Exciton-phonon coupling efficiency in CdSe quantum dots embedded in ZnSe nanowires

Abstract

Exciton luminescence of a CdSe quantum dot (QD) inserted in a ZnSe nanowire is strongly influenced by the dark exciton states. Because of the small size of these QDs (2--5 nm), exchange interaction between hole and electron is highly enhanced and we measured large energy splitting between bright and dark exciton states ($\ensuremath{\Delta}E\ensuremath{\in}[4,9.2]$ meV) and large spin-flip rates between these states. Statistics on many QDs showed that this splitting depends on the QD size. Moreover, we measured an increase of the spin-flip rate to the dark states with increasing energy splitting. We explain this observation with a model, taking into account the fact that the exciton-phonon interaction depends on the bright to dark exciton energy splitting, as well as on the size and shape of the exciton wave function. It also has consequences on the exciton line intensity at high temperature.