Up-conversion luminescence via a below-gap state in CdSe/ZnS quantum dots

We report on the efficient photoluminescence up-conversion in colloidally-prepared core-shell CdSe/ZnS quantum dots. We demonstrate that the efficiency of photon energy up-conversion and the magnitude of the spectral shift can be controlled by the thickness of the ZnS shell, the temperature and the excitation intensity. Based on the analysis of the experimental data we propose that intrinsic gap states are involved in the photoluminescence up-conversion as intermediate states rather than nonlinear two-photon absorption or Auger processes.