Influence of solvent environment and tip chemistry on the contact mechanics of tip-sample interactions in friction force microscopy of self-assembled monolayers of mercaptoundecanoic acid and dodecanethiol

Friction force microscopy measurements have been made for self-assembled monolayers of mercaptoundecanoic acid (C10COOH) and dodecanethiol (C11CH3) in different liquid media. In perfluorodecalin, the friction-load relationship was nonlinear and consistent with adhesion-controlled sliding. The effective range of the attractive force was controlled by using AFM tips functionalized with alkanethiols (chemical force microscopy). Like pairs of interacting molecules yielded data that were characterized by the Johnson-Kendall-Roberts model of contact mechanics, whereas the interaction between dissimilar pairs of molecules fitted the behavior predicted by the Derjaguin-Muller-Toporov model. In ethanol, the adhesive force was much smaller, and sliding was not adhesion-controlled. Under this condition of low adhesion, the friction force varied linearly with the applied load.