Fig. 1. (a) XRD patterns of Er-PTO nanoparticles. (b) Magnification of (110) diffraction peaks.

Fig. 2. SEM images of Er-PTO nanoparticles with different Er3+ doping concentrations. (a) PTO, (b) 1% Er-PTO, (c) 2% Er-PTO, (d) 3% Er-PTO, (e) 4% Er-PTO, respectively.

Fig. 3. (a) TEM, (b) HRTEM, and (c) STEM images and the corresponding EDS elemental mapping results of 1% Er-PTO sample.

Fig. 4. (a) Room temperature UCL spectra of Er-PTO (0, 1%, 2%, 3%, and 4%). The inset is the digital photographs of the 1% Er-PTO and 4% Er-PTO. (b) Plots of the integrated intensities versus the concentration of Er3+ ions. (c) Double logarithmic plots of pump power dependent emission intensities of 1% Er-PTO. (d) Energy levels of Er3+ ion and UCL mechanism of Er-PTO crystals.

Fig. 5. (a) Fluorescence emission spectra of 1% Er-PTO sample excited by 980 nm excitation with temperature from 333 K to 493 K; (b) temperature-sensing properties for 1% Er-PTO sample. The inset depicts the relationship between sensitivity and absolute temperature.