Nd 3+ , Yb 3+ :YF 3 Optical Temperature Nanosensors Operating in the Biological Windows.

This work is devoted to the study of thermometric performances of Nd 3+ (0.1 or 0.5 mol.%), Yb 3+ (X%):YF 3 nanoparticles. Temperature sensitivity of spectral shape is related to the phonon-assisted nature of energy transfer (PAET) between Nd 3+ and Yb 3+ ). However, in the case of single-doped Nd 3+ (0.1 or 0.5 mol.%):YF 3 nanoparticles, luminescence decay time (LDT) of 4 F 3/2 level of Nd 3+ in Nd 3+ (0.5 mol.%):YF 3 decreases with the temperature decrease. In turn, luminescence decay time in Nd 3+ (0.1 mol.%):YF 3 sample remains constant. It was proposed, that at 0.5 mol.% the cross-relaxation (CR) between Nd 3+ ions takes place in contradistinction from 0.1 mol.% Nd 3+ concentration. The decrease of LDT with temperature is explained by the decrease of distances between Nd 3+ 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 (Nd 3+ (0.5 mol.%), Yb 3+ (X%):YF 3 ) nanoparticles provides higher temperature sensitivity compared to the systems having one process (Nd 3+ (0.1 mol.%), Yb 3+ (X%):YF 3 ). The experimental results confirmed this suggestion. The maximum relative temperature sensitivity was 0.9%·K -1 at 80 K.