Ratiometric optical thermometry using deep red luminescence from 4T2 and 2E states of Cr3+ in ZnGa2O4 host

ZnGa2O4:Cr3+ is a well-known deep red (persistent) phosphor with R-lines due to the 2E→4A2 spin-forbidden transition of Cr3+ ions between 690 and 750 nm. Recently, this system has attracted a great deal of interest for in-vivo bio-imaging. In this study, we investigated the temperature dependence of photoluminescence (PL) spectrum of ZnGa2O4:Cr3+ and the possibility of using this material for thermometry. At 80 K, the zero-phonon lines and phonon-side bands of the 2E→4A2 transition were observed in the PL spectrum. At higher temperatures above 250 K, a broad PL band was observed at around 720 nm in addition to the sharp 2E→4A2 luminescence. After determination of the Stokes shift energy between the PL and PL excitation spectra and the increase of broad luminescence band intensity with increasing temperature, we attribute this additional broad luminescence band to the spin-allowed 4T2→4A2 transition of Cr3+ ions. The fluorescence intensity ratio of these emission transitions (4T2→4A2)/(2E→4A2) shows good linearity from 270 K to 700 K in the Arrhenius plot. The activation energy needed for excitation from the 2E state to the 4T2 state of Cr3+ was estimated to be 1744 cm−1. Based on our results, we suggest that the ZnGa2O4:Cr3+ system can work well as a ratiometric thermometer with a high relative sensitivity of 2.8 %K−1 (at 310 K) in a wide temperature range.