Growth and coverage dependent electronic structure of MgO on Ag(001)

Growth and coverage dependent electronic structure of MgO films on Ag(001) have been studied using low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and angle-resolved photoemission spectroscopic (ARPES) techniques. The appearance of mosaic structure in the LEED pattern has been observed around 10 ML coverage due to misfit dislocations in the metal-oxide interface. The XPS core level spectra show a substantial amount of binding energy shift relative to their bulk values, indicating the reduction of the on-site Coulomb (U) and change transfer (Delta) energies in oxide layers. This binding energy shift is mainly associated with the image charge screening effect of the highly polarizable metallic substrate. The effect of image charge screening is also reflected in the valence band electronic structure as the bands are shifted to lower binding energies with decreasing film coverage maintaining the bulk MgO band gap till 5 ML. However, for the film coverages below 5 ML, the band gap is reduced substantially in presence of O 2p-Ag 5sp hybridized bands with the finite size effects in proximity to the Ag(001) substrate. Furthermore, our measured valence band electronic structure has been compared with the theoretical calculations and a good agreement has been observed between them for the bulk MgO, however, a more detailed calculation is necessary to explain the monolayer MgO band structure properly.