Nanocasted mesoporous nanocrystalline ZnO thin films

Abstract

For the first time the formation of photoluminescent mesoporous ZnO thin films with high optical quality and low refractive index is reported. They have been thoroughly characterized via the use of a large set of modern characterisation techniques, GISAXS, HR-TEM, SEM-FEG, EEP and PL among them. These new materials gather the interesting photoluminescent properties of ZnO nanoparticle coatings with the high porosity and surface area of mesoporous network. Thin films were prepared via nanocasting using mesoporous SiO₂ thin film as a template. The process involves impregnation of a ZnCl₂ solution followed by thermally induced polycondensation and nanocrystallisation of ZnO and mild chemical removing of the silica mold. The resulting optically transparent 100 nm thick ZnO films are composed of 10 nm wurtzite photoluminescent nanoparticles, and exhibit low refractive index down to 1.19 (700 nm). Moreover due to a high accessible porosity (about 70%) the mesoporous ZnO films exhibit surface areas of 185 m²·cm⁻³, always interesting for catalytic photovoltaic and photonic properties. Indeed, high intensities of the UV and visible emission bands reveals a high amount of oxygen defects combined to small nanoparticles, which are well suited properties for applications as stimulated emitters, sensors, actuators, and lasing cavities. The presented strategy opens new avenues for the synthesis of complex mesoporous nanocrystalline metal oxides processed as optical thin films.