Structure and diffusion of small Ag and Au clusters on the regular MgO (100) surface

The lowest energy structures and the diffusion energy barriers of small M-N(N = 1 - 4) Ag and Au clusters absorbed on the regular MgO (100) surface are investigated via density-functional (DF) calculations, using two different xc-functionals (PBE and LDA). In agreement with previous work, it is found that the lowest-energy structures of Ag and Au clusters in this size-range exhibit a strong `metal-on-top' effect, by which the clusters are absorbed atop oxygen ions in a linear ( dimer) or planar ( trimer and tetramer) configuration perpendicular to the surface. The corresponding diffusion mechanisms range from monomer hopping, to dimer leapfrog (Ag-2) or hopping (Au-2), trimer walking, tetramer walking (Ag-4) or rocking and rolling (Au-4), exhibiting interesting differences between Ag and Au. An analysis of the corresponding energy barriers shows that trimers can diffuse at least as fast as monomers, while tetramers and ( especially in the case of gold) dimers present somewhat higher barriers, but are anyway expected to be mobile on the surface at the temperatures of molecular beam epitaxy (MBE) experiments. The calculated PBE diffusion energy barriers compare reasonably well with the values extracted from the analysis of recent MBE experimental data, with the LDA predicting slightly higher barriers in the case of gold.