Langmuir 2007, 23, 8299-8303
8299
Tribological Properties of Nanoparticle-Laden Ultrathin Films Formed by Covalent Molecular Assembly Sreenivasa Reddy Puniredd,† Yong Keng Wai,† N. Satyanarayana,‡ Sujeet K. Sinha,‡ and M. P. Srinivasan*,† Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, 4 Engineering DriVe 4, Singapore 117576, and Department of Mechanical Engineering, National UniVersity of Singapore, 9 Engineering DriVe 1, Singapore 117576 ReceiVed December 10, 2006. In Final Form: May 14, 2007 The tribological properties of ultrathin films containing nanoparticles encapsulated in immobilized dendrimers are investigated. The films were formed by covalent molecular assembly in supercritical carbon dioxide, and the Au nanoparticles were formed in aqueous solution. End-capping of the terminal amine groups of the dendrimer by fluorinated species resulted in a reduction in the size of the nanoparticles formed. The resulting film structure displayed a lower coefficient of friction when the nanoparticles were formed after fluorination. The observed improvement in the tribological properties is attributed to the reduction in agglomeration of the nanoparticles due to the presence of the fluorine moieties.
1. Introduction Ultrathin organic films have been of great interest in recent years because of their potential use as boundary lubricants in several technological applications. The interaction of solid surfaces with organic layers has also received much attention because of its importance in the reliability and durability of devices and applications based on hybrid organic/inorganic nanoscale systems.1-8 Dendrimers, as constituents of organic layers, are receiving attention because of their effect on adhesive and frictional behavior and particularly so as hosts for nanoparticles.9-11 The presence of metal nanoparticles within the dendrimer can markedly improve the nanomechanical behavior of nanostructured films. Recent studies have reported the mechanical properties and chemical interactions of Au, Al, Cr, Co, and Cu metal nanoparticles with physically adsorbed amineterminated dendrimer monolayers on the native oxide of a silicon wafer.12-19 * Corresponding author. E-mail:
[email protected]. Tel: +6565162171. Fax: +65-67791936. † Department of Chemical and Biomolecular Engineering. ‡ Department of Mechanical Engineering. (1) Bowden, F. P.; Tabor, D. The Friction and Lubrication of Solids; Clarendon Press: Oxford, U.K., 1986. (2) Suzuki, M.; Saotome, Y.; Yanagisawa, M. Thin Solid Films 1988, 160, 453. (3) Miyamoto, T.; Sato, I.; Ando, Y. In Tribology and Mechanics of Magnetic Storage DeVices, 2nd ed.; Bhushan, B., Eiss, N. S., Eds.; Springer-Verlag: New York, 1990. (4) Ando, E.; Goto, Y.; Morimoto, K.; Ariga, K.; Okahata, Y. Thin Solid Films 1989, 180, 287. (5) Bhushan, B.; Gupta, B. K. Handbook of Tribology: Materials, Coatings and Surface Treatments; McGraw-Hill: New York, 1991. (6) Ruhe, J.; Novotny, V. J.; Kanazawa, K. K.; Clarke, T.; Street, G. B. Langmuir 1993, 9, 2383. (7) Zarrad, H.; Clechet, P.; Belin, M.; Martelet, C.; Jaffrezic-Renault, N. J. Micromech. Microeng. 1993, 3, 222. (8) Bhushan, B.; Koinkar, V. N. J. Appl. Phys. 1994, 75, 5741. (9) Zhao, M.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1998, 120, 4877. (10) Balogh, L.; Tomalia, D. A. J. Am. Chem. Soc. 1998, 120, 7355. (11) Zhao, M.; Crooks, R. M. AdV. Mater. 1999, 11, 217. (12) Baker, A. B.; Zamborini, F. P.; Sun, L.; Crooks, R. M. Anal. Chem. 1999, 71, 4403. (13) Rar, A.; Zhou, J. N.; Liu, W. J.; Barnard, J. A.; Bennett, A.; Street, S. C. Appl. Surf. Sci. 2001, 175-176, 134. (14) Zhang, X.; Klein, J.; Sheiko, S.; Muzafarov, A. M. Langmuir 2000, 16, 3893. (15) Street, S. C.; Rar, A.; Zhou, J. N.; Liu, W. J.; Barnard, J. A.; Bennett, A. Chem. Mater. 2001, 13, 3669. (16) Tsukruk, V. V. AdV. Mater. 2001, 13, 95.
Requirements for effective boundary lubrication are strong adhesion of the molecules to the substrate, reduced friction, and high resistance to wear. However, when self-assembly and Langmuir-Blodgett (LB) techniques are employed, the weak interaction between the film and the substrate results in the molecules being worn away.20 Self-assembled aliphatic molecules such as alkanethiols and alkylsilanes are covalently bonded to the substrate and therefore are better candidates as lubricants.21-29 It is hypothesized that, within the self-assembled monolayers, the functional groups interact with the substrate surface via hydrogen bonding, dipole interaction, π stacking, or covalent attachment, which may enhance the mechanical integrity and stability.30 Direct covalent molecular assembly is advantageous in the sense that each layer in the assembled film can be covalently linked to its underlying layer and no excess deposition can take place because it is limited by the reactive sites on the layer surface. Multilayer films with covalent interlayer bonding31-33 are more advantageous because they are robust enough to withstand elevated temperatures, polar solvent attack, mechanical wear, and abrasion. Furthermore, interlayer bonding is useful for anchoring the lubricant layer to (17) Rar, A.; Curry, M.; Barnard, J. A.; Street, S. C. Tribol. Lett. 2002, 12, 87. (18) Xu, F. T.; Ye, P. P.; Curry, M.; Barnard, J. A.; Street, S. C. Tribol. Lett. 2002, 12, 189. (19) Li, X.; Curry, M.; Wei, G.; Zhang, J.; Barnard, J. A.; Street, S. C.; Weaver, M. L. Surf. Coat. Technol. 2004, 177-178, 504. (20) Bhushan, B.; Kulkarni, A. V.; Koinkar, V. N.; Boehm, M.; Odoni, L.; Martelet, C.; Belin, M. Langmuir 1995, 11, 3189. (21) Overney, R. M.; Meyer, E.; Frommer, J.; Gu¨ntherodt, H.-J. Langmuir 1994, 10, 1281. (22) Takano, H.; Fujihira, M. J. Vac. Sci. Technol., B 1996, 14, 1272. (23) Fujihara, M.; Morita, Y. J. Vac. Sci. Technol., B 1994, 12, 1609. (24) Xiao, X.; Hu, J.; Charych, D. H.; Salmeron, M. Langmuir 1996, 12, 235. (25) Lio, A.; Charych, D. H.; Salmeron, M. J. Phys. Chem. B 1997, 101, 3800. (26) Kim, H. I.; Graupe, M.; Oloba, O.; Koini, T.; Imaduddin, S.; Lee, T. R.; Perry, S. S. Langmuir 1999, 15, 3179. (27) Brewer, N. J.; Beake, B. D.; Leggett, G. J. Langmuir 2001, 17, 1970. (28) Ren, S.; Yang, S.; Zhao, Y. Langmuir 2003, 19, 2763. (29) Houston, J. E.; Doelling, C. M.; Vanderlick, T. K.; Hu, Y.; Scoles, G.; Wenzl, I.; Lee, T. R. Langmuir 2005, 21, 3926. (30) Song, S.; Ren, S.; Wang, J.; Yang, S.; Zhang, Y. Langmuir 2006, 22, 6010. (31) Sun, J.; Wang, Z.; Sun, Y.; Zhang, X.; Shen, J. Chem. Commun. 1999, 8, 693. (32) Zhang, F.; Srinivasan, M. P. Colloids Surf., A 2005, 257-258, 295. (33) Zhang, F.; Jia, Z.; Srinivasan, M. P. Langmuir 2005, 21, 3389.
10.1021/la0635707 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/04/2007
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enhance its stability during high disk rotation speeds for magnetic disk drives.34,35 Conventional deposition methods involve the use of a liquid solvent as a vehicle for the depositing species as well as for a rinse following deposition. Solvent retention or residual solvent in films has a detrimental effect on film properties.36,37 The solvent residues can negatively affect the tribological and/or chemical properties of the film.38 Moreover, the presence of residual solvent may enhance the molecular mobility and induce unpredictable frictional properties. When there is a strong interaction between the solute and solvent, it is often impossible to remove all the solvent in the films.39,40 In addition, conventional drying procedures for solvent removal may result in nonequilibrium drying and an uncontrolled increase in solute concentration. Strong hydrodynamic forces may be exerted in drying droplets that can change the morphology of structures and complicate reproducibility.41 Therefore, it would be desirable to develop protocols for the film deposition that minimizes the participation of liquid solvents. Thin film deposition from supercritical carbon dioxide (SCCO2) has received much attention because of the unique nature and properties of the solvent in the supercritical state. It has been used for the dissolution and deposition of thin perfluoropolyether films and the formation of self-assembled monolayers on solid substrates.42-44 Recently, we showed that SCCO2 could be used as a vehicle for building ultrathin films of oligoimide on silicon dioxide surface.45-47 The quality of the films in the supercritical fluids (SCF) medium was better than that for films formed in N,Ndimethylacetamide (DMAc), and the SCF-grown films were mechanically and thermally more stable. We have also investigated the formation of covalently bonded, multilayered, dendrimer-containing ultrathin films and the formation of Cu nanoparticles within the thin film matrix using supercritical carbon dioxide as a processing medium.48,49 In this work, we exploit the ability of layer-by-layer covalent molecular assembly to deliver robust, nanoparticle-laden ultrathin films for tribological applications. SCCO2 is employed as the processing medium to reduce the participation of conventional liquid solvents. We use fluorination as a means of capping the matrix to yield smaller nanoparticles and better tribological properties for the film-coated surface. (34) Gellman, A. J. Curr. Opin. Colloid Interface Sci. 1998, 3, 368. (35) Luzinov, I.; Julthongpiput, D.; Malz, H.; Pionteck, J.; Tsukruk, V. V. Macromolecules 2000, 33, 1043. (36) Duskova-Smrckova, M.; Dusek, K. J. Mater. Sci. 2002, 37, 4733. (37) Bistac, S.; Schultz, J. Prog. Org. Coat. 1997, 31, 347. (38) Popov, V. K.; Bagratashvili, V. N.; Krasnov, A. P.; Said-Galiyev, E. E.; Nikitin, L. N.; Afonicheva, O. V.; Aliev, A. D. Tribol. Lett. 1998, 5, 297. (39) Ponzio, E. A.; Echevarria, R.; Morales, G. M.; Barbero, C. Polym. Int. 2001, 50, 1180. (40) Briscoe, B. J.; Akram, A.; Adams, M. J.; Johnson, S. A.; Gorman, D. M. J. Mater. Sci. 2002, 37, 4937. (41) Gallymov, M. O.; Vinokur, R. A.; Nikitin, L. N.; Said-Galiyev, E. E.; Khokhlov, A. R.; Yaminsky, I. V.; Schaumburg, K. Langmuir 2002, 18, 6928. (42) (a) Fulton, J. L.; Pfund, D. M.; Romack, T. J.; Combes, J. R.; Samulski, E. T.; DeSimone, J. M.; Capel, M. Langmuir 1995, 11, 4241. (b) McClain, J. B.; Betts, D. E.; Canelas, D. A.; Samulski. E. T.; DeSimone, J. M.; Londono, J. D.; Cochran, H. D.; Wignall, G. D.; Chillura-Martino, D.; Triolo, R. Science 1996, 274, 2049. (c) Buhler, E.; Dobrynin, A. V.; DeSimone, J. M.; Rubinstein, M. Macromolecules 1998, 31, 7347. (43) Weinstein, R. D.; Yan, D.; Jennings, G. K. Ind. Eng. Chem. Res. 2001, 40, 2046. (44) Henon, F. E.; Camaiti, M.; Burke, A.; Carbonell, R. G.; DeSimone, J. M.; Piacenti, F. J. Supercrit. Fluids 1999, 15, 173. (45) Puniredd, S. R.; Srinivasan, M. P. Langmuir 2005, 21, 7812. (46) Puniredd, S. R.; Srinivasan, M. P. Langmuir 2006, 22, 4092. (47) Puniredd, S. R.; Zhang, F.; Srinivasan, M. P. Mater. Sci. Eng., B 2006, 132, 43. (48) Puniredd, S. R.; Srinivasan, M. P. J. Colloid Interface. Sci. 2007, 306, 118. (49) Puniredd, S. R.; Srinivasan, M. P. Ind. Eng. Chem. Res. 2007, 46, 464.
Letters
2. Experimental Section 2.1. Materials. p-Aminophenyltrimethoxysilane (APhS), second generation polyamidodiamine dendrimer (PAMAM), pentadecafluorooctyanoyl chloride, gold(III) chloride trihydrate (HAuCl4‚ 3H2O), and sodium citrate (HOC(COONa)(CH2COONa)2·2H2O) (all from Sigma-Aldrich) and 3-cyanopropyltrichlorosilane (CPS, Lancaster) were used as received. Trifluroacetic anhydride (from SigmaAldrich), carbon dioxide (SOXAL code P40J purified grade with
