Competition between zero-phonon and phonon-assisted luminescence in colloidal CdSe quantum dots

Abstract

We determine the low-temperature optical properties of dark-exciton states in CdSe/CdS nanocrystal quantum dots (NQDs). By using resonant laser excitation we distinguish zero-phonon from phonon-assisted photoluminescence. The NQDs show a decreasing zero-phonon intensity with decreasing temperature, resulting in a redshift of the nonresonant photoluminescence. This redshift is undone by application of a magnetic field. Our results show that dark-exciton luminescence originates from the intricate competition of phonon-assisted and zero-phonon transitions, the latter of which are enhanced by dark--bright-exciton mixing due to unpassivated surface states or a magnetic field.