Dynamic Friction of Nanoscale Sliding on a C-60 Deposited Film

We measured the frictional behavior acting on C-60 films at a small normal load as a function of oscillation amplitude using a MHz-range quartz crystal resonator and an optical AFM cantilever. The frictional behavior for C-60 films varies by the amplitude: For the amplitudes smaller than the C-C bond length of the C-60 molecule, the dynamic frictional force is directly proportional to the amplitude, while it becomes almost constant for larger amplitudes. Although the observation is in qualitatively the same manner as HOPG, the dynamic frictional force for small amplitudes is significantly larger than that of HOPG. This difference suggests that C-60 molecules easily tilt and/or rotate when the tip slides in C-60 films, and the dynamic friction arises from the tilting and/or rotating motion.