Facile nanopatterning of zirconium dioxide films via direct ultraviolet-assisted nanoimprint lithography

A novel ultraviolet (UV)-assisted imprinting procedure that employs photosensitive zirconyl 2-ethylhexanoate is presented for the fabrication of both amorphous and crystalline zirconium dioxide (ZrO₂) nanostructures. Upon annealing at 400 °C for 1 h, the lateral shrinkage and thickness shrinkage of ZrO₂ nanostructures were 69.8 and 66.7%, respectively, indicating an isotropic volume loss. During UV irradiation and annealing treatment, the refractive index of UV-irradiated ZrO₂ film is gradually increased by improvement in the packing density and crystallinity of the film. With increasing UV exposure time and annealing temperature, the optical band gap (E_g) of the UV-irradiated ZrO₂ film is red-shifted from 5.745 to 5.265 eV, due to the removal of organic groups and the resultant densification of the film during the photochemical reaction and the heat-induced increase in the crystallinity of the film. These results suggest that the refractive index and optical E_g of ZrO₂ nanostructures could be controlled by tuning the conditions of UV exposure time and annealing treatment. Nanopatterns of ZrO₂, fabricated by direct UV-assisted nanoimprint lithography, are potential candidates for protective coatings for optical mirrors and filters, e.g. high-reflectivity mirrors and broadband interference filters, as well as active electro-optical devices where ordered surface nanostructures are necessary.