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.