To investigate the influence of surface trapping and dark states on CdSe and CdSe/ZnS quantum dots (QDs), we studied the absorption, fluorescence intensity and lifetime by using one-and two-photon excitation, respectively. Experimental results show that both one- and two-photon fluorescence emission efficiencies of the QDs enhance greatly and the lifetime increase after capping CdSe with ZnS due to the effective surface passivation. The lifetime of one-photon fluorescence of CdSe and CdSe/ZnS QDs increase with increasing emission wavelength in a supralinear way, which is attributed to the energy transfer of dark excitons. On the contrary, the lifetime of two-photon fluorescence of bare and core-shell QDs decrease with increasing emission wavelength, and this indicates that the surface trapping is the dominant decay mechanism in this case.
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