Optical properties of Nb-doped TiO2 thin films prepared by sol–gel method

In this work, we studied optical properties of pure and Nb-doped TiO2 synthesized using a sol–gel method and deposited as thin films by spin-coating followed by annealing in air at 500 °C for 1 h. The surface elemental composition was derived from X-ray photoelectron spectra, while structure and surface morphology were investigated using X-ray diffraction and atomic force/scanning electron microscopy. Finally, the optical properties were investigated by means of UV–vis spectrophotometry and spectroscopic ellipsometry. The Nb content was determined from XPS measurements to vary between 1.8 and 4.3 at%. The XRD patterns of the deposited thin films, with a maximum thickness of about 56 nm, showed no diffraction peaks. As proven both by microscopy and spectroscopic ellipsometry studies doping TiO2 with Nb modified the surface morphology of the samples; the grain size is increasing while the surface roughness decreases with the increase in Nb content. This is accompanied by a decrease in the refractive index and an increase of the extinction coefficient.