Publication date: 3rd July 2020
Semiconductor nanocrystals (NCs) can exhibit narrow emission across the visible and NIR spectrum but suffer from photoluminescence intermittency (blinking) that is associated with irreversible photochemical damage.[1] Even under continuous excitation NC emission randomly switches between ON and OFF states, with both lacking characteristic lifetimes due to their power-law probability distributions.[2] It is a challenge to spectroscopically distinguish between proposed mechanisms due in part to the very different timescales of a single excitonic life-cycle and the governing intermittent dynamics. Particularly, simple first-order kinetics are unable to capture the observed ON- and OFF-time distributions. Blinking presents an intriguing fundamental challenge as mechanistic origins of the observed scale-free distributions remain unclear.
Here, we demonstrate all-optical two-colour modulation of blinking statistics in individual CdSe/ZnS NCs using sub-bandgap light. Motivated by single-NC measurements, we use a continuous wave source tuned to the stimulated emission transition to alter blinking statistics in individual CdSe/ZnS NCs. Accounting for background, we compare the modulated/unmodulated photon streams and discover that sub-bandgap modulation steepens the power-law slope of the ON-time probability distribution by ΔαON=0.46±0.09. This effect is characteristic (a consistent relative increase is observed across numerous NCs), selective (no changes with longer-wavelength, off-resonance modulation), and robust (data reduction and statistical truncation had negligible effects on the slope change). This slope change is also pervasive and is observed in 19/20 of the NCs examined at an irradiance of 2.6 kW/cm2. Intriguingly, we observe neither a concurrent change in the OFF-time slope, nor a significant effect on the truncation times of the ON- or OFF- probability distributions—as would be expected for a simple stochastic process. Our technique provides an all-optical method with an internal standard to monitor and modulate the emission from individual NCs and will facilitate mechanistic insight into the causes of blinking.
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