A theoretical study of the surface diffusion of large molecules. I. n-alkane-type chains on W(100)

In the present study the surface diffusion of model n-alkane-type chains adsorbed on a W(100) surface were simulated. The simulations were performed using molecular dynamics calculations where the thermal motion of the surface atoms was introduced via the generalized Langevin method. The potential function among the chain atoms used in these calculations described the nearest-neighbor interaction by a Morse potential while next-nearest-neighbors and next-next-nearest-neighbors interaction was described by a Lennard-Jones 12-6 and a repulsive exponential function, respectively. The length of the chains, N, considered were N = 3, 6, 10, and 20. For each value of N the chain diffusion at three or four surface temperatures was examined. For all values of N it was found that the diffusion coefficient could be described by an Arrhenius expression. It was found, in good agreement with the experimental results, that the activation energy for the diffusional motion scales with the chain length while the preexponential factors were practically independent of N. In addition, various static (e.g., average mean square end-to-end distance and average mean square radius of gyration) and dynamic (e.g., autocorrelation functions) properties of the simulated systems were computed. The results of these simulations were used to obtain a qualitative understanding of the mechanism by which such chains diffuse on a solid surface.