Dynamics of photoluminescence in medium-size CdSe quantum crystallites

We have measured time-resolved photoluminescence of high-quality CdSe nanocrystallites with diameters larger than the bulk exciton Bohr diameter, embedded in sodium borosilicate matrix. Such a weak-confinement situation yields sharper lines than those usually observed for much smaller nanocrystals, due to smaller size-distribution effects. Consequently, several contributions are spectrally separated, with different temporal behaviours. Three different mechanisms are identified. The first one, corresponding to near-band-edge emissions, at the highest photon energies, is very fast (tau similar to 30-50 ps) and involves 'free' excitons. The second one (tau similar to 150-300 ps) involves surface trapped excitons. The third one, at rather lower energy, is a slow regime (tau similar to 2-14 ns) involving radiative and non-radiative transitions between carriers trapped at the surface sites. The observed temporal and spectral behaviours are analysed in terms of the possibility of bleaching the absorption of such medium-size nanocrystals under high photoexcitation.