Exploiting Poly(dimethylsiloxane)-Modified Tips To Evaluate Frictional Behavior by Friction Force Microscopy Jeong Ho Cho, † Dae Ho Lee, † Hwa Sung Shin, † Sudip K. Pattanayek, † Chang Y. Ryu, ‡ and Kilwon Cho* ,† Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea, and Department of Chemistry, Rensselaer Polytech Institute, Troy, New York 12180 Received June 28, 2004. In Final Form: October 6, 2004 With the aim of investigating the effect of the surface properties on the friction behavior of self-assembled monolayers, we have modified tipless atomic force microscopy (AFM) cantilevers with a poly(dimethyl- siloxane) (PDMS) lens. The friction coefficient using the silicon tip is strongly influenced by the mechanical properties of the substrate monolayer because hard, sharp silicon tips penetrate the surface of organic monolayers. However, the friction coefficient obtained for the PDMS-modified AFM cantilever is mostly due to the surface properties of the monolayer functional end group, rather than the viscoelastic deformation of the monolayer. The use of the PDMS tip was demonstrated as a novel means to investigate the effect of surface properties on the frictional behavior of self-assembled monolayers with various functional groups with less mechanical deformation. Introduction Friction between two surfaces plays an important role in processes such as fluidic self-assembly and microelec- tromechanical systems (MEMS). Moreover, the patterning of organic monolayers on silicon substrates has emerged as an important modifying agent in MEMS devices. 1-6 The use of friction force microscopy to investigate the frictional properties of organic monolayers is a promising method for establishing a fundamental understanding of lubrication phenomena at the molecular level. 7,8 The effect of surface properties on the frictional properties of organic monolayers is well-documented; however, less well-known are the influences of certain mechanical properties which result in viscoelastic deformation of the organic monolayer chains in nanoscale. Recently, many research groups have documented the effects of end-group functionality on the frictional proper- ties of monolayers using chemically modified atomic force microscopy (AFM) tips. 9-15 However, the hardness and, indeed, sharpness of the AFM tips invariably result in the tip penetration of the monolayer when a normal force is applied. Frictional behavior using a large microbead such as glass and silica also results in the penetration of the probe as a result of the hardness of the microbead. Amonton’s law used in most references assumes that the true contact area is directly proportional to the normal load. 11-15 Therefore, to increase contact area between the probe and the monolayer surface with a normal load, the probe must penetrate on the monolayer surface. The corresponding frictional force is, therefore, a combination of both the mechanical and the surface properties of the organic monolayers. The mechanical properties of self-assembled monolayers (SAMs) vary according to the preparation conditions (i.e., reaction temperature, humidity, and substrate rough- ness). 16-18 In addition, the order-disorder transition temperatures associated with these organic monolayers change in accordance with the SAM functional end group, which in turn influences the mechanical properties. 19 Thus, any variation in the mechanical properties will result in molecular deformation of the organic monolayer chains and a concomitant change in the frictional behavior. However, the effect of viscoelastic deformation in mono- layers is often overlooked when it comes to investigating the frictional properties of SAMs with various end functional groups. In this paper, we describe our efforts to monitor the frictional properties of organic monolayers with less mechanical deformation of the monolayer surface, by fabricating an AFM cantilever with a soft poly(dimethyl- siloxane) (PDMS) elastomer probe at its apex. The probe has a soft and larger radius of curvature than conventional AFM silicon tips, which is expected to prevent surface penetration when a normal load is applied. * To whom correspondence should be addressed. E-mail: kwcho@ postech.ac.kr. † Pohang University of Science and Technology. ‡ Rensselaer Polytech Institute. (1) Tu, J. K.; Talghader, J. J.; Hadley, M. A.; Smith, J. S. Electron. Lett. 1995, 31, 1448. (2) Talghader, J. J.; Tu, J. K.; Smith, J. S. IEEE Photon. Technol. Lett. 1995, 7, 1321. (3) Hawe, R. T. J. Vac. Sci. Technol., B 1988, 6, 1809. (4) Fan, L. S.; Tai, Y. C. Sens. Actuators 1989, 20, 41. (5) Mehregancy, M.; Tai, Y. C. J. Micromech. Microeng. 1991, 1, 73. (6) Mehregancy, M. Circuits Devices 1993, 14. 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In preparation. 11499 Langmuir 2004, 20, 11499-11503 10.1021/la048409f CCC: $27.50 © 2004 American Chemical Society Published on Web 11/19/2004