Rapid annealing: minutes to enhance the green emission of the Tb³⁺-doped ZnGa₂O₄ nanophosphor with restricted grain growth

The microwave-assisted solvothermal technique (MAST) enabled the synthesis of an ultrafine nanocrystalline green-emitting Tb³⁺-doped ZnGa₂O₄ phosphor within minutes. Rapid annealing (RA) of these particles for a mere 15 min facilitated a dramatic increase in the emission intensity without grain growth. Their observed green emission corresponded to standard monochromatic primary green, which, combined with the small particle size (∼7 nm), points to their suitability for display applications. We show that the changes during RA are caused mainly during the rapid temperature excursion to the set temperature, followed by 10 min of soaking. Extended soaking or cyclic annealing improved the emission intensity. When the set temperature was 900 °C, the green emission (∼545 nm) was enhanced ∼50-fold, with the CIE coordinates moving towards green, while retaining a particle size of ∼7-9 nm. The lifetime measurements showed that the dopant exhibits two different coordination environments. Upon RA, their proportions could be tuned, leading to the observed intense green emission and an increase in the overall lifetime. Our work demonstrates a technologically viable technique for producing ZnGa_2−xTb_xO₄ - an ultrafine phosphor suitable for high-resolution display applications.