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⁺)

Abstract

Temperature-sensing media based on the fluorescence intensity ratio (FIR) of upconversion materials that suffer from low sensitivity owing to the small energy gap still have a need for new compounds with strong upconversion luminescence (UCL). In this work, a series of MSc₂O₄:Er³⁺/Yb³⁺ (M = Mg, Ca, Sr, and Ba) nanocrystals were prepared by a hydrothermal method using NaOH alkaline solution. The structure, morphology, and UCL characteristics of materials were investigated, and the red UCL of the CaSc₂O₄:Er³⁺/Yb³⁺ sample was dramatically enhanced by a factor of ∼12, ∼23, and ∼2000 compared with SrSc₂O₄, MgSc₂O₄, and BaSc₂O₄ samples, respectively. By adjusting 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-sensing behaviors are realized for CaSc₂O₄:Er³⁺/Yb³⁺ and SrSc₂O₄:Er³⁺/Yb³⁺ samples in the presence of Li⁺ ions, in which the maximum absolute sensitivity <i>S</i>_A values are about 0.0041 and 0.0036 K^-1 at 600 K and the corresponding relative sensitivity <i>S</i>_R values are expressed as 1197/T² and 1129/T² (the current optimal <i>S</i>_R = 1289/T²), respectively. The intense UCL and excellent <i>S</i>_A and <i>S</i>_R values indicate that CaSc₂O₄:Er³⁺/Yb³⁺ and SrSc₂O₄:Er³⁺/Yb³⁺ materials are promising candidates for application in high-temperature sensors working under 980 nm excitation.