Optical characterization of anatase TiO₂ films patterned by direct ultraviolet-assisted nanoimprint lithography

A direct ultraviolet (UV)-assisted nanoimprinting procedure using photosensitive titanium di-n-butoxide bis(2-ethylhexanoate) is employed in this study for the nanopatterning of anatase titanium dioxide (TiO₂) structure. Upon annealing at 400 °C for 1 h, the lateral shrinkage and thickness shrinkage of the TiO₂ nanostructure were 39.6% and 52.5%, respectively, which indicated an anisotropic volume loss. During UV irradiation and annealing treatment, the refractive index of UV-irradiated TiO₂ film is gradually increased by improvement in the packing density and crystallinity of the film. According to increasing UV exposure time and annealing temperature, the optical band gap (E_g) of UV-irradiated TiO₂ film is red-shifted from 3.73 to 3.33 eV due to the formation of lattice defects, vacancies and voids during the photochemical reaction and due to the effect of quantum confinement during annealing treatment. These results suggest that the refractive index and optical E_g of TiO₂ nanostructure could be controlled by tuning the UV exposure time and annealing treatment conditions. Nanopatterns of TiO₂ fabricated by direct UV-assisted nanoimprint lithography are potential candidates for use in protective coatings for optical mirrors and filters, high-reflectivity mirrors, broadband interference filters and active electro-optical devices where ordered surface nanostructures could be necessary.