Chapter 4 Ab Initio Calculations of Electronic Transitions and Photoabsorption and Photoluminescence Spectra of Silica and Germania Nanoparticles

Ab initio and density functional calculations of potential energy surfaces for the ground and excited electronic states of model clusters simulating various point defects, impurities, and their combinations in nanosized silica and germania materials are reported. The accurate geometric and electronic structures of these clusters, calculated photoabsorption and photoluminescence (PL) energies, and predicted absorption and PL spectra are obtained. Our calculations reproduced the experimental excitation energy (1.9–2.0 eV) for the non-bridging oxygen (NBO) defect in SiO 2 . The combined defects with oxygen vacancy (OV) in the first coordination sphere of NBO are shown to be unstable, but more distant OVs can coexist with NBO. The variety of the red PL bands observed experimentally at 1.1–1.9 eV has been assigned in terms of different arrangements of NBO and OV in the combined defects. The silanone and dioxasilyrane defects were found to be plausible candidates for green and red light-emitters. The PL band corresponding to the [AlO 4 ] 0 defect is predicted to contain few bands between 1.5 and 1.0 eV and in the range of 0.7–0.2 eV. In the –O–Ge≡ (NBO), –OO–Ge≡ (peroxy radical), O=Ge=, and (O 2 )Ge= defects in GeO 2 nanomaterials, UV-photoabsorption is shown to be accompanied by red or IR PL. The surface E′-center, –GeX 3 , and combined E′–OV, X 3 Ge–GeX 2 , defects are demonstrated to produce photoabsorption bands between 4 and 6 eV. The –GeX 3 defect is able to generate several PL bands in the UV (∼3 eV) and IR (1.2–1.4 and 0.5–0.6 eV) spectral ranges, whereas the X 3 Ge–GeX 2 defect gives a red/orange PL band at 2.0–2.1 eV.