Proceedings of nanoGe September Meeting 2015 (NFM15)
Publication date: 8th June 2015
The efficiency and stability of emission from semiconductor nanocrystal quantum dots (QDs) is negatively affected by ‘blinking’ on the single-nanocrystal level, i.e. random alternation of bright and dark periods. The timescales of these fluctuations can be as long as many seconds, orders of magnitude longer than typical lifetimes of exciton states in QDs. In this work we investigate photoluminescence from QDs ‘delayed’ over microseconds to milliseconds, thus connecting the timescales of exciton decay to those of blinking. With single-QD as well as ensemble measurements, we examine the dynamics and spectral characteristics of this delayed emission, and quantify radiative and nonradiative decay constants by systematically varying the photonic environment of the QDs. Our results indicate that delayed emission and blinking have the same physical origin, namely charge carrier separation, storage for microseconds to seconds, and eventual recovery of the exciton state. A new microscopic model is developed that connects our new results on delayed emission to exciton recombination and blinking. While existing models for blinking have in common that they seek an explanation for uninterrupted optical cycling during long ON ‘periods’ of up to many seconds, our results on delayed emission show that such long periods of uninterrupted cycling do not occur. The new blinking model, which assumes a significant (of order 10%) probability of charge carrier separation at all times during optical cycling, reproduces the photoluminescence decay dynamics of QDs, including the delayed component, as well as the blinking characteristics.
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