Abstract: The efficiencies of devices utilizing semiconductor nanocrystals (NCs) are predominantly regulated by nonradiative processes. One key process in this regard is hot exciton cooling, wherein a highly excited electron-hole pair undergoes nonradiative relaxation to give rise to a band-edge exciton. The timescale and mechanism of this cooling process are not comprehensively understood. The quantum confinement effect, resulting in a mismatch between electronic energy gaps and phonon frequencies, suggests an exceptionally slow cooling rate, often termed the “phonon bottleneck”. Conversely, the presence of heightened electron-hole interactions implies the potential for ultrafast cooling vi an “Auger-assisted” mechanism. To further add confusion, experimental measurements of the cooling timescale range over several orders of magnitude.
In this talk I will review our recent developments to describe phonon-mediated exciton dynamics and simulate the cooling of excitons in NCs of experimentally relevant sizes. These NCs typically contain thousands of atoms and tens of thousands of valence electrons with discrete spectra at low excitation energies, like atoms and molecules, that converge to the continuum bulk limit at higher energies. The initial segment of my talk will focus on introducing atomistic models based on the semiempirical pseudopotential approach. These models are fine-tuned through first-principle calculations and are specifically designed to elucidate the behavior of excitons and their interaction with lattice vibrations.
The latter part of my presentation will delve into the evolving timescales and mechanisms associated with phonon-mediated exciton cooling, specifically examining core and core-shell semiconductor NC of diverse sizes. I will illustrate that the process of hot exciton cooling in confined semiconductor NCs encompasses a complex interplay of factors, including electron-hole correlations, exciton-phonon coupling, the density of both "bright" and "dim" excitons, and multiphonon-mediated nonradiative transitions. These elements collectively play a pivotal role in overcoming the phonon bottleneck and facilitating the rapid relaxation of hot excitons to the band edge by a series of multiphonon-mediated transitions between excitonic states that are closely spaced in energy. The cooling timescale is predominantly dictated by the overall magnitude of exciton-phonon couplings, resulting in slower relaxation for larger cores and an order of magnitude slower relaxation for core-shell NCs. Interestingly, the phonon bottleneck can be restored inside an optical cavity with strong coupling to light.