Excitation Mechanism Rearrangement in Yb³⁺-Introduced La₂O₂S:Er³⁺/Polyacrylonitrile Photon-Upconverted Nanofibers for Optical Temperature Sensors

(La 1– x – y Er x Yb y ) 2 O 2 S polyacrylonitrile (LOS-EY/PAN) nanofibers enable the possibility of optical temperature sensing in microregions by introducing a dual temperature feedback fluorescence intensity ratio approach. A ratiometric strategy for the excitation of Er 3+ ions by a special upconversion (UC) process is proposed. The green and red UC emissions of Er 3+ single doping are confirmed to be unusual three-photon excitation processes attributed to cross-relaxation (CR) of Er 3+ ion pairs. When Yb 3+ is introduced, the CR process is forced to interrupt and energy is absorbed monopolistically by Yb 3+ due to the huge absorption cross section of Yb 3+ compared to that of Er 3+; therefore, energy transfer from Yb 3+ to Er 3+ plays a dominant role, and the excitation process is transformed into a conventional two-photon process. With the rearrangement of the excitation mechanism, the overall increase in Er 3+ emission is accompanied by an enhancement in the total quantum yield from 8.31 × 10 –5 to 1.23 × 10 –3 . Significant fluorescence sensitization improves the signal-to-noise ratio in practical applications and keeps the relative sensitivity at a high level of 1.29% K –1, further demonstrating the excellent self-feedback nano-temperature sensing function of LOS-EY/PAN fibers promising as a flexible sensing element in fabrics and biomedical devices.