SYSTEMATIC STUDIES OF THE FRICTIONAL-PROPERTIES OF FLUORINATED MONOLAYERS WITH ATOMIC-FORCE MICROSCOPY - COMPARISON OF CF3-TERMINATED AND CH3-TERMINATED FILMS

Molecular-level insight into the frictional properties of fluorinated self-assembled monolayers (SAMs) was achieved by combining two recentl y developed techniques that operate at the subnanometer scale: control of the interfacial composition through molecular self-assembly and tr ibological measurements performed with the atomic force microscope. To explore the origin of frictional forces in fluorinated films, the fri ctional properties of two classes of alkanethiols adsorbed on single c rystal gold were measured and compared. In these studies, films of equ ivalent chain length, packing density and packing energy, but differen t termination (methyl vs trifluoromethyl), were characterized and inve stigated. For these films, in which the only detectable difference was the outermost chemical structure composition, a factor of 3 increase in the frictional response was observed in going from the hydrogenated to the fluorinated film. These results support the conclusion that ch emical structure/composition alone plays an integral role in determini ng the frictional properties of an interface. We propose that the diff erence in friction arises predominantly from the difference in size of the methyl and trifluoromethyl groups.