Low-temperature impedance and dielectric relaxation of NiO nanocrystals

Low-temperatures impedance and dielectric properties of NiO nanocrystals as a function of particle size were investigated by alternative current impedance spectra. It is found that NiO nanocrystals showed distinct bulk and grain boundary conductions at low temperatures, which were almost the same within the experimental error during the decreasing and the subsequent increasing temperature processes. This result indicated that no further protons are formed during these temperature cycles. The conductivities are also highly dependent on the particle size. As the particle size reduced, the conductivities of NiO nanocrystals increased to show a maximum. Further particle size reduction less than 8.9 nm led to a sudden decrease in conductivity. The corresponding activation energies showed a minimum with varying the particle size. These observations were most likely due to the balance between the concentration of charge carriers, holes and protons.