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

the ‘‘on–off ’’ emission intensity fluctuations that occur on the timescale ofmicroseconds to minutes. Both of these QD phenomena, observed at cryogenicand room temperatures under continuous photoexcitation, give insightinto a rich array of electrostatic dynamics intrinsically occurring in andaround each individual QD.II.SINGLE-QUANTUM-DOT SPECTROSCOPYWe studied many individual QDs simultaneously using a home-built,epifluorescence microscope coupled with fast data storage and data analysis.This setup is also referred to as a wide-field or far-field microscope due to thediffraction-limited, spatial resolution of the excitation light source. The basiccomponents, shown in Fig. 1, are similar to most optical microscopes: a lightsource, microscope objective, stages for x-y manipulation and focusing of theobjective relative to the sample, a spectrometer, and coupled-charge device(CCD) camera. To access the emission from individual chromophores, a90% reflective silver mirror is placed as shown in Fig. 1 to allow for a smallfraction of the excitation light to pass into the objective and the collection of90% of the emitted light. Suitable optic filters are used to remove anyresidual excitation light. For all of the experiments discussed here, single-QDemission images and spectra were recorded with a bin size of 100 ms fordurations of 1 h under continuous wave (cw), 514-nm, Ar ion laser excitation.However, for strictly emission intensity measurements, time resolutionas fast as 500 As is possible using an avalanche photodiode. The lowtemperaturestudies were performed using a cold-finger, liquid-heliumcryostat with a long-working-distance air objective (N.A. 0.7), whereasroom-temperature studies were performed using an oil-immersion objective(N.A. 1.25). The raw data are collected in a series of consecutive images toform nearly continuous three-dimensional datasets as shown in Fig. 1b. Thedark spots represent emission from individual QDs spaced f1 Am apart. Anadvantage of a CCD camera over avalanche photodiode or photomultipliertube detection is that spectral data of single QDs can be obtained in oneframe using a monochromator. Moreover, all of the dots imaged on theentrance slit of the monochromator are observed in parallel. If only relativefrequency changes need to be addressed, then the entrance slit can beremoved entirely, allowing parallel tracking of emission frequencies andintensities of up to 50 QDs simultaneously. In cases where spectral informationis not needed, such as in Fig. 1b, up to 200 QDs can be imagedsimultaneously. The data analysis program then retrieves the time–frequency(or space)–intensity emission trajectories for all of these QDs. This highlyCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.