Ca-Doped Nanocrystals for Upconversion Luminescence Modulation and Their Application in Fluorescent Microspheres

This paper successfully synthesized NaGdF4:18%Yb³⁺/2%Er³⁺/x%Ca²⁺@NaYF₄ upconversion nanoparticles (UCNPs) with strong visible fluorescence using a solvothermal method. The upconversion luminescence intensity can be regulated by Ca²⁺ doping. Under 980 nm infrared excitation (at room temperature), the luminescence intensity gradually increases as the Ca²⁺ doping concentration increases from 0 mol% to 30 mol%. The intensity reaches its maximum at a doping concentration of 30 mol%, and then decreases gradually. Fluorescence spectra indicate that the emissions originate from two green light peaks at 540 nm and 520 nm. Transmission electron microscopy (TEM) was used to characterize the morphology of the samples, revealing that the synthesized NaGdF4:18%Yb³⁺/2%Er³⁺/x%Ca²⁺ @NaYF₄ upconversion nanocrystals have a hexagonal phase, uniform morphology, good particle dispersion, and an average particle size of approximately 60 nm. Based on these findings, nanocrystals with a 30 mol% doping concentration were selected for further processing and coating to enhance the luminescence intensity.