Unravelling the Size and Temperature Dependence of Exciton Lifetimes in Colloidal ZnSe Quantum Dots

We report on the temperature dependence of the band-edge photoluminescence decay of organically capped colloidal ZnSe quantum dots (QDs) in the size range from 4.0 to 7.5 nm. A similar trend is observed for all investigated sizes: the decay time is short (similar to 5 ns) above 20 K and increases sharply below 20 K, eventually reaching a constant value (270400 ns) at sufficiently low temperatures (<4 K). The temperature regime in which the decrease of lifetime occurs depends on the QD size and is lower for larger QDs. This behavior can be modeled by a Boltzmann distribution between a lower long-lived and a higher short-lived exciton states, with an energy separation ranging from 3.3 +/- 0.2 to 1.5 +/- 0.1 meV in the 4.0 +/- 0.3 to 7.5 +/- 0.5 nm size range. We show that this energy separation is consistent with coupling of the lowest exciton state to a confined acoustic phonon.