Spin pumping efficiency in room-temperature CdSe nanocrystal quantum dots

TL;DR

This study investigates how the size of CdSe nanocrystal quantum dots affects optical spin initialization efficiency at room temperature, revealing size-dependent features in the spin pumping process linked to band contributions.

Contribution

It provides a detailed analysis of size-dependent spin pumping efficiency in CdSe NCQDs using a simple effective mass model to explain experimental observations.

Findings

Larger NCQDs show two peaks in spin signal vs. pump energy.

Smaller NCQDs exhibit only a single peak before plateau.

Size influences the valence band state ordering and spin pumping behavior.

Abstract

To understand and optimize optical spin initialization in room temperature CdSe nanocrystal quantum dots (NCQDs) we studied the dependence of the time-resolved Faraday rotation signal on pump energy $E_p$ in a series of NCQD samples with different sizes. In larger NCQDs, we observe two peaks in the spin signal vs. $E_p$, whereas in smaller NQCDs, only a single peak is observed before the signal falls to a low, broad plateau at higher energies. We calculate the spin-dependent oscillator strengths of optical transitions using a simple effective mass model to understand these results. The observed $E_p$ dependence of the spin pumping efficiency (SPE) arises from the competition between the heavy hole (hh), light hole (lh) and split-off (so) band contributions to transitions to the conduction band. The two latter contributions lead to an electron spin polarization in the opposite direction from the former. At lower $E_p$ the transitions are dominated by the hh band, giving rise to the low energy peaks. At higher $E_p$, the increasing contributions from the lh and so bands lead to a reduction in SPE. The different number of peaks in larger and smaller NCQDs is attributed to size-dependence of the ordering of the valence band states.