Determination of the refractive index of β-NaYF4/Yb3+/Er3+/Tm3+ nanocrystals using spectroscopic refractometry

A method for measuring refractive index n of nanosize particles in the visible and near-IR spectral ranges is proposed. The method is based on comparing refractive index n colloid of a colloid solution of nanoparticles in several solvents with refractive indices n solvent of corresponding pure solvents and has an accuracy of ±2 × 10−4. Upconversion nanosize phosphors (UCNPs) are synthesized in the form of a β-NaYF4 crystalline matrix doped with Yb3+, Er3+, and Tm3+ rare earth ions. UCNPs have a doped core with a diameter of 40 ± 5 nm and undoped shell with a thickness of 3–5 nm. Synthesized nanocrystals possess intense photoluminescence in the blue, green, and red spectral ranges upon excitation by IR radiation with a wavelength of 977 nm. Using a spectroscopic refractometer, the dispersion of the refractive index of β-NaYF4/Yb3+/Er3+/Tm3+ nanocrystals was measured for the first time in the spectral range of 450–1000 nm with an accuracy of ±2 × 10−4.