Yb3+ Sensitized Er3+ Upconversion Phosphors for Noninvasive Luminescence-Based Optical Thermometry

Abstract

Rare earth ion doped Zn₂P₂O₇ phosphor was synthesized via a solid‑state reaction route. XRD measurements confirmed the formation of a single‑phase Zn₂P₂O₇ lattice. Sensitization of Er³⁺ (⁴I_13/2→⁴I_15/2) near‑infrared emission by Yb³⁺ ions via an energy transfer process under 980 nm excitation is discussed. Energy transfer upconversion (ETU) was also investigated in Er³⁺ and Er³⁺/Yb³⁺ co‑doped Zn₂P₂O₇ phosphors. Intense upconversion (UC) emissions were observed from Er³⁺ 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 Zn₂P₂O₇:Er³⁺,Yb³⁺ as a promising candidate for the next-generation photonic and sensors applications.