Imaging of electrical response of NiO_x under controlled environment with sub-25-nm resolution

The spatially resolved electrical response of polycrystalline NiO_x films, composed of 40 nm crystallites, was investigated under different relative humidity (RH) levels. The topological and electrical properties (surface potential and resistance) were characterized with sub-25-nm resolution using Kelvin probe force microscopy and conductive scanning probe microscopy under argon atmosphere with 0%, 50%, and 80% RH. The dimensionality of surface features obtained through autocorrelation analysis of topological maps increased linearly with increased RH, as water was adsorbed onto the film surface. Surface potential decreased from 280 to 100 mV and resistance decreased from 5 GΩ to 3 GΩ, in a nonlinear fashion when RH was increased from 0% to 80%. Spatially resolved surface potential and resistance of the NiO_x films was found to be heterogeneous throughout the film, with distinct surface features that grew in size from 60 to 175 nm at 0% and 80% RH levels, respectively. The heterogeneous character of the topological, surface potential, and resistance properties of the polycrystalline NiO_x film observed under dry conditions decreased with increased RH, yielding nearly homogeneous surface properties at 80% RH, suggesting that the nanoscale potential and resistance properties converge with the mesoscale properties as water is adsorbed onto the NiO_x film.