Lifetime Blinking in Non Blinking Quantum Dots

Photoluminescence (PL) blinking is a common property of nanoscale light emitters. Nanocrystal quantum dots have often been used as model systems in studies of this intriguing phenomenon. Here, we use recently developed thick-shell CdSe/CdS NQDs to demonstrate a new regime of blinking where discrete fluctuations in the PL lifetime (``lifetime blinking'') occur without appreciable changes in the PL intensity. Single-dot measurements under controlled electrochemical charge injection [1] yield the PL lifetimes of neutral and charged excitons. We show that the observed ``lifetime blinking'' are due to random charging/discharging of the nanocrystal [2]. Indeed, the injection of electrons does not appreciably modify the PL quantum yield, which explains the coexistence of a nonblinking intensity with a ``blinking'' lifetime. At higher excitation power, charged excitons dominate the PL emission. We build a quantitative model showing that nanocrystal charging is caused by Auger-assisted ejection of a hole, producing negatively charged species. Importantly, Auger recombination that involves excitation of an electron is suppressed while hole-based processes remain efficient.\\[4pt] [1] Galland \textit{et al.}, Nature \textbf{479}, 203-207 (2011)\\[0pt] [2] Galland \textit{et al}., Submitted (2011)