UC Berkeley

Semiconductor nanocrsytals (NCs) have been of much interest over the past several decades due to their highly tunable optoelectronic properties, which make thempromising materials for applications ranging from solar energy conversion to quantum information. These optoelectronic properties depend significantly on electron-hole and exciton-phonon interactions, which are enhanced in NCs due to quantum confinement. A fundamental understanding of these interactions as well as processes, such as exciton decay and dephasing, is key to developing rational design principles for NCs with decreased thermal losses and increased quantum yields. Yet the description of excited carriers in NCs remains a great challenge for modern computational science. In this dissertation, we address this gap by developing a unified, atomistic model to accurately describe excitonic properties and dynamics in experimentally relevant NC systems with thousands of atoms and tens of thousands of electrons.

We develop an approach for calculating exciton-phonon couplings (EXPCs) and validate them by computing the reorganization energy, which is a measure of EXPC and is relevant for optical Stokes shifts, charge transfer processes, and NC-based device efficiencies. This microscopic theory allows us to delineate the dependence of the reorganization energy on NC size and structure as well as on phonon frequency and localization, resolving questions regarding the role of quantum confinement on EXPC. Additionally, we use this EXPC framework to perform quantum dynamics simulations of hot exciton cooling to address the longstanding controversy of the phonon bottleneck, which hypothesized slow nonradiative relaxation of hot carriers to the band edge in confined semiconductor nanostructures. Contrary to the phonon bottleneck but in agreement with recent experimental measurements, we find that cooling in CdSe NCs occurs in tens of femtoseconds. We show that this ultrafast timescale is governed by both electron-hole correlations and efficient multiphonon emission processes. Finally, we employ these tools in collaboration with experimentalists to understand lattice heating and polaron formation, photoluminescence, and charge trapping and transfer in various NC systems. The atomistic theories and calculations presented in this dissertation bridge our understanding of molecular and bulk systems to provide fundamental insight into exciton-phonon interactions and nonradiative processes at the nanoscale.