Enhancement of the sensitivity of single band ratiometric luminescent nanothermometers based on Tb3+ ions through activation of the cross relaxation process

Abstract

The description of luminescent processes and their thermally induced changes, that may be also influenced by the optically active ions concentration, and thus by the various inter-ionic processes, is the key to the improved development of luminescence thermometry. A phosphor doped with only trivalent terbium ions was described, which, by using two excitation lines fitted to the 7F6 → 5D3 and 7F5 → 5D3 transitions, shows a luminescent signals with the opposite characteristics of intensity changes as a function of temperature. By modifying the concentration of Tb3+ ions, the probability of {5D3, 7F6} ↔ {5D4, 7F0} cross-relaxation was being altered, which turned out to have a beneficial effect on the properties of the described nanothermometers. The ratio of intensities for both excitations was found to be temperature dependent, which resulted in high relative sensitivities of temperature readout reaching 3.2%/°C for 190 °C and not reaching values below 2%/°C in the broad range of the temperature. Extensive decay time measurements for 5D3 and 5D4 emissive levels were presented and the variability of both rise- and decay times as a function of terbium concentration and temperature was investigated. Thanks to this, conclusions were drawn regarding thermally dependent optical processes occurring in a given and similar systems.