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 diam eter, embedded in sodium borosilicate matrix. Such a weak-confinement situation yields sharper lines than those usually observed for much sm aller nanocrystals, due to smaller size-distribution effects. Conseque ntly, several contributions are spectrally separated, with different t emporal behaviours. Three different mechanisms are identified. The fir st one, corresponding to near-band-edge emissions, at the highest phot on energies, is very fast (tau similar to 30-50 ps) and involves 'free ' excitons. The second one (tau similar to 150-300 ps) involves surfac e trapped excitons. The third one, at rather lower energy, is a slow r egime (tau similar to 2-14 ns) involving radiative and non-radiative t ransitions between carriers trapped at the surface sites. The observed temporal and spectral behaviours are analysed in terms of the possibi lity of bleaching the absorption of such medium-size nanocrystals unde r high photoexcitation.