Optical and EPR properties of point defects at a crystalline silica surface: Ab initio embedded-cluster calculations

We have studied the structure and spectroscopic properties of the paramagnetic nonbridging oxygen hole center and of the E-gamma' center at the hydroxylated silica surfaces using density functional theory and an embedded-cluster model. To investigate the influence of the local environment on the defect properties, three crystalline polymorphs of SiO2 were considered: the edingtonite (100), beta-cristobalite (111), and alpha-quartz (0001) surfaces. The calculations reproduce the essential properties of the centers: the g tensor and the optical transitions between well localized defects states are in excellent agreement with the experimental data for the silica surface. The hyperfine coupling constants agree well with the experimental findings. The overall absorption spectra are in qualitative agreement with the experiment. We discuss the advantages and limitations of our approach in describing the properties of hydroxylated amorphous silica surface.