Nucleation and growth of Ni clusters on regular sites and F centers on the MgO(0 0 1) surface

The stability and growth of Ni clusters on the MgO surface was studied performing density functional calculations on model systems. Ni4, Ni5, Ni8, Ni9 and Ni12 clusters were deposited on regular and defect sites, in particular on neutral and charged oxygen vacancies (Fs and Fs+ centers) of the MgO(0 0 1) surface, and optimized under C4v symmetry constraint. The MgO substrate was represented by clusters of ions embedded in arrays of point charges and effective core potentials. The study was aimed at elucidating the role of point defects in nucleation and growth. In particular, we considered the energy gain resulting from the addition of Ni atoms to a pre-existing Ni cluster as function of the site where the cluster had grown. Oxygen vacancies at the MgO surface act as anchoring sites preventing cluster diffusion, but do not result in a larger energy release when new Ni atoms are added compared to Ni clusters grown on regular MgO terrace sites.