Structural and optical properties of WO3 sputtered thin films nanostructured by laser interference lithography

A study of the influence of annealing temperature on the structural, morphological and optical properties of WO3 thin films is presented. The coatings are deposited by RF reactive magnetron sputtering and characterized by XRD analysis and FESEM. The XRD diagrams of the samples show a phase transition from tetragonal to monoclinic when the annealing temperature is raised from 800 to 900 °C. Moreover, the increase of the annealing temperature to 800 °C favors the presence of a granular structure on the surface of the film. A decrease in the optical energy band gap (3.65-3.5 eV and 3.5-3.05 eV for direct and indirect transitions respectively) with annealing temperature has been measured employing Tauc's relation. Furthermore, WO3 thin films are processed by laser interference lithography (LIL) and periodic nanostructures are obtained. The processed films are characterized by a hexagonal symmetry with a period of 340 nm and the diameter of the nanostructured holes of 150 nm. These films show improved morphological properties of interest in several applications (gas sensors, photonic crystals, etc.) independent of the annealing temperature.