A new microwave approach for the synthesis of green emitting Mn²⁺-doped ZnAl₂O₄: A detailed study on its structural and optical properties

A simple recipe for synthesizing green emitting Mn²⁺-doped ZnAl₂O₄ phosphor has been developed. Metal-organic complexes, with their unique properties, were employed as precursors to obtain phase-pure, nanocrystalline material in the as-prepared form within just 5 min of microwave irradiation. The Mn²⁺ doping concentration that showed the highest photoluminescence (PL) intensity was optimized and a comprehensive investigation of the structural and optical properties were made for various annealing temperatures. Rietveld refinement of the samples annealed at 1200 °C and 1400 °C, showed that the cationic inversion in the spinel decreased from 3.4 to 2.1% and this change was validated by the X-ray photoelectron spectroscopy results. XPS confirmed that the inversion for Zn²⁺_, Al³⁺, and Mn²⁺ cations decreased with annealing temperature, despite of which, inversion remained at 20%, 10%, and 15%, respectively for the sample annealed at 1400 °C, emphasizing the fact that synthesis plays an important role in controlling the amount of inversion in an otherwise normal spinel. Electron paramagnetic resonance spectra of the as-prepared and the samples annealed at high temperatures confirmed that the Mn²⁺ hyperfine spectrum was not just a function of the crystal field environment but also strongly depends on the doping concentration. The PL spectrum taken at different annealing temperatures, comprised of the characteristic ⁴T₁ (G) → ⁶A₁ (S) spin-forbidden Mn²⁺ transitions, showed that the emission intensity depends on the material crystallinity. The sample annealed at 1400 °C displayed a significantly higher PL intensity compared to those annealed at lower temperatures. The variation of PL spectrum of this sample was investigated between 9 K and 300 K to determine the origins of the asymmetry at room temperature and the vibrational sidebands at lower temperatures. The energy levels of the Mn²⁺ dopant, calculated theoretically and verified experimentally, were used to determine the spectroscopic parameters such as the Racah B and C values and the crystal field energy, Dq. These values showed that the Mn²⁺ was in a weak tetrahedral field. This work demonstrates a technologically important, green, and swift technique in synthesizing phosphors for various applications in displays, bioimaging, solid state lighting, etc.