Temperature dependence of photoluminescence dynamics in colloidal CdS quantum dots

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.