Band Edge Dynamics in CdSe Nanocrystals Observed by Ultrafast Fluorescence Upconversion

Fluorescence upconversion spectroscopy has been performed on high-quality, low size dispersion CdSe nanocrystals synthesized using the CdO precursor with tri-n-octylphosphine oxide and hexadecylamine cosurfactants. These measurements reveal that an increase in nanocrystal diameter, from 25 to 60 Å, is accompanied by an increase in both the short-lived (τ1) and long-lived (τ2) components of the fluorescence lifetime at the band edge. The increase in τ1 is explained by the decrease in accessible trap sites through a reduction in surface-to-volume ratio. The addition of hexadecylamine as a cosolvent, through interaction with dangling selenium bonds on the nanocrystal surface, is shown to increase τ1 as compared to nanocrystals synthesized only in tri-n-octylphosphine oxide. These results indicate that carrier dynamics are preparation dependent.