Regulating the upconversion luminescence properties of Tm3+/Yb3+-codoped ZrScW2PO12 microparticles with a negative thermal expansion effect through thermal stimulation for optical thermometry

Abstract

Tm³⁺/Yb³⁺-codoped ZrScW₂PO₁₂ microparticles were prepared in order to solve the problems of the severe thermal quenching and unsatisfactory thermometric properties of most luminescent materials. The synthesized materials exhibit a rarely observed negative thermal expansion (NTE) effect, which was verified by in situ X-ray diffraction experiments, performed under high temperature conditions. Upon excitation with a 980 nm laser, bright blue upconversion (UC) emissions originating from Tm³⁺ were observed. Moreover, owing to the promoted energy transfer, cross-relaxation and non-radiative decay processes at high temperatures triggered by the NTE effect, the observed UC emissions arising from ¹G₄ and ³F_2,3 levels show non-monotonic responses to temperature. By analysing the temperature-dependent luminescence intensity ratio of these UC emissions originating from the non-thermally coupled levels of Tm³⁺ (¹G₄ and ³F_2,3), the thermometric properties of the prepared microparticles were investigated in detail. Interestingly, the maximum absolute and relative sensitivities of the synthesized compounds are 0.09 and 1.45% K⁻¹, respectively, which are independent of Yb³⁺ content, but they can be manipulated by employing different sensing modes. Our results manifest that the exploitation of the NTE effect is an efficient way to control the UC luminescence features of rare earth ions and to realize high performance optical thermometry.