Local charging effects in nanocrystalline CdSe films

We performed conductance atomic force microscopy (C-AFM) measurements on nanocrystalline CdSe films prepared by the chemical bath deposition technique. We found a decrease in the local conductance of these films in the vicinity of every spot on the surface to which negative voltage was locally applied via the AFM tip, which we attribute to charging. We found that the currents, measured under such a constant negative bias, decay with time and that the otherwise stable and reproducible positive (tip-bias) currents, measured around the same area, are consequently reduced. This alteration typically extends to a radius of several hundred nanometers around the point of negative-bias application, and it was found to fade on time scales of tens of minutes. We interpret these results as being due to an accumulation of electrons in the vicinity of the tip-sample contact, also yielding nonlinear I-V characteristics for positive tip voltages. This suggests that the electron transport mechanism in these films involves frequent trapping-detrapping events. The comparison with charging phenomena previously reported for various colloidal CdSe nanocrystal arrays indicates that this mechanism is generic to nanocrystalline CdSe systems, independent of structural details and method of preparation.