Semiconductor nanocrystals (NCs) are promising for a variety of applications due to their highly tunable optoelectronic properties. Exciton-phonon coupling (EXPC) plays a key role in these NC properties and is central to understanding the nonradiative decay and dephasing of electronic excited states. Despite its importance, a microscopic picture of EXPC is still lacking. Challenges associated with accurately measuring or modeling EXPC have led to a set of outstanding questions: the scaling of EXPC with system size, the relative coupling strengths of excitons to acoustic versus optical modes, and the role of the NC surface in EXPC. Here, we develop and validate an atomistic approach for describing EXPC in NCs of experimentally relevant sizes. We demonstrate that exciton formation distorts the NC lattice primarily along the coordinates of low-frequency acoustic modes that are delocalized over the entire NC. Modes at the NC surface play a significant role in smaller CdSe NCs, which can be mitigated through the growth of CdS shell. Next, we use this EXPC framework to perform quantum dynamics simulations of hot exciton cooling in order to address the longstanding controversy of the phonon bottleneck in NCs. Contrary to the hypothesized phonon bottleneck, we find that cooling in CdSe NCs occurs within ~30 fs, in agreement with recent experiments, via a cascade of relaxation events due to efficient multiphonon emission processes. These simulations provide the first unified, microscopic theory for hot exciton cooling in nanoscale systems that reconciles previous experimental discrepancies and that provides design principles for NCs with tuned EXPC and cooling timescales.
