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

investigated. For optical experiments, such samples are ideal. In particular,the intensity of deep-trap emission, which dominates the luminescencebehavior of dots prepared by many other methods, is very weak in thesesamples. Although the true origin of this emission is unknown, it is generallyassumed to arise from surface defects which are deep in the bandgap. Insteadof deep-trap emission, the newer nanocrystals exhibit strong band-edgeluminescence with quantum yields measured as high as 90% at 10 K. Atroom temperature, the quantum yield is typically 10%. However, by encapsulatingthe CdSe nanocrystals in a higher-bandgap semiconductor, such asZnS or CdS, the quantum yield can be further improved [61–63]. Emissionefficiencies greater than 50% at room temperature have been reported.B. Spectroscopic MethodsSamples obtained from these new synthetic procedures provided the firstopportunity to study the size dependence of the electronic structure in detail.However, because even the best samples contain residual sample inhomogeneitieswhich can broaden spectral features and conceal transitions, severaloptical techniques have been used to reduce these effects and maximize theinformation obtained. These techniques include transient differential absorptionspectroscopy (TDA), photoluminescence excitation spectroscopy (PLE),and fluorescence line-narrowing spectroscopy (FLN), which are describedfurther in this subsection. More recently, single-molecule spectroscopy [64],which can remove all inhomogeneities from the sample distribution, has beenadapted to nanocrystals and many exciting results have been observed [65].However, because single quantum-dot spectroscopy will be described elsewherein this volume and these methods have mostly provided informationabout the emitting state (i.e., not the electronic-level structure), it will not beemphasized here.From a historical perspective, the most common technique to obtainabsorption information has been TDA, also called pump-probe or holeburningspectroscopy [8,47,48,52–54,66–73]. This technique measures theabsorption change induced by a spectrally narrow pump beam. TDAeffectively increases the resolution of the spectrum by optically exciting anarrow subset of the quantum dots. By comparing the spectrum with andwithout this optical excitation, information about the absorption of thesubset is revealed, with inhomogeneous broadening greatly reduced. Becausethe quantum dots within the subset are in an excited state, the TDA spectrumwill reveal both the absence of a ground-state absorption (a bleach) andexcited-state absorptions (also called pump-induced absorptions). Unfortunately,when pump-induced absorption features overlap with the bleachfeatures of interest, the analysis becomes complicated and the usefulness ofthe technique diminishes.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.