Li, Al, and Ni Substitutional Doping in MgO Ultrathin Films on Metals: Work Function Tuning via Charge Compensation

The properties of MgO/Mo(100) ultrathin films where the Mg2+ cation is substituted with Li, Al, or Ni impurities have been investigated by means of density functional theory calculations. While on bare MgO the dopants affect the oxide electronic structure by creating holes or excess electrons, for supported ultrathin films the metallic support provides a reservoir to compensate charges via electron transfer through the oxide layer. The charge transfer has direct consequences on the work function (phi) of the combined oxide/metal system. As MgO/Mo(100) films are characterized by a very low work function, doping with monovalent Li atoms results in an increase of phi. On the contrary, a small reduction of phi can be obtained with low concentration of trivalent Al dopants. A more sizable reduction of phi is obtained for Al-doped MgO/Ag(100) films, where the work function of the undoped system is higher. In the case of Ni, the impurity atoms can assume different oxidation states, Ni2+, Ni+, and Ni-0, depending on the position within the film. For all configurations considered, substitutional Ni leads to an increase of the work function.