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Microfrictional Behavior of C60 Particles in Different C60 LB Films Studied by AFM/FFM Pingyu Zhang, Jinjun Lu, Qunji Xue,* and Weimin Liu State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China Received July 24, 2000. In Final Form: November 20, 2000
C60 was supposed to be a super lubricant on the basis of its unique spherical and highly symmetric structures. In the present work, efforts have been made to give some experimental approaches and evidences that strongly support the supposition of the “rolling effect” of C60. For this purpose, three kinds of C60 LB films were prepared designated for the predictable effects with or without “rolling”. By studying the AFM morphologies and their corresponding frictional images, some evidences supporting the “rolling effect” of C60 were observed in C60/BA (behenic acid) and C60/AA (arachidic acid)/OA (octadecylamine), while for the C60 derivative LB film, results indicated a “nonrolling effect” related to a high frictional force.
Introduction Carbon is probably the most interesting and magical element ever known. The fact is greatly imposed on one’s mind especially since the first synthesis and characterization of C60 in 1985 by Kroto and his colleagues.1 The attraction of C60 comes from its low energy, spherical and highly symmetric structure, and some unknown properties.2 In the past, graphite has been one of the dominant solid lubricants for many years. However, more attention is paid to what is called the “tribology of C60” in recent years.3-7 If C60 is proved to be a real super lubricant, a new birth is given to carbon in tribology. As a prediction, the unique properties of C60 in dimension and topology allow one to design different nanodevices and molecular machinery parts. In reviewing the former research on the tribology of C60, some interesting results should be mentioned. Bhushan et al.5,6 found that low friction and wear could be obtained for C60 film. Additionally, by using ion bombardment on C60 film, prolonged wear lifetime of the film can be obtained. Yan and Xue7 found phase transformation of C60 during sliding. Zhang8 proposed a model of plain molecular bearing to explain the tribological behavior of C60 LB films. The key points of his model lie in two aspects. The first is the role of long-chain molecules in reducing friction and localizing the C60 molecules. The other aspect is the load-bearing ability of C60. Apparently, * To whom correspondence should be addressed: Tel 86 931 8278209; Fax 86 931 8277088; E-mail
[email protected]. (1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162. (2) Hou, J. G.; Yang, J. L.; Li, H. Q.; et al. Phys. Rev. Lett. 1999, 83, 3001. (3) Kratschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354. (4) Brenner, D. W.; Harrison, J. A.; White, C. T.; Colton, R. J. Thin Solid Films 1991, 206, 220. (5) Bhushan, B.; Gupta, B. K.; Cleef, G. W. V.; Capp, C.; Coe, J. V. Tribol. Trans. 1993, 36, 573. (6) Bhushan, B.; Ruan, J. B.; Gupta, K. J. Phys. D: Appl. Phys. 1993, 26, 1319. (7) Yan, F. Y.; Xue, Q. J. J. Phys. D: Appl. Phys. 1997, 30, 781. (8) Xue, Q. J.; Zhang, J. Tribol. Int. 1995, 28, 287.
due to lack of experimental evidence in microscale, additional evidences are needed to support Zhang’s model. The authors believe that C60 LB film is an ideal model system for the research of “tribology of C60” because (a) it is easy to fabricate different types of C60 LB films with different fatty acids and/or amides and (b) it is easy to fabricate multilayer C60 LB films with different configurations. As we know, atomic force microscopy (AFM) is an increasingly popular technique to assess film roughness, fractal dimension, or power spectral density.9 In addition to AFM, recently, lateral force microscopy (LFM) or friction force microscopy (FFM) has been gaining importance for characterization of surfaces in micro-tribological systems. AFM together with FFM may provide useful information, such as anisotropy of friction and wear, etc. Hence, AFM with FFM is considered as an ideal tool in investigating the microfrictional behavior of C60. The main purposes of the present research are (1) to fabricate different kinds of C60 LB films and (2) to investigate their microfrictional behaviors using AFM and FFM. Finally, comparison on the microfrictional behavior of the films will be put forward. Experimental Section Three kinds of C60 LB films were prepared. We have three choices to fabricate a C60 LB film. Apparently, C60/BA (behenic acid) LB film is one choice. According to our former work conducted in the 1990s,8 AA (arachidic acid)/OA (octadecylamine) should be better served as “sockets”. Therefore, C60/AA/OA is selected. The C60 derivative LB film is selected as the third sample. The structure of the C60 derivative was reported in Huang’s paper.10 The above-mentioned C60 LB films on Si (100), known as C60/BA (behenic acid) and C60/AA (arachidic acid)/OA (octadecylamine) as the first group as well as C60 derivative LB film, were prepared on the Atemeta LB-105 trough made in (9) Lileys, M.; et al. Science 1998, 280, 273. (10) Huang, Y. Y.; et al. J. Colloid Interface Sci. 1998, 204, 277.
10.1021/la0010424 CCC: $20.00 © 2001 American Chemical Society Published on Web 03/09/2001
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Figure 1. (A) AFM image of C60/BA LB film. High heights correspond to light shades of gray. (B) FFM image of C60/BA LB film. Low frictional forces correspond to dark shades of gray.
Zhang et al.
Figure 3. (A) AFM image of C60 derivative LB film. High heights correspond to light shades of gray. (B) FFM image of C60 derivative LB film. High frictional forces correspond to light shades of gray. cally. The film was spread over an aqueous solution pH 6.5, at a temperature of 20 °C, and pulled or dipped at a speed of 0.3 cm/min. The microfrictional behavior of the C60 LB films was investigated on a SPM-9500 AFM produced by Shimadzu Corporation under “constant force” mode. Using “constant force” mode, the constant applied normal force is maintained. Hence, by measuring the frictional force between the tip and sample, the variation in height of the sample as well as the frictional force could be determined accordingly. In our experiment, the cantilever is made of Si3N4 and has a tip radius less than 20 nm. The operating normal force, which is given as voltage, is 2 V. The scanner with XY ) 30 µm and Z ) 10 µm is selected. The results indicate that the agglomeration of C60 particles could not be avoided in the three films. For example, what appear to be clusters of C60 are seen in the AFM of C60/AA/OA film. The existence of C60 particles is often in the presence of “clusters” where there are hundreds or thousands of C60 molecules. Despite these, it might be useful when considering the radius of the tip (20 nm) in AFM. For our samples, the size of the “C60 cluster” is about 100 nm. In other words, the size of agglomerated C60 particles “matches” the size of the tip. Were it too small for the “cluster”, the resolution of the AFM would not be enough.
Results and Discussion Figure 2. (A) AFM image of C60/AA/OA LB film. High heights correspond to light shades of gray. (B) FFM image of C60/AA/OA LB film. Low frictional forces correspond to dark shades of gray. France. The trough was mainly composed of a film balance and a dipping mechanism. The deposition was controlled automati-
According to the AFM morphologies of the three different C60 LB films obtained (see Figure 1A, Figure 2A, and Figure 3A), some interesting points can be found. The bright contrast in Figures 1A-3A corresponds to the C60 clusters, while for the smooth areas of the surfaces, it is more likely that it was the substrate modified by the BA (Figure 1A) or AA/OA (Figure 2A) layer. For
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Figure 2A, it can be seen clearly that C60 clusters in AA/ OA are better dispersed than that in BA. In other words, C60 clusters in Figure 2A are “islands” in the “sea” of AA/ OA layers. Referring to the height distribution bar on the right side of Figures 1A-3A, it is found that the highest height is obtained in Figure 2A while the lowest height is obtained in Figure 3A. In addition, according to the height distribution, it can be determined that the C60 clusters in Figures 1A-3A are slim in the z dimension. When taking Figure 1A,B as a whole, the frictional force image exhibited a low value corresponding to the high peak of its morphology. Likewise, it is the same for Figure 2A,B. To see it more clearly, the variation of height and frictional force in the same straight line from Figure 1 is shown in Figure 4A. The low frictional force could only be attributed to the “rolling” of C60, even if the contribution is not made individually. As discussed, these results indicate some indirect evidences for the “rolling effect” of C60. Conversely, Figure 4B originating from Figure 3A,B showed that the high frictional force to high peak is the result of no “rolling effect” presented. As such, these results support the existence of the “rolling effect” of C60 and its contribution to the frictional force. Figure 4. (A) Heights and corresponding frictional force of C60/BA LB film originated from Figure 1A,B. (B) Heights and corresponding frictional force of C60 derivative LB film originated from Figure 3A,B.
Acknowledgment. The authors are grateful to the Chinese Natural Science Foundation (Contracts 59735110 and 50023001) for support of this research.
Figure 3A,the smooth areas of the surfaces are blank substrate Si (100). When comparing Figure 1A with
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