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