ZnGa_2-xEu_xO₄ nanoparticles: 10 minutes microwave synthesis, thermal tuning of Eu³⁺ site distribution and photophysical properties

Microwave-assisted synthesis of red-emitting ZnGa_2-xEu_xO₄ (x = 0.005, 0.008, 0.010, 0.012, 0.015) nanoparticles is reported. Salient features of this method are: 10 min are sufficient to obtain well-crystallized, phase-pure nanoparticles; synthesis upto 2 g/batch can be accomplished with high yields (>90%) at a low temperature of 200 °C; the obtained nanoparticles are near-spherical and of ∼7 nm size as seen by HR-TEM; as-prepared nanoparticles show intense red emission at 615 nm due to the ⁵D₀ → ⁷F₂ transition, when excited at 395 nm. The structural and optical properties of the ZnGa_2-xEu_xO₄ were studied by XRD, HR-TEM, FE-SEM, FT-IR and time-resolved photoluminescence (PL) spectroscopy. ZnGa_1.99Eu_0.01O₄ nanoparticles, which showed the most intense red emission, were studied for the effect of annealing (in air) on their optical properties. Annealing at up to 1000 °C (i) enhanced crystallinity and the crystallite size increased from ∼7 nm in the as-prepared material to ∼46 nm (ii) enhanced emission intensity, and moved the CIE coordinates towards red. Photoluminescence decay curves displayed two lifetimes, implying the distribution of Eu³⁺ (i) on particle surface (ii) within the host lattice; the distribution gradually changes with annealing. The peak fitting of the emission spectra implies C_2v site symmetry around Eu³⁺. A detailed Judd–Ofelt analysis has been presented. The samples annealed at 600–800 °C showed ∼58% quantum efficiency. The color purity could be increased to 87% by varying the Eu³⁺ content. Our report demonstrates the efficacy of swift microwave-assisted synthesis (10 min) in providing a red phosphor of high color purity suitable for LEDs and display applications.