Organic mediated synthesis of highly luminescent Li⁺ ion compensated Gd₂O₃:Eu³⁺ nanophosphors and their Judd-Ofelt analysis

Highly luminescent red emitting Gd₂O₃:Eu³⁺, Li⁺ nanophosphor has been synthesized by the solvothermal combustion of the metal-citrate complex in diethylene glycol medium. The morphology and luminescence properties of these nanophosphors are found to be highly sensitive to the extent of lithium ion compensation. It is found that lithium ions promote grain growth and alter the morphology of the Gd₂O₃:Eu³⁺ nanophosphor from nearly spherical to cobblestone like. A significant enhancement in intensity of luminescence and quantum efficiency is observed in lithium compensated nanophosphors. The highest emission intensity is observed for the Gd_1.75Eu_0.1Li_0.15O₃ nanophosphor, about 1.83 times that of Gd_1.9Eu_0.1O₃ and is attributed to the enhanced intra 4f-4f emission transitions arising from the modifications of the crystal field and distortion of the local symmetry around the europium ions. The luminescence decay profiles are found to be single exponential in nature and the lifetime measured was 1.36 ms for the Gd_1.75Eu_0.1Li_0.15O₃ nanophosphor. The chromaticity coordinates of these nanophosphors indicated high colour purity. Judd-Ofelt intensity parameters indicated that lithium compensation increases the polarization of the local environment and an increase of covalency and asymmetry around the europium ions.