Effect of Surface Stoichiometry on Blinking and Hole Trapping Dynamics in CdSe Nanocrystals

We measure the photoinduced carrier dynamics as the surface composition of CdSe nanocrystals is systematically varied from metal rich (similar to 80% surface Cd) to nearly stoichiometric (similar to 50% surface Cd). Using time-resolved optical spectroscopy, we determine that the luminescence lifetime is controlled by the rate of hole trapping at the newly exposed surface selenium atoms. However, the increased rate of the photoluminescence decay is not sufficient to explain the decreased photoluminescence quantum yield, and requires a growing proportion of nanocrystals in a dark or "OFF" state to explain the data. A global kinetic model is proposed that relates the fraction of selenium sites to the rate of hole trapping. A linear relationship between the rate of hole trapping and the fraction of exposed Se sites (x(Se)) is observed within the range of accessible stoichiometries (x(Se) = 0.5.-0.2). Extrapolation to higher surface cadmium fractions suggests that not all Se sites serve as effective hole traps. These results explain the strong nonlinear dependence of the fluorescence yield on the nanocrystal stoichiometry.