Mechanisms of exchange diffusion on fcc(111) transition metal surfaces

Abstract

Exchange diffusion on unreconstructed fcc (111) transition metal surfaces has been studied through a combination of molecular dynamics with a friction force (quenched molecular dynamics) and the nudged elastic band method. Four homogeneous systems [$\mathrm{Au}∕\mathrm{Au}(111)$, $\mathrm{Pt}∕\mathrm{Pt}(111)$, $\mathrm{Ni}∕\mathrm{Ni}(111)$, and $\mathrm{Cu}∕\mathrm{Cu}(111)$] provide evidence for the occurrence of exchange via the formation of a transient dimer (concerted exchange) when the following conditions are met. First, adatom and substrate are of the same nature, second, the exchange involves the original adatom and a surface atom in a nearest-neighbor position on the surface (short range exchange). Consideration of two heterogeneous systems, $\mathrm{Co}∕\mathrm{Pt}(111)$ and $\mathrm{Co}∕\mathrm{Au}(111)$, reveals that an alternative mechanism (two-steps exchange) may take place provided the tensile stress of the substrate is sufficiently high. This is the case in $\mathrm{Co}∕\mathrm{Au}(111)$ and in $\mathrm{Co}∕\mathrm{Pt}(111)$, when this latter is submitted to in-plane expansion. By focusing on mechanisms involving surface atoms next-nearest neighbors of the original adatom, we have highlighted two different kinds of medium range diffusion via exchanges. The first amounts to a concerted movement of the adatom and two surface atoms, one of them simply sliding on the surface in a position intermediate between the remaining two, moving upward and downward, respectively. The second occurs through a pair of two correlated short range exchanges. We found that exchange diffusion barriers are lowered by the medium range mechanism on $\mathrm{Au}(111)$ substrates. An atomic-scale rationale explaining this behavior is presented.