Hydrothermal Synthesis and Properties of Yb 3+ /Tm 3+ Doped Sr 2 LaF 7 Upconversion Nanoparticles.

We report the procedure for hydrothermal synthesis of ultrasmall Yb 3+ /Tm 3+ co-doped Sr 2 LaF 7 (SLF) upconversion phosphors. These phosphors were synthesized by varying the concentrations of Yb 3+ (x = 10, 15, 20, and 25 mol%) and Tm 3+ (y = 0.75, 1, 2, and 3 mol%) with the aim to analyze their emissions in the near IR spectral range. According to the detailed structural analysis, Yb 3+ and Tm 3+ occupy the La 3+ sites in the SLF host. The addition of Yb 3+ /Tm 3+ ions has a huge impact on the lattice constant, particle size, and PL emission properties of the synthesized SLF nanophosphor. The results show that the optimal dopant concentrations for upconversion luminescence of Yb 3+ /Tm 3+ co-doped SLF are 20 mol% Yb 3+ and 1 mol% Tm 3+ with EDTA as the chelating agent. Under 980 nm light excitation, a strong upconversion emission of Tm 3+ ions around 800 nm was achieved. In addition, the experimental photoluminescence lifetime of Tm 3+ emission in the SLF host is reported. This study discovered that efficient near IR emission from ultrasmall Yb 3+ /Tm 3+ co-doped SLF phosphors may have potential applications in the fields of fluorescent labels in bioimaging and security applications.