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

structure of energy states leads to the discrete absorption spectrum of QDs(schematically shown by vertical bars in Fig. 1c), which is in contrast to thecontinuous absorption spectrum of a bulk semiconductor (Fig. 1c).Semiconductor QDs bridge the gap between cluster molecules and bulkmaterials. The boundaries among molecular, QD, and bulk regimes are notwell defined and are strongly material dependent. However, a range fromf100 to f10,000 atoms per particle can been considered as a crude estimateof sizes for which the nanocrystal regime occurs. The lower limit of this rangeis determined by the stability of the bulk crystalline structure with respect toisomerization into molecular structures. The upper limit corresponds to sizesfor which the energy level spacing is approaching the thermal energy kT,meaning that carriers become mobile inside the QD.Semiconductor QDs have been prepared by a variety of ‘‘physical’’and ‘‘chemical’’ methods. Some examples of physical processes, characterizedby high-energy input, include molecular-beam-epitaxy (MBE) andmetalorganic-chemical-vapor-deposition (MOCVD) approaches to quantumdots [1–3] and vapor–liquid solid (VLS) approaches to quantum wires[4,5]. High-temperature methods have also been applied to chemical routes,including particle growth in glasses [6,7]. Here, however, we emphasize‘‘soft’’ (low-energy input) colloidal chemical synthesis of crystalline semiconductornanoparticles that we will refer to as nanocrystal quantum dots(NQDs). NQDs comprise an inorganic core overcoated with a layer oforganic ligand molecules. The organic capping provides electronic andchemical passivation of surface dangling bonds, prevents uncontrolledgrowth and agglomeration of the nanoparticles, and allows NQDs to bechemically manipulated like large molecules with solubility and reactivitydetermined by the identity of the surface ligand. In contrast to substrateboundepitaxial QDs, NQDs are ‘‘freestanding.’’ In this discussion, weconcentrate on the most successful synthesis methods, where success isdetermined by high crystallinity, adequate surface passivation, solubility innonpolar or polar solvents, and good size monodispersity. Size monodispersitypermits the study and, ultimately, the use of materials size effects todefine novel materials properties. Monodispersity in terms of colloidalnanoparticles (1–15-nm size range) requires a sample standard deviation ofj V 5%, which corresponds to F one lattice constant [8]. Because colloidalmonodispersity in this strict sense remains relatively uncommon, preparationsare included in this chapter that achieve approximately j V 20%, inparticular where other attributes, such as novel compositions or shapecontrol, are relevant. In addition, we discuss ‘‘soft’’ approaches to NQDchemical and structural modification, as well as to NQD assembly intoartificial solids or artificial molecules.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.