Growth pattern of Ag(n) (n = 1-8) clusters on the α-Al2O3(0001) surface: a first principles study

We report an extensive first-principles study of the structure and electronic properties of Ag(n) (n = 1-8) clusters isolated in gas phase and deposited on the α-Al(2)O(3) surface. We have used the plane wave based pseudopotential method within the framework of density functional theory. The electron ion interaction has been described using projector augmented wave (PAW), and the spin-polarized GGA scheme was used for the exchange correlation energy. The results reveal that, albeit interacting with support alumina, the Ag atoms prefers to remain bonded together suggesting an island growth motif is preferred over wetting the surface. When compared the equilibrium structures of Ag clusters between free and on alumina substrate, a significant difference was observed starting from n = 7 onward. While Ag(7) forms a three-dimensional (3D) pentagonal bipyramid in the isolated gas phase, on alumina support it forms a planar hexagonal structure parallel to the surface plane. Moreover, the spin moment of the Ag(7) cluster was found to be fully quenched. This has been attributed to higher delocalization of electron density as the size of the cluster increases. Furthermore, a comparison of chemical bonding analysis through electronic density of state (EDOS) shows that the EDOS of the deposited Ag(n) cluster is significantly broader, which has been ascribed to the enhanced spd hybridization. On the basis of the energetics, it is found that the adsorption energy of Ag clusters on the α-Al(2)O(3) surface decreases with cluster size.