Band edge recombination in cadmium selenide, cadmium sulfide and cadmium sulfide-selenium alloy nanocrystals observed by ultrafast fluorescence upconversion: The effect of surface trap states

The change in carrier dynamics of cadmium chalcogenide nanocrystals associated with varying ligands and surface composition has been explored through femtosecond fluorescence upconversion. The observed size-dependent changes on the fluorescence lifetime at the band edge were explained by a decrease in accessible trap sites with increasing nanocrystal size. The addition of hexadecylamine as a cosolvent to CdSe nanocrystals was shown to decrease the effects of surface trapping as compared to nanocrystals synthesized only in tri-n-octylphosphine oxide, indicating that carrier dynamics are preparation dependent. Additionally, in the CdSxSe1-x alloy nanocrystal system, compositional variations in carrier dynamics were observed and largely attributed to increased carrier trapping on the nanocrystal surface with addition of sulfur. These results indicate that carrier dynamics are not only size dependent, but also strongly composition dependent.