Red and near-infrared photoluminescence from silica-based nanoscale materials: Experimental investigation and quantum-chemical modeling

Experimental study of room temperature photoluminescence (PL) from silica nanoparticles and mesoporous silicas induced by ultraviolet and visible laser light (lambda (EXC)=266 and 532 nm or 4.66 and 2.33 eV, respectively) reveals several well-defined PL bands in the red- and near-infrared spectral range, which are peaked at 1.905, 1.78, 1.61, 1.40, 1.27, and 1.14 eV. The relative intensities of the bands depend on the specimen heat pretreatment temperature and excitation wavelength. The band at 1.905 eV shows all conceivable characteristics of nonbridging oxygen (NBO) defects in bulk silica, so it can be assigned to the same species in nanometer-sized SiO2 fragments. The more slowly decayed 1.78-eV band was assigned to NBOs incorporated into distorted SiO4 tetrahedrons on the surface. The 1.14-1.61 eV PL bands can be observed mainly with 2.33-eV excitation for heat-pretreated specimens (T-ht=873 and 1173 K) and can also be associated with NBOs. The red shift of these bands is explained in terms of formation of combined defects involving NBO as an electron acceptor and an additional point defect in NBO vicinity serving as a donor, with the electronic energy level somewhat higher than the bottom of the forbidden band gap. Using sophisticated quantum-chemical modeling [geometry optimization of model clusters containing up to 60 Si and O atoms employing two-layered integrated molecular orbital ONIOM method [I. Komaromi , J. Mol. Struct.: THEOCHEM 461, 1 (1999)], followed by time-dependent density functional calculations of excitation and PL energies] we have shown that combined defects including NBO and an oxygen vacancy (OV) in various positions exhibit the suitable properties (both energies and oscillator strengths) to be responsible for the observed PL bands. These combined defects are proposed to occur in extremely thin (similar to1 nm) nonequilibrium substoichiometric silicon oxide (SiOx, x <2) layers. The emphasis is on the PL band shift induced by one, two, and three OVs in the nearest vicinity of NBO, and the influence of the Si-Si bond relaxation in OVs as well as the orientation of NBOs and distances between them and OVs on transition energies and oscillator strengths. The results of calculations closely match the PL peaks observed. Since silicon nanoscale materials are typically covered by nonequilibrium substoichiometric passivating oxide layers as well, our findings may also be helpful to clarify the nature of light emission from these materials. (C) 2002 American Institute of Physics.