Exciton spin dynamics and photoluminescence polarization of CdSe/CdS dot-in-rod nanocrystals in high magnetic fields

TL;DR

This study investigates exciton spin dynamics and polarization in CdSe/CdS nanocrystals under high magnetic fields, developing a theoretical model that explains experimental polarization behaviors and highlights phonon-mediated spin relaxation mechanisms.

Contribution

The paper introduces a comprehensive theory for polarization properties in nanocrystals under magnetic fields, incorporating phonon coupling and dielectric effects, and applies it to experimental data.

Findings

Magnetic field induces circular polarization dependent on nanocrystal geometry.

Phonons significantly influence dark exciton spin relaxation.

Surface and interface effects impact spin relaxation channels.

Abstract

The exciton spin dynamics and polarization properties of the related emission are investigated in colloidal CdSe/CdS dot-in-rod (DiR) and spherical core/shell nanocrystal (NC) ensembles by magneto-optical photoluminescence (PL) spectroscopy in magnetic fields up to 15 T. It is shown that the degree of circular polarization (DCP) of the exciton emission induced by the magnetic field is affected by the NC geometry as well as the exciton fine structure and can provide information on nanorod orientation. A theory to describe the circular and linear polarization properties of the NC emission in magnetic field is developed. It takes into account phonon mediated coupling between the exciton fine structure states as well as the dielectric enhancement effect resulting from the anisotropic shell of DiR NCs. This theoretical approach is used to model the experimental results and allows us to explain most of the measured features. The spin dynamics of the dark excitons is investigated in magnetic fields by time-resolved photoluminescence. The results highlight the importance of confined acoustic phonons in the spin relaxation of dark excitons. The bare core surface as well as the core/shell interface give rise to an efficient spin relaxation channel, while the surface of core/shell NCs seems to play only a minor role.