Molecularly specific studies of the frictional properties of monolayer films: A systematic comparison of CF3-, (CH3)(2)CH-, and CH3-terminated films

The origin of frictional forces in self-assembled monolayers (SAMs) was inv estigated through systematic correlation of the frictional properties with the chemical structure/composition of the films. Atomic force microscopy wa s used to probe the frictional properties of the SAMs formed by the adsorpt ion of methyl-, isopropyl-, and trifluoromethyl-terminated alkanethiols on Au(lll) surfaces. The frictional properties of mixed monolayers composed of varying concentrations of the methyl- and trifluoromethyl-terminated thiol s were also studied. Polarization modulation infrared reflection adsorption spectroscopy was used to measure the vibrational spectra of each of these monolayers and in turn to determine that each was characterized by a well-p acked backbone structure. For these films, which differed only in the natur e of the outermost chemical functionality, a substantial enhancement in the frictional response was observed for films with isopropyl- and trifluorome thyl-terminal groups and for mixed monolayers containing small concentratio ns of the trifluoromethyl-terminated component. These results strongly supp ort the model that the difference in friction in such systems arises predom inantly from the difference in the size of the terminal groups. Larger term inal groups in films of the same lattice spacing give rise to increased ste ric interactions that provide pathways for energy dissipation during slidin g.