Langmuir 1997, 13, 2333-2339
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Nanotribological Properties of Octadecyltrichlorosilane Self-Assembled Ultrathin Films Studied by Atomic Force Microscopy: Contact and Tapping Modes M. Garcia-Parajo,*,† C. Longo,‡,§ J. Servat,‡ P. Gorostiza,†,‡ and F. Sanz‡ Unitat de Te` cniques Nanome` triques, Serveis Cientı´fico-Te` cnics, Universitat de Barcelona, Lluı´s Sole´ y Sabarı´s, 1-3, 08028 Barcelona, Spain, and Departament de Quı´mica-Fı´sica, Universitat de Barcelona, Av. Diagonal 647, 08028 Barcelona, Spain Received May 28, 1996. In Final Form: February 12, 1997X Atomic force microscopy, working in contact and tapping modes, has been used to probe the mechanical response of octadecyltrichlorosilane, CH3-(CH2)17-SiCl3 (OTS), self-assembled ultrathin films grafted on silicon-treated surfaces. To assess the adhesion hysteresis and the elasticity of the films, we have performed loading and unloading experiments in contact mode. The effect of compression is observed while the load is being applied to the film. Once the load is fully removed, the film remains compressed showing a clear deviation from perfect elasticity, with relaxation times in the range of 0.1 s. In the tapping mode, the loading/unloading experiments show again the deviation from perfect elastic recovery of the films. Upon loading, three different regions are encountered. These can be associated with an initial disorder of the chains produced by the indenting tip, followed by a smooth compression and a final hardening effect due to repulsive forces between the compressed chains. The tapping mode is also used to provide strong evidence of a “cushioning” effect. The tribological properties of our OTS layers seem to be influenced by molecular disorder of the alkyl chains.
1. Introduction It has long been recognized that monomolecular organic layers, particularly those with long hydrocarbon chains, work exceedingly well as boundary lubricants, having an important effect on the macroscopic tribological properties of the substrate upon which the film is attached.1-3 The two most widely studied self-assembled monolayer (SAM) systems have been organothiolates grafted on Au surfaces and organosilanes grafted on SiO2 surfaces, the latter having more interest to the microelectronic industry.4 The tribological properties of the organothiolate/Au system have been extensively studied by several groups5,6 and are fairly well understood. However, the investigation of the organosilane/SiO2 system has been mostly focused on the film/substrate preparation7-10 and the SAM quality * Author to whom correspondence should be addressed at Department of Applied Physics, MESA Institute, Applied Optics Group, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands. Fax: (31)-53-489 11 05. Tel: (31)-53-489 38 71. e-mail address:
[email protected]. † Unitat de Tecniques Nanometriques, Serveis Cientifico-Te ` cnics, Universitat de Barcelona. ‡ Departament de Quı´mica-Fı´sica, Universitat de Barcelona. § On leave from The Instituto de Quı´mica, Universidade de Sa ˜o Paulo, CP. 26077, 05599 Sa˜o Paulo, Brazil. X Abstract published in Advance ACS Abstracts, March 15, 1997. (1) Adams, The Physics and Chemistry of Surfaces; Oxford University Press: Oxford, 1941. (2) Bowden, F. P.; Tabor, D. The Friction and Lubrication of Solid Surfaces; Clarendon: Oxford, 1986. (3) Bhushan, B.; Israelachvili, J.; Ladman, U. Nature 1995, 374, 607 and references therein. (4) Deng, K.; Collins, R. J.; Mehran M.; Sukenik, C. J. Electrochem. Soc. 1995, 142, 1278. (5) (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.; Houston, J. E.; Michalske, T. A.; Crooks, R. M. Science 1993, 259, 1883. (6) (a) Salmeron, M.; Neubauer, G.; Folch, A.; Tomitori, M.; Ogletree, D. F.; Sautet, P. Langmuir 1993, 9, 3600. (b) Liu, G.-Y.; Salmeron, M. Langmuir 1994, 10, 367. (c) Du, Q.; Xiao, X.-D.; Charych, D.; Wolf, F.; Frantz, P.; Shen, Y. R.; Salmeron, M. Phys. Rev. B 1995, 51, 7456. (7) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (8) Parikh, A. N.; Allara, D.; Ben Azouz, I.; Rondelez, F. J. Phys. Chem. 1994, 98, 7577 and references therein. (9) Fujii, M.; Sugisawa, S.; Fukada, K.; Kato, T.; Seimiya, T. Langmuir 1995, 11, 405. (10) Tripp, C. P.; Hair, M. L. Langmuir 1995, 11, 149.
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characterization by various techniques.11-15 This has been so because, in spite of the apparent easiness for the alkylsilanes to graft on the oxide surface, a complete understanding of the mechanism for film formation is missing as manifested in the variety of procedures used to prepare supposedly similar films and the significant discrepancies in the reported film structures. While the formation of uniform, solid-like monolayers is desirable for many fundamental studies, there is not much understanding on how the quality of the grafted layer will affect adhesion, friction, wear, and other mechanical properties. It has been inferred3,16,17 that the tribological performance of the grafted films depends on their molecular properties, such as the chain length, orientation of the alkyl molecules, dynamic film structure, and monolayer coverage, on the surface quality to which the molecules are grafted to, and on the method of preparation. Techniques like the surface force apparatus (SFA),18 the interfacial force microscope (IFM),19 and atomic force microscopy (AFM)20 have been used to gain more understanding on the process. Molecular dynamics simulations have been also used to probe the response of SAMs under compression exerted either by a flat surface21 (11) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. Soc. 1989, 111, 5852. (12) (a) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1120. (b) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1961. (c) Tripp, C. P.; Hair, M. L. Langmuir 1995, 11, 1215. (d) Tripp, C. P.; Veregin, R. P. N.; McDougall, M. N. V.; Osmond, D. Langmuir 1995, 11, 1858. (13) (a) Bierbaum, K.; Kinzler, M.; Wo¨ll, Ch.; Grunze, M.; Ha¨hner, G.; Heid, S.; Effenberger, F. Langmuir 1995, 11, 512. (b) Bierbaum, K.; Grunze, M.; Baski, A. A.; Chi, L. F.; Schrepp, W.; Fuchs, H. Langmuir 1995, 11, 2143. (14) Allara, D. L.; Parikh, A. N.; Rondelez, F. Langmuir 1995, 11, 2357. (15) Hoffmann, H.; Mayer, U.; Krischanitz, A. Langmuir 1995, 11, 1304. (16) Yoshizawa, H.; Chen, Y.-L.; Israelachvili, J. J. Phys. Chem. 1993, 97, 4128. (17) Bhushan, B.; Kulkarni, A. V.; Koinkar, V.; Boehm, M.; Odoni, L.; Martelet, C.; Belin, M. Langmuir 1995, 11, 3189. (18) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1985. (19) Joyce, S. A.; Houston, J. E. Rev. Sci. Instrum. 1991, 62, 710. (20) Bining, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930.
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or by single asperities.22,23 In the latter case, the sharp tip of the atomic force microscope is a convenient model of a single asperity. Added to the capability of AFM to investigate the mechanical properties of materials, it is also possible to image the surface either by contact or in the tapping regime with little perturbation during the process itself.24,25 The work to be reported in here has the purpose of shedding some light on the tribological properties of octadecyltrichlorosilane (OTS) thin layers grafted on Si substrates. Conventional contact and tapping AFM techniques have been used for the characterization of the films. To test the adhesion hysteresis and the elasticity of the films, loading and unloading experiments in contact mode using soft cantilevers were performed. In the tapping regime, the loading/unloading experiments show again the nonperfect elastic behavior of the films. 2. Experimental Section 2.1. Film Preparation. Unless specified, the whole procedure to be described was performed in clean room conditions (class 100, T ) 21 ( 1 °C, and relative humidity (RH) ) 44 ( 5%). Before film preparation the Si(100) substrates were properly hydroxylated using a H2SO4/H2O2 (7:3) solution and exposing the hydrophilic surface to UV radiation in humid conditions. For film preparation, the substrates were immersed under dry conditions (glovebox with RH
