Indium on Cu(100) from first principles: Energetics, complex formation, and diffusion of adsorbates and vacancies on terraces and at steps

Indium (In) on the Cu (001) surface is a paradigmatic adsorption system exhibiting two important effects: first, In has been reported to foster the layer-by-layer growth of this surface; second, In diffuses anomalously and possibly in concert with surface vacancies, and has indeed been used as a tracer of vacancy surface diffusion. Detailed knowledge of the energetics and diffusion barriers of In on Cu (001), adsorbed, coadsorbed with Cu adatoms, or in the presence of surface vacancies, as can be extracted from first-principles calculations, is an important ingredient in the understanding of these effects. Surprisingly, only a very limited amount of theoretical work exists on this system, providing rather disparate results. Here we present ab initio calculations of the adsorption and clustering energetics, and diffusion and kinetic barriers for a large selection of possible systems and motions involving Cu vacancies and adatoms, In adsorbates, as well as clusters and complexes thereof. We address possible modes of vacancy creation and In incorporation involving kink sites at steps, as well as the interlayer diffusion of Cu with and without In at the step edge. Among the results shedding light on the diffusion mode of In:Cu(100) as well as on the surfactant action of In, we find that In diffuses via a concerted motion with vacancies, and that In at a step lowers drastically the exchange downstep diffusion barrier for Cu, thus favoring two-dimensional growth.