Size effect in self-trapped exciton photoluminescence fromSiO2-based nanoscale materials

Direct evidence for a size effect in self-trapped exciton (STE) photoluminescence (PL) from silica-based nanoscale materials as compared with bulk type-III fused silica is obtained. Two kinds of mesostructures were tested: (1) silica nanoparticle composites with primary particle size of 7 and 15 nm, (2) ordered and disordered mesoporous silicas with pore size ranging from ∼2 to ∼6 nm and wall thickness ∼1 nm. The PL was induced by the two-photon absorption of focused 6.4 eV ArF laser light with intensity $\sim {10}^6{\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{-}2}$ and measured in a time-resolved detection mode. Two models are applied to examine the blue shift of the STE PL (STEPL) band with decreasing size of nanometer-sized silica fragments. The first model is based on the quantum confinement effect on Mott-Wannier-type excitons developed for semiconductor nanoscale materials. However, the use of this model leads to a contradiction showing that the model is completely unusable in the case of wide-band-gap nanoscale materials (the band-gap of bulk silica $E_g\cong 11\mathrm{eV}).\mathrm{}$ In order to explain the experimental data, we propose a model that takes into account the laser heating of Frenkel-type free excitons (FE’s). The heating effect is assumed to be due to the FE collisions with the boundary of nanometer-sized silica fragments in the presence of an intense laser field. According to the model, laser heating of FE’s up to the temperature in excess of the activation energy required for the self-trapping give rise to the extremely hot STE’s. Because the resulting temperature of the STE’s is much higher than the lattice temperature, the cooling of STE’s is dominated by the emission of lattice phonons. However, if the STE temperature comes into equilibrium with the lattice temperature, the absorption of lattice phonons becomes possible. As a result, the blue shift of the STEPL band is suggested to originate from the activation of hot (phonon-assisted) electronic transitions. Good agreement between experimental data and our FE laser heating model has been obtained.