Strong red upconversion luminescence of Yb3+/Er3+ co-doped Bi2O3 phosphors for optical thermometry

Bi 2 O 3 phosphors co-doped with Yb 3+ and Er 3+ ions were successfully synthesized using a high-temperature solid-state method. Under 980 nm near-infrared laser excitation, Bi 2 O 3 : Yb 3+ /Er 3+ phosphors emit strong red light at 658 nm and weak green light at 551 nm. The optimal doping concentrations of Yb 3+ and Er 3+ are 0.10 and 0.02, respectively. This composition results in a red-to-green intensity ratio of 157.6. The red emission is primarily driven by energy transfer, promoting Er 3+ from the 4 I 13/2 level to the 4 F 9/2 level [Yb 3+ ( 2 F 5/2 ) + Er 3+ ( 4 I 13/2 ) → Yb 3+ ( 2 F 7/2 ) + Er 3+ ( 4 F 9/2 )]. The cross-relaxation process lowers the photon population in the 4 F 7/2 level and raises the photon population in the 4 F 9/2 level, leading to the occurrence of a single-band red emission. This phosphor also demonstrates promising potential for optical temperature sensing. It exhibits a maximum relative sensitivity of 3.02 % K −1 at 298 K.