Investigation on the upconversion luminescence and ratiometric thermal sensing of SrWO4:Yb3+/RE3+ (RE = Ho/Er) phosphors

SrWO4 phosphors doped with Ho3+(Er3+)/Yb3+ are successfully prepared by a high temperature solid-state reaction method. The upconversion (UC) luminescence properties of all the samples have been investigated under 980 nm excitation. Strong green emissions are obtained in the SrWO4:Yb3+/Ho3+ and SrWO4:Yb3+/Er3+ samples with the naked eyes. In a temperature range going from 303 K to 573 K, the UC emission spectra of the phosphors have been measured. Then the temperature sensing properties also have been discussed via fluorescence intensity ratio (FIR) technology. For the SrWO4:Yb3+/Ho3+ phosphor, the FIR technologies based on thermal coupling levels (TCLs)(5F4,5F5) and non-thermal coupling levels (non-TCLs)(5S2, 5F4/5F5) are used for investigating the sensitivity. The results show that the maximum absolute sensitivity reaches 0.0158 K−1 with non-TCLs. As for Yb3+/Er3+ codoped SrWO4 phosphor, the maximum absolute sensitivity reaches 0.013 K−1 with TCLs (2H11/2,4S5/2) at a temperature of 513 K. These significant results demonstrate that the SrWO4:Ho3+(Er3+)/Yb3+ phosphors are robust for optical temperature sensors.