Size- and temperature-dependence of exciton lifetimes in CdSe quantum dots

In this work we have investigated the temperature-dependence of the band-edge photoluminescence decay of efficiently luminescing organically capped CdSe quantum dots (QDs) with diameters ranging from $1.7\text{to}6.3\mathrm{nm}$ over a broad temperature range $(1.3–300\mathrm{K})$. The overall trend is similar for all the investigated sizes, consisting of different temperature regimes. The low-temperature regime (below $\sim 50\mathrm{K}$) is characterized by purely radiative decay and can be modeled by a thermal distribution between a lower dark and a higher bright exciton state, with a size-dependent energy separation (viz., from $0.7\text{to}1.7\mathrm{meV}$) and dark exciton lifetime (viz., from $0.3\text{to}1.4\mu \mathrm{s}$ for QDs ranging from $6.3\mathrm{nm}\text{to}1.7\mathrm{nm}$ in diameter). Nonradiative relaxation processes become increasingly important above $\sim 50\mathrm{K}$ until the temperature antiquenching regime is reached, leading to a decrease in the nonradiative contributions and photoluminescence intensity recovery above $\sim 200\mathrm{K}$.