Temperature and size dependence of nonradiative relaxation and exciton-phonon coupling in colloidal CdTe quantum dots

We report on the temperature and size dependence of the photoluminescence of core CdTe colloidal quantum dots (QDs). We show that at temperatures lower than 170 K a thermally activated transition between two different states separated by about 12-20 meV takes place. At temperatures higher than 170 K, the main nonradiative process is thermal escape assisted by multiple longitudinal optical (LO) phonons absorption. Moreover, we show that quantum confinement affects both the exciton-acoustic phonons and the exciton-LO phonons coupling. The coupling constant with acoustic phonons is strongly enhanced in QDs (up to 31 mu eV/K) with respect bulk CdTe (0.7 mu eV/K). On the contrary, the exciton-LO phonons coupling constant decreases as the dot size decreases (down to 14 meV with respect 24.5 meV in the bulk).