Highly-sensitive lifetime optical thermometers based on Nd3+, Yb3+:YF3 phosphors

Abstract

Optical thermometers based on Nd3+ (0.5 mol.%), Yb3+ (1.0, 2,0, 4.0, and 8.0 mol.%):YF3 nano-phosphors were studied in the 80–320 K temperature range. The Nd3+, Yb3+:YF3 phosphors were nanoparticles having 48 ± 1 nm average diameter with the orthorhombic crystal structure. The effective luminescence lifetime (τeff) of 4F3/2 of Nd3+ (4F3/2 – 4I9/2 transition, λem = 866 nm) was taken as a temperature-dependent parameter. The excitation wavelength λex = 355 nm corresponds to 4I9/2 – 4D3/2 absorption band of Nd3+ (non-resonant excitation). It was revealed that the τeff decreases with the temperature decrease in the 80–260 K range. In turn, τeff temperature dependence in the 260–320 K range is less pronounced. The temperature-dependent X-ray diffraction (XRD) experiments revealed that the XRD patterns shift toward fewer angles as temperature increases. This phenomenon was explained by the temperature expansion of the crystal lattice. At lower temperatures the distance between Nd3+ and Yb3+ is less in comparison to the distance at higher temperatures. At shorter distances the quenching of τeff Nd3+ is more efficient hence the τeff is less. It was suggested that the contribution of phonon-assisted energy transfer between Nd3+ and Yb3+ has less impact on the τeff temperature dependence. Finally, the Nd3+, Yb3+:YF3 phosphors demonstrated the highest performances in the 100–250 K range. Particularly, Nd3+ (0.5%), Yb3+ (8.0%):YF3 and Nd3+ (0.5%), Yb3+ (1.0%):YF3 samples showed the highest Sr values (0.80 μ%/K (at 80 K) and 0.58%/K (at 148 K), respectively.