Nd³⁺, Yb³⁺:YF₃ Optical Temperature Nanosensors Operating in the Biological Windows

This work is devoted to the study of thermometric performances of Nd³⁺ (0.1 or 0.5 mol.%), Yb³⁺ (X%):YF₃ nanoparticles. Temperature sensitivity of spectral shape is related to the phonon-assisted nature of energy transfer (PAET) between Nd³⁺ and Yb³⁺). However, in the case of single-doped Nd³⁺ (0.1 or 0.5 mol.%):YF₃ nanoparticles, luminescence decay time (LDT) of ⁴F_3/2 level of Nd³⁺ in Nd³⁺ (0.5 mol.%):YF₃ decreases with the temperature decrease. In turn, luminescence decay time in Nd³⁺ (0.1 mol.%):YF₃ sample remains constant. It was proposed, that at 0.5 mol.% the cross-relaxation (CR) between Nd³⁺ ions takes place in contradistinction from 0.1 mol.% Nd³⁺ concentration. The decrease of LDT with temperature is explained by the decrease of distances between Nd³⁺ with temperature that leads to the increase of cross-relaxation efficiency. It was suggested, that the presence of both CR and PAET processes in the studied system (Nd³⁺ (0.5 mol.%), Yb³⁺ (X%):YF₃) nanoparticles provides higher temperature sensitivity compared to the systems having one process (Nd³⁺ (0.1 mol.%), Yb³⁺ (X%):YF₃). The experimental results confirmed this suggestion. The maximum relative temperature sensitivity was 0.9%·K^-1 at 80 K.