Color-Tunable Upconversion Luminescence and Multiple Temperature Sensing and Optical Heating Properties of Ba₃Y₄O₉:Er³⁺/Yb³⁺ Phosphors

Upconversion (UC) luminescence materials doped with rare earth ions are extensively investigated as optical temperature probes by the fluorescence intensity ratio technique. However, most Er 3+ -doped materials are still suffering from low sensing sensitivity. In the present study, we attempt to develop high-sensing-sensitivity Er 3+ -doped materials based on the thermally coupled energy levels (TCLs) from Stark sublevels as well as the properties at subzero temperatures, for which there is continuous lack of research. Er 3+ /Yb 3+ codoped Ba 3 Y 4 O 9 (BYO) phosphors were produced via a solid-state reaction. Excited by 980 nm, various output colors, including bright green, yellow, and red, in BYO:Er 3+ /Yb 3+ phosphors as well as the relative emission intensities could be regulated through altering Yb 3+ concentrations. Subsequently, on the basis of all 12 pairs of TCLs especially from Stark sublevels of 2 H 11/2, 4 S 3/2, and 4 F 9/2 of Er 3+ ions, multiple temperature-sensing performances are evaluated over a wide range of 73–573 K. The results show that the maximum sensitivity of the 2 H 11/2 and 4 S 3/2(1) levels is approximately 1-fold higher than that of traditional TCLs of 2 H 11/2 / 4 S 3/2 at elevated temperature and the maximum sensitivity based on the 2 H 11/2(1) and 2 H 11/2(2) levels is more than 12 times higher than that of the traditional TCLs of 2 H 11/2 / 4 S 3/2 at subzero temperature. Therefore, it is expected to realize high-sensitivity temperature detection from subzero to elevated temperatures by combining two pairs of different TCLs. In addition, the potential of Er 3+ /Yb 3+ codoped BYO phosphors to be used as an optical heater is studied. The generated temperature can be accurately monitored by BYO:Er 3+ /Yb 3+ phosphors and regulated by adjusting the excitation power, which indicate that BYO:Er 3+ /Yb 3+ phosphors can be used as an optical heating device.