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

parallel form of data acquisition is vital for a proper statistical sampling ofthe entire population.The CdSe QDs are prepared following the method of Murray et al. [17]and protected with ZnS overcoating [18,19], whereas the CdTe samples wereprepared following Ref. 20. All single QD samples are highly diluted and spincastin a 0.2–0.5-Am thin film of poly(methyl methacrylate) (PMMA) on acrystalline quartz substrate.III.SPECTRAL DIFFUSION AND FLOURESCENCEINTERMITTENCYFigures 2 and 3 showcase the typical, phenomenological behavior of fluorescenceintermittency and spectral dynamics observed. An illustrative 3000-stime trace of fluorescence intermittency is shown for a CdSe/ZnS QD at 10 Kand at 300 K in Figs. 2a and 2b, respectively. At first glance, there are cleardifferences between the blinking behaviors at these different temperatures.The QD appears to be emitting considerably more often at low temperatureand the QD appears to turn on and off more frequently at room temperature(RT). However, by expanding a small section of the time trace, thesimilarities between these traces at different temperatures and the selfsimilarityof the traces on different timescales can be observed. The spectrallyresolved time traces shown in Figs. 3b and 3c compare the spectral shiftingfor QDs at 10 K and at RT, respectively. At RT, the emission spectral peakwidths range from 50 to 80 meV, whereas at 10 K, the characteristic phononprogression shown in Fig. 3a verifies the presence of CdSe QDs. Ultranarrowpeak widths for the zero-phonon emission as small as 120 AeV have beenpreviously observed at 10 K [21]. At either temperature, spectral shifts aslarge as 50 meV were observed in our experiments. Figure 4 shows the largevariation in spectrally dynamic time traces from three QDs observedsimultaneously at 10 K. The spectrum in Fig. 4a shows sharp emission lineswith nearly constant frequency and intensity. The spectrum in Fig. 4b showssome pronounced spectral shifts and a few blinking events; the spectrum inFig. 4c is fluctuating in frequency and shows a number of blinking events on amuch faster timescale.Early investigations of blinking and spectral diffusion have shed somelight regarding these novel properties. The blinking of QDs showed a dependenceon the surface overcoating, temperature, and excitation intensity.Individually or in any combination, increased thickness of ZnS overcoating,lower temperatures, and lower excitation intensity all decreased the blinkingrate [12,22]. However, these earlier experiments were restricted to smallnumbers of QDs studied—one QD at a time—using confocal microscopy.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.