We present a study of the kinetics of photoluminescence (PL) and cathodolum
inescence (CL) degradation of semiconductor quantum dot composites, formed
by highly luminescent (CdSe)ZnS core-shell nanocrystals embedded in a ZnS m
atrix. The photoluminescence and cathodoluminescence spectra indicate that
both emissions originate from the same near band-edge state of the nanocrys
tals. We observe a strong decrease in the PL and CL intensities with time.
Photoluminescence experiments carried out at high laser fluences (0.5-10 mJ
/cm(2) per pulse) show that the PL intensity decay with time depends on the
size of the nanocrystals and the nature of the surrounding matrix. For ins
tance, close-packed films showed a much slower decay than composite films.
The cathodoluminescence intensity degradation is enhanced at lower temperat
ures. Partial recoveries of the CL signal have been achieved after thermal
annealing at temperatures around 120 degrees C, which indicates that activa
tion of trapped carriers can be induced by thermal stimulation. We attribut
e the CL and PL decay in the composite films to photo- and electroionizatio
n of the nanocrystals, and subsequent trapping of the ejected electrons in
the surrounding semiconductor matrix. (C) 2000 American Institute of Physic
s. [S0021-8979(00)05412-8].