Fluorescence intensity ratio analysis of upconversion Gd 2 O 2 S: Yb 3+ , Er 3+ phosphor for non-contact temperature sensing

Abstract Ratiometric optical thermometry based on thermally coupled green emission levels of Er 3+ ions in upconversion (UC) phosphors provides a sensitive approach for non-contact temperature measurement. In this work, a gadolinium oxysulfide (Gd 2 O 2 S) phosphor co-doped with 9% Yb 3+ and 1% Er 3+ is proposed for UC-based temperature sensing. Under 980 nm excitation, strong UC emissions corresponding to multiple Er 3+ transitions are observed, with intensities that are highly temperature dependent. The fluorescence intensity ratio (FIR), derived from integrated emission intensities of selected transition pairs, was analyzed as a function of temperature. By systematically analyzing FIR relationships across all observable emission pairs, only the thermally coupled levels, 2 H 11 / 2 → 4 I 15 / 2 and 4 S 3 / 2 → 4 I 15 / 2 , were found to exhibit linear Boltzmann behavior, thereby confirming the applicability of the FIR model for ratiometric thermometry. The phosphor exhibits a maximum relative thermal sensitivity of 1.71% K −1 at 273 K, a temperature uncertainty of 0.35 K at 300 K, and an FIR model error of 2.069% based on the fitted and experimentally measured energy gaps over the investigated temperature range of 273–873 K. These results indicate that the proposed phosphor is a promising candidate for high-precision, non-contact temperature sensing applications.