Role of Graphene Surface Ripples and Thermal Vibrations in Molecular Dynamics of C₆₀

Nanocars are artificial molecular machines with chassis, axles, and wheels designed for nanoscale transport at materials' surfaces. Understanding the dependence of surface dynamics of nanocars on the substrate's physicochemical properties is critical to the design of the transport properties of such man-made nanoscale devices. Among the multitude of potential substrates for the nanotransporters, graphene exhibits intrinsic ripples on its surface, which may affect the surface dynamics of nanocars. In this work, we report our molecular dynamics study of motion of C₆₀, a popular nanocar wheel, on the graphitic substrates with systematically controllable surface ripples. We find that surface ripples increase the amplitude of fullerene fluctuation in the direction normal to surface, which leads to decrease of the desorption temperature from 650 K on a double-layer graphite system with less ripples to 550 K on single-layer graphene with more ripples. The surface diffusion of C₆₀ follows the rare hops mechanism for temperatures up to 30 K. It switches to continuous semiballistic motion at 150 K. The surface ripples do not significantly affect the diffusion coefficients, but change the anomaly parameter, especially at low temperatures. The ripples exhibit no major effect on the rotational dynamics of C₆₀, which is attributed to very small energy barriers for C₆₀ rotation on graphitic surfaces.