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 $1.7\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}6.3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ over a broad temperature range $(1.3--300\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. The overall trend is similar for all the investigated sizes, consisting of different temperature regimes. The low-temperature regime (below $\ensuremath{\sim}50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) 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 $0.7\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1.7\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$) and dark exciton lifetime (viz., from $0.3\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1.4\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$ for QDs ranging from $6.3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1.7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in diameter). Nonradiative relaxation processes become increasingly important above $\ensuremath{\sim}50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ until the temperature antiquenching regime is reached, leading to a decrease in the nonradiative contributions and photoluminescence intensity recovery above $\ensuremath{\sim}200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.