Enhanced Deep-Blue Emission and Quantum Yield of Ca₀.₉₉₅YGaO₄:0.5%Bi³⁺ via Lithium-Based Flux Additives

Abstract

In this work, Bi³⁺-doped CaYGaO₄ phosphors were synthesised through solid-state reaction and systematically optimised using lithium-based fluxes to enhance their photoluminescence quantum yield (PLQY) in the blue region. Comparative studies between Li₂O and Li₂CO₃ fluxes revealed distinct structural and optical impacts. Rietveld refinement confirmed effective Bi³⁺ incorporation, with samples treated using 2% Li₂CO₃ and 1% Li₂O exhibiting superior crystallinity. SEM analysis showed molten-like particle morphologies and increased porosity in Li₂CO₃-treated samples features absent in those treated with Li₂O. XPS results highlighted a notable reduction in oxygen-related defects in Li₂CO₃-modified phosphors, correlating with improved PLQY performance. Photoluminescence measurements demonstrated strong blue emission at 430 nm under 330 nm excitation, alongside additional red (720 nm) and near-infrared (780 nm) emissions from stabilised Bi²⁺ species, attributed to lithium-induced lattice rearrangements. The sample treated with Li₂CO₃ achieved a PLQY of 70%, surpassing flux-free and Li₂O-assisted counterparts, reinforcing the pivotal role of flux chemistry in fine-tuning emission efficiency. In particular, lithium carbonate flux profoundly influences Bi³⁺ doped CaYGaO₄ phosphors, establishing flux chemistry as a versatile tool for engineering efficient blue emitters for photonic applications.