Anti-stokes photoluminescence in II-VI colloidal nanocrystals

For the first time highly efficient photon energy up-conversion (anti-Stokes photoluminescence) has been observed in colloids of CdTe nanocrystals (NCs) and CdSe NCs covered by ZnS shell. The maximum magnitude of the up-converted blue shift ΔE_UC^max = 335 meV was obtained for CdSe/ZnS NCs under excitation by a low-power He-Ne laser. The maximum efficiency for energy conversion was estimated to be about η_max = 2.7 × 10^-2 % at room temperature. The intensity of the anti-Stokes photoluminescence increases with temperature and shows a linear dependence on the excitation intensity. We suggest that subgap surface states are involved as intermediate states in the up-conversion process rather than nonlinear two-photon absorption or Auger processes.