Enhanced upconversion luminescence and tuned red-to-green emission ratio of LiGdF₄ nanocrystals: Via Ca²⁺ doping

LiGdF₄ nanoparticles (NPs) with different Ca²⁺ contents were synthesized by a thermal decomposition method. The as-prepared NPs were able to emit green and strong red light when excited by a 980 nm laser through a two-photon process and they were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) spectroscopy. Rietveld refinement was performed to explore the structure and phase composition of the product. It was found that Ca²⁺ ions were vital to the successful synthesis of LiGdF₄ NPs and an appropriate Ca²⁺ content (ca. 19%) yielded the product with highest purity. The purity of the obtained product was highly correlated with the PL intensity and red/green light intensity ratio.