Circumventing the phonon bottleneck by multiphonon-mediated hot exciton cooling at the nanoscale

Nonradiative processes govern efficiencies of semiconductor nanocrystal (NC)-based devices. A central process is hot exciton cooling, or the nonradiative relaxation of a highly excited electron/hole pair to form a band-edge exciton. Due to quantum confinement effects, the timescale and mechanism of cooling are not well understood. A mismatch between electronic energy gaps and phonon frequencies has led to the hypothesis of a phonon bottleneck and extremely slow cooling, while enhanced electron-hole interactions have suggested ultrafast cooling. Experimental measurements of the cooling timescale range six orders of magnitude. Here, we develop an atomistic approach to describe phonon-mediated exciton dynamics and simulate cooling in NCs of experimentally relevant sizes. We find that cooling occurs on similar to 30 fs timescales in CdSe NCs, in agreement with the most recent measurements, and that the phonon bottleneck is circumvented through a cascade of multiphonon-mediated relaxation events. Furthermore, we identify NC handles for tuning the cooling timescale.

Automated summary

Nonradiative processes control the efficiencies of semiconductor nanocrystal-based devices, and hot exciton cooling is a central process in which highly excited electron/hole pairs relax to form band-edge excitons through nonradiative means. However, the timescale and mechanism of cooling in nanocrystals are not well understood due to quantum confinement effects. Experimental measurements of the cooling timescale range six orders of magnitude.