Two types of luminescence blinking revealed by spectroelectrochemistry of single quantum dots

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Abstract
The phenomenon of fluorescence intermittency (blinking between ON/OFF states) has been observed for both naturally occurring fluorophores (such as organic dyes and biomolecules) and artificial nanostructures (such as carbon nanotubes and semiconducting nanocrystal quantum dots). This study aims to resolve the long-standing controversy surrounding the origin of photoluminescent blinking in semiconductor nanocrystals, also known as quantum dots. Researchers usually evoke the Auger, or A-type, mechanism in which a separation of charges yields to the OFF state, but recent observations have raised doubts about this explanation. Galland et al. describe a second mechanism (called B-type) in which an excited, or hot, electron becomes trapped in the shell for a time before being released to the emitting core. By controlling various parameters, such as applied voltage potential and shell thickness, the authors can control the frequency of blinking, or suppress it completely. Photoluminescence blinking—random switching between states of high (ON) and low (OFF) emissivities—is a universal property of molecular emitters found in dyes1, polymers2, biological molecules3 and artificial nanostructures such as nanocrystal quantum dots, carbon nanotubes and nanowires4,5,6. For the past 15 years, colloidal nanocrystals have been used as a model system to study this phenomenon5,6. The occurrence of OFF periods in nanocrystal emission has been commonly attributed to the presence of an additional charge7, which leads to photoluminescence quenching by non-radiative recombination (the Auger mechanism)8. However, this ‘charging’ model was recently challenged in several reports9,10. Here we report time-resolved photoluminescence studies of individual nanocrystal quantum dots performed while electrochemically controlling the degree of their charging, with the goal of clarifying the role of charging in blinking. We find that two distinct types of blinking are possible: conventional (A-type) blinking due to charging and discharging of the nanocrystal core, in which lower photoluminescence intensities correlate with shorter photoluminescence lifetimes; and a second sort (B-type), in which large changes in the emission intensity are not accompanied by significant changes in emission dynamics. We attribute B-type blinking to charge fluctuations in the electron-accepting surface sites. When unoccupied, these sites intercept ‘hot’ electrons before they relax into emitting core states. Both blinking mechanisms can be electrochemically controlled and completely suppressed by application of an appropriate potential.