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

exposure to only room light. In our model, after cw laser excitation andexciton formation, an electron or hole from the exciton localizes near thesurface of the QD, leaving a delocalized charge carrier inside the QD core.Following this initial charge localization or ionization, (1) the delocalizedcharge carrier can also be localized leading to a net neutral QD core, (2) If theQD environment is decorated by charges following process 1, then aftersubsequent ionization, a charge localized in the QD’s environment can relaxback into the QD core, recombining with the delocalized charge carrier, or (3)Coulombic interaction can lead to a permanent reorganization of thelocalized charge carriers present in the QD environment even after the samecharge relaxes back into the core and recombines with initial delocalizedcharge. Mechanisms 1–3 would create, if not alter, a surface dipole and lead toa net change in the local electric field. The single-QD spectra express thischange as a large Stark shift in the emission frequency. However, the modeldoes not necessarily require that a blinking event be followed always by a shiftin emission frequency. If the dark period was produced and removed by alocalization and recapture of the same charge without a permanent reorganizationof charges in the environment (4), the emission frequency does notchange. Any changes in the emission frequency during this mechanism wouldbe entirely thermally induced and such small spectral shifts are observed.Indeed, this pathway for recombination dominates very strongly, as mostdark periods are not accompanied by large frequency shifts.V. ‘‘POWER-LAW’’ BLINKING STATISTICSThe small number of QDs sampled and the short duration of each time tracelimited early studies into the statistics of blinking in single CdSe QDs. Recently,Kuno et al. found that room-temperature fluorescence intermittencyin single QDs exhibited power-law statistics—indicative of long-range statisticalorder [31]. By analyzing the power-law statistical results within aphysical framework, we begin the dissection of the complex mechanism forblinking in these QDs. The statistics of both on- and off-time distributions areobtained under varying temperature, excitation intensity, size, and surfacemorphology conditions.As schematically shown in Fig. 7a, we define the on time (or off time) asthe interval of time when no signal falls below (or surpasses) a chosenthreshold intensity value. The probability distribution is given by the histogramof on or off events:PðtÞ ¼ P events of length tð1Þwhere t refers to the duration of the blink-off or blink-on event. The off-timeprobability distribution for a single CdSe/ZnS QD at room temperature isCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.