Structural and optical properties of multilayered un-doped and cobalt doped TiO2 thin films

•An interesting study on optical properties of multilayer coating of TiO2 thin films.•Structural uniformity of the films inspite of multilayer coatings.•Refractive index and transmittance of Co-doped TiO2 layers layers are significantly more.•Significant blue emission in Co-doped multilayered TiO2 films. The present investigation reports the effect of multilayers (1, 3 and 7 layers) and cobalt (Co) doping on structural and optical properties like transmittance, refractive index, extinction coefficient, etc. of TiO2 thin films prepared by a sol–gel spin coating technique. X-ray diffraction (XRD) and Raman spectroscopy were used to investigate the phase and structure of the prepared films which confirm the formation of single phase anatase TiO2 structure of the films. Morphology study by SEM indicate development of cracks with increase in number of layers at large scale but in the small (nano) scale the films are compact and smooth. Thickness study by cross sectional SEM shows increase in thickness with increase in number of layers. Energy Dispersive X-ray spectra were used to study the presence of cobalt in doped films. UV–Visible spectroscopy was used to study the transmittance of the films and spectroscopic ellipsometry was used to study the optical constants like refractive index and extinction coefficient which indicate the highest refractive index and the lowest extinction coefficient for single layered cobalt doped TiO2 thin films while 7 layered films show the lowest refractive index and the highest transmittance. Spectroscopic ellipsometry studies indicate increase in packing density of coated layers in Co-doped TiO2 films due to which refractive index as well as transmittance of these films are significantly more in comparison to undoped films. Photoluminescence spectra show increase in oxygen vacancies in Co-doped multilayered TiO2 films indicating increase in blue emission. Thus, the enhanced values of refractive index as well as transmittance and blue emission in Co-doped TiO2 multilayers films are promising for optical device applications.