Investigation on the upconversion luminescence and ratiometric thermal sensing of SrWO 4 :Yb 3+ /RE 3+ (RE = Ho/Er) phosphors.

SrWO 4 phosphors doped with Ho 3+ (Er 3+ )/Yb 3+ 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 SrWO 4 :Yb 3+ /Ho 3+ and SrWO 4 :Yb 3+ /Er 3+ 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 SrWO 4 :Yb 3+ /Ho 3+ phosphor, the FIR technologies based on thermal coupling levels (TCLs)( 5 F 4 , 5 F 5 ) and non-thermal coupling levels (non-TCLs)( 5 S 2 , 5 F 4 / 5 F 5 ) are used for investigating the sensitivity. The results show that the maximum absolute sensitivity reaches 0.0158 K -1 with non-TCLs. As for Yb 3+ /Er 3+ codoped SrWO 4 phosphor, the maximum absolute sensitivity reaches 0.013 K -1 with TCLs ( 2 H 11/2 , 4 S 5/2 ) at a temperature of 513 K. These significant results demonstrate that the SrWO 4 :Ho 3+ (Er 3+ )/Yb 3+ phosphors are robust for optical temperature sensors.