Thermally activated photoluminescence in lead selenide colloidal quantum dots.

The thermal activation processes in PbSe colloidal quantum dots and their influence on the ground-state exciton emission are discussed. Activation of a dark exciton occurs at 1.4-7 K, assisted by an acoustic phonon coupling. Activation of a bright exciton occurs at 100-200 K, which appears as a sudden change in the photoluminescence band intensity, energy, and full width at half maximum. This activation overcomes the dark-bright-state splitting, when the activation temperature increases with the decrease of the dots' size. The dark exciton lifetime is found to be approximately 6-12 micros at 1.4 K, while the bright exciton lifetime at 300 K evaluated as 450 ns varies slightly with the change in the size of the dots. In addition, the emission quantum yield of these dots, measured at a variety of temperatures when dissolved in various solvents, reveals information about the influence of the environment on the recombination processes.