Atomic and electronic structure of Cu clusters on MgO

We have studied the structural and electronic properties of Cu_n (n ≤ 13) aggregates on defect-free MgO(100) using density functional theory. The calculations include full relaxation of the surface layer and of all adatoms. The preferred site of adsorption for the copper adatom is the oxygen site. Using the Perdew-Wang gradient corrected exchange correlation functional, we found the adsorption energy and distance to be 0.99 eV and 2.04 A, respectively. Binding occurs through Cu 3d-4s/O 2p band overlap. The saddle point for surface diffusion is the hollow site with a diffusion barrier of 0.45 eV. The adsorption energy per Cu atom decreases with increasing cluster size, while the Cu-Cu cohesive energy rapidly becomes more important than the adsorption energy. For n ≥ 5, three-dimensional cluster formation is preferred to two-dimensional coverage of the surface.