Upconversion Properties and Temperature-Sensing Behaviors of Alkaline-Earth-Metal Scandate Nanocrystals Doped with Er³⁺/Yb³⁺Ions in the Presence of Alkali Ions (Li⁺, Na⁺, and K⁺)

Temperature-sensing media based on thefluorescence intensity ratio (FIR) ofupconversion materials that suffer from low sensitivity owing to the small energy gap still have a needfor new compounds with strong upconversion luminescence (UCL). In this work, a series ofMSc2O4:Er3+/Yb3+(M = Mg, Ca, Sr, and Ba) nanocrystals were prepared by a hydrothermal methodusing NaOH alkaline solution. The structure, morphology, and UCL characteristics of materials wereinvestigated, and the red UCL of the CaSc2O4:Er3+/Yb3+sample was dramatically enhanced by a factorof similar to 12,similar to 23, and similar to 2000 compared with SrSc2O4, MgSc2O4, and BaSc2O4samples, respectively. Byadjusting alkali ions (Li+,Na+,K+), the UCL intensities of CaSc2O4:Er3+/Yb3+and SrSc2O4:Er3+/Yb3+samples were further improved, especially in the presence of Li+ions. Excellent temperature-sensingbehaviors are realized for CaSc2O4:Er3+/Yb3+and SrSc2O4:Er3+/Yb3+samples in the presence of Li+ions, in which the maximum absolute sensitivitySAvalues are about 0.0041 and 0.0036 K-1at 600 Kand the corresponding relative sensitivitySRvalues are expressed as 1197/T2and 1129/T2(the currentoptimalSR= 1289/T2), respectively. The intense UCL and excellentSAandSRvalues indicate thatCaSc2O4:Er3+/Yb3+and SrSc2O4:Er3+/Yb3+materials are promising candidates for application in high-temperature sensors workingunder 980 nm excitation