| Summary: | We report the procedure for hydrothermal synthesis of ultrasmall Yb<sup>3+</sup>/Tm<sup>3+</sup> co-doped Sr<sub>2</sub>LaF<sub>7</sub> (SLF) upconversion phosphors. These phosphors were synthesized by varying the concentrations of Yb<sup>3+</sup> (x = 10, 15, 20, and 25 mol%) and Tm<sup>3+</sup> (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<sup>3+</sup> and Tm<sup>3+</sup> occupy the La<sup>3+</sup> sites in the SLF host. The addition of Yb<sup>3+</sup>/Tm<sup>3+</sup> 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<sup>3+</sup>/Tm<sup>3+</sup> co-doped SLF are 20 mol% Yb<sup>3+</sup> and 1 mol% Tm<sup>3+</sup> with EDTA as the chelating agent. Under 980 nm light excitation, a strong upconversion emission of Tm<sup>3+</sup> ions around 800 nm was achieved. In addition, the experimental photoluminescence lifetime of Tm<sup>3+</sup> emission in the SLF host is reported. This study discovered that efficient near IR emission from ultrasmall Yb<sup>3+</sup>/Tm<sup>3+</sup> co-doped SLF phosphors may have potential applications in the fields of fluorescent labels in bioimaging and security applications.
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