Electronic Structure and Optical Properties of the Surface F-Centers in MgO: A Theoretical Analysis by DFT Approach

Electronic structure of Mg9O9 and Mg9O8 clusters modeling nano-crystalline powders of magnesium oxide has been analyzed within the frames of the density functional theory (DFT). In the framework of time-dependent DFT method (TD-DFT), the relationship between the surface and bulk properties of nano-crystals is analyzed based on variations in the density of electronic states (DOS) and changes of electronic spectra. The spectroscopy of spatial defects like low-coordinated oxygen ions and of surface point defects like F⁺- and F-centers is investigated. Optical properties of the nano-sized crystalline magnesium oxide are characterized by a spectrum of absorption bands in the range of 1-5 eV. Point defects such as F-centers absorb light in the range of 1.2-1.5 eV. Spatial defects O_LC in nano-crystals generate absorption bands in the range of 2.5-5.0 eV. According to calculations, there is no direct relation between coordination numbers of surface ions and excitation energies. Theoretical excitation energies are compared with experimental optical properties of the F⁺- and F-centers.