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Circumventing the Phonon Bottleneck by Multiphonon-Mediated Hot Exciton Cooling at the Nanoscale

In semiconductor materials, hot exciton cooling is the process by which highly excited carriers nonradiatively relax to form a band edge exciton. While cooling plays an important role in determining the thermal losses and quantum yield of a system, the timescales and mechanism of cooling are not well understood in confined semiconductor nanocrystals (NCs). A mismatch between electronic energy gaps and phonon frequencies in NCs has led to the hypothesis of a phonon bottleneck, in which cooling would be extremely slow, while enhanced electron-hole interactions in NCs have been used to explain cooling that would occur on ultrafast timescales. Here, we develop an atomistic approach for describing phonon-mediated exciton dynamics, and we use it to simulate hot exciton cooling in NCs of experimentally relevant sizes. Our framework includes electron-hole correlations as well as multiphonon processes, both of which are necessary to accurately describe the cooling process. We find that cooling occurs on timescales of tens of femtoseconds in CdSe cores, in agreement with experimental measurements, through a cascade of relaxation events that are mediated by efficient multiphonon emission. Cooling timescales increase with increasing NC size due to decreased exciton-phonon coupling (EXPC), and they are an order of magnitude larger in CdSe-CdS core-shell NCs because of reduced EXPC to low- and mid-frequency acoustic modes.