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Influence of Solvent Environment and Tip Chemistry on the Contact Mechanics of Tip-Sample Interactions in Friction Force Microscopy of Self-Assembled Monolayers of Mercaptoundecanoic Acid and Dodecanethiol Tracie J. Colburn and Graham J. Leggett* Department of Chemistry, UniVersity of Sheffield, Brook Hill, Sheffield S3 7HF, U.K. ReceiVed July 31, 2006. In Final Form: January 23, 2007 Friction force microscopy measurements have been made for self-assembled monolayers of mercaptoundecanoic acid (C10COOH) and dodecanethiol (C11CH3) in different liquid media. In perfluorodecalin, the friction-load relationship was nonlinear and consistent with adhesion-controlled sliding. The effective range of the attractive force was controlled by using AFM tips functionalized with alkanethiols (chemical force microscopy). Like pairs of interacting molecules yielded data that were characterized by the Johnson-Kendall-Roberts model of contact mechanics, whereas the interaction between dissimilar pairs of molecules fitted the behavior predicted by the Derjaguin-Muller-Toporov model. In ethanol, the adhesive force was much smaller, and sliding was not adhesion-controlled. Under this condition of low adhesion, the friction force varied linearly with the applied load.
Introduction In recent years, friction force microscopy (FFM) has emerged as a powerful analytical tool with the capacity to provide quantitative information about both the chemical composition and the molecular organization of a material’s surface. FFM has also found application in fundamental studies of adhesion, lubrication, and wear.1-11 However, whereas FFM is undoubtedly a powerful tool for monitoring tribological behavior at the molecular level, gaps still remain in our understanding of the tip-surface interaction. In particular, the nature of the contact mechanics has yet to be established unequivocally. A number of groups have published FFM data that show a linear dependence of the friction force upon the applied load.12-23 Such data are normally quantified using Amontons’ law * Corresponding author. E-mail:
[email protected]. (1) Liang, Q.; Li, H.; Xu, Y.; Xiao, X. J. Phys. Chem. B 2006, 110, 403. (2) Strawhecker, K.; Asay, D. B.; McKinney, J.; Kim, S. H. Tribol. Lett. 2005, 19, 17. (3) Bhushan, B.; Israelachvili, J. N.; Landman, U. Nature 1995, 374, 607. (4) Hu, J. J.; Zabinski, J. S. Tribol. Lett. 2005, 18, 173. (5) Vakarelski, I. U.; Brown, S. C.; Rabinovich, Y. I.; Moudgil, B. M.; Langmuir 2004, 20, 1724. (6) Pidduck, A. J.; Smith, G. C. Wear 1997, 212, 254. (7) Lee, D. H.; Oh, T.; Cho, K. J. Phys. Chem. B 2005, 109, 11301. (8) Houston, J. E.; Doelling, C. M.; Vanderlick, T. K.; Hu, Y.; Scoles, G.; Wenzl, I.; Lee, T. R. Langmuir 2005, 21, 3926. (9) Park, J. Y.; Ogletree, D. F.; Salmeron, M.; Jenks, C. J.; Thiel, P. A. Tribol. Lett. 2004, 17, 629. (10) Surtchev, M.; de Souza, N. R.; Jerome, B. Nanotechnology 2005, 16, 1213. (11) Stevens, F.; Langford, S. C.; Dickinson, J. T. J. Appl. Phys. 2006, 99, 023529/1. (12) Beake, B. D.; Leggett, G. J. Phys. Chem. Chem. Phys. 1999, 1, 3345. (13) Brewer, N. J.; Beake, B. D.; Leggett, G. J. Langmuir 2001, 17, 1970. (14) Brewer, N. J.; Leggett, G. J. Langmuir 2004, 20, 4109. (15) Green, J.-B. D.; McDermott, M. T.; Porter, M. D.; Siperko, L. M. J. Phys. Chem. 1995, 99, 10960. (16) Kim, H. I.; Koini, T.; Randall Lee, T.; Perry, S. S. Langmuir 1997, 13, 7192. (17) Kim, H. I.; Graupe, M.; Oloba, O.; Koini, T.; Imaduddin, S.; Randall Lee, T.; Perry, S. S. Langmuir 1999, 15, 3179. (18) McDermott, M. T.; Green, J.-B. D.; Porter, M. D. Langmuir 1997, 13, 2504.
FF ) µFN...
(1)
where FF is the friction force and FN is the load. The constant of proportionality between the friction and the applied load, µ, is the coefficient of friction. This relationship is based on a macroscopic model for friction involving contacts between multiple asperities on the sliding surfaces. Intuitively, one might expect that FFM would be more accurately modeled by singleasperity contact mechanics on the basis of the assumption that the sharp FFM tip represents an idealized single asperity.24,25 The Hertz model
R 2/3 A ) π FN ... K
( )
(2)
describes a single sliding asperity in the absence of adhesion.26,27 Here, A is the contact area between the tip and the sample surface, and FN is the applied load. However, for molecular materials, adhesion occurs between the tip and the sample surface, and this must also be accounted for. A single-asperity model that allows for adhesion, such as the Johnson-Kendall-Roberts (JKR) model, therefore seems more promising. In JKR theory, the contact area, A, between a sphere of radius R and a plane has a finite value at zero load:28 (19) Noy, A.; Frisbie, C. D.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1995, 117, 7943. (20) Shon, Y.; Lee, S.; Colorado, R.; Perry, S. S.; Lee, T. R. J. Am. Chem. Soc. 2000, 122, 7556. (21) van der Vegte, E. W.; Hadziioannou, G. Langmuir 1997, 13, 4357. (22) Vezenov, D. V.; Noy, A.; Rozsnyai, L. F.; Lieber, C. M. J. Am. Chem. Soc. 1997, 119, 2006. (23) Wong, S.; Takano, H.; Porter, M.D. Anal. Chem. 1998, 70, 5209. (24) Urbakh, M.; Klafter, J.; Gourdon, D.; Israelachvili, J. Nature 2004, 430, 525. (25) Gao, J.; Luedtke, W. D.; Gourdon, D.; Ruths, M.; Israelachvili, J. N.; Landman, U. J. Phys. Chem. B 2004, 108, 3410. (26) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992. (27) Hertz, H. R. J. Reine. Angew. Math. 1881, 92, 156. (28) Johnson, K. L.; Kendall, K.; Roberts, A. D. Proc. R. Soc. London, Ser. A 1971, 324, 301.
10.1021/la062259m CCC: $37.00 © 2007 American Chemical Society Published on Web 03/22/2007
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A)π
R 2/3 (FN + 3πγR + x6πγRFN + (3πγR)2)2/3... K
()
Colburn and Leggett
(3)
Bowden and Tabor29 proposed that the frictional force is proportional to the real area of contact between two sliding bodies, and this idea was further developed for scanning probe applications by Carpick et al.30 The surface tension, γ, can be thought of as encompassing all attractive interactions.31,32 Along with the Derjaguin-Muller-Toporov (DMT) model,33 JKR theory has been widely used in the interpretation of data from scanning probe studies. The JKR model applies in the case of strong adhesion and short-range attractive forces.28,34,35 In contrast, the DMT model describes single junction contacts in which the elastic deformation of an asperity is small compared to the range of the adhesive force.33-35 Paradoxically, both Amontons’ law and JKR theory have found application in the FFM literature to quantify friction data and determine the extent of adhesion between the tip and the sample, respectively.19,21 The resolution of this inconsistency is now essential in order to utilize FFM meaningfully in quantitative applications. Gao et al. presented data obtained with the surface force apparatus (SFA) and by FFM on a variety of adhesive and nonadhesive materials.25 The sliding of low-adhesion singleasperity contacts was shown to result in a linear dependence of friction upon applied load. They also concluded that the concept of the “real” area of contact was nonfundamental, although it might act as a scaling factor for more fundamental properties such as the number of intermolecular bonds made and broken at the sliding interface. Recent work in this laboratory on a polymer system has lent support to the hypothesis of Gao et al. by demonstrating an influence of the liquid medium on the contact mechanics.36 However, in the earlier study, the chemistry of the tip-sample interaction was not controlled. Here we report studies of the frictional behavior of self-assembled monolayers (SAMs) using SAM-functionalized probes (chemical force microscopy, CFM19,22,23,37,38) in media of different dielectric constants. SAMs of alkanethiols on gold have been widely used for the study of friction at the molecular level. Variations in both the surface composition14,16,17 and the molecular organization of these materials12,18,20 have been shown to influence the frictional properties of the interface. SAMs are thus ideal systems to use in further exploring the relationship between adhesion and contact mechanics. Here we demonstrate clear correlations between the strength of adhesion and the fit of data to particular contact mechanics models. Experimental Section Friction Force Microscopy. Friction force measurements were performed on a Digital Instruments Multimode Nanoscope IIIa (Digital Instruments, Cambridge, U.K.) operating in contact mode. The probes were silicon nitride nanoprobes (Digital Instruments, (29) Bowden, F. P.; Tabor, D. The Friction and Lubrication of Solids; Clarendon Press: Oxford, England, 1950; Part 1. (30) Carpick, R. W.; Ogletree, D. F.; Salmeron, M. Appl. Phys. Lett. 1997, 70, 1548. (31) Carpick, R. W.; Agrait, N.; Olgetree, D. F.; Salmeron, M. Langmuir 1996, 12, 3334. (32) Carpick, R. W.; Agrait, N.; Olgetree, D. F.; Salmeron, M. J. Vac. Sci. Technol., B 1996, 14, 1289. (33) Derjaguin, B. V.; Muller, V. M.; Toporov, Y. P. J. Colloid Interface Sci. 1975, 53, 314. (34) Carpick, R. W.; Ogletree, D. F.; Salmeron, M. J. Colloid Interface Sci. 1999, 211, 395. (35) Garcia, R.; Perez, R. Surf. Sci. Rep. 2002, 47, 197. (36) Hurley, C. R.; Leggett, G. J. Langmuir 2006, 22, 4179. (37) Vezenov, D. V.; Noy, A.; Ashby, P. J. Adhes. Sci. Technol. 2005, 19, 313. (38) Vancso, G. J.; Hillborg, H.; Schonherr, H. AdV. Polym. Sci. 2005, 182, 55.
Cambridge, U.K.) with a nominal force constant of 0.06 N m-1 for FFM and 0.12 N m-1 for pull-off force measurements. The calibration of normal forces involved two steps. First, the photodetector sensitivity was calibrated by measuring a force curve for a very stiff sample. Mica was used because, relative to the very flexible lever, the stiffness of the mica is sufficiently large that it may be assumed that all deflection during the force measurement will be in the lever. Under these circumstances, the photodetector sensitivity is the gradient of a plot of photodetector signal versus displacement while measuring repulsive forces. Second, the spring constants of the levers were determined from their thermal spectra using a routine implemented within the microscope software (on our instrument, it is contained within the Digital Instruments PicoForce software) and based on the method of Hutter and Bechhoefer.39 This approximates the cantilever as a harmonic oscillator, the motion of which is driven by thermal noise. Applying the equipartition theorem, Hutter and Bechoefer derived a relationship between the spring constant and the power spectrum of the cantilever response. Experimentally, the laser spot was focused on the apex of the cantilever, and the thermal fluctuations of the cantilever were measured and used to derive the power spectrum. Representative bare and gold-coated probes were characterized by scanning electron microscopy (SEM). The morphology of the gold coating was found to be smooth, with no evidence, within the resolution limits of SEM, of either the formation of protrusions or blunting of the probes. Tip radii were also estimated by examining gold colloids with a narrow size distribution and a radius of 19.5 nm (Agar, Cambridge, U.K.). Convolution of the tip profile with the topography of the small colloidal particles leads to broadening in the AFM image, and the dimensions of the features so produced in the image may be used to estimate the extent of variation on the tip radius.40 It was found that the radii of the tips were 22.5 ( 1.7 and 36.0 ( 1.7 nm, respectively, for the bare probes and the gold-coated ones. The nominal tip radius quoted by the manufacturer was 20 nm for the bare probes. Friction data were acquired with the fast scan direction perpendicular to the long axis of the cantilever. All measurements were taken under fluid using a liquid cell fitted with a silicone O-ring. The solvents used were ethanol (HPLC grade, Fischer) and perfluorodecalin (Sigma-Aldrich). FFM measurements were taken from friction loops acquired by obtaining forward-reverse scan cycles along a single line with the microscope in scope mode. The friction signal is obtained by subtracting the mean signals in both directions, giving a resultant force that is twice the frictional force. For these experiments, the scan area was 3 µm × 3 µm, the scan rate was 3.05 Hz, and the number of data points per line was 512. In each case, the load was first minimized to zero and then increased stepwise to 30 nN (ethanol) or 20 nN (perfluorodecalin) before being decreased in increments of 0.5 nN until the tip pulled free of the surface. Care was taken to ensure that the lateral force deflection was zero at zero load to ensure that alignment errors did not contribute to the data. Multiple repeat measurements were made with different probes on different occasions to verify that the experimental system was stable and provided reproducible data. Care was taken in the optimization of the sum signal during laser alignment. Good reproducibility between different cantilevers was achieved and was attributed both to care in laser alignment and to the implementation of rigorous procedures during the preparation of samples and coated probes. The use of a liquid environment also minimized errors due to variations in ambient humidity. The loads quoted are the product of the deflection set point, the cantilever spring constant, and the photodetector sensitivity. FFM data were acquired at five locations on the sample surface. Force curves were obtained at a minimum of 300 locations on the sample surface for each tip and liquid (39) Hutter, J. L.; Bechhoeffer, J. ReV. Sci. Instrum. 1993, 64, 1868, 3342. (40) The expression used to determine the “real” radius of an AFM tip is r ) R2/(4Rt), where Rt is the tip radius of curvature, r is the real radius of the feature imaged (9.75 nm for the colliodal gold), and R is its apparent radius in the AFM image. Roberts, C. J.; Sekowski, M.; Davies, M. C.; Jackson, D. E.; Price, M. R.; Tendler, S. J. B. Biochem. J. 1992, 283, 181.
Contact Mechanics in FFM of SAMs
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Table 1. Mean Adhesive Forces Obtained from Pull-Off Force Measurements for Acid- and Methyl-Terminated SAMs with Bare and Functionalized Probes in Ethanol and Perfluorodecalin SAM C10COOH
C11CH3
tip chemistry
ethanol
PFD
ethanol
PFD
bare (Si3N4) C10COOH C11CH3
0.34 ( 0.037 nN 0.349 ( 0.027 nN 0.083 ( 0.061 nN
4.32 ( 0.144 nN 5.57 ( 0.537 nN 2.226 ( 0.144 nN
0.081 ( 0.027 nN 0.138 ( 0.087 nN 0.319 ( 0.035 nN
2.118 ( 0.069 nN 1.05 ( 0.045 nN 3.68 ( 0.116 nN
combination. Pull-off forces were then extracted from these using Carpick’s Toolbox.41 The purity of the perfluorodecalin was 95%. Mass spectrometric analysis revealed that the 5% of material that was not perfluorodecalin was composed of partially fluorinated hydrocarbon material. There was no evidence of contamination by polar material, and the degree of fluorination of the non-perfluorodecalin components was high. Therefore, the impurities are unlikely to modify the dielectric constant of the fluid significantly. Extensive tests were carried out to examine whether there was a possibility that either the probe or the sample was damaged during imaging. For loads of up to ca. 60 nN, there was no evidence of any modification to either of the SAMs studied here, even after repeated scanning, under any conditions. For short-chain adsorbates (mercaptopropanoic acid and butanethiol), there was some evidence of surface modification following repeated scanning (reflected in changed friction contrast) at loads greater than 30 nN in air but not under fluid. However, this was observed to relax slowly, suggesting that plastic deformation had not occurred even then. There is thus no evidence of any modification to the substrate at the modest loads (99%, 0.5 mm, Goodfellow metals), chromium chips (99.99%, 0.7-3.5 mm, agar), and absolute ethanol (HPLC grade, Fischer) were all used as received. Thiol compounds 1-dodecanethiol (C11CH3) and mercaptoundecanoic acid (C10COOH) were obtained from Aldrich and used as received. All glassware used in sample preparation was cleaned by submersion in piranha solution, a mixture of hydrogen peroxide and concentrated (95%) sulfuric acid in a 3:7 ratio for a minimum of 30 min. (Caution! Piranha solution is an extremely strong oxidizing agent that has been known to detonate spontaneously upon contact with organic material.) Following treatment with piranha solution, the glassware was rinsed thoroughly with deionized water and dried in an oven at approximately 80 °C. SAMs were prepared on glass cover slips (BDH, 22 mm × 64 mm, no. 1.5 thickness) covered in a thin film of evaporated gold. The gold films were prepared using an Edwards Auto 306 bell jar vacuum coater system with a base pressure of 10-6 mbar. Chromium and gold were both deposited onto the glass substrate via thermal evaporation from resistively heated 70 A Mo boats. A thin layer (ca. 3 nm) of Cr was deposited onto the glass first, at a rate of 0.01 nm s-1, to act as an adhesion promoter for the deposition of gold onto the cover slip. Au was deposited at a rate of 0.01 nm s-1 to a thickness of ca. 50 nm. This technique has been shown to favor the formation of polycrystalline Au films with the (111) surface exposed. Freshly prepared gold substrates were immediately sealed in clean 33 mL sample vials fitted with polyethylene stoppers and containing 1 mM solutions of the thiols in degassed ethanol, for approximately 18 h at room temperature. After this time, the samples were removed (41) http://mandm.engr.wisc.edu/faculty_pages/carpick/toolbox.htm.
from solution, rinsed with copious amounts of degassed ethanol, and dried in a stream of N2 gas. Silicon nitride AFM probes (Digital Instruments Ltd) were chemically functionalized by deposition of a self-assembled monolayer of either dodecanethiol (C11CH3) or mercaptoundecanoic acid (C10COOH). An Edwards Auto 306 bell jar vacuum coater system was first used to deposit a 1-nm-thick layer of chromium, the deposit rate being in the region of 3 nm s-1. This rate represents a compromise between the slow deposition rate required to form a reasonably flat layer of Cr and the need to ensure that the cantilevers do not bend in response to long exposure to elevated temperatures.19,21 Following deposition of the adhesion layer, the system was allowed to cool for approximately 20 min prior to deposition of a 10-nm-thick layer of gold, again at a typical deposition rate of 0.03 nm s-1. Once cool, the cantilevers were immersed in a 1 mM solution of C11CH3 in degassed ethanol and left for a minimum of 18 h to ensure complete formation of the monolayer. Prior to use, the functionalized tips were rinsed with clean degassed ethanol and gently dried in a stream of nitrogen.
Results Pull-Off Forces. Pull-off forces were measured for both C11CH3 monolayers and C10COOH monolayers under ethanol and perfluorodecalin (Table 1). In perfluorodecalin, the pull-off forces were significantly larger than those measured in ethanol for all tip-surface combinations. Pull-off forces measured under ethanol were typically CH3/CH3 > COOH/CH3, leading the authors to conclude that interactions between similar functionalities are stronger than those between dissimilar ones. This is consistent with the results presented in this article. Here, the interaction between an acid-modified tip and an acid surface under perfluorodecalin yielded FFM data consistent with JKR contact mechanics: strong adhesion and short-range attractive forces. The CH3/CH3 pairing gave data that were also modeled by JKR contact mechanics. Dissimilar pairings (including the interaction of a bare tip with the surface) produced FFM data consistent with the DMT model, characterized by low adhesion (comparably smaller pull-off forces) and long-range attractive interactions. These findings suggest that single asperity contact mechanics are observed in media with low dielectric constants, where dispersion forces are very strong and frictional interactions are adhesion-controlled. In liquids with large dielectric constants, sliding is not adhesion-controlled, and linear friction-load behavior is observed. In addition, using CFM to vary the nature of the intermolecular force between the tip and the sample allowed the strength and nature of the interaction to be varied systematically. These data support the hypothesis of Gao et al.25 that the real area of contact between the tip and the surface is not a fundamental quantity but rather a convenient scaling factor for the number of intermolecular bonds made and broken during sliding. This interpretation implies a change in our understanding of friction, from one grounded in mechanical concepts toward one based upon molecular interactions, making a more obvious connection with chemical concepts.
Conclusions By fine tuning the solvent environment of the FFM experiment, it is possible to switch between adhesion-controlled contact mechanics, modeled by the JKR and DMT equations, and Amontons’ law. In perfluorodecalin, a liquid with a low dielectric constant, the friction-load relationship fits the behavior predicted using the Johnson-Kendall-Roberts model for like pairs of interacting molecules and the Derjaguin-Muller-Toporov model for unlike molecules. The sliding of the AFM probe across the SAM is therefore adhesion-controlled. In contrast, FFM measurements obtained in ethanol, a liquid with a larger dielectric constant, demonstrate a linear dependence of friction force on the applied load, and the data are fit to Amontons’ law. Acknowledgment. T.J.C. acknowledges the Royal Society of Chemistry (RSC) and the Engineering and Physical Sciences (51) Thomas, R. C.; Houston, J. E.; Crooks, R. M.; Kim, T.; Michalske, T. A. J. Am. Chem. Soc. 1995, 117, 3830.
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Research Council (EPSRC) for a research studentship. G.J.L. thanks the EPSRC and the RSC Analytical Division for support. Supporting Information Available: Variation in the photodetector signal with FN2/3 for carboxylic acid-terminated and methylterminated SAMs in perfluorodecalin and in ethanol. Fitting of the photodetector-load relationship in perfluorodecalin carried out using
Colburn and Leggett the appropriate contact mechanics model. Linear friction-load relationships observed for SAMs in ethanol. Histograms of pull-off forces measured with bare, carboxylic acid-functionalized, and methylfunctionalized tips. This material is available free of charge via the Internet at http://pubs.acs.org. LA062259M
