Exciton spin dynamics of colloidal CdTe nanocrystals in magnetic fields

The recombination and spin dynamics of excitons in colloidal CdTe nanocrystals (NCs) are studied by timeresolved 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 zeromagnetic field and 4.2-K temperature. This long component shortens to 150 ns at 15 T due to themagnetic-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 -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.