Yb 3+concentration influence on NIR and upconversion emission and temperature sensing properties of Er 3+/Yb 3+co-doped Ta 2 O 5 particles

We synthesized Er 3+ and Yb 3+ co -doped tantalum oxide (Ta 2 O 5 ) spherical sub -micrometric particles by a facile method involving formation of tantalum glycolates after pouring in acetone. Then, we investigated how the annealing temperature and Yb 3+ concentration (0-30 mol %) affect their structural and luminescence properties. Particle sizes ranged from 146 to 258 nm for the samples annealed at 800 degrees C. XRD analysis revealed the orthorhombic L-Ta 2 O 5 crystalline structure. The different symmetry sites that the rare earth ions can occupy within the Ta 2 O 5 structure led to intense and broad emission bands centered around 1530 nm. Up -conversion measurements and the calculated number of photons for intense green and red emissions obtained with 980 and 1550 nm excitations helped us to elucidate the mechanisms involved at both excitation wavelengths. We investigated how the 0.5 mol % Er 3+ /1.5 mol % Yb 3+ co -doped Ta 2 O 5 sample performs as a primary thermometer by using the Boltzmann distribution law to predict the absolute temperature from the ratio between the Er 3+ 2 H 11/2 -> 4 I 15/2 and 4 S 3/2 -> 4 I 15/2 transitions. We obtained a maximum relative thermal sensitivity of 0.97 +/- 0.04 % K -1 and a minimum delta T of 2.41 K. The calculated repeatability was above 95%. These results show that promising nanothermometer based on Ta 2 O 5 with controlled size and morphology can be designed.