Scanning Force Microscopic Exploration of the Lubrication Capabilities of n-Alkanethiolate Monolayers Chemisorbed at Gold:  Structural Basis of Microscopic Friction and Wear

This paper explores the ability of n -alkanethiolates chemisorbed at Au(111) to function as boundary lubricants at microscopic length scales as probed by scanning force microscopy (SFM). Through an examination of the influence of alkyl chain length, we show that the macroscopic structure of this system, as developed from insights into the chain-packing density via infrared reflection spectroscopy, greatly influences the observed friction and wear. That is, the longer chain monolayers exhibit a markedly lower friction and a reduced propensity to wear than the shorter chain monolayers, a situation that reflects the more extensive cohesive interactions between chains. From the combined weight of these findings, we examine the frictional process within the context of an activation mechanism that involves pressure and shear activation volumes. The ability of longer chain alkanethiolate monolayers to lubricate features that arise from changes in substrate topography is also presented, and the resulting mechanistic issues are discussed.