Polarized emission of CdSe nanocrystals in magnetic field: the role of phonon-assisted recombination of the dark exciton

TL;DR

This study investigates the spin polarization and recombination dynamics of dark excitons in CdSe nanocrystals under high magnetic fields, revealing phonon-assisted processes that explain unusual polarization behaviors.

Contribution

The paper introduces a theoretical model explaining the polarization features of dark exciton emission via phonon-assisted recombination in CdSe nanocrystals, resolving previous contradictions.

Findings

Polarization degree depends on spectral features and magnetic field.

Inverted PL maxima ordering results from phonon-assisted emission.

Inhomogeneous broadening exceeds optical phonon energy, affecting emission.

Abstract

The recombination dynamics and spin polarization of excitons in CdSe nanocrystals synthesized in a glass matrix are investigated using polarized photoluminescence in high magnetic fields up to 30 Tesla. The dynamics are accelerated by increasing temperature and magnetic field, confirming the dark exciton nature of the low-temperature photoluminescence (PL). The circularly polarized PL in magnetic fields reveals several unusual appearances: (i) a spectral dependence of the polarization degree, (ii) its low saturation value, and (iii) a stronger intensity of the Zeeman component which is higher in energy. The latter feature is the most surprising being in contradiction with the thermal population of the exciton spin sublevels. The same contradiction was previously observed in the ensemble of wet-chemically synthesized CdSe nanocrystals, but was not understood. We present a theory which explains all the observed features and shows that the inverted ordering of the circular polarized PL maxima from the ensemble of nanocrystals is a result of competition between the zero phonon (ZPL) and one optical phonon (1PL) assisted emission of the dark excitons. The essential aspects of the theoretical model are different polarization properties of the dark exciton emission via ZPL and 1PL recombination channels and the inhomogeneous broadening of the PL spectrum from the ensemble of nanocrystals exceeding the optical phonon energy.