Ca7Mg1.8Zn0.2Ga6O18:Bi3+/Mn4+ phosphor: energy transfer enables tunable colour emission and high sensitivity in optical thermometry

Abstract

Recent advancements in optical thermometry utilizing the fluorescence intensity ratio (FIR) technology have attracted tremendous attention in real-time temperature measurements due to its numerous advantages, such as high sensitivity. In this work, we rationally designed an FIR-type thermometer by utilizing the distinctive thermal responses of the blue and far-red emissions from Bi3+ and Mn4+ activators, respectively. To achieve a broad range of FIR values, we first optimized the blue emission of Ca7Mg2-xZnxGa6O18:0.05Bi3+ (CMZxGO:0.05Bi3+) through Zn2+-to-Mg2+ substitution and then utilized Bi3+/Mn4+ codoping into the optimal composition to realize dual emission. The blue emission of CMZxGO:0.05Bi3+ was improved significantly, which is associated with complex spectroscopic red and blue shifts due to the competitive effect of crystal field splitting and centroid shift for Bi3+-emitters. Additionally, Bi3+/Mn4+-codoped CMZ0.2GO:0.05Bi3+/yMn4+ phosphors exhibit tunable colour emission from blue to deep-red, arising from an increased energy transfer efficiency from Bi3+ to Mn4+ upon increasing the Mn4+-content. Benefitting from this energy transfer, Bi3+ and Mn4+ activators display severe thermal quenching and high thermal stability, respectively, thereby resulting in a high relative sensitivity (Sr) of 1.64% K-1 to temperature measurement. This Sr value surpasses that of most Bi3+/Mn4+- and Bi3+/Eu3+-codoped optical thermometers, implying the promising application potential of CMZ0.2GO:0.05Bi3+/0.001Mn4+ as an optical thermometer.