P. Lefebvre et al., DYNAMICS OF PHOTOLUMINESCENCE IN MEDIUM-SIZE CDSE QUANTUM CRYSTALLITES, Semiconductor science and technology, 12(8), 1997, pp. 958-965
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.