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.