Size-Dependent Photoionization in Single CdSe/ZnS Nanocrystals

Fluorescence intermittency in single semiconductor nanocrystals has been shown to follow power law statistics over many decades in time and in probability. Recently, several studies have shown that, while “off” dwell times are insensitive to almost all experimental parameters, “on” dwell times exhibit a pump-power dependent exponential truncation at long times, suggestive of enhanced biexciton photoionization probabilities at high excitation powers. Here we report the dependence of this on-time truncation on nanocrystal radius. We observe a decrease in the per-pulse photoionization probability from 1.8(2) × 10^–4 to 2.0(7) × 10^–6 as the CdSe core radius increases from 1.3 to 3.5 nm, with a radius scaling for the probability for charge ejection arising from biexciton formation P_ionize(r) ∝ 1/r^3.5(5). Effective mass calculations of the exciton wave functions show that the product of fractional electron and hole probabilities in the trap-rich ZnS shell scale similarly with nanocrystal radius. Possible charge ejection mechanisms from such a surface-localized state are discussed.