Multifunctional dual-color BaYAlZn₃O₇: Bi³⁺/Eu³⁺ phosphors: A green route to luminescence thermometry, plant-growth LEDs and forensic applications

Bi³⁺ doped and Eu³⁺ co-doped BaYAlZn₃O₇ (BYAZO) phosphors are synthesized via an eco-friendly solution combustion method utilizing Spinach leaf extract as a natural fuel source. Structural characterization confirmed the formation of a hexagonal phase with a P6₃mc (186) space group. The optimized phosphor composition, BYAZO:0.03Bi³⁺/0.03Eu³⁺, exhibited dual photoluminescence (PL) emission behaviour under 305 nm excitation, producing characteristic Bi³⁺ and Eu³⁺ emissions at 410 nm and 610 nm, respectively. Notably, this sample achieved an internal quantum efficiency of 80.97 % and a correlated color temperature (CCT) of 2822 K, indicating strong potential for white light-emitting diode (w-LED) applications. The energy transfer efficiency from Bi³⁺ to Eu³⁺ is determined to be 87 %, facilitated by dipole–dipole interactions. Furthermore, the co-doped phosphors demonstrated remarkable temperature sensing capabilities, with a maximum relative sensitivity (S_R) of 1.56 %K⁻¹ over a temperature range of 303–573 K. A visible thermochromic shift from pink to blue is observed with increasing temperature, highlighting their suitability for thermal indicator applications. When incorporated into LED devices, the phosphors exhibited stable luminescence and proved effective in promoting cowpea seed growth, resulting in enhanced biomass and yield. Additionally, these materials are employed in the preparation of security inks with tunable emission across various substrates under ultra-violet (UV) 365 nm light, affirming their utility in anti-counterfeiting (AC). The phosphors also enabled high-resolution latent fingerprints (LFPs) visualization on multiple surfaces, including glass, leaves, and plastic cards. Owing to their multifunctional performance, high efficiency, and eco-friendly synthesis, these phosphors present a promising platform for use in sensing, solid-state lighting, security, and forensic applications.