SURFACE-DIFFUSION ON HETEROGENEOUS SURFACES - THE EFFECT OF STEPS

Monte Carlo modeling was utilized for determining the tracer and chemi cal surface diffusion coefficients of adsorbates. Simulations were car ried out on a square array of 64 x 64 lattice sites applying periodic boundary conditions. In order to analyze the effects of steps on surfa ce diffusion different atomistic models of steps were introduced. For the simplest model steps were assumed to affect the site energies of a single row of lattice sites located on the down-side of the step. Ano ther set of simulations was performed for steps affecting the site ene rgies on the up-side of the step as well. For attractive interactions between adsorbates and step sites surface diffusion is slowed down wit h respect to the noninteracting Langmuir gas on a plain surface. Steps cause diffusion to be anisotropic as crossing of steps costs an extra activation energy. Additional calculations are performed in order to analyze the effect of the saddle points of the periodic potential on s urface diffusion.