CHAIN-LENGTH DEPENDENCE OF THE FRICTIONAL-PROPERTIES OF ALKYLSILANE MOLECULES SELF-ASSEMBLED ON MICA STUDIED BY ATOMIC-FORCE MICROSCOPY

We show that the frictional properties of alkylsilane monolayers self- assembled on mica in contact with Si3N4 tips depend strongly on the le ngth of the alkyl chains. Friction is particularly high with short cha ins of less than eight carbons. We attribute this to the large number of dissipative modes in the less ordered short chains. Longer chains, stabilized by van der Waals attractions form more compact and rigid la yers and act as much better lubricants. This lubricating action is los t at a certain threshold load, where wear of the molecular layer occur s, leading to much higher friction force values. The results presented here clearly indicate that the chemical identity of the exposed end g roups is not sufficient to determine the frictional properties of mono layer films. The increased number of energy dissipation modes facilita ted by the presence of molecular disorder (e.g., rotations about a C-C axis), in fact dominates the frictional behavior of monolayers with s hort chains.