Large and constant absorption coefficient in Nb _x Ti _{1− x} O ₂ thin films throughout the visible range

We investigated the optical absorption properties of Nb _x Ti _1−x O ₂ thin films, a solid solution combining the reduced titanium and niobium oxide phases TiO ₂ and NbO _2. The optical absorption properties of Nb _x Ti _1−x O ₂ thin films prepared by pulsed laser deposition at 600 °C were shown to be large in magnitude at an almost constant value of ≈17 µm ⁻¹ . Because this large absorption coefficient is nearly independent of incident photon energy in the visible range (400–700 nm), the Nb _x Ti _1−x O ₂ thin films appear optically black. Flat and homogenous, optically black coatings like these are desirable for color isolation in flat panel displays. The origin of flat wavelength dispersion in Nb _x Ti _1−x O ₂ is the coexistence of semiconducting absorption and metallic light absorption mechanisms. Localized Nb-Nb dimers in the metallic Nb _x Ti _1−x O ₂ phase open an optical band gap which gives rise to semiconducting behavior. We show here that reduction results in strong visible light sensitization in normally transparent Nb-Ti oxides.