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

The approaches to fabrication of semiconductor quantum dots can bedivided into two main classifications: In the top-down approach, nanolithographyis used to reduce the dimensionality of a bulk semiconductor.These approaches are presently limited to structures with dimensions on theorder of tens of nanometers [17]. In the bottom-up approach, two importantfabrication routes of QDs are presently used: molecular beam epitaxy(MBE) deposition utilizing the strain-induced growth mode [18,19], andcolloidal synthesis [20–24]. In this chapter, we focus on colloidal-grownnanocrystal QDs. These samples have the advantage of continuous sizecontrol, as well as chemical accessibility due to their overcoating withorganic ligands. This chemical compatibility enables the use of powerfulchemical or biochemical means to assemble nanocrystals in a controlledmanner [25–27]. Artificial solids composed of nanocrystals have beenprepared, opening a new domain of physical phenomena and technologicalapplications [28–30]. Nanocrystal molecules and nanocrystal–DNA assemblieswere also developed [31].Colloidal synthesis has been extended to several directions, allowingfurther powerful control, in addition to size, on optical and electronic propertiesof nanocrystals. Heterostructured nanocrystals were developed, wheresemiconductor shells can be grown on a core [22,32]. One important classof such particles, are core–shell nanocrystals [33–38]. Here, the core is overcoatedby a semiconductor shell with a gap enclosing that of the core semiconductormaterials. Enhanced fluorescence and increased stability can beachieved in these particles, compared with cores overcoated by organic ligands.Recently, shape control was also achieved in the colloidal synthesisroute [39]. By proper modification of the synthesis, rod-shaped particlescan be prepared—quantum rods [40,41]. Such quantum rods manifest thetransition from zero-dimensional (0D) quantum dots to 1D quantum wires[42].From the early work on the quantum-confinement effect in colloidalsemiconductor nanocrystals, electronic levels have been assigned according tothe spherical symmetry of the electron and hole envelope functions [6,43]. Thesimplistic ‘‘artificial atom’’ model of a particle in a spherical box predictsdiscrete states with atomiclike state symmetries (e.g., s and p). To probe theelectronic structure of II–VI and III–V semiconductor nanocrystals, a varietyof ‘‘size-selective’’ optical techniques have been used, mapping the sizedependence of dipole allowed transitions [44–48]. Theoretical models basedon an effective mass approach with varying degree of complexity [45,49] aswell as pseudopotentials [50,51] were used to assign the levels.Tunneling transport through semiconductor nanocrystals can yield complementarynew information on their electronic properties, which cannot beCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.