The dual-model up/down-conversion green luminescence of NaSrGd(MoO4)3: Er3+ and its application for temperature sensing

Abstract

Er³⁺-doped phosphors are widely used as dual-functional optical thermometers due to their distinctive up/down-conversion luminescence and the thermally coupled energy states (²H_11/2 and ⁴S_3/2) of Er³⁺. The development of high-performance Er³⁺-activated optical thermometers is both an intriguing subject and a formidable challenge in the field. This article investigates the up/down-conversion (UC and DC) photoluminescence properties of NaSrGd(MoO₄)₃ (NSGM): Er³⁺. When excited at 375 and 975 nm, the phosphors emit peaks at 530, 550, and 657 nm, corresponding to the ²H_11/2, ⁴S_3/2, and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺, with the ⁴S_3/2 → ⁴I_15/2 transition displaying the highest intensity. The optical properties are comprehensively studied through UV-visible absorption, PL spectroscopy, and PLE spectroscopy. Optimal luminescence intensity is achieved at an Er³⁺ concentration of 4% mol. The resulting chromatic coordinates (x, y) and high correlated color temperature (CCT) values of the doped phosphors yield thermally stable cold emissions in the green region, boasting color purities of approximately 98.76% and 80.74% for DC and UC conversion, respectively. The optical temperature sensing properties of thermally coupled energetic states are explored based on the fluorescence intensity ratio principle. NSGM: 0.04Er³⁺, under 375 nm light excitation, demonstrates the maximum relative sensitivity of 0.87%/K⁻¹ at 298.15 K, spanning a wide temperature range from 298.15 to 488.15 K. Conversely, under 975 nm light excitation, NSGM: 0.04Er³⁺ exhibits the maximum relative sensitivity of 0.63%/K⁻¹ over the same temperature range, with temperature uncertainty (δT) less than 0.50 K and repeatability (R) (more than 98%). These findings position this material as a promising candidate for optical thermometer applications. The optical heating capacity of the synthesised phosphor is also determined using optical thermometry results, and heat generation up to approximately 457 K is found, indicating that NSGM: 0.04Er³⁺ could be useful for photo-thermal therapy.