Temperature dependence of the local conductance in nanocrystalline CdSe films

We have studied the local transport properties of nanocrystalline CdSe films by conductance atomic force microscopy (C-AFM) in the temperature range of 25-300 K. Surprisingly, the conductance measured under positive AFM-tip bias for the more resistive films was found to increase with decreasing temperature (i.e., "metallic-like behavior"), whereas the opposite temperature dependence was observed for the more conductive samples consisting of larger crystallites. We explain the results by considering the crystallite-size dependence of quantum-confinement and electron delocalization, which accounts also for the current decay-rate under the application of negative tip-bias.