A Common Mechanism Underlies the Dark Fraction Formation and Fluorescence Blinking of Quantum Dots

CdSe quantum dots (QDs) are known to exhibit both power-law blinking dynamics and a dark fraction. A complete description of the mechanistic origins of these properties is still lacking. We show that a change in the pH of the QD environment systematically changes both the dark fraction and the blinking statistics. As pH is lowered, shorter "on" times and longer "off" times, as well as an increase in the permanent dark fraction, are observed. The increase in the dark fraction is preceded by a decrease in the emission intensity of a single QD. Interestingly, the form of the probability distribution function describing blinking changes when the QDs are taken from an air-exposed environment into an aqueous one. These results are used to propose a coupled role for H(+) ions by which they first reduce the intensity of the emitting state as well as affect the probabilities of the QD to switch between "on" and "off" states and eventually trap the QD in a permanent "off" state. We discuss and extend two theoretical blinking models to account for the effect of H(+) ions as well as to highlight their common principle of a diffusion-controlled mechanism governing blinking.