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

Films of passivated CdSe nanocrystals are sufficiently robust to be incorporatedinto thin-film devices, and their transport properties are described in thissection.Many of the factors governing charge transport in arrays of metalnanocrystals are clearly important to transport in these arrays of nanocrystallinesemiconductors, but there are also significant differences between transportin the two systems. The most obvious and important difference is thepresence of the semiconductor bandgap. In wide-bandgap semiconductors,the thermally generated charges which dominate the transport properties ofmost metal particle arrays will be almost entirely suppressed, and theconductivity will be dominated by carriers injected from the electrodes [33],created by photoexcitation [21], or perhaps through chemical doping [104].Another difference is that metal nanoparticles have a near continuum ofstates, modified only by the Coulomb charging energies, into which electronscan tunnel. For semiconductor nanocrystals, the spacing between the quantizedconduction-band states is on the order of a few hundred milli-electronvolts, and both the intrinsic density of states and Coulomb effects will determinethe tunneling density of states and the charge transport properties.Furthermore, the charge carrier wave functions do not extend far beyond thesurfaces of CdSe dots, and strong electronic overlap between dots is unlikelyto occur, even at high pressure [105]. Finally, the charge screening length willbe significantly longer in an array of semiconductor particles than it is in anarray of metal particles. It can be expected that long-range Coulombinteractions and space-charge effects [33] will play a much greater role inarrays of semiconductor dots.In the dark, charge carriers must be injected from an electrode beforethey can be transported through an array of CdSe nanocrystals. Because theparticles are both undoped and covered with an insulating monolayer,injection can be viewed as a tunneling problem at a metal–insulator–semiconductorinterface. For the case of electrons, injection must therefore occurby tunneling from states near, or below, the Fermi level of the metal contactinto the conduction band states of the nanocrystal. If the Fermi level of themetal lies lower in energy than the lowest quantized conduction-band states inthe nanocrystals, then no tunneling can occur. Heath and co-workers utilizedthis behavior to fabricate tunnel diodes from monolayers of CdSe nanocrystals[106]. From the I–V characteristics of their devices, they estimatedthat the lowest quantum-confined conduction-band state lies f3.6 eV belowthe vacuum level for a 3.8-nm-diameter nanocrystals, in fair agreement withthe estimate of f4.0 eV obtained via Eq. (17). We have measured theefficiency of electron injection into 200-nm-thick close-packed CdSe filmssandwiched between electrodes of several different metals [33]. Results from asimilar set of our experiments are depicted in Fig. 15. It can be seen that theCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.