Diffusion of rhodium layers on the stepped surface of a tungsten micro-crystal

We have investigated the spreading of rhodium at coverages of 0.25, 0.5, 1, 2 and 3 ML over the curved surface of a field emitter tip using field electron microscopy. We have found that the activation energy of spreading as well as the prefactor for the diffusivity depend strongly on the thickness of the layer diffusing, due to a change in interactions in the adsorbate-substrate system. The derived average activation energy for spreading first decreases from E_dif = 1.32 eV/atom at Θ = 0.25 ML to E_dif = 0.71 eV at Θ ≈ 2 ML and than rises again to E_dif = 1.20 eV at Θ ≈ 3 ML. The prefactor for the diffusivity D₀ also decreases with increasing coverage from 0.5 to 1 ML, and stays almost constant for multilayer diffusion in a range of few orders of magnitude lower than for single atom diffusion. We register typical spreading behavior with a sharp moving boundary in the (0 1 1)-(0 0 1) zone of the tip and an unusual picture of diffusion in the (0 1 1)-(1 1 2) region of the tip. In the second region diffusion proceeds without a sharp boundary, independent of the thickness of the moving layer. We think that such an unusual picture can be caused by the change in composition of the second and next layers of adsorbing material due to the early stage of faceting observed in this region of the tip at higher temperature. The results are compared with data for diffusion of individual Rh atoms and small clusters; to understand the observed diffusion we propose taking account of the atomic surface structure of the substrate, modified by strong interactions of the Rh adsorbate with the W micro-crystal surface.