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

Using density functional theory (DFT) calculations we have investigated the electronic structure and stability of diamagnetic F 0 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 F 0 defect state is very close to the metal Fermi level, like in MgO films on Ag(100), the doubly occupied F 0 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, F 0 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.