Abstract
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 ( - 50 ps) and involves `free' excitons. The second one (150 - 300 ps) involves surface trapped excitons. The third one, at rather lower energy, is a slow regime (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.