Adhesion, Friction and Mechanical Properties of CF₃- and CH₃-Terminated Alkanethiol Monolayers: A Comparative Study

Self-assembled monolayers (SAMs) have received considerable recent attention as molecular-level lubricants in, for example, micro-electro-mechanical systems (MEMS).[1] 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.[2] 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[3] (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”.[4]