Optical constants and band structure of trigonal NiO

Using spectroscopic ellipsometry and transmission measurements, the authors determined the optical constants (absorption coefficient, complex refractive index, and dielectric function) of bulk trigonal NiO from 0.08 to 6.5 eV. By careful discussion of the data, elastic scattering by oxygen bubbles was ruled out and the effects of surface roughness were removed numerically to obtain an accurate dielectric function of NiO. A direct band gap of 0.85 eV was found from transmission and assigned to direct interband transitions from the Ni-O hybrid valence band states to the Ni 4s conduction band at the center of the Brillouin zone. At 4 eV, the authors find the well-known charge transfer gap from the lower to the upper Hubbard band. Several intermediate sharp peaks were also found. The temperature dependence of the NiO charge transfer gap is similar to the E₁ gap of Si between 100 and 700 K. At higher temperatures, heating NiO in vacuum leads to sublimation, which has drastic irreversible consequences for the pseudodielectric function of the sample, including a strong Ni nanoparticle plasmon peak at 2 eV.