DFT studies of the early stages of growth of Nb on MgO(100)

Using DFT calculations of binding and adsorption energies for various sizes and shapes of Nb clusters on MgO(100) surfaces, we have examined the effects of cluster shape and a neutral O vacancy on the energies and stability of Nb[100] and [110] island structures. Similarly to other cases of metal adsorbates on MgO(100) surfaces, O-vacancy sites tend to act as nucleation sites for Nb adatoms, while the effect of a nearby O vacancy on the binding energy of a Nb cluster is much weaker. In particular, we find that the binding energy for a Nb monomer at an O site (O-vacancy site) is 1.52 eV (2.2 eV) while the energy barrier for Nb monomer diffusion is 0.58 eV. In addition, although both isolated 4-atom Nb [100] and 5-atom Nb [110] islands are isotropic with a slightly higher binding energy for the [100] island, for larger clusters an anisotropic Nb [110] structure is more stable than a square Nb [100] structure, which is in good agreement with a recent experimental result [1]. \\[4pt] [1] M. Krishnan et al., Phys. Rev. ST. Accel. Beams 15, 032001 (2012).