Enhanced optical thermometry performance based on ⁴S_3/2/⁴F_7/2 thermally-coupled levels in SrLa₂S₄:Er³⁺ phosphors

A novel SrLa₂S₄:Er³⁺ high-efficiency upconversion phosphor was prepared using a gas-solid reactive strategy with high sensitivity optical thermometry. Under excitation at 980 and 1550 nm, it exhibited bright pure green emission due to the ²H_11/2/⁴S_3/2→⁴I_15/2 transitions of Er³⁺. The green-to-red fluorescence intensity ratio I_G/I_R exceeded 120, the highest value reported to date for Er³⁺ doped samples. Additionally, under 980 nm excitation, a strong blue-green emission from Er³⁺ ions ⁴F_7/2→⁴I_15/2 transition was unexpectedly detected, a phenomenon rarely observed in normal Er³⁺ doped phosphors. Based on this discovery, a novel temperature sensor strategy utilizing thermal coupling level (⁴F_7/2/⁴S_3/2) is designed in this work, which significantly differs from conventional non-contact optical thermometry measurement based on thermal coupling level of Er³⁺ (²H_11/2/⁴S_3/2). It has been verified that thermal coupling levels ⁴F_7/2/⁴S_3/2 also obey the Boltzmann distribution and therefore can be utilized for fluorescence ratiometric thermometry. The energy gap between ⁴F_7/2 and ⁴S_3/2 level is notably larger than that between ²H_11/2 and ⁴S_3/2 level. Consequently, the temperature sensor based on the ⁴F_7/2/⁴S_3/2→⁴I_15/2 transition demonstrates significantly higher relative sensitivity with the relative sensitivity of 3.07 % K⁻¹, and absolute sensitivity of 18.27 × 10⁻³ K⁻¹ at 300 K. In contrast, the temperature sensor based on ²H_11/2/⁴S_3/2 level only achieves relative sensitivity of 1.37 % K⁻¹ and absolute sensitivity of 6.02 × 10⁻³ K⁻¹.