Nanotribological Properties of Composite Molecular Films: C₆₀ Anchored to a Self-Assembled Monolayer

Abstract

Tribological properties of molecular films composed of a fullerene monolayer chemically attached to the functional surface of self-assembled monolayers (C60−SAM) were studied by friction force microscopy. We observed very high wear stability of composite fullerene films. The friction coefficient (μ) 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 μm/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 C60−SAM films of 0.04−0.15 at various velocities and loads is much lower than μ 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, N3−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: μCH3-SAM < μN3-SAM < μC60-SAM ≤ μC60 < μSiO2. 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, θCH3-SAM (110°) > θN3-SAM (84°) > θC60-SAM (70°) ≫ θSiO2 (≈0°), and variation of the adhesive forces, ΔFCH3-SAM (4 nN) ≤ ΔFN3-SAM (5 nN) < ΔFC60-SAM (9 nN) < ΔFSiO2 (12 nN).