Figure 1. Relationship between charging and blinking in thin-shell and thick-shell CdSe/CdS NQDs.

(a) In thin-shell NQDs, the high-emissivity 'bright' state corresponds to an exciton in a neutral dot (X⁰). The dominant recombination channel is radiative decay (orange arrow) with rate γ_r. Any random fluctuation towards a charged state (here a negative trion X⁻) renders the dot essentially non-emissive (the dot blinks off) because the rate of nonradiative Auger recombination (black arrows; rate γ_A) is orders of magnitude higher than γ_r. (b) The core/shell NQDs with very thick shells (≥15 monolayers) are essentially nonblinking (the PL intensity remains stable over minutes and even hours). This behaviour can be either due to a complete suppression of charge fluctuations (the dot is always in the neutral state) or owing to the fact that the PL quantum yield of charged excitons is similar to that of neutral excitons. The latter situation can only be realized if the Auger recombination rate, γ_A, of a charged exciton is much lower than its radiative decay rate. (c) The expected qualitative form of the PL decay traces (semi-log scale) for the neutral and the charged exciton. Because the number of radiative recombination pathways is increased in the presence of an additional electron, the radiative lifetime of a trion is twice as short as that of a neutral exciton (radiative rate 2γ_r). Further, because of the increase in the emission rate, the early time PL amplitude of the trion is also increased by a factor of two compared with that of the neutral exciton. By analysing lifetime variations, one can, in principle, determine whether different charged states contribute to the measured emission even when the PL intensity is nonblinking.