Upconversion Luminescence and Temperature-Sensing Characteristic of Yb(III), Er(III), and Tm(III) Codoped 12CaO·7Al₂O₃ Single Crystals

Abstract

Noncontact optical thermometers have attracted interest of scientific researchers due to their reliability and accuracy. However, the use of optical thermometers based on the luminescence intensity ratio of two thermally coupled energy levels (TCELs) is limited by the Boltzmann factor of the energy difference between them. Here, we propose a strategy to improve temperature sensing by utilizing the nonthermally coupled energy levels (NTCELs) of Tm3+ and Er3+ ions. We studied the upconversion luminescence (UCL) and temperature-sensing properties of Yb3+/Er3+/Tm3+-codoped 12CaO·7Al2O3 single crystals. The UCL color of these crystals changes from white to green as the temperature increases from 323 to 623 K, and they exhibit a large temperature dependence. The maximum relative sensitivity based on NTCELs (Tm:1G4/Er:3H11/2) is 3.073% K–1, which is approximately six times higher than that achieved using TCELs (Er3+:2H11/2/4S3/2). The maximum absolute sensitivity-based luminescence intensity ratio of NTCELs (I 800/870) is 12.5% K–1 at 323 K, which is the highest absolute sensitivity for Er3+ and Tm3+ ions. This work provides a promising strategy to realize an accurate and effective temperature sensor via tuning of dual-mode upconversion in a single crystal.