Using the upconversion luminescence of the CaWO4:Yb3+-X3+ (X = Er/Ho/Tm) phosphors for ratiometric thermal sensing

Based on the luminescence ratiometric method, here we systematically investigate the relative sensitivity (Sr) of three pairs of typical thermally coupled energy levels (TCELs), that are, the 2H11/2/4S3/2 levels of Er3+, the two sub-Stark 5F5 levels of Ho3+, and the 3F3/3H4 levels of Tm3+, embedded in the CaWO4 host. On the basis of the emission spectra of the CaWO4:Yb3+-X3+ (X = Er/Ho/Tm) phosphors, the gaps that separate the 2H11/2/4S3/2, the two sub-Stark 5F5, and the 3F3/3H4 TCELs are calculated to be 719, 402, and 1785 cm−1, respectively. According to the expression Sr = ΔE/kT2 (where ΔE is the energy gap, k is the Boltzmann constant and T is the absolute temperature), the maximum relative sensitivities of these three pairs of TCELs are expected to be 1.12, 0.63, and 2.79% K−1 at room temperature 303 K. However, the practical relative sensitivities 1.03, 0.23, and 0.22% K−1 are achieved at 303 K for these three pairs of TCELs accordingly. It is then demonstrated that not only at 303 K but over the whole experimental temperature range, the relative sensitivities of these three pairs of TCELs are confirmed to be overestimated to some degree. The work not only presents the practical relative sensitivities of these three pairs of TCELs, but also probably discloses a quite common phenomenon that the real relative sensitivity of TCELs is often overestimated. Therefore, it is expected to provide guidance for the practical applications of the luminescence ratiometric thermometry in the future.