Yb ³⁺ Sensitized Er ³⁺ Upconversion Phosphors for Noninvasive Luminescence‐Based Optical Thermometry

Rare earth ion doped Zn 2 P 2 O 7 phosphor was synthesized via a solid‑state reaction route. XRD measurements confirmed the formation of a single‑phase Zn 2 P 2 O 7 lattice. Sensitization of Er 3+ ( 4 I 13/2 → 4 I 15/2 ) near‑infrared emission by Yb 3+ ions via an energy transfer process under 980 nm excitation is discussed. Energy transfer upconversion (ETU) was also investigated in Er 3+ and Er 3+ /Yb 3+ co‑doped Zn 2 P 2 O 7 phosphors. Intense upconversion (UC) emissions were observed from Er 3+ in the green (520–570 nm) and red (640–680 nm) regions under 980 nm diode laser excitation. Fluorescence intensity ratio (FIR)‐based thermometry was explored using both thermally coupled levels (TCL) and non‑thermally coupled levels (NTCL). The material exhibits a consistent temperature‑dependent luminescence response with high relative sensitivity ( S R ) over the investigated temperature range. The experimentally determined temperature resolution reaches 1 K at room temperature and 2.1 K at 390 K under practical measurement conditions, demonstrating reliable thermometric performance over a wide temperature range. These results provide critical insights into temperature‐dependent luminescence characteristics and establish Zn 2 P 2 O 7 :Er 3+ ,Yb 3+ as a promising candidate for the next‐generation photonic and sensors applications.