Langmuir 1996, 12, 3905-3911
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Nanotribological Properties of Composite Molecular Films: C60 Anchored to a Self-Assembled Monolayer Vladimir V. Tsukruk* Materials Science and Engineering Program, College of Engineering & Applied Sciences, Western Michigan University, Kalamazoo, Michigan 49008
Mark P. Everson Physics Department, Ford Research Laboratory, Dearborn, Michigan 48121
Lorraine M. Lander and William J. Brittain Maurice Morton Institute of Polymer Science, The University of Akron, Akron, Ohio 44325 Received February 21, 1996. In Final Form: May 17, 1996X Tribological properties of molecular films composed of a fullerene monolayer chemically attached to the functional surface of self-assembled monolayers (C60-SAM) were studied by friction force microscopy. We observed very high wear stability of composite fullerene films. The friction coefficient (µ) for these films varies in a wide range from 0.04 ( 0.02 at high loads and 0.06 ( 0.02 at the highest velocities tested ( θN3-SAM (84°) > θC60-SAM (70°) . θSiO2 (≈0°), and variation of the adhesive forces, ∆FCH3-SAM (4 nN) e ∆FN3-SAM (5 nN) < ∆FC60-SAM (9 nN) < ∆FSiO2 (12 nN).
Introduction Fullerene (C60) compounds have been recently introduced as prospective solid lubricants with intriguing tribological properties such as a relatively low friction coefficient at high loads (in the range of 0.07-0.18) and high thermal and chemical stability.1-5 Their highly symmetric molecular shape and high bulk elastic modulus make fullerene materials promising soft solid lubricants. It has been shown that the dissipation energy and shear strength of fullerene films are 1 order of magnitude lower than typical values for boundary lubricants.3 However, testing of the frictional properties of fullerenes has revealed low mechanical stability of fullerene films physically adsorbed at a solid surface and questioned the potential of these materials as superior lubricants. Progressive wear and transfer of fullerene materials were observed with various mating materials.1 Friction force microscopy observations revealed that adsorbed fullerene films deteriorate at approximate pressure as low as 0.1 GPa.6 The formation of mechanically stable, ordered monomolecular films of fullerenes firmly attached to a * To whom correspondence should be addressed: fax, 616-3876517; e-mail,
[email protected]. X Abstract published in Advance ACS Abstracts, July 1, 1996. (1) Bhushan, B.; Gupta, B. K.; Van Cleef, G. W.; Capp, G.; Coe, J. V. Tribol. Trans. 1993, 4, 573. (2) Bhushan, B; Ruan, J. J. Mater. Res. 1993, 8, 3019. (3) Lu¨thi, R.; Meyer, E.; Haefke, H.; Howald, L.; Gutmannsbauer, W.; Gu¨ntherodt, H.-L. Science 1994, 266, 1979. (4) Thundat, T.; Warmack, R. J.; Ding, D.; Compton, R. N. Appl. Phys. Lett. 1993, 63, 891. (5) Lu¨thi, R.; Meyer, E.; Haefke, H.; Howald, L., Gutmannsbauer, W.; Guggisberg, M.; Bammerlin, M.; Gu¨ntherodt, H.-L. Surf. Sci. 1995, 338, 247.
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solid substrate is considered to be a necessary step in the exploitation of their potential tribological properties. Stable fullerene molecular monolayers covalently attached to self-assembled monolayers with terminal amine or pyridine groups have been reported.7 In previous work from these laboratories, fullerene molecules have been covalently bonded to the surface of self-assembled monolayers prepared on a silicon substrate.8 The chemical composition and microstructure of these composite monolayers as shown in Figure 1 was confirmed by water contact angles, ellipsometry, X-ray photoelectron spectroscopy, atomic force microscopy (AFM), and UV-vis spectroscopy as was reported in our previous publications.8 AFM shown smooth, homogeneous areas of monomolecular fullerene films with a microroughness of 0.5 nm with rare surface microcrystallites. In these monolayers, fullerene molecules are packed in an ordered lattice corresponding to the {h00} faces of a face centered cubic unit cell with edge length a ) 1.4 nm. More recently, Mirkin and co-workers9 have prepared C60 layers by chemisorption of thiol-functionalized C60 to gold surfaces and observed hexagonal packing of fullerene molecules using AFM. We account for the difference by (6) Schwarz, U. D.; Allers, W.; Gensterblum, G.; Weisendanger, R. Phys. Rev. 1995, B52, 14976. (7) Caldwell, W. B.; Chen, K.; Mirkin, C. A.; Babinec, S. J. Langmuir 1993, 9, 1945. Chen, K.; Caldwell, W. B.; Mirkin, C. A. J. Am. Chem. Soc. 1993, 115, 1193. Chupa, J. A.; Xu, S.; Fischetti, R. F.; Strongin, R. M.; McCauley, J. P., Jr.; Smith, A. B., III; Blaisie, J. K.; Peticolas, L. J.; Bean, J. C. J. Am. Chem. Soc. 1993, 115, 4383. (8) Tsukruk, V. V.; Lander, L. M.; Brittain, W. J. Langmuir 1994, 10, 996; Lander; L. M.; Brittain; W. J.; Tsukruk, V. V. Polym. Prepr. 1994, 35, 488. (9) Shi, X.; Caldwell, B.; Chen, K.; Mirkin, C. A. J. Am. Chem. Soc. 1994, 116, 11598.
© 1996 American Chemical Society
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Figure 1. Schemes of molecular structures of self-assembled monolayers (refs 8 and 11): CH3-SAM (a), N3-SAM (b), and C60-SAM (c).
the nature of attachment; Mirkin tethered C60 to gold while we used a preformed alkylsilane SAM as the template for native C60.8 These types of molecular surfaces have attracted attention as potential boundary lubricants with superior tribological properties. In contrast to physically adsorbed fullerene films, no wear and material transfer between sliding surfaces was observed while using a pinon-disk friction apparatus.10 In the present communication we report the results of investigations of nanotribological behavior of composite molecular films composed of C60 monolayers chemically anchored to azide-terminated self-assembled monolayers on a silicon surface (Figure 1).8 We focus on tribological behavior of these films on a submicrometer scale in comparison with tribological properties of conventional self-assembled monolayers of alkylsilanes. The friction coefficient, µ, at various sliding conditions, loading and unloading behavior, and wear of these films are probed by scanning force microscopy (SFM) that includes combined atomic force and friction force modes (AFM/FFM). Experimental Section We studied a set of self-assembled monolayers composed of methyl-, azide-, and fullerene-terminated monolayers; the chemical structures are shown in Figure 1. The methyl-terminated monolayers (CH3-SAM) were prepared by the chemisorption of hexadecyltrichlorosilane.11a Preparation and characterization of the azide- and fullerene-terminated films have been reported earlier.8 In Figure 1 we depicted the most probable type of ringopened form of the azafulleroids on the surfaces. We based this model on the results of investigations of the chemical reaction of alkyl azides and fullerene solution because direct spectroscopic studies of composite monolayers are inconclusive.8 Studies of solution reaction11b shown ring-opened azafulleroid products. We presumed that the reaction of C60 adsorbed on the azideterminated SAMs proceeds in a similar fashion.8 AFM and FFM images of the films were obtained at ambient temperature with two microscopes: the Autoprobe LS (Park Scientific Instruments) and the Nanoscope III (Digital Instruments, Inc.) according to well-established procedures.12 Relative humidity was within 35 ( 10%. Scan sizes ranged from 200 nm to 20 µm with the normal load in the range of 1-100 nN (Autoprobe) and 20-1000 nN (Nanoscope). Despite modest normal loads applied in this study, the local pressure produced by the sharp SFM tip is very significant. Assuming local elastic deformation less than 10-15% for monolayers of 3 nm thickness and an average radius of curvature of the SFM tip of 30 nm, we estimated the surface pressures in our study to be in the 1 GPa range. The estimated area of tip-surface contact for this level of elastic deformation is about 100 nm2. The sliding velocity of (10) Lander, L. M.; Brittain, W. J.; DePalma, V. A.; Girolmo, S. R. Chem. Mater. 1995, 7, 1437. (11) (a) Lander, L. M.; Siewierski, L. M.; Brittain, W. J.; Vogler, E. A. Langmuir 1993, 9, 2237. (b) Prato, M.; Li, Q. C.; Wudl, F.; Lucchini, V. J. Am. Chem. Soc. 1993, 115, 1148. (12) Frommer, L. Angew. Chem. Int. Ed. Engl. 1992, 31, 1298. Tsukruk, V. V.; Reneker, D. H. Polymer 1995, 36, 1791.
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Figure 2. AFM (left) and FFM (right) images of C60-SAM film showing surface domain morphology, 2 µm × 2 µm. the SPM tip was varied from 0.01 to
