Blue emission of ZnGa₂O₄ nanoparticles dispersed in fluoride thin films via sol-gel process

Nanocomposite thin films where ZnGa₂O₄ nanoparticles were dispersed in fluorides, MgF₂ or LaF₃, were prepared by a sol-gel method using metal acetates and nitrates as starting reagents and trifluoroacetatic acid as a fluorine source. ZnGa₂O₄ and the fluorides such as MgF₂ and LaF₃ were formed simultaneously after the heat treatment of the spin-coated gel films as evidenced by X-ray diffraction. The films were transparent in the visible region. With decreasing the ZnGa₂O₄ content, absorption edges showed blue-shift. All the films exhibited a broad-band blue emission around 430 nm by ultra-violet excitation at 254 nm. The intensity of the blue emission in the PL spectra increased with decreasing the ZnGa₂O₄ content. These phenomena were attributed to the quantum size effects (the increased band-gap) and the surface effects (the increased luminescent center) in the nano-sized ZnGa₂O₄. The mechanism of the emission was discussed in view of surface states in the oxide nanoparticles.