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

discussed briefly in Section IV. Charge transport in that system exhibits anumber of unusual nonexponential relaxation phenomena which have beenanalyzed in the context of a continuous-time random walk [85]. In theprototypical CdSe system, the efficiency of nanocrystal–nanocrystal chargetransport is central to the viability of the polymer–nanocrystal-based lightemittingand photovoltaic diodes discussed in Section VI. We will eventuallyfocus on transport in close-packed CdSe nanocrystal films, but we first turnour attention to the better studied topic of transport through arrays of chemicallyprepared metal nanoparticles to introduce many concepts relevant totransport through films of ligand-separated quantum dots.A. Metallic Nanocrystals: Charging, Disorder, CouplingAlthough a metallic nanoparticle may not exhibit an inheret HOMO–LUMOgap unless it is f1 nm in diameter or smaller [86], the capacitance of a metalparticle and the quantization of charge will still create a ‘‘Coulomb gap’’ inthe single-particle density of states and endow metal nanoparticles withsingle-electron Coulomb blockade properties. Although more detailed reviewsof these effects are available elsewhere [74,87], we briefly mention theseproperties here. Figure 13 depicts a single metal particle connected to twoidentical electrodes by tunnel junctions. An electron can only tunnel betweenstates of identical energy. However, if the total capacitance between theparticle and its surroundings is C, then the energy of a single additional chargeon the particle will be e 2 /2C away from the Fermi level of the electrons in themetal nanoparticle. Thus, a bias must be applied sufficient to raise the energyof the electrons in the lead to the energy they would occupy on the charged dotbefore any can tunnel onto the particle. Furthermore, when one electron doestunnel onto the dot, its presence will prevent the addition of a second charge,either until the first charge tunnels off or until the bias voltage is raised:additional electrons are thus ‘‘blockaded’’ by the presence of the first. Thetwo-terminal current–voltage curve will exhibit discontinuous increases asthe bias is made large enough to allow larger numbers of electrons to exist onthe dot at the same time. Finally, the addition of a third terminal to thestructure in Fig. 13 allows the energy levels of the particle to be gated so as toallow or prohibit single-electron tunneling events, thus forming a singleelectrontransistor. As already mentioned, the preceding discussion is onlyrelevant when k B Te 2 2C. At higher temperature, the thermal distributionof carriers in the leads will mask the discrete structure. Coulomb blockadeeffects have been the subject of extensive experimental study in both lithographically[74,87] and chemically [88–91] defined nanostructures.Transport through ensembles of particles can exhibit more complexphenomena. Middleton and Wingreen calculated that in the CoulombCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.