Electrical conductivities of nanosheets studied by conductive atomic force microscopy

The electrical conductivities of single nanosheets of titanium oxide (TiO2), manganese oxide (MnO2), double-layered titanium perovskite oxide (GdEuTiO), niobium oxide (NbO), and graphite oxide (GO) adsorbed on HOPG were studied by conductive atomic force microscopy (C-AFM) with a Pt–Ir tip. The conduction mechanism for different types of nanosheets could be clarified by using electrodes (HOPG and Pt–Ir tip) having different work functions. While the TiO2, GdEuTiO, and NbO nanosheets showed asymmetric (rectifying) current/voltage (I/V)-profiles, those for the MnO2 and GO nanosheets were symmetric (nonrectifying). The differences in the I/V-profile indicated that the dominant electron transfer mechanism in case of TiO2, GdEuTiO, and NbO nanosheets was tunneling under reverse bias like an n-type semiconductor and that for MnO2 and GO nanosheets, having a defected structure, was hopping. Among all these nanosheets, MnO2 exhibited the highest conductivity.