Semiconductor nanocrystals (NCs) are inorganic materials with fascinating electrophysical and photophysical properties, such as tunable band gaps, high extinction coefficient, bright photoluminescence (PL) emission, accessible to optical orientation effects, etc., making them great and versatile candidates for next generation of materials in lighting, solar energy harvesting, photo-catalysis, sensing, bio-imaging and spintronics applications. As these applications draw most of the interest to NCs ability to interact with light, the excited NCs are plagued by complex photodynamics, expressed as phenomena like multi-exponential PL decays, delayed PL emission, PL intermittency, etc. As these phenomena hinders the performance of NCs in applications discussed above, understanding the underlying photophysics is the key to rationally design NCs based devices. Air stable free radicals such as nitronyl nitroxides (NNs) are suitable probes for excited state NCs, as they can efficiently quench the PL of NCs through energy transfer process, with a rate comparable to the intrinsic recombination of NCs, making such a perturbation of PL decays easily measurable. Comparing the unquenched PL decays of NCs to the classical 2-states emitter PL formalism, an ultrafast trapping process is found necessary to explain the loss of intensity at t = 0. While the quenched PL can only be analyzed with complex models, a phenomenological log-normal model reveals a serial kinetics, which is interpreted as exciton trapping-detrapping-recombination/transfer. A trap "storage" model that describes such photophysics is proposed, of which the trap states with normal distribution of energies can "store" the trapped hole carrier that can be thermally re-populated to the band edge after delayed time. Such a model can successfully fit/predict all the currently observed photophysics. Another similar system employing tetramethylpiperidine oxide (TEMPO) derivatives as a charge acceptor is studied and found to be able to undergo an ultrafast (sub-picosecond timescale) hole extraction from photoexcited NCs. These results and models lead to a better understanding of excited state NCs, a rethinking of NCs trap states, and a series of interesting future directions of research on NCs photophysics.
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