No abstract

This study measures the x-ray-absorption spectra of a series of nanodiamond thin films with grain diameters ranging from 3.5 nm to 5 mm at the C K-edge using the sample drain current mode at room temperature. Resonance peaks resembling the C 1s core exciton are observed. The exciton state and conduction band edge are found to shift to higher energies with the decrease of the grain size indicative of the presence of the quantum confinement effect. [S0031-9007(99)

Direct evidence for a size effect in self-trapped exciton ͑STE͒ photoluminescence ͑PL͒ from silica-based nanoscale materials as compared with bulk type-III fused silica is obtained. Two kinds of mesostructures were tested: ͑1͒ silica nanoparticle composites with primary particle size of 7 and 15 nm, ͑2͒ ordered and disordered mesoporous silicas with pore size ranging from ϳ2 to ϳ6 nm and wall thickness ϳ1 nm. The PL was induced by the two-photon absorption of focused 6.4 eV ArF laser light with intensity ϳ10 6 W cm Ϫ2 and measured in a time-resolved detection mode. Two models are applied to examine the blue shift of the STE PL ͑STEPL͒ 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 Х11 eV͒. In order to explain the experimental data, we propose a model that takes into account the laser heating of Frenkel-type free excitons ͑FE's͒. 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-assisted͒ electronic transitions. Good agreement between experimental data and our FE laser heating model has been obtained.