Understanding the transport mechanism of charge carriers in films comprised of nanoparticles is important for the development of technological applications for nanoparticle assemblies. In many cases, these nanoparticles can be thought of as conducting grains embedded within an insulating matrix. If the carrier concentration in the conducting grains is above the Mott transition, then such composite materials can be modeled as granular metals. Beloborodov et al. have established a rigorous theory that relates the charge carrier transport mechanism within the film, to the contact resistance between particles.1 We have experimentally established control over the contact resistance by controlling the contact radius between particles using atomic layer deposition (ALD).
Our initial studies have focused upon charge transport mechanisms in thin films comprised of 7 nm ZnO nanoparticles abutted against one another and embedded in an Al2O3 matrix. The contact radius between ZnO particles was controlled by coating the nanocrystal assembly with a small amount (0.2 to 3.0 nm) of ZnO by ALD before filling in the porous film with Al2O3. When the contact radius between particles was much smaller than the Bohr donor radius, the contact resistance between particles was much larger than the quantum resistance, and therefore charge transport is expected to occur in the dielectric regime. When the contact radius was much larger than the Bohr donor radius, the contact resistance was much smaller than the quantum resistance and therefore charge transport occurs in the disordered metallic regime.
The theoretical framework of Beloborodov et al. for granular metals was experimentally tested by measuring the resistivity temperature dependence for films comprised of nanocrystals that were expected to be in different regimes of charge transport, as determined from the interparticle contact resistance at room temperature. In the insulating regime, wherein the interparticle contact resistance is greater than the quantum resistance, the resistivity is expected to follow the stretched exponential temperature dependence of Efros-Shklovskii variable range hopping. When the interparticle contact resistance decreases to less than the quantum resistance, the film behaves as a disordered metal. In the metallic regime, the resistivity is expected to have a weaker, logarithmic temperature dependence and have finite value at 0 K. Our experimental results are in surprisingly good agreement with the predictions of this theory. The experimental methods used to reach this conclusion, as well as the details of the model, will be presented.
1. Beloborodov, I. S.; Lopatin, A. V.; Vinokur, V. M.; Efetov, K. B., Granular electronic systems. Reviews of Modern Physics 2007, 79, (2), 469-518.
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