Exciton spin dynamics in spherical CdS quantum dots

TL;DR

This study combines experimental and theoretical approaches to understand exciton spin dynamics in spherical CdS quantum dots, revealing that both electron and hole wavefunctions are of S-type, and matching relaxation rates with size-dependent theory.

Contribution

The paper provides a detailed analysis of exciton states and spin relaxation in spherical CdS quantum dots, challenging previous assumptions about dark excitons and validating size-dependent theoretical models.

Findings

Electron and hole wavefunctions are S-type in lowest exciton state.

The size dependence of spin relaxation rates matches theoretical predictions.

Experimental techniques effectively separate size-specific effects from general material properties.

Abstract

Exciton spin dynamics in quasi-spherical CdS quantum dots is studied in detail experimentally and theoretically. Exciton states are calculated using the 6-band k.p Hamiltonian. It is shown that for various sets of Luttinger parameters, when the wurtzite lattice crystal field splitting and Coulomb interaction between the electron-hole pair are taken into account exactly, both the electron and hole wavefunction in the lowest exciton state are of S-type. This rules out the spatial-symmetry-induced origin of the dark exciton in CdS quantum dots. The exciton bleaching dynamics is studied using time- and polarization-resolved transient absorption technique of ultrafast laser spectroscopy. Several samples with a different mean size of CdS quantum dots in different glass matrices were investigated. This enabled the separation of effects that are typical for one particular sample from those that are general for this type of material. The experimentally determined dependence of the electron spin relaxation rate on the radius of quantum dots agrees well with that computed theoretically.