Zn2+- and Mn4+-Codoped K2LiB1–xB′xF6 (B, B′ = Al, Ga, In) Fluorescent Nanoparticles: Implications for Micro-LED Color Conversion and Anticounterfeiting Inks

To address issues such as uneven particle size, agglomeration, low luminescence efficiency, and poor stability in the nanosizing of Mn4+-doped fluoride red phosphors, this study combines solid solution engineering and aliovalent charge compensation to prepare high-performance K2LiB1–xB′xF6:Mn4+ (B, B′ = Al, Ga, In) nanoparticles (FNPs). Using an HF-free coprecipitation method for the synthesis of solid-solution matrices, followed by a ligand-assisted reprecipitation step that employs a small amount of HF to dissolve K2MnF6 to rapidly and uniformly incorporate Mn4+ into the matrices, and the particle size is reduced from 33.32 to 16.05 nm with improved dispersibility by adjusting the volume ratio of oleic acid (OA) to oleylamine (OAm) ligands; through the gradual solid solution of Al3+, Ga3+, and In3+, the regulatory mechanism of lattice distortion and local crystal field on luminescence is revealed, with optimal luminescence intensity achieved at specific solid-solution ratios and the emission peak blue-shifted from 636 to 632 nm. Zn2+ codoping for charge compensation enhances the luminescence intensity of the 0.9% Mn4+-doped sample by 120%, with the internal quantum efficiency (IQE) increased to 40.01%, and the fluorescence lifetime extended to 4.11 ms. The FNPs have good thermal stability (56% retention at 150 °C) and moisture resistance (85% retention after 240 h in water). Packaged white light-emitting diode (WLED) exhibits a luminous efficacy of 20.54 lm W–1, and the fluorescent ink enables red–green anticounterfeiting display under UV excitation, providing insights into applications in high-end display and lighting fields, especially micro-LED color conversion layers, WLED packaging, and anticounterfeiting/encryption inks.