Diffusion of short-chain molecules on metal surfaces

In this paper, we review our recent theoretical and simulation studies of the surface diffusion of n-alkanes, ranging in size from ethane to hexadecane, physically adsorbed on Pt(111). The model system exhibits many features seen experimentally. Through both animation of the molecular trajectories and determination of the minimum-energy path for nearest-neighbor hopping, we find that the shorter molecules (ethane through octane) all have similar diffusion mechanisms, involving coupled translation and rigid rod-like rotation in the surface plane. In addition, the diffusion energy barriers for these molecules increase nearly linearly with chain length in both the static and dynamic calculations. The diffusion of decane and hexadecane does not adhere to the trends for the shorter molecules and a decrease can be observed in the dynamical diffusion energies for these molecules. The diffusion of the longer molecules involves hops, with unique mechanisms, to second and third neighbor sites. Our static analysis has indicated, for decane, that the diffusion-energy barrier for third-neighbor hopping is lower than that for nearest-neighbor hopping and is in agreement with the trend seen in the dynamical diffusion barriers. Even though there is agreement between theoretical and simulated diffusion energy barriers for many of the molecules, the motion observed in the MD simulations does not agree with the assumptions of the hopping model. A model that can incorporate the influence of long flights would provide a more realistic description of the motion.