Anisotropy of diffusion along steps on the (111) faces of gold and silver

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Abstract
We present a molecular-dynamics simulation of adatom diffusion along the two close-packed steps on the (111) surfaces of gold and silver. Both metals are modeled by employing many-body potentials derived within the second-moment approximation to the tight-binding model. The simulation predicts very different behaviors for the two metals. For Au, the diffusion is much faster along the step with (111) microfacets (step B), whereas for Ag the diffusion is faster along the step with (100) microfacets (step A). The difference between the diffusion coefficients along the steps is more marked in gold and, for both metals, the Arrhenius plots show a dynamical lowering of the activation barriers with respect to the static potential barriers; no evidence of an inversion of the anisotropy of diffusion is obtained. As the diffusion along steps is quasi-one-dimensional, the results of the simulations have been compared to those based on the Fokker-Planck equation in a one-dimensional periodic potential. The agreement between the model and the simulations is remarkable for B steps both in gold and silver; the model predicts the temperature dependence of the rate, the correct proportion of long jumps, and the details of the behavior of the mean-square displacement. In A steps, the agreement is satisfactory for Ag and qualitative for Au; in the latter case, the diffusion path is rather different from a straight line and the application of a one-dimensional model may be questionable.