Spin blockades to relaxation of hot multi-excitons in nanocrystals

TL;DR

This study reveals a spin blockade effect in nanocrystals that delays hot carrier relaxation, challenging the assumption of rapid relaxation and suggesting a new mechanism for multiexciton energy dissipation.

Contribution

It demonstrates that spin blockade can hinder hot carrier relaxation in nanocrystals, providing a novel insight into exciton dynamics.

Findings

Only half of the photoexcited electrons relax immediately

Blocked electrons flip spin after ~15 ps to relax

Spin blockade delays carrier relaxation significantly

Abstract

The rates of elementary photophysical processes in nanocrystals, such as carrier cooling, multiexciton generation, Auger recombination etc., are determined by monitoring the transient occupation of the lowest exciton band. The underlying assumption is that hot carriers relax rapidly to their lowest quantum level. Using femtosecond transient absorption spectroscopy in CdSe/CdS nanodots we challenge this assumption. Results show, that in nanodots containing a preexisting cold exciton "spectator", \emph{only half of the photoexcited electrons}relax directly to the band-edge and the complementary half is blocked in an excited state level due to Pauli exclusion. Full relaxation occurs only after \textasciitilde 15 ps, as the blocked electrons flip spin. This novel spin-blockade effect may offer the key for the long-sought-for bottleneck mechanism for multiexciton energy dissipation.