Self-assembled monolayers (SAMs) have received considerable recent attention as molecular-level lubricants in, for example, micro-electro-mechanical systems (MEMS). Of particular interest as tribological films have been SAMs terminated by fluorocarbon groups, because of their inert nature and enhanced thermal stability. Surprisingly however, fluorocarbon films were shown to actually produce higher coefficients of friction (relative to CH3-terminated films) in atomic force microscopy (AFM) studies. Subsequent work has concluded that the increased van der Waals radius of the fluorine groups (∼45%) causes a steric disruption of the order of the molecular surface giving rise to an increased friction. We present results from a direct comparison of the adhesive, mechanical and frictional properties of SAMs terminated by CF3 and CH3 groups using both Interfacial Force Microscopy (IFM) and the AFM. IFM results are shown for a two micron tungsten tip interacting with C16 alkylthiol molecules assembled on Au(111) single-crystal surfaces. AFM results involve a ∼20 nm tip interacting with the same two molecules assembled on Au films deposited on mica surfaces. A direct AFM comparison is accomplished by using a “nanografting technique”.
Adhesion, Friction and Mechanical Properties of CF3- and CH3-Terminated Alkanethiol Monolayers: A Comparative Study
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Houston, JE, Doelling, CM, Vanderlick, TK, Hu, Y, Scoles, G, Wenzl, I, & Lee, TR. "Adhesion, Friction and Mechanical Properties of CF3- and CH3-Terminated Alkanethiol Monolayers: A Comparative Study." Proceedings of the World Tribology Congress III. World Tribology Congress III, Volume 1. Washington, D.C., USA. September 12–16, 2005. pp. 345-346. ASME. https://doi.org/10.1115/WTC2005-63769
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