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Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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Figure 8 (a) Average off-time probability distribution for 25-Å-radius CdSe(ZnS)QD at 300 K (5), 10 K (4), 30 K ( w ), and 70 K (5). The a values are 1.41, 1.51,1.37, and 1.45, respectively. (b) Average off-time probability distributions for 39CdSe(ZnS) QDs of radius 15 A˚ , (5) and 25-A˚ -radius CdSe(ZnS) QD (w ) and 25-A˚ -radius CdSe QD (4) at RT. The a values are 1.54, 1.59, and 1.47, respectively.trace, as seen in Fig. 2; that is, fewer on–off events are observed and the ontimes are longer.As shown in Figs. 9c and 9d, varying the cw average excitation power inthe range 100–700 W/cm 2 at 300 K and 10 K shows on-time probability distributionchanges, consistent with earlier qualitative observations. We alsocompared the on-time statistics for QDs differing in size (15-A˚ versus 25-A˚core radius) and QDs with and without a six-monolayer shell of ZnS overcoatingshown in Fig. 9. With reduced excitation intensity, lower temperature,or greater surface overcoating, the truncation sets in at longer wait times andthe power-law distribution for the on time becomes more evident.Given that the exponent for the on-time power-law distribution is nearlythe same for all of our samples, then a measure of the average truncation point(or maximum on time) is possible <strong>by</strong> comparing ‘‘average on times’’ for dif-<strong>Copyright</strong> <strong>2004</strong> <strong>by</strong> <strong>Marcel</strong> <strong>Dekker</strong>, <strong>Inc</strong>. <strong>All</strong> <strong>Rights</strong> <strong>Reserved</strong>.

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