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

Figure 2 (a) Simple model of a nanocrystal (quantum dot) as a semiconductorinclusion embedded in an insulating matrix. (b) Potential well formed in any onedimension (x, y, or z) in the conduction and valence bands. The energy levels of theexcited carriers (electrons and holes) become quantized due to the finite size of thesemiconductor nanocrystal.[1], spectra from semiconductor nanocrystals exhibit a series of discreteelectronic transitions between these quantized levels. Accordingly, semiconductornanocrystals are sometimes referred to as artificial atoms. Furthermore,because the energies of the electron and hole levels are quite sensitive tothe amount of confinement, the optical spectra of nanocrystals are stronglydependent on the size of the crystallite.To review recent progress in utilizing optical spectroscopy to understandthis size dependence, we begin in Section II with a discussion of the basictheoretical concepts necessary to understand electronic structure in nanocrystals.Then, experimental data from the prototypical direct-gap semiconductorsystem, cadmium selenide (CdSe), is described in Section III. As thefirst system to be successfully prepared with extremely high quality [7], CdSehas been extensively studied. Indeed, it was the first system where the sizedependence of the electronic structure was understood in detail [8,9]. Furthermore,this understanding led to the resolution of a long-standing mysteryCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.