Single molecule studies of quantum dot fluorescence intermittency: evidence for both dark and light-assisted blinking dynamics

The present work investigates single CdSe QD blinking behaviour in the absence of illumination, in order to elucidate the role of photoexcitation on the long time dynamics. The probability of a QD transitioning in the dark from a high quantum yield ('on') state to a low quantum yield ('off') state is found to be extremely low and essentially independent of dwell time from 100 ms to minute time scales. The absence of a dark pathway is consistent with many models, for example, in which 'on' to 'off' blinking is ascribed to photo-assisted Auger ionization and charging of the QD. On the other hand, blinking recovery statistics (from 'off' to 'on' state) reveal an approximate power law dependence on dwell time, consistent with non-photoactivated and highly distributed kinetics in the dark. Most importantly, such dark QD recovery is orders of magnitude slower than predicted by previous power law kinetic studies under constant illumination conditions, highlighting the presence of both (i) light-activated and (ii) dark blinking recovery pathways. Furthermore, the kinetic analysis indicates that nearly all single CdSe QD studies to date have been performed under conditions where blinking recovery is dominated by the light-activated channel to date.