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

the electron–hole pair, the carriers are confined by the boundaries of thematerial. This phenomenon, which is known as the quantum-size effect, leadsto atomic-like optical behavior in nanocrystals as the bulk bands becomequantized (see Fig. 1b). Because, at the atomic level, the material remainsstructurally identical to the bulk crystal, this behavior arises solely due to itsfinite size. Therefore, by revealing this atomic-like behavior, simple opticaldata (e.g., absorption spectra) can give useful information about the nanometer-sizeregime.In many nanocrystal systems, this effect can be quite dramatic. Consequently,these materials, sometimes referred to as colloidal quantum dots,provide an easily realizable system for investigation of the nanometer-sizeregime. Once this was realized, early research endeavored to explain theunderlying phenomenon [2–6]. It was shown (see below) that by modeling thequantum dot as a semiconductor inclusion embedded in an insulating matrix,as illustrated in Fig. 2a, the basic physics could be understood. Photoexcitedcarriers reside in a three-dimensional potential well, as shown in Fig. 2b. Thiscauses the valence and conduction bands to be quantized into a ladder of holeand electron levels, respectively. Therefore, in contrast to the bulk absorptionspectrum, which is a continuum above the bandgap of the semiconductor (E s g)Figure 1 (a) Band diagram for a simple two-band model for direct-gap semiconductors.(b) Optical transitions in finite-size semiconductor nanocrystals are discretedue to the quantization of the bulk bands.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.