Temperature dependence of the photoluminescence properties of colloidal CdSe/ZnS core/shell quantum dots embedded in a polystyrene matrix

We report on the temperature dependence of the photoluminescence (PL) spectrum and of the PL relaxation dynamics for colloidal CdSe/ZnS core/shell quantum dots (QDs) embedded in an inert polystyrene matrix. We demonstrate that the confinement energy in the QDs is independent of the temperature. The coupling with both acoustic and optical phonons is also studied. Quantum confinement results in a strong increase of the exciton-acoustic-phonon coupling constant, up to 71 mu eV/K, and in a reduced exciton-longitudinal-optical (LO)-phonon coupling constant, down to 21 meV, with respect to bulk CdSe. In addition, we demonstrate that the main nonradiative process that limits the quantum efficiency of the QD at room temperature is the thermal escape from the dot assisted by scattering with four LO phonons. Thermally activated trapping in surface states is also observed at low temperature, with an activation energy of about 15 meV.