Ultra-wide band near-infrared (NIR) optical thermometry (12–673 K) performance enhanced by Stark sublevel splitting in Er³⁺ ions near the first biological window in the PbZr_0.53Ti_0.47O₃:Er³⁺/Yb<s

The fluorescence intensity ratio (FIR) approach, which relies on thermally coupled levels (TCLs), is significantly important for optical thermometry at room temperature and above, but was found to be impractical for low temperature sensing due to limited population density (thermal) or lack of spectrum at extremely low temperatures. Herein, we report a wide temperature range (12-673 K) sensing capability of the PbZr_0.53Ti_0.47O₃:Er³⁺/Yb³⁺ (C1:PZT) phosphor utilising the bandwidth of Stark sublevel split near-infrared (NIR) emission bands as one sensing parameter and FIR as another. Motivated by our previous studies on upconversion (UC) and the promising thermometry performance of the C1:PZT phosphor for real time nanothermometer monitoring (using visible TCLs), this work extends to the same thermometry application using UC-NIR emission as TCLs. The temperature-dependent UC spectra were measured across the ranges of 12-313 K and 313-673 K under 980 nm excitation, and their sensing capabilities were thoroughly evaluated. An enveloped single emission band, comprising multiple peaks in the NIR region, was observed and subsequently deconvoluted using Gaussian fitting. These individual peaks were analyzed in relation to Stark sublevel splitting, which was particularly evident at 12 K, and the variations in their full width at half maximum (FWHM) were compared across temperatures up to 313 K. Based on the temperature-dependent bandwidth, two prominent peaks ∼11 655 cm^-1 (858 nm) and 11 454 cm^-1 (873 nm), were identified as TCL levels, and a sensitivity (<i>S</i>_r) of 0.68 ± 0.01% K^-1 at 673 K was observed, making it a suitable thermometer for reading low temperatures using NIR bands.