Tb3+,Mn3+ co-doped La2Zr2O7 nanoparticles for self-referencing optical thermometry

Abstract

In optical thermometry based on fluorescence intensity ratio (FIR) of two thermally coupled energy states, the sensitivity and signal discriminability are limited by the energy gap between two thermally coupled excited states. Here, we have employed the strategy of combining rare earth ion Tb3+ and transition metal ion Mn3+ with completely different thermal-quenching behaviors to obtain high thermometric sensitivity using uniform La2Zr2O7:Tb3+,Mn3+ nanoparticles (NPs) synthesized by a molten salt synthesis method. We have proposed a model to describe the electronic transitions of Mn3+ ion doped in the compound. Based on Tanabe-Sugano diagram, the excitation peaks centered at 345 and 450 nm are assigned to the 5E → 3E and 1E transitions. Due to the strong Jahn-Teller effect, the 5E ground state is split into two components, namely 5E′ and 5E΄΄. The excitation at upper 3E state is followed by two emission bands to the split ground state (3E → 5E′ and 5E″) which are dominated upon heating. Additionally, two transitions from lower excited level of 1E to the split ground states are possible (1E → 5E′ and 5E″) which are strengthened upon cooling. Mn3+ ion featuring d-d transitions and Tb3+ with f-f transitions have high and low temperature dependencies, which allow their use as luminescence probe and reference, respectively. The emissions intensity ratio of Tb3+ at 543 nm to Mn3+ at 719 nm is explored over the temperature range of 98–338 K. The maximum relative sensitivity value from our La2Zr2O7:Tb3+,Mn3+ NPs is obtained as high as 1.82%·K−1 with a minimum temperature uncertainty of ~0.27K at 298 K. The temperature-dependent FIR has also shown repeatable results which indicate that the La2Zr2O7:Tb3+,Mn3+ NPs are a promising material for dual activator ratiometric optical thermometry.