Nanotribological properties of composite molecular films: C-60 anchored to a self-assembled

Tribological properties of molecular films composed of a fullerene monolayer chemically attached to the functional surface of self-assembled monolayers (C-60-SAM) were studied by friction force microscopy. We observed very high wear stability of composite fullerene films. The friction coefficient (mu) for these films varies in a wide range from 0.04 +/- 0.02 at high loads and 0.06 +/- 0.02 at the highest velocities tested (< 1000 mu m/s) to 0.15 +/- 0.03 at intermediate velocities and low loads. This non-monotonic velocity behavior is a striking feature of fullerene films as compared to steadily rising friction forces for alkylsilane monolayers (CH3-SAM) and may be related to exceeding dissipation of energy during structural rearrangement of fullerene molecules. The friction coefficient of C-60-SAM films of 0.04-0.15 at various velocities and loads is much lower than mu for silicon surfaces (0.1-0.6) and is comparable to the friction coefficient of CH3-SAM (0.02-0.05) and an azide-terminated monolayer, N-3-SAM (0.04-0.07), measured at the same conditions. This value is within an interval found in literature data for sublimed fullerene films (0.07-0.2). Our data allow ranking of the friction coefficients of the various surfaces studied here in the following order: mu(CH3-SAM) < mu(N3-SAM) < mu(C60-SAM) less than or equal to mu(C60) < mu(SiO2). This ranking is consistent with macroscopic pin-on-disk measurements on the same films,(10) variation of water contact angles reflecting a trend of increasing hydrophilicity, theta(CH3-SAM) (110 degrees) > theta(N3-SAM) (84 degrees) > theta(C60-SAM) (70 degrees) much greater than theta(SiO2) (approximate to 0 degrees), and variation of the adhesive forces, Delta F-CH3-SAM(4 nN) less than or equal to Delta F-N3-SAM (5 nN) < Delta F-C60-SAM (9 nN) < Delta F-SiO2 (12 nN).