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

narrow size distribution (less then 10%) [21,23]. Furthermore, InAs is anarrow-gap semiconductor (E g = 0.418 eV) with a large Bohr radius a 0 of 340A˚ (as compared to CdSe, with E g = 1.84 eV and a 0 = 55 A˚ ) and serves as aprototypical system for the study of quantum-confinement effects. Forscanning tunneling spectroscopy (STS), the narrow gap allows one to probeexcited levels in a highly charged state, as will be described in Section III.B. Itis also important to note that InAs is presently perhaps the only system thatcan be fabricated both by epitaxial growth techniques as well as by colloidalchemistry techniques, thus providing an important point of comparison.A. Photoluminescence Excitation SpectroscopyThe InAs QDs were prepared using a solution-phase pyrolitic reaction oforganometallic precursors. These nanocrystals are nearly spherical in shape,with size controlled between 1 and 4 nm in radius and size distribution betterthan 10% [21,23]. As a demonstration of the size and shape homogeneity ofthese samples, we present in Fig. 4 the transmission electron microscopic(TEM) images of superlattices of InAs nanocrystals. The nanocrystal surfaceis passivated by organic ligands. For the low-temperature optical experiments,dilute samples were embedded in free-standing, optically clear films ofpolyvinylbutyral and cooled to 10 K.Figure 4 Transmission electron microscopic images of superlattices of InAs QDs(upper frames). Three different facets of an face-centered cubic (fcc) structure of thesuperlattice can be identified, as can be seen from the optical diffraction of the TEMnegatives shown in the lower frames.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.