  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 (2-5nm), exchange interaction between hole and electron is highly
enhanced and we measured large energy splitting between bright and dark exciton
states ($\Delta E\in [4, 9.2 ]$ 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.
