Exciton spin dynamics of colloidal CdTe nanocrystals in magnetic fields

Abstract

The recombination and spin dynamics of excitons in colloidal CdTe nanocrystals (NCs) are studied by time-resolved photoluminescence in high magnetic fields up to 15 T and at cryogenic temperatures. The recombination decay shows a nonexponential temporal behavior, with the longest component corresponding to the dark excitons having 260-ns decay time at zero magnetic field and 4.2-K temperature. This long component shortens to 150 ns at 15 T due to the magnetic-field-induced mixing of the bright- and dark-exciton states. The spin dynamics, assessed through the evolution of the magnetic-field-induced circular polarization degree of the photoluminescence, has a fast component shorter than 1 ns related to the bright excitons and a slow component of 5--10 ns associated with the dark excitons. The latter shortens with increasing magnetic field, which is characteristic for a phonon-assisted spin-relaxation mechanism. The relatively low saturation level of the associated magnetic-field-induced circular polarization degree of $\ensuremath{-}30%$ is explained by a model that suggests the CdTe NCs to constitute an ensemble of prolate and oblate NCs, both having a structural quantization axis. The exciton $g$ factor of 2.4--2.9 evaluated from fitting the experimental data in the frame of the suggested approach is in good agreement with the expected value for the dark excitons in CdTe NCs.