Sensitivity Enhancement of the Tb3+-Based Single Band Ratiometric Luminescent Thermometry by the Metal-to-Metal Charge Transfer Process

An improvement of the thermometric performance of a single band ratiometric luminescent thermometer by the boosting of its relative sensitivity can be executed either by an increase of the probability of the excited state absorption or by the enhancement of the luminescence thermal quenching upon ground state absorption. In this article, to achieve the latter, the metal-to-metal charge transfer process has been involved. To evaluate the utility of this approach, spectroscopic properties of the KGd(WO4)2 tungstate doped with different concentrations of Tb3+ ions were investigated in a wide temperature range. It was found that the Tb3+ luminescence was efficiently quenched at elevated temperatures because of the metal-to-metal charge transfer process despite the large energy gap between the emitting level 5D4 and the level below 7F0 (14800 cm–1). The probability of the metal-to-metal charge transfer process was found to be dependent on dopant amount, and the maximal relative sensitivity as high as 1.63%/°C was obtained for an optimal 80% of Tb3+ concentration. The use of the metal-to-metal charge transfer phenomenon and the study of the influence of the dopant ion concentration on this process is an idea that can also significantly improve single band ratiometric nanothermometers based on other optically active ions.