Thermal enhancement of the upconversion luminescence of Zn2+ ion-doped KYb(MoO4)2:Er3+ phosphors for multimode temperature sensing

Abstract

Pure phase KYb(MoO₄)₂:Er³⁺ phosphors doped with different concentrations of Zn²⁺ ions were prepared by a high temperature solid phase method. Based on XRD refinement results, some Yb³⁺ ions and K⁺ ions were replaced by Zn²⁺ ions. The upconversion luminescence (UCL) spectra of 980 nm laser excitation show that KYb(MoO₄)₂:Er³⁺ emits single near-infrared light and KYb(MoO₄)₂:Er³⁺,Zn²⁺ presents multi-modal UCL emission. The optimal doping concentration of Zn²⁺ ions was 0.08. Meanwhile, the energy transfer process of UCL was revealed by optical characterization methods. The temperature sensing characteristics of the phosphor were tested in the temperature range of 298–523 K. The phosphor exhibits thermal enhancement. Variable temperature XRD from 298 K to 523 K resulted in a slight enlargement and expansion in the refinement of cell parameters and volume. This indicates that thermal enhancement is attributed to the lattice structure distortion caused by doping of Zn²⁺ ions, not caused by phase transformation and negative thermal expansion. In addition, the relative and absolute sensitivities of the phosphor were 1.1% K⁻¹ at 298 K and 1.73% K⁻¹ at 348 K, respectively. This provides a new approach and opens up new avenues for the theory of thermally enhanced luminescence.