Bonding of Pd, Ag, and Au atoms on MgO(100) surfaces and MgO/Mo(100) ultra-thin films: A comparative DFT study

We have studied by means of density functional theory (DFT) plane-wave calculations the adsorption properties of ultrathin MgO films grown on Mo(100) as function of the thickness of the oxide film, and compared the results with those of unsupported slabs representing a single-crystal MgO(100) surface. The presence of a metal substrate at the interface with the MgO film results in the charging of adsorbed atoms with high electron affinity, like Ag and Au, while Pd is basically unperturbed. As a consequence, while the properties of Pd are the same on MgO(100) or MgO/Mo(100), for Ag and Au substantial differences are found. On MgO/Mo(100) films the Ag(5s) and the Au(6s) levels fall below the Fermi level of Mo, leading to negative charging of the metal atom and to a different bonding mechanism; the 5s level of Pd, on the contrary, is just above the Fermi level of Mo, and no charge transfer occurs. The effect tends to decrease for thicker MgO films and is closely related to the change in work function of the Mo(100) surface when a thin MgO film is deposited on it and to the appearance of metal-induced gap states at the metal-insulator interface. The results are of relevance to understand the properties of ultrathin oxide films. The charging effect is absent on single-crystal or polycrystalline MgO surfaces.