Rapid synthesis of nanocrystalline ZnGa 2O 4 phosphor at low temperature

A novel microwave-assisted synthesis technique was used for the rapid preparation of nanocrystalline ZnGa ₂O ₄ at two different temperatures. The crystalline spinel oxide is formed at temperatures as low as 100°C within few minutes, at a high yield of 96%, requiring no post-synthesis annealing. The as-prepared samples are polycrystalline and phase-pure as verified by XRD, with a crystallite size of ∼5 nm. Polycrystalline ZnGa ₂O ₄ substituted with Mn ²⁺, Cr ³⁺, Cu ²⁺, and Co ²⁺ was also similarly prepared. All samples are highly monodispersed, as measured by TEM. The ZnGa ₂O ₄ nanocrystals without further surface modification can be readily dispersed in chloroform to form a fully transparent colloidal solution, using which the bandgap of ZnGa ₂O ₄ was determined to be ∼4.5 eV. The entire synthesis procedure, including solution preparation, microwave irradiation, and centrifugation takes about 30 minutes, which is faster than any procedure reported for a complex oxide like ZnGa ₂O ₄, as well as one with a small thermal budget. Photoluminescence shows a broad emission extending from 330 nm to 800 nm, which is surmised to be due to the defect structure in the oxide produced.