Optical Constants of Electrodeposited Mixed Molybdenum−Tungsten Oxide Films Determined by Variable-Angle Spectroscopic Ellipsometry

Mixed molybdenum−tungsten oxides of varying stoichiometry (Mo x W 1- x O 3, 0 < x < 1) prepared by cathodic electrodeposition from aqueous peroxo-polymolybdotungstate solutions on indium tin oxide (ITO) coated glass substrates were evaluated by variable-angle spectroscopic ellipsometry (VASE) and transmission measurements from 200 to 1000 nm (1.24−6.2 eV). A Tauc−Lorentz dispersion model was used to determine the real and imaginary components of the complex refractive index for Mo x W 1- x O 3 films as a function of Mo fraction. The optical band gaps were also estimated from Tauc plots. The refractive index increased (2.07−2.20 at 800 nm) while the optical band gap decreased (3.38−2.95 eV) in a linear fashion for Mo x W 1- x O 3 films with increasing Mo fraction. These trends correlate chiefly with Mo-doping-induced changes in film structure and grain size as supported by X-ray diffraction measurements.