Friction force microscopy of self-assembled monolayers: Influence of adsorbate alkyl chain length, terminal group chemistry, and scan velocity

Friction force measurements have been made using friction force microscopy for a series of self-assembled monolayers (SAMs) of alkanethiols of varying chain lengths and terminal groups adsorbed on gold. The chemistry of the tip was controlled by the deposition of a SAM. When carboxylic acid terminated tips were employed, the friction coefficients of hydroxyl and carboxylic acid terminated SAMs were found to be greater than those of methyl terminated SAMs. However, although the friction coefficients of short-chain methyl terminated SAMs were significantly greater than those of long-chain methyl terminated SAMs, there was not a significant difference between the values determined for short- and long-chain SAMs with polar terminal groups. When methyl terminated tips were employed, there was no difference between the behavior observed for hydroxyl and carboxylic acid terminated SAMs of equal chain length, but methyl terminated SAMs exhibited increased friction coefficients which were larger for short-chain adsorbates. Friction-velocity plots exhibited markedly different behavior for polar and nonpolar SAMs when carboxylic acid terminated tips were employed. These observations are explained in terms of the stabilization of the adsorbates by intermolecular hydrogen bonding in SAMs with polar terminal groups.