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 1.70.3em0exto0.3em0ex6.30.3em0exnm over a broad temperature range (1.3--3000.3em0exK). The overall trend is similar for all the investigated sizes, consisting of different temperature regimes. The low-temperature regime (below ~500.3em0exK) 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 0.70.3em0exto0.3em0ex1.70.3em0exmeV) and dark exciton lifetime (viz., from 0.30.3em0exto0.3em0ex1.40.3em0exμs for QDs ranging from 6.30.3em0exnm0.3em0exto0.3em0ex1.70.3em0exnm in diameter). Nonradiative relaxation processes become increasingly important above ~500.3em0exK until the temperature antiquenching regime is reached, leading to a decrease in the nonradiative contributions and photoluminescence intensity recovery above ~2000.3em0exK.