Theoretical optical properties of composite metal–NiO films

This paper reports theoretical optical properties of composite metal (Cu, Ag, Au, Ni, Pd and Pt) nickel oxide thin films, aiming to create a new electrochromic material. Selective absorption was verified only in composites with metals belonging to the IB column of the periodic table. In particular, emphasis was given to the spectral transmission, reflection and absorption, between 0.4 and 2.5 µm, of Au–NiO composite thin films, that showed the narrowest absorption band in the visible spectrum. Au–CoO films presented similar results. The calculations were based on the Maxwell–Garnett effective medium theory, with Au fill factors, f, varying from 0.0 to 0.2. The Au configurations were modelled to be isolated single spheres (ISS), isolated double spheres, single linear chain (SLC), double linear chain and fcc clusters (FCCs). Independent of the aggregate state, the increase in the fill factor enhances the selective absorption and broadens the absorption band of the films. Films with different thicknesses (0.1–0.3 µm), but with the same amount of Au particles per unit area and aggregate state, showed distinct interference patterns in the transmission and reflection spectra. The absorption spectra were almost insensitive to thickness, being very dependent on the gold content and configuration. The tailoring of the optical properties in the visible region of the Au–NiO films was achieved by controlling the Au configuration. A combination of ISS (25%), SLC (25%) and FCC (50%) was also simulated.