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

2Electronic Structure inSemiconductor Nanocrystals *David J. NorrisUniversity of Minnesota, Minneapolis, Minnesota, U.S.A.I. INTRODUCTIONOne of the primary motivations for studying nanometer-scale semiconductorcrystallites, or nanocrystals, is to understand what happens when a semiconductorbecomes small. This question has been studied not only for its fundamentalimportance but also for its practical significance. Because objectsare rapidly shrinking in modern electronic and optoelectronic devices, wewish to understand their properties. To address this question, a variety ofsemiconductor nanocrystals have been investigated over the past two decades.Throughout these studies, one of the most important and versatile toolsavailable to the experimentalist has been optical spectroscopy. In particular, ithas allowed the description of how the electronic properties of these nanocrystalschange with size. The purpose of this chapter is to review progress inthis area.The usefulness of spectroscopy stems from the inherent properties ofbulk semiconductor crystals. Direct-gap semiconductors can absorb a photonwhen an electron is promoted directly from the valence band into theconduction band [1]. In this process, an electron–hole pair is created in thematerial, as depicted in Fig. 1a. However, if the size of the semiconductorstructure becomes comparable to or smaller than the natural length scale of* Portions of this chapter have been adapted (with permission) from D. J. Norris, Ph.D. thesis,MIT, 1995.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.