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 length of the alkyl chains. Friction is particularly high with short chains 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 layers and act as much better lubricants. This lubricating action is lost at a certain threshold load, where wear of the molecular layer occurs, leading to much higher friction force values. The results presented here clearly indicate that the chemical identity of the exposed end groups is not sufficient to determine the frictional properties of monolayer films. The increased number of energy dissipation modes facilitated by the presence of molecular disorder (e.g., rotations about a C-C axis), in fact dominates the frictional behavior of monolayers with short chains.