Tunable upconversion in ZnAl2−xGaxO4:Er,Yb phosphors by modulating the Al/Ga ratio and application in optical thermometry

Abstract

Non-contact optical thermometers based on upconversion luminescence (UCL) with high efficiency are in high demand in the areas of biomedicine, space technology, detectors and several other applications. It will be an added advantage if the same materials can display color tunability with change in composition as well as change in laser power. Due to the low sensitivity and poor discrimination ability of the thermally coupled levels, here the emission response from non-thermally coupled levels has been used for optical thermometry. Under NIR excitation, ZnAl_2−xGa_xO₄:Er,Yb phosphors exhibited UCL in the green and red region and downconversion in the NIR-II region. The most intense upconversion and downconversion as well as excited state lifetime were observed in ZnGa₂O₄:Er,Yb. Furthermore, higher oxygen vacancies in gallium rich samples (ZnGa₂O₄) and lower cation vacancies compared to Al-rich samples (ZnAl₂O₄) aids in better UCL of the former compared to the latter. Gallium insertion gradually leads to beautiful color tunability from pure red in ZnAl₂O₄:Er,Yb to orange and yellow in solid solution of intermediate composition and then finally to green in ZnGa₂O₄:Er,Yb. The maximum absolute sensitivity is found to be 11.10 × 10⁻³ K⁻¹ at 302 K, whereas the relative sensitivity is 5.02 × 10⁻³ K⁻¹ at 302 K indicating the applicability of ZnGa₂O₄:Er,Yb as an optical temperature sensor in the temperature range of 302–473 K.