Temperature dependence of radiative recombination in CdSe quantum dots with enhanced confinement

We studied in details the recombination dynamics and its temperature dependence in epitaxially grown neutral CdSe/ZnSSe quantum dots with additional wide-band gap MgS barriers. Such design allows to preserve a very high quantum yield and track the radiative recombination dynamics up to room temperature. A fast initial decay of $\sim0.6$ ns followed by a slow decay with a time constant $\sim30-50$ ns is observed at low temperature $T<50$ K. The fast decay gradually disappears with increasing temperature while the slow decay shortens and above 100 K predominantly a single-exponential decay is observed with a time constant $\sim1.3$ ns, which is weekly temperature dependent up to 300 K. To explain the experimental findings, a two-level model which includes bright and dark exciton states and a temperature dependent spin-flip between them is considered. According to the model, it is a thermal activation of the dark exciton to the bright state and its consequent radiative recombination that results in the long decay tail at low temperature. The doubling of the decay time at high temperatures manifests a thermal equilibrium between the dark and bright excitons.