We have investigated the temperature dependence of photoluminescence (PL) dynamics in CdS quantum dots (QDs) prepared by a colloidal method. A size-selective photoetching process and a surface modification technique with a Cd(OH) 2 layer enabled us to prepare size-controlled CdS QDs with high PL efficiency. The PL decay profiles became slower with an increase in temperature, contrary to an ordinary behavior. We have revealed that such anomalous temperature dependence of the PL-decay profile is explained by a three-state model consisting of a ground-state and two excited states: a lower-lying bound-exciton state and a higherlying free-exciton state (the "dark-exciton state") having an optically inactive triplet nature.

We have investigated the effects of surface modification on photoluminescence (PL) properties of CdS quantum dots (QDs) prepared by a reverse-micelle method. It is demonstrated that the modification of QD surface with a Cd(OH)2 layer leads to a strong enhancement of the band-edge PL intensity of CdS QDs in reverse micelles. The PL-decay profile before the surface modification exhibits a fast decay component less than 50ps. After the modification, the fast-decay component disappears. The drastic change of the PL-decay profiles before and after the surface modification corresponds to the marked increase of the PL intensity. This suggests that the strong enhancement of the band-edge PL intensity originates from the remarkable reduction of nonradiative recombination processes, which usually result in a very short decay time, due to surface defects of QDs.

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