Modeling adsorption on fcc(nnm) surfaces

Abstract

In general, fcc(nnm) surfaces consist of very long armchair (111) terraces separated by steps. The number M of atomic sites in the width of the terraces depends on the Miller indices (nnm). The model presented here considers values of M ≤ 6, with adsorbate-substrate interaction energy on step-sites different from those on bulk sites, takes into account first- and second-neighbor adsorbate-adsorbate interactions, and specializes to the case of attractive first-neighbors. We obtain the complete low temperature, 3-dimensional, energy phase diagrams. The occupational configurations of the phases exhibit features similar to those of the phases obtained in the infinite-width limit case, or flat fcc(111) surfaces. This yields a classification of the phases into types, and, within each type, the phases are grouped into families. This suggests a number of generalizations for any value of M beyond 6, leading to a better understanding of the competing interaction energies and of the evolution of the phase diagrams with increasing width of the terraces. The relevance of these results to experiments is discussed within the context of preferential adsorption on step sites and applied to the water adsoption on Pt(335).