Investigation on anomalous thermal enhancement and temperature sensing properties of Zn3Mo2O9:Yb3+/RE3+ (RE = Er/Ho) phosphors

Abstract

In this work, Yb³⁺/RE³⁺ (RE = Er/Ho) co-doped Zn₃Mo₂O₉ phosphors were synthesized by high-temperature solid-state reactions. Under 980 nm excitation, the upconversion (UC) luminescence thermal enhancement was obtained for Zn₃Mo₂O₉:Yb³⁺/RE³⁺ phosphors. The green emission intensity of the Zn₃Mo₂O₉:Yb³⁺/Er³⁺ sample was increased 5 times from 373 to 573 K. The red emission intensity of the Zn₃Mo₂O₉:Yb³⁺/Ho³⁺ sample was enhanced 7.92 times. The anomalous thermal enhancement of UC emission was induced by the negative thermal expansion (NTE) of the Zn₃Mo₂O₉ host. The energy transfer rate from the sensitizer (Yb³⁺) to the activator (RE³⁺) was enhanced because of the lattice contraction and distortion for NTE materials. Compared with the UC emission of Er³⁺single doped Zn₃Mo₂O₉ sample, the luminescence thermal enhancement was absent, which contributed to proving the physical mechanism. The temperature sensing properties of the Zn₃Mo₂O₉:Yb³⁺/Er³⁺ and Zn₃Mo₂O₉:Yb³⁺/Ho³⁺ samples were also investigated based on the fluorescence intensity ratio (FIR) technology. The absolute sensitivity (S_A) and relative sensitivity (S_R) of Zn₃Mo₂O₉:Yb³⁺/Er³⁺ phosphor reached 0.0060 K⁻¹ and 0.72% K⁻¹, which is based on the thermal coupling levels (²H_11/2, ⁴S_3/2) FIR of Er³⁺ ions. In addition, the S_A and S_R of Zn₃Mo₂O₉:Yb³⁺/Ho³⁺ phosphor reached 0.0119 K⁻¹ and 0.86% K⁻¹, that is based on the non-thermal coupling levels (⁵S₂/⁵F₄, ⁵F₅) FIR of Ho³⁺ ions. The research results indicate that the Zn₃Mo₂O₉ host shows NET. The Yb³⁺/RE³⁺ co-doped Zn₃Mo₂O₉ phosphors are good materials for highly sensitive optical temperature measurement, which can be used to develop thermally enhanced ratiometric optical thermometers.