Defect-Engineered Mn2+, Ga3+ Co-Doped Li2ZnGeO4 Phosphors Exhibiting Long-Lasting Green Luminescence for Advanced Optoelectronic Applications

Abstract

Persistent phosphors capable of long-lasting emission after excitation are crucial for emerging display, lighting, and sensing technologies. In this study, manganese and gallium co-doped lithium zinc germanate (Li₂ZnGeO₄) phosphors were synthesized via a high-temperature solid-state route, exhibiting bright and thermally stable green persistent luminescence. Comprehensive characterization using X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) confirmed the phase purity, morphology, and chemical states of the phosphor. The oxidation state and local coordination of Ga ions were further confirmed by synchrotron-based X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses. The incorporation of gallium ions (Ga³⁺) introduced defect states within the host lattice, as supported by density functional theory (DFT) calculations, revealing a defect-assisted mechanism responsible for the prolonged afterglow. Manganese ions (Mn²⁺) occupying tetrahedral coordination sites generated intense green emission, while Ga³⁺ co-doping enhanced both the persistence duration and quantum efficiency without compromising colour purity or thermal stability. Thermoluminescence (TL) studies further validated the role of increased trap centers on Ga³⁺ co-doping in enhancing the persistent luminescence. The optimized Li₂ZnGeO₄:10% Ga³⁺,0.25% Mn²⁺ phosphor exhibited an exceptional afterglow lasting over 1800 seconds and demonstrated strong applicability in thermally stable and very bright green phosphor-converted light-emitting diodes (pc-LEDs). Overall, this work establishes a comprehensive co-doping and defect-engineering strategy, supported by synchrotron and theoretical insights, for developing next-generation, thermally stable persistent phosphors for advanced optoelectronic applications.defect states within the host lattice, as confirmed by synchrotron-based EXAFS and supported by DFT calculations, revealing a defect-assisted mechanism responsible for the long afterglow. Mn²⁺ doping produced intense green emission, while Ga³⁺ co-doping enhanced both persistence duration and quantum efficiency without compromising color purity or stability. The optimized LZGGO:0.25%Mn²⁺phosphor displayed an exceptional afterglow exceeding 1800 seconds and demonstrated strong applicability in phosphor-converted LEDs. These findings establish a robust co-doping and defect-engineering approach for designing next-generation, thermally stable persistent phosphors for advanced optoelectronic applications.