Intrinsically slow cooling of hot electrons in CdSe nanocrystals compared to CdS

TL;DR

This study compares the intrinsic hot electron relaxation times in CdSe and CdS nanocrystals, revealing that electrons in CdSe relax much more slowly due to subtle electronic and phononic differences, impacting optoelectronic applications.

Contribution

It provides a theoretical analysis explaining the large disparity in electron relaxation times between CdSe and CdS nanocrystals based on intrinsic material properties.

Findings

Hot electrons in CdSe have 5-6 orders of magnitude longer lifetimes than in CdS.

Differences arise from small variations in energy gaps, phonon frequencies, and couplings.

Phonon interactions are dominated by a Frohlich mechanism in charged systems.

Abstract

The utilization of excited charge carriers in semiconductor nanocrystals (NCs) for optoelectronic technologies has been a long-standing goal in the field of nanoscience. Experimental efforts to extend the lifetime of excited carriers have therefore been a principal focus. To understand the limits of these lifetimes, in this work, we theoretically study the timescales of pure electron relaxation in negatively charged NCs composed of two prototypical materials: CdSe and CdS. We find that hot electrons in CdSe have lifetimes that are five to six orders of magnitude longer than in CdS when the relaxation is governed only by the intrinsic properties of the materials. Although these two materials are known to have somewhat different electronic structure, we elucidate how this enormous difference in lifetimes arises from relatively small quantitative differences in electronic energy gaps, phonon frequencies, and their couplings. We also highlight that the effect of the phonons on electron relaxation is dominated by a Fr\"ohlich-type mechanism in these charged systems with no holes.