0 20 40 60 80 100 120 0 100 200 300 400 500 600 700 800 normalized intensity time [s] 0 2 4 6 8 10 12 10 1 10 2 10 3 10 4 time [ns] normalized intensity (a) (b) FIG. 1: (a) Intensity time trace of a single CdSe/CdS/ZnS QD showing fluorescence intermittency (bin width = 60ms). The ‘on’ and ‘off’ intensity thresholds are also shown. The excitation power is adjusted to an excitation rate of about 0.2, taking into account the radiative lifetime and absorption cross section [21]. (b) ‘On’ (red) and ‘off’ (blue) PL decay curves. The dotted line represents the dark count level of the detector. 1 /e decay time to each data set. Histograms of these decay times are presented in Fig. 2 for all three sizes of QDs. The observed distributions demonstrate a broad peak at about 250ps and are evidently size-independent. This is in stark contrast with the significant variance in the biexciton Auger decay lifetimes, which are approximately 30ps, 65ps and 350ps respectively for the three sizes [11, 22]. Since both the slow and the fast components of the ‘off’ state decay curve contribute to the 1 /e decay time, the lifetimes of the slower components are significantly longer than biexciton Auger recombination lifetimes, particularly for the smaller QDs (for these, they are even longer than the expected trion Auger lifetime [23]). This fact, in combination with the lack of size dependence in the observed rates, leads us to the conclusion that the nonradiative recombination process causing the intermittent darkening of QDs is not Auger recombination. Further information on the ‘off’ state decay can be extracted from intensity dependent measurements, performed here on the 3 . 8 nm QDs. For each QD the excitation rate (i.e. the average number of absorption events per exciton lifetime) was assessed by saturation of the ‘on’ state emission. In addition, a clear indication of the average excitation rate approaching unity is the emergence of a fast multiexciton transient feature in the ‘on’ decay curve. The measurements of the ‘off’ and ‘on’ decay transients on the same QD at varying excitation rates reveal a strong correlation between the excitation intensity and the ‘off’ decay rate. While the effect is general, it is most dramatically observed in QDs with slow 1 /e decay 5