Anti-Stokes photoluminescence in colloidal semiconductor quantum dots

We report anti-Stokes photoluminescence (photon energy up-conversion) from size-quantized CdSe and InP nanocrystalline colloids. The observed up-conversion is highly efficient and occurs at very low excitation intensities. With low temperatures the intensity of the up-converted photoluminescence decreases while that of the usual Stokes photoluminescence increases; the up-converted photoluminescence is also restricted to energies corresponding to the band gaps of the quantum dots that are present in the colloid ensemble. The anti-Stokes photoluminescence is explained by a model that involves surface states.