Visualizing current flow at the mesoscale in disordered assemblies of touching semiconductor nanocrystals

Abstract

The transport of electrons through assemblies of nanocrystals is important for the performance of these materials in optoelectronic applications. The transport of electrons has primarily been studied by focusing on single nanocrystals or transitions between pairs of nanocrystals. There is a gap in knowledge of how large numbers of nanocrystals in an assembly behave collectively, and how this collective behavior manifests at the mesoscale. In this work, the transport of electrons in assemblies of touching, heavily doped ZnO nanocrystals was visualized as a function of temperature at the mesoscale theoretically using the model of Skinner, Chen and Shklovskii (SCS); and experimentally by conductive atomic force microscopy on ultrathin films only a few particle layers thick. Agreement was obtained between the model and experiments, with a few notable exceptions. The SCS model predicts that a single network within the nanocrystal assembly, comprised of sites connected by small resistances, dominates conduction - namely the well-known optimum band from variable range hopping theory. However, our experiments revealed that in addition to the optimum band, there are subnetworks that appear as additional peaks in the resistance histogram; which were not observed in the model calculations. Furthermore, the connections of these subnetworks to the optimum band changes in time. As time proceeds, some isolated subnetworks become connected to the optimum band; while some of the connected subnetworks become disconnected and then isolated from the optimum band. The subset of nanocrystals comprising the optimum band is dynamic.