Optical Interactions and Photoluminescence Properties of Wide-Bandgap Nanocrystallites

The UV-photoluminescence (PL) properties of GaN and ZnO nanocrystallites and nanocrystallite ensembles were studied utilizing micro-photoluminescence. We address the origin of the light emissions of the nanocrystallite as to whether it is due a bandgap or excitonic recombination process. The other topic presented here focuses on the interaction of the laser with a collective of crystallites; we address the phenomena of intensity saturation at a high density of laser excitations as well as the impact of the vacuum state on the PL characteristics. Our analysis indicates that the PL of both GaN and ZnO nanocrysallites is excitonic-like and very similar to the behavior of the free exciton in bulk materials. Additionally, we attribute the intensity saturation of GaN and ZnO to the laser heating and heat trapping which takes place in the enclosure of the nanocrystallite ensemble. In vacuum the PL energy was found to exhibit a strong PL energy redshift relative to the PL in air. We attribute the observed shift to a thermal effect and analyze it in terms of the conditions enabling a convective cooling in the ensemble: the mean free path of air in atmospheric pressure and in vacuum relative to the interparticle separation inside the ensemble.