Nanostructure and Optical Nonlinearity of Cobalt Oxide Thin Films

Abstract

Nanostructures and nonlinear optical properties of cobalt oxide (Co₃O₄, CoO) thin films were studied. Both the Co₃O₄ and CoO thin films consisted of grains with a 7.5 nm average grain size. Complex refractive index (ñ=n-ik) of the Co₃O₄ thin film changed from 1.91-0.85i to 2.70-0.65i by laser beam irradiation at a wavelength of 405 nm, and it also changed from 3.31-1.15i to 3.05-0.60i for λ=650 nm irradiation. On the other hand, ñ of the CoO thin films irradiated at λ=405 and 650 nm were 1.75-0.32i and 2.41-0.18i, respectively, and they were constant as a function of laser intensity. Co₃O₄ had band gap energies of 2.0 and 1.3 eV, while that of CoO was about 3.7 eV. Therefore, Co₃O₄ absorbed the irradiated laser beams and interacted with them, but CoO did not because it was transparent to them. The reversible change of ñ of the Co₃O₄ thin films by thermal treatment was also observed, and a band structural change of the Co₃O₄ thin film induced by thermal treatment was identified. The band gap energy near 2.0 eV did not show any significant changes when treated. However, the other band gap energy near 1.3 eV had a large shift, and absorption coefficient, α₀, decreased greatly when thermally treated. Therefore Co²⁺O₄ ^2- tetrahedrons in Co₃O₄ normal spinel did not change remarkably, but a drastic deformation of Co³⁺O₆ ^2- octahedrons occurred on thermal treatment, which caused the large change of refractive index of the Co₃O₄ thin film.