Emission dynamics of dark and bright excitons in CdSe/ZnS nanocrystals

The CdSe/ZnS nanocrystals (NCs) forming the wurtzite structure were prepared by using a colloidal chemical method for the investigation of the temperature dependence of the band edge emission. The size of CdSe NCs was estimated with the energy band gap obtained from the absorption measurement. The PL spectra and the time-resolved spectra were measured over a broad temperature range from 4 K to 290 K. There was a red shift of 50 meV in energy band gap in the temperature range due to the temperature dependent electron lattice interaction and thermal expansion. The time-resolved spectra in the low temperature regime up to 50 K exhibited a single exponential decay of the band edge emission from thermal equilibrium between the bright and the dark exciton levels, of which the decay rate becomes steeper as temperature rises. The energy gap between the lowest two excition levels was obtained by analyzing the dynamics of band edge emission with a model based on a three-level system. The model described well the dynamics of energy relaxation in the temperature regime up to 50 K. The emissions in the high temperature regime above 50 K exhibited both the initially populated carrier recombination and the surface-related emission.