a technique that uses a contact area at nanoscale dimensions. Scanning force microscopy (SFM) has recently shown a remarkable capability for measuring adhesion10 and friction11-15 at nanometer length scales, a situation that provides a pathway to characterize the frictional properties (8) (a) DePalma, V.; Tillman, N. Langmuir 1989, 5, 868. (b) Timsit, R. S. In Fundamentals of Friction: Macroscopic and Microscopic Processes; Singer, I. L., Pollock, H. M., Eds.; Kluwer Academic: Dordrect, The Netherlands, 1992; p 287. (9) Chidsey, C. E. D.; Loiacono, D. N.; Sleator, T.; Nakahara, S. Surf. Sci. 1988, 200, 45. (10) (a) Thomas, R. C.; Houston, J. E.; Crooks, R. M.; Kim, T.; Michalske, T. A. J. Am. Chem. Soc. 1995, 117, 3830. (b) Moy, V. T.; Florin, E.-L.; Gaub, H. E. Science 1994, 266, 257. (c) Thomas, R. C.; Tangyunyong, P.; Houston, J. E.; Michalske, T. A.; Crooks, R. M. J. Phys. Chem. 1994, 98, 4493. (d) Florin, E.-L.; Moy, V. T.; Gaub, H. E. Science 1994, 264, 415. (e) Pierce, M.; Stuart, J.; Pungor, A.; Dryden, P.; Hlady, V. Langmuir 1994, 10, 3217. (f) Lee, G. U.; Chrisey, L. A.; Colton, R. J. Science 1994, 266, 771. (g) Fujihira, M.; Kawate, H.; Yasutake, M. paper presented at the 183rd Meeting of the Electrochemical Society, Honolulu, HI, May 18, 1993. (h) Nakagawa, T.; Ogawa, K.; Kurumizawa, T.; Ozaki, S. Jpn. J. App. Phys. 1993, 32, L294. (i) Hoh, J. H.; Cleveland, J. P.; Prater, C. B.; Revel, J.-P.; Hansma, P. K. J. Am. Chem. Soc. 1992, 114, 4917. (11) (a) Graphite: Mate, C. M.; McClelland, G. M.; Erlandsson, R.; Chiang, S. Phys. Rev. Lett. 1987, 59, 1942. Baselt, D. R.; Baldeschwieler, J. D. J. Vac. Sci. Technol. B 1992, 10, 2316. Mate, C. M. Wear 1993, 168, 17. (b) Mica: Erlandsson, R.; Hadziioannou, G.; Mate, C. M.; McClelland, G. M.; Chiang, S. J. Chem. Phys. 1988, 89, 5190. (c) Gold: Cohen, S. R.; Neubauer, G.; McClelland, G. M. J. Vac. Sci. Technol. A 1990, 8, 3449. Nie, H. Y.; Mizutani, W.; Tokumoto, H. Surf. Sci. 1994, 311, L649. (d) NaCl: Meyer, G.; Amer, N. M. Appl. Phys. Lett. 1990, 57, 2089. (12) (a) Mate, C. M. Phys. Rev. Lett. 1992, 68, 3323. (b) Haugstad, G.; Gladfelter, W. L.; Weberg, E. B.; Weberg, R. T.; Weatherill, T. D. Langmuir 1994, 10, 4295. (13) Meyer, E.; Overney, R.; Brodbeck, D.; Howald, L.; Lu¨thi, R.; Frommer, J.; Gu¨ntherodt, H.-J. Phys. Rev. Lett. 1992, 69, 1777. (14) Liu, Y.; Wu, T.; Evans, D. F. Langmuir 1994, 10, 2241.

© 1997 American Chemical Society

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of alkanethiolates at gold. To such ends, SFM frictional characterizations for a number of uncoated surfaces have been reported.11 Relevant to the work herein, SFM has been used to measure the friction at surfaces modified with polymers,12 Langmuir-Blodgett (LB) films,13 surfactant monolayers,14 and other types of ordered films.15 Related studies have shown that friction is sensitive to surface composition, and several laboratories have exploited this sensitivity to map via SFM the chemical composition of surfaces at micrometer16,17 and submicrometer length scales.18 In this paper, we characterize the friction and wear of n-alkanethiolate monolayers chemisorbed at gold as a function of chain length. This approach minimizes the influence of variations in interfacial composition, facilitating the development of a mechanistic picture of tribological properties that is based solely on film architecture. Through a study of the chain length dependence, we show that the macroscopic structure of this system greatly influences its microscopic friction, as recently found for alkylsilane monolayers deposited at mica.15b Our results build upon those of the latter investigation through a direct comparison of the SFM-based friction and the macroscopic structural descriptions of alkanethiolate monolayers developed from infrared spectroscopic studies.19 From these findings, we show that our results are consistent with an activation model and discuss a qualitative frictional mechanism based on activation volumes. The ability of longer chain alkanethiolate monolayers to lubricate features that arise from changes in substrate topography is also demonstrated, and the resulting mechanistic issues are discussed. Experimental Section Sample Preparation. Substrates were prepared by the vapor deposition of 300 nm of gold at 0.3-0.4 nm/s onto cleaved mica or Tempax glass (Berliner Glas) at ambient temperatures. Both types of substrates were then annealed at 300 °C for 4 h. The Tempax substrates were further annealed in a H2 flame. These substrates were subsequently immersed for 12-24 h into n-alkanethiol-containing (0.5-1.0 mM) ethanolic solutions to form the corresponding thiolate monolayers.1 Prior to imaging, the samples were rinsed copiously with ethanol and dried under a stream of argon. Ethanol (Quantum, punctilious grade) and all of the n-alkanethiols (Aldrich), except octadecanethiol, were used as received. Octadecanethiol (Aldrich) was recrystallized twice from ethanol. Docosanethiol was a gift from A. Ulman (Brooklyn Polytech.). Friction Measurements. A MultiMode NanoScope III SFM (Digital Instruments), equipped with a 1-µm scanner and operated in the laboratory atmospheric environment, was utilized for our measurements. Triangular Si3N4 cantilevers with pyramidal tips (Digital Instruments) were used after a methanol rinse. Values for the normal bending force constants (kN ∼ 0.06 (15) (a) Bhushan, B.; Kulkarni, A. V.; Koinkar, V. N.; Boehm, M.; Odoni, L.; Martelet, C.; Belin, M. Langmuir 1995, 11, 3189. (b) Xiao, X.; Charych, D. H.; Salmeron, M. Langmuir 1996, 12, 235. (16) (a) Overney, R. M.; Meyer, E.; Frommer, J.; Gu¨ntherodt, H.-J.; Fujihira, M.; Takano, H.; Gotoh, Y. Langmuir 1994, 1281. (b) Overney, R. M.; Meyer, E.; Frommer, J.; Brodbeck, D.; Lu¨thi, R.; Howald, L.; Gu¨ntherodt, H.-J.; Fujihira, M.; Takano, H.; Gotoh, Y. Nature 1992, 359, 133. (17) (a) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. (b) Wilbur, J. L.; Biebuyck, H. A.; MacDonald, J. C.; Whitesides, G. M. Langmuir 1995, 11, 825. (c) Noy, A.; Frisbie, C. D.; Razsnyai, L. F.; Wrighton, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1995, 117, 7943. (d) Sinniah, S. K.; Steel, A. B.; Miller, C. J.; Reutt-Robey, J. E. J. Am. Chem. Soc. 1996, 118, 8925. (18) (a) Green, J.-B.; McDermott, M. T.; Porter, M. D.; Siperko, L. M. J. Phys. Chem. 1995, 99, 10960. (b) Green, J.-B.; McDermott, M. T.; Porter, M. D. J. Phys. Chem. 1996, 100, 13342. (19) (a) Stole, S. Ph.D. Dissertation, Iowa State University, 1990. (b) Chang, S.-C.; Chao, I.; Tao, Y.-T. J. Am. Chem. Soc. 1994, 116, 6792.

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Figure 1. Friction loops for (a) uncoated Au(111) and (b) ODT/ Au(111). In both cases, the data were recorded at the top of an atomically flat crystallite. Scan rate ∼1 µm/s and FN ∼ 10 nN. N/m) of the cantilevers were calculated as described,20 using the physical and material specifications provided by Digital Instruments. The normal force between tip and sample, FN, was estimated from force vs z-displacement curves (i.e., force curves). The values of FN represent the total normal force, with FN ) 0 nN defined as the point where the tip breaks contact with the surface. Thus, FN is the sum of capillary forces, molecular interactions (e.g., hydrogen-bonding and van der Waals interactions), and applied load. Frictional data and imaging were conducted with the fast scan axis of the sample movement perpendicular to the principal axis of the cantilever while systematically varying FN and correcting for subsequent displacements of the tip. Frictional forces, f , were measured from plots of the detector signal vs the lateral displacement of the sample during 50-nm trace-retrace cycles (i.e., friction loops11c) along a single scan line at ∼1000 nm/s. The sensitivity of the detector to lateral tip displacements was determined from the slopes at the stationary turnaround points of 5-nm friction loops,14 with the torsional force constants (kt ∼ 80 N/m) calculated by approximating the cantilever as two parallel beams.20 Hysteresis in the measured friction was observed in only a few instances when cycling FN; these results are not included in this paper. Image Acquisition. All images were acquired concurrently in a constant force mode at FN ∼10 nN and a temperature of 23 ( 2 °C. The instrument was equilibrated thermally for ∼1 h after mounting a sample. Vertical displacements of the sample were calibrated using the heights of monoatomic Au(111) steps (0.236 nm).

Results and Discussion General Observations. Microscopic Friction and Wear. We first examined the friction and wear of longchain n-alkanethiolates formed at Au(111) to evaluate the potential utility of this adsorbate-substrate combination as a model boundary lubricant. The ability of a monolayer of octadecanethiolate chemisorbed at Au(111) (ODT/Au(111)) to reduce friction and wear at microscopic length scales is demonstrated in Figures 1 and 2. In the case of the former, the average value of f can be determined from the friction loops11c in Figure 1. Curve (a) in Figure 1 is a friction loop at uncoated Au(111)21 and curve b at ODT/Au(111). Both loops were acquired with the tip centered on large (∼100-nm-diameter), atomically smooth Au(111) terraces, an approach that eliminates topographic contributions from the underlying substrate to the mea(20) O’Shea, S. J.; Welland, M. E.; Wong, T. M. H. Ultramicrosc. 1993, 52, 55. (21) The term “uncoated Au(111)” is used to indicate gold surfaces not purposely exposed to the thiol-containing solutions. Because these measurements were performed under ambient conditions, a contaminant layer of organic material and water is likely to be contributing to the observed friction and wear properties at the Au(111) surface.

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Figure 2. Topographic images collected with FN ∼ 25 nN and a scan rate of 10 Hz. (A) 250- × 250-nm image of an uncoated Au(111) crystallite. (B) Image of the same crystallite as in A after allowing the tip to scan the entire image 5 times at FN ∼ 80 nN. (C) 350- × 350-nm image of a Au(111) crystallite coated with a monolayer of ODT. (D) Image of the same crystallite as in C after 20 scans at FN ∼ 100 nN. Z scales in all images are 0-5 nm.

surement.22 As is evident, the magnitude of f at ODT/ Au(111) (∼1 nN) is considerably less than that at uncoated Au(111) (∼35 nN), demonstrating the lubricating ability of long-chain alkanethiolate monolayers at microscopic length scales. In addition to lowering f, an effective lubricant must reduce wear. The results of such tests are presented in the topographic images in Figure 2. Figure 2A is an image of unmodified Au(111), showing ∼200-nm-sized terraces separated by identifiable monoatomic steps. The shapes of the step edges in this image remain unchanged when scanning repetitively at FN ∼ 25 nN. Figure 2B shows the same region after five successive scans at FN ∼ 80 nN. A tip-induced wear of the surface is evident from the changes in the shapes of the step edges. Interestingly, the wear detected in Figure 2B, which is confined to step edges, occurs at a much lower value of FN than required to induce wear at locations further away from step edges.23 Together, these findings suggest that wear originates preferentially near substrate defects like step edges. The images in parts C and D of Figure 2 demonstrate that wear can be markedly reduced by the chemisorption (22) Greenwood, J. A. In Fundamentals of Friction: Macroscopic and Microscopic Processes; Singer, I. L., Pollock, H. M., Eds.; Kluwer Academic: Dordrect, The Netherlands, 1992; p 57. (23) Goss, C. A.; Brumfield, J. C.; Irene, E. A.; Murray, R. W. Langmuir 1993, 9, 2986.

of an alkanethiolate monolayer at Au(111). Figure 2C is an image at FN ∼ 10 nN for ODT/Au(111), and Figure 2D is an image of the same area after more than 20 scans at FN ∼ 100 nN. The two images are indistinguishable. These results show that the ODT monolayer efficiently disperses the force of the tip, protecting the underlying Au(111) surface from tip-induced wear. It follows that an ODT monolayer has sufficient mechanical strength to maintain an effective separation between the tip and underlying surface at values of FN up to ∼100 nN. We attribute this situation to the strong binding between sulfur and gold and the strong cohesive interactions between neighboring alkyl chains.24 These assertions are supported by our observations that under similar test conditions, longer chain length monolayers (n g 11, where n is the number of methylene groups in the chain) are also effective in reducing wear, whereas the monolayers with shorter chains (n ) 2-7) exhibit wear at much lower values of FN (∼0 to 100 nN). Our observations also correlate with the results from recent indentation studies with the interfacial force microscope which have shown that long-chain alkanethiolate monolayers can elastically support loads up to 15 µN.25 Thus, the ability to reduce both friction (24) (a) Alves, C. A.; Smith, E. L.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 1222. (b) Alves, C. A.; Porter, M. D. Langmuir 1993, 9, 3507.

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Figure 3. Plots of friction force (f) vs normal force (FN) for n-alkanethiolate monolayers of varying chain length. The values to the right of each curve indicate the number of CH2 groups. The points in each curve were determined from friction loops collected over a 50-nm scan line on an atomically flat crystallite at a scan rate of ∼1 µm/s. Curves for n ) 5, 15, and 21, which have been omitted for clarity, follow the trend shown.

and wear indicates that longer chain alkanethiolate monolayers formed at Au(111) are effective as boundary lubricants. Chain-Length Dependence. A more detailed picture of the microscopic tribological properties of this monolayer system develops from the dependencies of f on FN and on n. These data are presented in Figure 3 where n ranges from 2 to 21. (The plots for n ) 5, 15, and 21 have been omitted for clarity). The plots in Figure 3 illustrate two important properties of this system. First, f increases linearly with FN for each chain length, including those not shown. These dependencies can, as shown in our earlier report,18 be functionally described by eq 1, where

f ) RFN + f0

(1)

R and f0 are the slope and the y-intercept of each plot, respectively.26 This type of dependence, as discussed later, is consistent with the Eyring activation model27 which attributes the deformation of solids to the movement of small molecular domains within a solid.6,13 Importantly, eq 1 has successfully been used to characterize the experimental5-8,13-15 and theoretical28 findings for the friction at several monolayer and multilayer systems. Second, the performance of these monolayers as lubricants degrades as n decreases. This degradation is evident from the increase in the slopes of the plots for f vs FN as n decreases. For example, the frictional force at n ) 21 is virtually undetectable and results in an effectively frictionless sliding contact between tip and sample under our test conditions. However, f shows a much stronger dependence on FN for shorter chains (e.g., n ) 6), reaching a value of ∼60 nN at FN ) 50 nN. Thus, n governs the quantitative effectiveness of these films as lubricants. These observations parallel those found for alkylsilane monolayers adsorbed to mica.15b (25) (a) Joyce, S. A.; Thomas, R. C.; Houston, J. E.; Michalske, T. A.; Crooks, R. M. Phys. Rev. Lett. 1992, 68, 2790. (b) Thomas, R. C.; Houston, J. E.; Michalske, T. A.; Crooks, R. M Science 1993, 259, 1883. (26) Equation 1 is derived from the linear relationship between shear stress and contact pressure from Eyring’s model27 and the relationship between friction force and shear stress, f ) ScA, where Sc is the critical stress required to shear the junction and A is the contact area [Bowden, F. P.; Tabor, D. The Friction and Lubrication of Metals, Pt. 1; Oxford University: Oxford 1950]. In Eyring’s model, S ) S0 + Rp, where S is the shear stress and p is the pressure at the contact. Substituting S for Sc, we obtain f ) (S0 + Rp)A. It follows that since p ) FN/A, then f ) RFN + S0A, which is eq 1 if we define f0 as S0A. (27) (a) Eyring, H. J. Chem. Phys. 1935, 3, 107. (b) Eyring, H. J. Chem. Phys. 1936, 4, 283. (28) Tupper, K. J.; Brenner, D. W. Thin Solid Films 1994, 253, 185.

Figure 4. Plots of R and ∆ν1/2 vs the number of methylene units for n-alkanethiolate monolayers. Values of R were determined from linear regression analysis of the f vsFN curves in Figure 3. The peak widths at half-maximum (∆ν1/2) of the asymmetric CH2 stretching modes were obtained from external reflection infrared absorption spectra from ref 19a.

Figure 4 details further the relationship between R and n. Interestingly, R exhibits a bimodal dependence on n. From n ) 15 to 21, the change in R is comparatively small, with all values less than ∼0.1. In contrast, the effect of n on R is markedly greater below a n between 11 and 14. Furthermore, the change in f as n decreases results in an R at n ) 2 that is about 2 orders of magnitude larger than those for longer chains (e.g., n ) 17). We note that our findings yield values of R (0.01-0.05) for longer chain monolayers that are similar to those found for alkyl chain LB films (R ∼ 0.04)6 and other self-assembled systems,15a as well as to that predicted in a molecular dynamics simulation of friction at a hexadecanethiolate monolayer on gold (R ∼ 0.02).28 These results, as discussed in more detail in the next section, clearly demonstrate that the frictional properties of these monolayers have a strong dependence on n. Structural Correlations. Insight into the dependence of R on n can be gained from the qualitative descriptions of the relative order in the chain structure of these monolayers. Macroscopic structural characterizations have shown that the architecture of these monolayers is strongly affected by n.29,30 The combined weight of these studies indicates that the chain structures of monolayers prepared with longer chain lengths are more ordered and more densely packed in comparison to those of monolayers prepared with shorter chain lengths. To assess in more detail the influence of chain packing on f , we have examined the structural insights that have developed from infrared reflection spectroscopic characterizations of this system. As previously shown,31 the bandwidth of the methylene stretching mode (νa(CH2)) exhibits a qualitative correlation with the packing density of the chains. Crystalline n-C20H42 has a full width at half-maximum (∆ν1/2) for νa(CH2) of 11 cm-1 as compared to a broader ∆ν1/2 of (18 cm-1) for n-alkanes with a more liquidlike packing density.31 Accordingly, Figure 4 also contains a plot for ∆ν1/2 of νa(CH2) for these monolayers (29) For characterizations utilizing infrared reflection spectroscopy, optical elipsometry, and electrochemical methods, see, for example: Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (30) Other characterizations include (a) wettability (Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164) (b) low-energy helium diffraction (Chidsey, C. E. D.; Liu, G.-Y.; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989, 91, 4421) and (c) X-ray diffraction (Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. Rev. Lett. 1993, 70, 2447). (31) (a) Wood, K. A.; Snyder, R. G.; Strauss, H. L. J. Chem. Phys. 1989, 91, 5255. (b) Evans, S. D.; Goppert-Berarducci, K. E.; Uranker, E.; Gerenser, L. J.; Ulman, A.; Snyder, R. S. Langmuir 1991, 7, 2700.

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as a function of n.19a This plot exhibits a bimodal dependence, with a transition at n ∼ 12. Thus, the longer chain monolayers (n g 12) have ∆ν1/2 values (11-13 cm-1) that are diagnostic of a more densely packed chain structure. Below n ∼ 12, ∆ν1/2 begins to increase with decreasing n, arguing that the packing density of the chains decreases.32 This conclusion agrees with the considerations of the positions of the methylene stretching modes, which are also sensitive to the intermolecular environment of the chains.29,33 Both interpretations are consistent with the energetic considerations in that the cohesive interactions between neighboring chains increase by ∼0.8 kcal/mol per methylene group.34 Importantly, the profile of ∆ν1/2 vs n mirrors that of R vs n. This similarly indicates that the microscopic friction of these monolayers is strongly dependent on the chain-packing density. Following the above discussion, the results in our and other laboratories have shown that both R and f0 are dependent on the functional group at the terminus of longer chain alkanethiolate monolayers at Au(111).18 While Figure 3 indicates that f is linearly dependent on FN for all values of n, it is clear that R is also governed by the length and therefore the packing density of the chains. In contrast to the noted results for functionalized monolayers, f0 for n-alkanethiolate monolayers is ∼0 nN for all chain lengths. This situation is indicative of comparatively low interfacial free energies and, in turn, negligibly small adhesive interactions at the tip-sample contact. Thus, the microscopic friction at alkanethiolate monolayers not only depends on the chemical identity of the end group17,18 but also on the packing density of the chain structure. The next section examines the importance of this conclusion at a mechanistic level. Frictional Mechanism. The correlation of the macroscopic structure and microscopic friction is crucial to a mechanistic understanding of the lubrication properties of these monolayers. Based on an Eyring analysis of friction,27 R reflects the interplay between the pressure activation volume (Ω) and stress activation volume (φ) such that R ) Ω/φ.6 Although a fully viable physical description of both terms remains problematic at both qualitative and quantitative levels, φ can be viewed as the size of the molecular structure that moves during shear and Ω as the local increase in volume that accompanies shear. It follows that a description of the frictional mechanism entails insight into the displacement of small groups of molecules during shear.6 In this section, we qualitatively compare the relative magnitudes of φ and Ω for long- and short-chain n-alkanethiolate monolayers based on the correlations in Figure 4 between chain structure and observed friction. The results presented in Figures 1-4 clearly demonstrate that longer chain (n ) 15-21) n-alkanethiolate monolayers are markedly more effective lubricants than shorter chain monolayers. At a molecular level, a lower value of R within the context of an Eyring analysis reflects the motion of a relatively large domain of molecules (i.e., large φ) that is accompanied by a small volume change (i.e., small Ω).6 Such a mechanism is consistent with a long-chain monolayer exhibiting strong intermolecular interactions that hold a large block of molecules together during shear without an effective loss of molecular order. The insight developed in the last section from the infrared (32) The frequency of the νa(CH2) mode and the frequency and bandwidth of the νs(CH2) mode also follow the same general trend with n as ∆ν1/2 shown in Figure 2B. (33) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (34) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558.

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spectroscopic data indicates that the longer chain monolayers exhibit the type of structural architecture conducive to lower values of R. That is, the long-chain monolayers are more tightly packed because of the greater cohesive interactions between chains. Interestingly, the noted molecular dynamics simulation predicts that the frictional process at a hexadecanethiolate monolayer involves an oscillation in the tilt angle of the chains with an overall preservation of packing and molecular scale order.28 Retention of molecular scale order, importantly, would inhibit the formation of structural defects like gauche kinks that would increase the free volume (i.e., Ω) during shear. Thus, the frictional mechanism for longer chain monolayers (n ) 15-21) appears to involve the motion (i.e., precession and tilt) of relatively large blocks of adsorbates. These blocks then move in response to shear while effectively retaining molecular order, resulting in a small relative volume change during shear. Extending the above argument, the lower packing density of the chains for the shorter n-alkanethiolate monolayers (n ) 2-11) argues that the extent of the cohesive binding between chains is less than that of the longer chain monolayers. It follows, in contrast to the longer chain monolayers, that the frictional process of the shorter chain monolayers does not involve the motion of as large a block of adsorbates under shear (i.e., smaller φ) and that sliding is more likely to induce gauche conformers in the chain structure (i.e., larger Ω). As a consequence, the measured values of R are larger for the shorter as opposed to the longer chain monolayers. The descriptions in the above analysis are consistent with the energy-dissipation aspects of friction. When two macroscopic surfaces slide against one another, energy dissipation occurs via an inelastic and plastic deformation of surface asperites.35 At a microscopic scale, as is the case when a SFM tip moves across a surface, energy is dissipated through surface phonons. The noted molecular dynamics study argues that energy dissipation in a longchain alkanethiolate monolayer is associated with vibrational energy through an oscillation in the tilt angle of the chains.28 The shear-induced disorder of the chains in the shorter monolayers therefore results in a lousy contact because of energy dissipation through bond rotations and vibrations, leading to a higher microscopic friction. Further, we often observe wear in the monolayers for n e 5 at moderate values of FN (e.g., ∼50 nN), which is also indicative of a contribution from plastic deformation to the higher friction observed for the short-chain monolayers. In summary, the microscopic frictional mechanism at n-alkanethiolate monolayers formed at Au(111) is consistent with the general features of an Eyring activation model. We have not, however, been able to develop quantitative aspects (e.g., values for φ and Ω) of the analysis that are consistent with expectations of likely structural delimiters. For example, we have attempted to utilize the size of domains observed in scanning tunneling microscopic (STM) studies of short-chain (n < 9) alkanethiolate monolayers36 as a structural basis for assigning quantitative values to φ. However, due to the lack of a clear correlation between domain size and chain length and the inability to observe domains via STM for longer chain length systems, these efforts have provided limited insight. These investigations are continuing. (35) Tabor, D. In Fundamentals of Friction: Macroscopic and Microscopic Processes; Singer, I. L., Pollock, H. M., Eds.; Kluwer Academic: Dordrect, The Netherlands, 1992; p 3. (36) (a) McDermott, C. A.; McDermott, M. T.; Green, J.-B.; Porter, M. D. J. Phys. Chem. 1995, 99, 13257. (b) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 10966.

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Figure 5. 300- × 300-nm topographic (A, C, E, G) and lateral force (B, D, F, H) images of uncoated and n-alkanethiolate-coated Au(111). (A and B) Uncoated Au(111), FN ∼ 10 nN; (C and D) heptanethiolate/Au(111), FN ∼ 15 nN (E and F) decanethiolate/ Au(111), FN ∼ 15 nN; (G and H) octadecanethiolate/Au(111), FN ∼ 25 nN.

Frictional Damping of Topographic Structures. The data in Figures 1-4 show that the longer chain monolayers are effective as boundary lubricants at atomically smooth Au(111) terraces. The results in Figure 5 demonstrate the ability of these films to function as lubricants at rougher, more effective models of technologically relevant surfaces. Parts A and B of Figure 5 are respective topographic and lateral force images at length scales that encompass large roughness changes in an unmodified Au(111) substrate and were collected with a

left-to-right scan direction. The topographic image shows that the surface of these substrates is composed of 100200-nm-sized crystallites separated by grain boundaries of varied width and depth. The steps and terraces on each crystallite in Figure 5A appear “unfocused” because of the effectively large contact area (∼3 nm2) between tip and sample.37,38 On the other hand, steps and terraces are easily identified in the lateral force image of Figure 5B. Both the steps and the boundaries between crystallites generally appear as localized regions of increased

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Table 1. Lateral Force Responses Observed at Monoatomic Steps and at Grain Boundaries for Uncoated Au(111) and Alkanethiolate Monolayers of Differing Alkyl Chain Lengths at Au(111) ∆f,a nN surface uncoated Au(111) PT/Au(111) HT/Au(111) HPT/Au(111) DT/Au(111) ODT/ Au(111)

n

steps

grain bound

2 5 6 9 17

22 ( 12 (N ) 45) 12 ( 5 (N ) 8) 5 ( 4 (N ) 15) 4 ( 2 (N ) 8) 1 ( 0.5 (N ) 10) 0 (N ) 15)

40 ( 9 (N ) 9) 18 ( 5 (N ) 4) 10 ( 5 (N ) 7) 11 ( 2 (N ) 3) 3 ( 1 (N ) 12) 0 (N ) 9)b

a N equals the number of steps or boundaries examined. b Values of ∆f at grain boundaries at ODT/Au(111) were below detection in all but two occasions where one boundary measured at 0.5 nN and another at 2 nN.

lateral force, whereas the terraces appear as regions of constant friction. For example, as the tip moves up an atomic step that separates two large-sized terraces, the lateral force increases by ∼13 nN. This force transient reflects the torsional loading of the cantilever as the tip “trips” up a step.11d,12b In contrast, movement of the tip across the boundaries between crystallites can induce frictional changes (∆f) up to ∼35 nN. Thus, the interaction of the tip with topographic features as small as a single atomic step can result in a notable increase in the detected friction. The images of Figure 5C-H show that the lateral force detected at steps and grain boundaries can be manipulated by modifying gold with n-alkanethiolate monolayers. Parts C and D of Figure 5 are the respective topographic and friction force images of heptanethiolate (HPT) formed at Au(111). The ∆f values at the steps in Figure 5D are ∼2 nN, whereas the grain boundaries induce a ∆f of ∼10 nN; both values are notably lower than the respective responses at unmodified Au(111). For a monolayer of decanethiolate (DT) at Au(111) (Figure 5E,F), the values of ∆f at steps and grain boundaries are ∼0.5 and ∼2 nN, respectively. At ODT/Au(111) (Figure 5G,H), the values of ∆f at steps are below our detection capability (∼0.2 nN). Grain boundaries at ODT/Au(111) generally yield values of ∆f that are also undetectable but can occasionally produce responses of 0.5-2 nN. Qualitatively, these data show that as n increases, the friction images become increasingly featureless because the contributions from topographic changes to the observed friction decrease. In a more detailed manner, Table 1 lists ∆f values at steps and grain boundaries for a range of chain lengths, including those in Figure 5. These data were obtained using several different samples of each monolayer and several different tips. Although the size and shape of the tip play a crucial role in “sensing” topographic features, we believe that an explanation of the observations in Figure 5 can be constructed within the framework of the (37) Gewirth, A. A.; LaGraff, J. R. In The Handbook of Surface Imaging and Visualization; Hubbard, A. T., Ed.; CRC: Boca Faton, FL, 1995; Chapter 2. (38) We have estimated the upper limit of the radius of our contact between the SFM tip and unmodified Au(111) using a Hertzian model (Timoshenko, S. P.; Goodier, J. N. Theory of Elasticity; McGraw-Hill: New York, 1970). We have assumed that the contact region of our tip is defined by a sphere with a radius of curvature of 30 nm. Values for the Poisson ratio and Young’s modulus for both Au and Si3N4 were taken from ref 23.

structural and frictional correlations developed from Figure 4. That is, the molecular scale mechanism by which n-alkanethiolate monolayers damp the SFM-measured friction at steps can be understood by comparing the length of the chain with the height of a Au(111) step and the resulting effect on film structure. For example, the thickness of an ODT layer (∼2.6 nm) is significantly greater than the height of a Au(111) step. Although a step will induce an offset at the ends of the chains above and below a step edge, we suspect that there is sufficient interplay over the remaining portion of the chains to form a functionally ordered structure from a frictional perspective at a step. As a consequence, the molecular parameters φ and Ω remain relatively constant, resulting in a frictional process at steps that is not detectably different from that at terraces. Stated differently, the longer chain length monolayers effectively disperse both the normal and frictional force of the tip at the steps, which translates to a more uniform “flow” of the tip across a step between two large terraces and likely inhibits a tip-induced wear as demonstrated in Figure 2C,D. In contrast to the long-chain monolayers, the height of a Au(111) step for a short-chain monolayer is a larger fraction of the total chain length. We believe that this offset further weakens the cohesive interactions between chains at steps, increasing Ω. Thus, shear at the tipmonolayer junction at steps disrupts the structure of shortchain-length monolayers to the extent that a tip “senses” the presence of a step. Analogous arguments can be made for the responses found at grain boundaries since the edge of a crystallite consists of descending or ascending arrays of closely spaced steps. Conclusions We have utilized SFM to examine the molecular scale frictional properties of n-alkanethiolate monolayers chemisorbed at gold. These films can reduce the friction between an SFM probe and modified Au(111) terraces by a factor of ∼35 and inhibit tip-induced wear. These results demonstrate the potential value of this adsorbatesubstrate combination as a model boundary lubricant. The microscopic frictional properties of this system were also correlated with descriptions of the chain-packing density as a function of the length of the alkyl chain. These correlations revealed that that the observed friction was governed by the packing density of the film, arguing that the ability of the longer chain monolayers to retain molecular scale order during shear leads to a lower observed friction. Our findings were also analyzed within the context of an Eyring activation model, which views the fundamental frictional process as a mixing of pressure and stress activation volumes. Experiments to test both the qualitative and quantitative applicability of the Eyring model are presently being designed. Acknowledgment. This work was supported by the Chemical Sciences Division of the Office of Basic Research of the U.S. Department of Energy and by the Institute for Physical Research and Technology of Iowa State University through an IPRT Postdoctoral Fellowship (M.T.M.). The Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. W-7405-eng-82. LA962099M