Challenge to the Charging Model of Semiconductor-Nanocrystal Fluorescence Intermittency from Off-State Quantum Yields and Multiexciton Blinking

Semiconductor nanocrystals emit light intermittently; i.e., they “blink,” under steady illumination. The dark periods have been widely assumed to be due to photoluminescence (PL) quenching by an Auger-like process involving a single additional charge present in the nanocrystal. Our results challenge this long-standing assumption. Close examination of exciton PL intensity time traces of single CdSe(CdZnS) core(shell) nanocrystals reveals that the dark state PL quantum yield can be 10 times less than the biexciton PL quantum yield. In addition, we observe spectrally resolved multiexciton emission and find that it also blinks with an on/off ratio greater than $10:1$. These results directly contradict the predictions of the charging model.

Figure 1

(a) Blinking time trace of a single NC and (b) an expanded view of the baseline showing off periods with count rate as low as $\sim 300\mathrm{cps}$.

Figure 2

(a) Transient PL of ensemble CdSe(CdZnS) core(shell) NCs in hexane. The black spectrum is averaged over the first 400 ps after the excitation pulse, containing both $1P_e$ and band edge emission. The red spectrum is averaged over 750 ps after 1 ns delay following the excitation pulse, and is only from the X band edge emission. The orange spectrum is obtained by subtracting the X emission contribution from the black spectrum, representing the $1P_e$ emission. (b) Schematic illustration of the experimental setup. (c) Cross correlation of signals in the two channels. (d)–(e) PL traces of a single NC excited at (d) $10\mu \mathrm{J}·{\mathrm{cm}}^{-2}$ and (e) $400\mu \mathrm{J}·{\mathrm{cm}}^{-2}$. The black dashed line in the channel 2 trace in (d) denotes the average background counts at the same excitation power. The average count rate of each time trace after background subtraction is labeled in red. The integrated intensity ratio channel 2 to 1 is 0.46% in (d) and 1.38% in (e). (f) Correlation coefficient of the time traces in (e).

Figure 3

(a)–(d) Schematics of the various recombination processes of interest for our experiment: (a) charged $X$ (“trion”) Auger decay as in the off state of blinking NCs; (b) neutral BX; (c) neutral TX; (d) charged TX. (e)–(f) Schematics of alternative blinking mechanisms: (e) trapping multiple charges; (f) trap-assisted recombination.