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Chain Length Dependence of the Frictional Properties of Alkylsilane Molecules Self-Assembled on Mica Studied by Atomic Force Microscopy Xudong Xiao,† Jun Hu,‡ Deborah H. Charych, and Miquel Salmeron* Materials Sciences Division, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720 Received September 15, 1995. In Final Form: October 30, 1995X We show that the frictional properties of alkylsilane monolayers self-assembled on mica in contact with Si3N4 tips depend strongly on the length of the alkyl chains. Friction is particularly high with short chains of less than eight carbons. We attribute this to the large number of dissipative modes in the less ordered short chains. Longer chains, stabilized by van der Waals attractions form more compact and rigid layers and act as much better lubricants. This lubricating action is lost at a certain threshold load, where wear of the molecular layer occurs, leading to much higher friction force values. The results presented here clearly indicate that the chemical identity of the exposed end groups is not sufficient to determine the frictional properties of monolayer films. The increased number of energy dissipation modes facilitated by the presence of molecular disorder (e.g., rotations about a C-C axis), in fact dominates the frictional behavior of monolayers with short chains.
Introduction Boundary lubrication is important to many modern technologies, including magnetic storage and micromachines.1,2 Criteria for good boundary lubricants include strong adhesion to the substrate and reduction of the friction between the two moving surfaces. In general, molecules in self-assembled monolayers form strong chemical bonds with the substrates and therefore are good candidates for model studies of boundary lubrication. These molecular assemblies can be used to test a variety of simple, yet important questions about the nature of frictional interactions. For example, the role of adhesion can be tested by using similar molecules with chemically different terminal groups.3,4 Other important questions include the effect of the lateral order within the layer, the presence of heteroatoms (O, N, etc.) or branches inside the main chain and the existence of double and triple bonds that change the rigidity of the molecules. Often these characteristics are not isolated; for example, changing the terminal group might also change the order in the layer. In this paper we address the simple question of how the chain length of alkylsilane monolayers having the same terminal group, methyl, affects friction. By having the same terminal group, we want to exclude the contribution from local chemical effects3,5 between the two contacting surfaces to the energy dissipation processes. If local chemical effects were the determining factor, one might expect that alkylsilane monolayers with different chain lengths will exhibit similar adhesion and friction properties as long as their terminal groups were CH3. The same † Also with the Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong. ‡ Permanent address: Shanghai Institute of Nuclear Research, Academia Sinica, Post Office Box 800-204, Shanghai 201800, People’s Republic of China X Abstract published in Advance ACS Abstracts, December 15, 1995.
(1) Zarrad, H.; Chovelon, J. M.; Clechet, P.; Jaffrezic-Renault, N.; et al. Sens. Actuators, A 1995, 47, 598. (2) Deng, K.; Collins, R. J.; Mehregany, M.; Sukenik, C. N. J. Electrochem. Soc. 1995, 142, 1278. (3) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. (4) Overney, R.; Meyer, E. MRS Bull. 1993, May, 27. (5) Harrison, J. A.; White, C. T.; Colton, R. J.; Brenner, D. W. Phys. Rev. B 1992, 46, 9700.
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would hold true for alkylsilane and alkanethiol monolayers on silica or gold substrates, respectively. By elimination or great reduction of the local chemical effects, the contribution to energy dissipation due to the excitation of rotational and vibrational modes in the molecules can contribute strongly to the energy transfer to the substrate and thus to the frictional properties. To test these ideas, we have studied the frictional properties of self-assembled monolayers of alkylsilanes on mica substrates as a function of their chain length. Our results using atomic force microscopy (AFM) show that the friction force is strongly dependent on chain length with about 1 order of magnitude change between the long chain molecules (C18) and the shorter chain ones (C3, C6). Materials and Methods The measurement technique has been described elsewhere.6 Briefly, a homemade AFM instrument equipped with microfabricated cantilevers was used. Cantilever motions were measured by the laser deflection method with a quadrant photodiode detector to measure the normal and torsional forces. The torsional force exerted on the cantilever is a measure of the friction between the tip and the monolayer lubricant. External load is calculated as the product of the z displacement of the lever and the nominal cantilever force constant (0.58 N/m). Since the torsional force constant is not known, only arbitrary units will be used in these measurements. As a reference, the friction characteristics of freshly cleaved mica using the same tip and lever were also studied. All the experiments reported here were performed with Si3N4 tips acquired from Digital Instruments. Unless otherwise specified, all data were taken with the same cantilever/tip, which is very important for meaningful quantitative comparisons. To obtain friction data, the tip was scanned back and forth in the x direction in contact with the sample at a constant load while the lateral deflection of the lever was measured. The load was increased (or decreased) linearly in each successive scan line. These sets of data were displayed graphically in a “friction image”, with x being the scan direction and y the load axis. The difference in the lateral deflection or friction signal between back and forth motions is proportional to the friction force. Although in principle the tip repeatedly scans the same line on the sample, thermal and other drifts slowly move the tip into new areas of the mica. (6) Lio, A.; Morant, C.; Ogletree, D. F.; Salmeron, M. Submitted for publication in Langmuir.
© 1996 American Chemical Society
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Figure 2. Extended friction curve for a C18 alkylsilane monolayer on mica. Four distinct regimes exist: (I) elastic regime friction; (II) distortion and displacement of the alkylsilane molecules; (III) tip in contact with mica substrate; (IV) wear of the mica substrate.
Figure 1. Representative friction vs load curves for C3, C6, C8, and C18. A friction curve from bare mica is also shown for comparison. All the data were obtained using the same tip. Alkylsilanes with different chain lengths, Me3Si(CH2)nCH3 with n ) 2, 5, 7, 17, were used to form the monolayers in this study; the molecules are hereafter referred to as C3, C6, C8, and C18, respectively. A modification of the procedure by Kessel and Granick was used.7 Briefly, 0.20 g of the appropriate silane and 0.20 g of 1 N HCl were added to 25 mL of tetrahydrofuran in a cleaned ehrlenmeyer flask. The hydrolysis solution was stirred for 5 days at room temperature. Just prior to self-assembly on mica, the hydrolysis solution was filtered through a 0.20 µM Teflon membrane (13 mm). The filtrate was diluted 1:20 in cyclohexane and the diluted solution added to a cleaned petrie dish containing the freshly cleaved mica samples. Deposition was carried out at room temperature with stirring. Deposition time was varied depending on the chain length of the silane as follows: C18, 10 min; C8, 10 min; C6, 13 min; C3, 15 min. All samples were prepared in duplicate. All glassware were cleaned with Nochromix solution.
Results Figure 1 shows some representative results of friction vs load curves for C3, C6, C8, and C18 monolayers on mica. For reference, data from freshly cleaved mica obtained with the same tip are also included. Measurements taken at different spots on the sample show a deviation of (15% from the data shown in Figure 1. The nonzero value of the friction signal at zero external load is due to the attractive forces that cause the jump-tocontact instability during approach of the tip to the sample surface. It can be seen that the friction force curves for the C3 and C6 monolayers are similar in shape and magnitude over the entire range of loads. The frictional force is consistently larger than that for mica at the same load. We verified that no pinholes or wear occurred in the scanned area by subsequent imaging of the same area at very low load. The friction force at all loads for the C8 monolayer is significantly smaller than that of either the shorter chain layers or the bare mica. The friction curve is convexshaped (i.e., with a decreasing slope), in the low load region up to about 30 nN. After that, the friction force increases faster than the linear extrapolation of the low load part of the curve. Finally, at a load of ∼70 nN a more (7) Kessel, C. R.; Granick, S. Langmuir 1991, 7, 532.
substantial increase occurs, which brings the friction force to values similar to and above those of the bare mica. The magnitude of the friction force for the C18 monolayer is smaller than that of the C8 and much smaller than that for C3 and C6, by about 1 order of magnitude. The lubricating effect of the C18 monolayer for mica is therefore apparent and significant. Similar to the case of the C8 layer, a slow initial increase of the frictional force is observed. The curve again has a convex curvature up to a load of about ∼60 nN. The friction then departs from the linear extrapolation of the low load part of the curve and is soon followed by a much faster increase when the load exceeds ∼120 nN. Figure 2 shows a similar study with a C18 layer with a different tip but extending to much higher loads. In the low load range, we observed the same effects as described above. The rapid increase in friction observed between 100 and 150 nN is followed by a slower, nearly linear increase up to 270 nN. Above this value, the friction becomes unstable with large oscillations. As already shown in a previous study,8 this behavior corresponds to the layer-by-layer wear of the mica substrate. Discussion It is clear from the data in Figure 1 that the chemical nature of the surface, with the layers terminating in -CH3, does not alone determine the frictional properties of an organic lubricant. The explanation for the observed chain length dependence is to be sought in the fact that there is a substantial disorder in the layers formed by molecules with short chains. This disorder facilitates the presence of numerous kinks, gauche defects, etc. that provide many excitation modes (rotations about C-C axis, bending, etc.) to efficiently absorb energy and thus give rise to a high value of friction. Distortions in the short chain molecules are in part due to thermal excitation. They can also be a result of the pressure exerted by the tip, which more easily affects the shorter chain molecules than the longer ones. We have recently presented spectrocopic evidence, using nonlinear optical techniques,9 that one effect of pressure is the disordering of the end groups of molecules with alkyl chains via formation of terminal gauche defects. (8) Hu, J.; Xiao, X.-d.; Ogletree, D. F.; Salmeron, M. Surf. Sci. 1995, 327, 358. (9) Du, Q.; Xiao, X.-d.; Charych, D.; Wolf, F.; Frantz, P.; Shen, Y. R.; Salmeron, M. Phys. Rev. B 1995, 51, 7456.
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An important stabilizing factor in organic monolayers films is the attractive forces from van der Waals interactions between chains, which play an essential role in ensuring good close packing and self-organization of the molecules. The rapid saturation of this stabilization energy with chain length can be simply illustrated by computing the energy per CH2 unit in an ideal close packed layer of alkyl molecules as the length of the chain increases.10 Starting with the CH3 end groups of a C1 monolayer, each one is attracted by the six CH3 groups in the nearest neighboring molecules, which already provide 97% of the total energy. To this, we add the contributions from the second, third, etc., nearest neighbors in the same plane. Since the van der Waals energy decays with an r-6 law, the sum converges rapidly. For a C2 molecule, there is the additional stabilization from the CH2 groups of the second layer, which contribute an energy of ∼83% of that provided by the first layer alone. For a C3 molecule, the third layer adds another ∼54% of the energy provided by the first layer. The fourth layer in a C4 molecule would add 28% more energy. The stabilization energy contributed by additional layers continues to decrease and saturates at the length of roughly the C8-C10 molecules. At this point, the stabilization energy per CH2 group is ∼ 7 kJ/mol, using the van der Waals parameters in ref 10. Since at room temperature kT is ∼2.5 kJ/mol, we can see that a substantial contribution to the molecular stability is provided by the van der Waals attraction alone. This explains the well-known fact that for chain lengths only above ∼8, well-ordered and densely packed layers are formed for both alkylsiloxanes and alkanethiols.11-13 Another manifestation of the saturation of the film properties with chain length is provided by a study of the water contact angle, by Wasserman et al.14 These authors found that it increases from about 80° to 110°, where it saturates for n ≈ 6. The convex-shaped friction curves found for C3, C6, and bare mica, and for C8 and C18 at low loads, are typical of most materials in the elastic contact regime, with a constant shear force and a contact area that increases with load with a power law dependence of 270 nN, as seen in Figure 2. It is interesting to note that the response of alkanethiol layers of the same length on Au(111) to high tip pressure is drastically different. As we have shown, sharp tips (radius
