Feb 26, 2024: Eran Rabani (UC Berkeley)
Hot exciton cooling in nanocrystals quantum dots: Why exciton under confinement relax rapidly?
Understanding the mechanisms governing the nonradiative decay of electronic excited states in semiconductor nanocrystals (NCs) is crucial for advancing NC-based technologies with reduced thermal losses and enhanced device efficiencies. When a NC is excited by a photon with energy exceeding the NC band gap, the absorbed photon generates a highly excited electron-hole pair. This, so called “hot exciton” then relaxes to form a band edge exciton by nonradiative relaxation emitting phonons.
In bulk semiconductors, efficient hot exciton cooling, typically occurring on timescales of ≈1ps or less, is facilitated by Fröhlich and deformation potential interactions between excitons and phonons, as well as continuous densities of electronic and phonon states. However, in semiconductor NCs, confinement alters the nature of exciton-phonon coupling (EXPC) and leads to the discretization of both electronic and phonon states, giving rise to the hypothesis of a phonon bottleneck and potentially slow cooling. On the other hand, enhanced electron-hole interactions suggest ultrafast cooling. Experimental measurements of the cooling timescales span several orders of magnitude, indicating the presence of competing effects and possibly multiple channels. While significant progress has been made, there are several unresolved questions regarding the timescales and mechanism of hot exciton cooling in confined semiconductor materials.
In this presentation, I will discuss our recent progress in developing an atomistic approach to describe phonon-mediated exciton dynamics and simulate cooling in NCs of experimentally relevant sizes. Our findings reveal that cooling occurs on ultrafast timescales (<100fs) in CdSe NCs, consistent with the recent 2D measurements. Furthermore, we demonstrate that the phonon bottleneck is bypassed through a cascade of multiphonon-mediated relaxation events, which surprisingly occur rapidly. We explore the proposed Auger-assisted mechanisms and identify NC parameters that can be tuned to control the cooling timescale. Additionally, time permitting, I will present strategies for restoring the phonon bottleneck by strongly coupling the NC to light in a cavity.