Exploiting bandgap engineering to finely control dual-mode Lu2(Ge,Si)O5:Pr3+luminescence thermometers

It was proved quite recently that luminescence thermometry may benefit from utilizing the 5d -> 4f/4f -> 4f intensity ratio of Pr(3+)transitions. This paper presents a comprehensive study of Lu-2(Ge-x,Si1-x)O-5:Pr phosphors in the full range of Ge concentrations (x= 0-1) for luminescence thermometry. Silicon substitution by germanium allows effective management of their thermometric properties through bandgap engineering. The Ge/Si ratio controls the range of temperatures within which the 5d -> 4f Pr(3+)luminescence can be detected. This, in turn, defines the range of temperatures within which the 5d -> 4f/4f -> 4f emission intensity ratio can be utilized for thermometry. Altogether, the bandgap engineering allows widening the operating range of thermometers (17-700 K), fine-tunes the range of temperatures with the highest relative sensitivity, and reduces the inaccuracy of the measurements. The kinetics of the 5d -> 4f luminescence is also controlled by bandgap engineering and can be also used for luminescence thermometry. The Lu-2(Ge-x,Si1-x)O-5:Pr phosphors were, thus, designed as dual-mode luminescence thermometers exploiting either the inter- and intra-configurational intensity ratios or the 5d -> 4f decay time. The highest relative thermal sensitivity, 3.54% K-1, was found at 17 K for Lu-2(Ge-0.75,Si-0.25)O-5:Pr and at 350 K for Lu2SiO5:Pr and it was combined with a very low (<0.03 K) temperature uncertainty. Herein, we proved that bandgap engineering is a promising and effective approach to developing high-performance luminescence thermometers.