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

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 Au∕Au (111), Pt∕Pt (111), Ni∕Ni (111), and Cu∕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, Co∕Pt (111) and Co∕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 Co∕Au (111) and in Co∕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 Au (111) substrates. An atomic-scale rationale explaining this behavior is presented.