Time-Resolved Intraband Relaxation of Strongly-Confined Electrons and Holes in Colloidal PbSe Nanocrystals

TL;DR

This study investigates the ultrafast relaxation dynamics of electrons and holes in colloidal PbSe nanocrystals, revealing picosecond relaxation times that challenge existing theories and suggest alternative mechanisms bypassing the phonon bottleneck.

Contribution

It provides the first time-resolved measurements of intraband relaxation in strongly-confined PbSe nanocrystals, highlighting discrepancies with current theoretical models.

Findings

Electrons and holes relax on a picosecond timescale despite large energy level spacings.

Current theories cannot explain the observed rapid relaxation.

Discussion of potential mechanisms bypassing the phonon bottleneck in IV-VI quantum dots.

Abstract

The relaxation of strongly-confined electrons and holes between 1P and 1S levels in colloidal PbSe nanocrystals has been time-resolved using femtosecond transient absorption spectroscopy. In contrast to II-VI and III-V semiconductor nanocrystals, both electrons and holes are strongly confined in PbSe nanocrystals. Despite the large electron and hole energy level spacings (at least 12 times the optical phonon energy), we consistently observe picosecond time-scale relaxation. Existing theories of carrier relaxation cannot account for these experimental results. Mechanisms that could possibly circumvent the phonon bottleneck in IV-VI quantum dots are discussed.