Abstract
A solvothermal method was used to prepare a series of Yb
3+
/Tm
3+
/Ca
2+
co-doped NaYF
4
nanoparticles with different Ca
2+
contents. Strong upconversion blue fluorescence could be observed under 980 nm laser excitation of the samples. The effect of different Ca
2+
contents on the luminescence intensity was investigated, and it was found that the UV-vis upconversion luminescence increased and then decreased with an increasing Ca
2+
concentration during the increase of the Ca
2+
content from 0 mol% to 25 mol%, reaching the strongest fluorescence at 15 mol%, which was up to about 28 times stronger than that without Ca
2+
doping. Furthermore, the mechanism was investigated, and it was found that the doping of Ca
2+
disrupted the symmetry of the crystal field, resulting in a significant enhancement of the overall fluorescence. Applied to fluorescence intensity ratio thermometry, the absolute and relative sensitivities are as high as 0.0418 K
−1
and 2.31% K
−1
, respectively, with a minimum temperature resolution of 0.0129 K.