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

function of the density of localized states at the Fermi level and thus, thedisorder of the nanocrystal superlattice. Beverly et al. [66] also found theconductivity increased as the nanocrystal size distribution decreased.Sampaio et al. [65] detailed the effect of interparticle spacing on theelectron transport through a nanocrystal superlattice. By lifting films of Agnanocrystals off of a LB trough at different surface pressures, they were ableto vary the interparticle spacing while keeping the nanocrystal size constant.The conductivity of the nanocrystal superlattices increased as interparticleseparation decreased. Also, the temperature of the transition from a metallikeelectron transport to a thermally activated hopping mechanism decreased asthe interparticle spacing decreased, and, along the same lines, the activationenergy in the thermally activated hopping region decreased as the interparticleseparation decreased. All of these results are consistent with an electron transportprocess that is dominated by the single-nanocrystal charging energy.Electron transport through three-dimensional nanocrystal superlatticeshas also been studied. [68]. The size and size distribution both have an effecton the conductivity. Figure 14 shows a typical interdigitated array electrodefabricated by optical lithography used to measure electron transport throughthe nanocrystal superlattice. The electrode spacing in this image is 10 Am.Compare this to the electrodes with a 100-nm separation defined by electronbeam lithography shown in Fig. 15. The silver nanocrystals were drop cast onthe substrate to form an incomplete monolayer. Doty et al. [68] observed ametal–insulator transition in the temperature-dependent dc conductivity,defined as a change in sign of the slope of the resistance versus temperature,occurred for size-monodisperse nanocrystals ordered into 3D superlattices(Fig. 16). This behavior was later confirmed for monolayers of Ag nanocrystalsas well [65]. Doty et al. [68] found that the metal-insulator transitiondoes not occur for thin films with large size distributions and topologicaldisorder (>20%). Instead, the conductivity showed semiconducting conductivityversus temperature behavior throughout the entire temperature rangestudied, as shown in Fig. 16. The TEM images in Fig. 17 show the significantdifference in nanocrystal order in the monolayer for monodisperse andpolydisperse nanocrystals. The qualitative difference in electron transportthrough spatially ordered and disordered superlattices occurs because thequantum mechanical exchange interactions between nanocrystals cannotovercome the disorder induced by the nanocrystal size distribution, and thedisordered superlattice is an Anderson insulator. The authors also found thatthe ‘‘metal–insulator transition’’ temperature decreased with increasingnanocrystal diameter, as did the activation energy for transport in the thermallyactivated hopping regime. Deposition of 3D nanocrystal superlatticeswith specified film thicknesses and height, however, is difficult and quantitativedevice-to-device comparisons are hard to make.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.