F and F+ Centers on MgO/Ag(100) or MgO/Mo(100) Ultrathin Films: Are They Stable?

Abstract

Using density functional theory (DFT) calculations we have investigated the electronic structure and stability of diamagnetic F0 and paramagnetic F+ centers in perfect and stepped ultrathin MgO films on Ag(100) and Mo(100) supports. The results show that when the one-electron energy of an F0 defect state is very close to the metal Fermi level, like in MgO films on Ag(100), the doubly occupied F0 center is metastable and can transform spontaneously into the paramagnetic F+ center by losing one of its electrons to the metal support. The phenomenon is enhanced when the vacancy is closer to the interface or is located at a step edge. On the MgO/Mo(100) films, where the Fermi level is at higher energy, F0 centers are stable and do not show a tendency to transform into F+, but F+ centers are unstable and capture one electron from the Mo metal to generate the corresponding doubly occupied vacancy.