Temperature-dependent upconversion luminescence and spectra characteristic of Er³⁺/Yb³⁺ co-doped fluorotellurite glasses

One of the fundamental parameters of measurement of these sensors is the fluorescent intensity ratio (FIR) technique. The measurement of FIR requires a comparison of two emissions coming from thermally coupled levels (TCL) [14,17]. The temperature causes a variation in the FIR by inducing a redistribution of the population among the TCL, which are energetically close.

Previous studies on temperature measurement materials have mostly focused on the thermal coupling level pairs (TCL) of rare earth ions. Based on a special 4f energy level structure, phosphors that have recently been doped with Er3+ or co-doped with other ions are often selected as candidates for temperature sensor materials, for example, Er3+/Yb3+ co-doped TeO2–WO3–La2O3–Na2O (TWLN) glasses (a maximum sensitivity of 86.7 × 10−4 K−1 at 553 K) [6], Er3+/Yb3+ co-doped fluorotellurite glasses (a maximum sensitivity of 54.09 × 10−4 K−1 at 531 K) [7], Er3+/Yb3+ co-doped BaMoO4 phosphors (a maximum sensitivity of 110.7 × 10−4 K−1 at 493K) [8],Yb3+/Er3+ co-doped BaMgF4 phosphors (a maximum sensitivity of 83.2 × 10−4 K−1 at 583 K) [9], Yb3+/Er3+ co-doped Ba3Y4O9 (a maximum sensitivity of 45.8 × 10−4 K−1 at 573 K) [10], Yb3+/Er3+ doped LuVO4 (a maximum sensitivity of 0.82% at 423 K) [11], Yb3+/Er3+ doped BiPO4 (a maximum sensitivity of 0.00413 K−1 at 573 K) [12]. Unfortunately, The small energy gap between TCS (for example, 500–900 cm−1 between 2H11/2 and 4S3/2 states of Er3+) limits the improvement of relative temperature sensitivity, which leads to the deviation between the measured FIR and the actual value, leading to a larger detection error.

To acquire the high temperature sensitivity, most conventional FIR materials are concentrated on lanthanide doped phosphors, whose emissions originate from their abundant energy levels, especially the thermally coupled level (TCL) pairs [6,7], are utilized as the temperature label since the population in the upper and lower levels would change oppositely to induce the variation of their emission intensities with the changing temperature. On the basis of special 4f energy level structure, with the excitation of 980 nm laser, Er3+ doped phosphors are recently selected as possible candidates of temperature sensors materials, which illustrate a maximum sensitivity of 83.29 × 10−4 K−1 at 583 K (323–593 K) in Yb3+/Er3+ co-doped BaMgF4 with [8], 45.8 × 10−4 K−1 at 573 K (298–573 K) in Yb3+/Er3+ co-doped Ba3Y4O9 [9], 0.82% at 423 K (303–423 K) in Yb3+/Er3+ doped LuVO4 [10], 0.00413 K−1 at 573 K (313–573 K) in Yb3+/Er3+ doped BiPO4 [11], 0.65% K−1 at 498 K (313–573 K) in Yb3+/Er3+ doped Ba2In2O5 phosphors [7], 54.09 × 10−4 K−1 at 531 K (298–568 K) in Er3+/Yb3+ co-doped fluorotellurite glasses [12]. However, the narrow energy gap with the TCL pairs (e.g., 2H11/2 and 4S3/2 levels for Er3+) in these materials would induce overlap of the two monitored emission peaks, resulting in an inferior signal discriminability to limit the temperature sensitivity of materials for actual application.