Thermal sensing in NIR to visible upconversion of Ho3+ and Yb3+ doped yttrium oxyfluoride phosphor

Rare-earth (RE) oxyfluorides compatible for RE doping make a path for efficient near-infrared (NIR) to visible upconversion (UC). This paper describes the efficient use of YOF:Ho3+/Yb3+ phosphor nanoparticles synthesised by the Pechini sol-gel method as thermal sensors using UC in the visible region. Material characterisations such as X-ray diffraction, Raman spectroscopy and scanning electron microscopy (SEM), revealed the structural information of the material and its prominence for RE doping. Average size of 44 nm obtained from SEM with maximum phonon energy of 480 cm− 1 observed from Raman measurements assure the use of RE doped YOF for UC. Excitation of YOF:Ho3+/Yb3+ with NIR (980 nm) revealed intense UC emission in the visible and DS in the NIR region. Although Ho3+ and Yb3+ ions do not have resonance energy levels at 980 nm, three two-photon UC emission bands centred at 538, 655 and 755 nm from 5 F4+5 S2→5 I8, ( 5 F5→5 I8+5 F3→5 I7) and 5 F4+5 S2→5 I7 transitions were obtained through phonon-assisted energy transfer. For the evaluation of thermal sensitivities using 980 nm excitation, fluorescence intensity ratio (FIR) was used based on the thermally non-coupled levels in the visible UC bands between the temperatures 20–300 K. The relative sensitivities (SR) which is the measure of temperature sensing ability of YOF materials with 1 mol % Ho3+ and 10 mol % Yb3+ was found to be 0.32 for UC in visible region was achieved with 980 nm excitation at 300 K.