Highly thermally stable upconversion in copper(II)-doped LiYF4:Yb,Er microcrystals toward ultrahigh temperature (micro)thermometers

Lanthanide-based optical ratiometric thermometer is a promising temperature detecting tool for noninvasive temperature measurements. However, owing to the thermal quenching of upconversion luminescence (UCL), the traditional temperature sensing range is limited to less than 600 K. In this work, highly thermally stable UCL is observed through introduced appropriate Cu 2+ ions into LiYF 4 :Yb/Er (20/2 mol%) microcrystals (MCs) and on which are further used for temperature sensing applications. As gradually increases the doping of Cu 2+ ions, the octahedron morphology of LiYF 4 MCs with smooth surfaces gradually aggregates smaller particles on their surface forming diamond-like micro-clusters. Based on the single-particle spectroscopy technique , the temperature-dependent UCL properties were systematically investigated and demonstrated. Doping of Cu 2+ ions can efficiently adjust and enhance the UCL in the LiYF 4 :Yb/Er (20/2 mol%) MCs. The UCL thermal quenching temperature is less than ~670 K for Cu 2+ -free MCs. In contrast, the quenching temperature extends to as high as ~870 K when doping with 2 mol% Cu 2+ ions. Furthermore, these highly thermal stable MCs are utilized to detect the temperature in a wide temperature ranging from 298 K to 873 K. A maximum relative sensitivity ( S r ) of 1.23%K −1 at 298 K is obtained and temperature uncertainty δ T < 0.091 K is determined under the measured temperature for the 2 mol% Cu 2+ -doped MCs. The highly thermally stable UCL properties and excellent thermometer performance of these MCs, particularly conducted at single-particle spectroscopy technique , can be potentially applied to micro-scale thermometers.