(160d) Anomalous Hall Effect in Conducting Nanoparticle Networks

To characterize the electronic performance of a material, key properties such as mobility, conductivity, and majority carrier type must to be measured. Hall Effect measurement is a powerful technique that allows determination of majority carrier concentration and type, as well as calculation of mobility if the conductivity is known. Hall Effect measurements have been very successful in characterizing ordered materials, and this technique is now quite routine. However, there is evidence to suggest that the classical relationship between Hall coefficient and carrier concentration does not always hold for conducting nanoparticle networks.    

We have developed an experimental method to prepare nanocrystal networks with controlled interparticle contact resistance, and therefore controlled charge carrier transport mechanism.  The method involves coating a ligand-free, ZnO nanoparticle assembly with a small amount (0.2 to 3.0 nm) of the same material (i.e. ZnO on ZnO) by atomic layer deposition (ALD), and then filling in remaining pores with Al₂O₃, which is also deposited by ALD.  The resulting embedded ZnO nanocrystals have local carrier concentration above the Mott transition, as measured by infrared absorbance.  The interparticle contact resistance is thus controlled by the number of ZnO ALD cycles carried out prior to Al₂O₃ infill.  By controlling the interparticle contact resistance, films can be prepared wherein the electron transport mechanism is variable range hopping, or disordered metallic conduction. 

Hall Effect measurements were performed as a function of temperature on films comprised of heavily doped ZnO nanocrystals (7 nm diameter) that had different interparticle contact resistances and different electron transport mechanisms.   During the course of these experiments we observed an anomalous Hall Effect that appeared when the transport mechanism was variable range hopping.  There are two features of this anomalous Hall Effect. First, the Hall coefficient increases super-linearly in magnitude with decreasing temperature in a range where no changes in carrier concentration are expected.  Second, the sign of the Hall coefficient, which is indicative of carrier type in ordered materials, depends on spatial direction in the film.  Surprisingly, if a few more ZnO ALD cycles were applied to decrease the contact resistance between particles such that the transport mechanism shifted into the metallic regime, the anomalous Hall Effect disappeared.  In the metallic regime, the Hall Effect fits the expectation for a disordered metal, specifically a spatially isotropic Hall coefficient that is independent of temperature.  We will present these results and additional experiments performed to identify the source of this anomalous behavior.