Publication date: 3rd July 2020
Band-edge excitons of semiconductor nanocrystals feature a complex set of energy sublevels. Those sublevels affect the light-emission properties of the nanocrystals. At low temperatures, when the thermal energy of the system is comparable to the energy differences between those sublevels, multi-exponential decay dynamics of the fluorescent emission is probed with time-resolved measurements [1]. Often temperature-dependent decay studies are used to gain insight into the transition rates between sublevel states, recombination rates, and energetic ordering of the sublevels. However, such studies can sometimes yield multiple interpretations on the dynamics of the band-edge exciton. Here we show that control of the local density of optical states (LDOS) of the nanocrystal environment is another technique to probe exciton fine-structure states. As initially shown by Drexhage on europium complexes [2], the LDOS modifies the rate of radiative transitions according to Fermi’s golden rule. This is experimentally achieved by placing the nanocrystals at different distances from a reflecting surface, therefore exposing them to a different LDOS. We record low-temperature time-resolved photoluminescence, which reflects changes in radiative decay rates. Using CdSe-based nanocrystals, we show that our method provides a complementary tool to investigate the dynamics between excited-state energy sublevels and to estimate the radiative recombination efficiency of the energy sublevels in the low-temperature regime. Our approach is also easily extended to other fluorescent colloidal nanocrystals.
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