Solvothermal synthesis of lanthanide-doped CaF₂upconversion nanoparticles using liquid-solid-solution approach

Upconversion nanoparticles (UCNPs) capable of emitting pure green, red, blue, and white emissions have grabbed a lot of attention for bioimaging, security printing, and optoelectronic devices. Ho³⁺/Yb³⁺, Er³⁺/Yb³⁺, Tm³⁺/Yb³⁺, and Er³⁺/Tm³⁺/Yb³⁺ double- and triple-doped CaF₂ nanoparticles (NPs) were synthesized by the solvothermal method. The nanoparticleswere characterized by XRD, FTIR, HR-TEM, Raman, and photoluminescence spectroscopy. Under 980 nm, UCNPs emit red, green, blue, yellow, and white emissions by the appropriate doping of lanthanide ions.The CIE-1931 chromaticity study was carried out to determine the color coordinate and color purity of the UCNPs. The color purity of red, green, blue, and yellow emissions exceeds 90%, showing the excellent chromatic performance. The energy transfer mechanismin the sensitizer Yb³⁺ions withthe activators Ho³⁺, Er³⁺, and Tm³⁺ was successfully studied. Er³⁺/Yb³⁺ co-doped CaF₂ UCNPs were used to study the potential scope in latent fingerprint detection and security applications.