Dual-mode ratiometric optical thermometry in NaCaY(MoO4)3:Ho3+/Nd3+/Yb3+ phosphor via simultaneous downshifting and upconversion

Abstract

Dual-mode down-/up-conversion (DC/UC) phosphors are promising substances for non-contact optical thermometry. This study generated a NaCaY(MoO₄)₃ (NCYM) phosphor tri-doped with Ho³⁺/Nd³⁺/Yb³⁺ by the sol–gel process and assessed its efficacy as a ratio-metric thermometer within the temperature range of 303 to 543 K. The crystal structure and microstructure were analyzed, and the luminescence characteristics and energy-transfer mechanisms were examined under 325 nm (DC) and 980 nm (UC). With excitation 325 nm excitation, the emission peak exhibit minimal variation with temperature; however, the relative intensities are modified owing to thermal quenching. DC temperature sensing was accomplished by two intensity ratios: Ho³⁺ (I₅₄₂/I₆₆₀) and Yb³⁺/Ho³⁺ (I₉₇₅/I₆₆₀), yielding maximal relative sensitivities of approximately 0.44% K⁻¹ and 1.25% K⁻¹, respectively. For UC thermometry, two supplementary ratios were selected: a Ho³⁺-based visible ratio (I₆₆₀/I₅₄₂) and Nd³⁺-based NIR-I ratio (I₈₀₆/I₈₇₃). The UC mode was found to enhance the thermometric performance, achieving maximum relative sensitivities of around 0.50% K⁻¹ for Ho³⁺ and approximately 1.06% K⁻¹ for Nd³⁺, with a temperature uncertainty δT < 1.42 K throughout the 303–543 K range. In addition, the phosphor exhibited excellent thermal stability and repeatability over successive heating–cooling cycles, with a repeatability value reaching 98%. These results demonstrate that NCYM:Ho³⁺/Nd³⁺/Yb³⁺ is a robust dual-mode, multi-ratiometric phosphor for accurate optical thermometry in the 303–543 K range.