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

(La1–x–yErxYby)2O2S 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 Er3+ ions by a special upconversion (UC) process is proposed. The green and red UC emissions of Er3+ single doping are confirmed to be unusual three-photon excitation processes attributed to cross-relaxation (CR) of Er3+ ion pairs. When Yb3+ is introduced, the CR process is forced to interrupt and energy is absorbed monopolistically by Yb3+ due to the huge absorption cross section of Yb3+ compared to that of Er3+; therefore, energy transfer from Yb3+ to Er3+ 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 Er3+ 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.