Unraveling the thermometric behaviour of Tm,Yb:LiLuF4

(Nano)thermometry is a fast-developing field of research due to the need of remote, precise, reliable, and non-invasive temperature sensing in real time. This is both important for research and development as well as for industrial applications. The most common Ln3+ pair used for ratiometric (nano)thermometry is the Er,Yb upconversion (UC) system. However, this system operates outside of the first Biological Window (BW-I) (650-900 nm), in which tissue scattering and autofluorescence is minimized. Here, we present (nano)thermometers based on the Tm,Yb UC system in an inorganic LiLuF4 host. The advantage of this system is that the ratio of the 3F2,3 3H6 electronic transition (680 nm) to the 3H4 3H6 electronic transition (900 nm) of Tm3+ can be utilized, therefore operating in the BW-I. So far, an understanding of the excited state dynamics of the proposed thermometer based on the Yb, Tm UC system is limited, especially the role of the activator Tm3+ and possible cross-relaxation processes that additionally introduce doping concentration as a control parameter for the performance of the luminescent thermometer. itself. All experimental results are understandable in terms of a multi-phonon relaxation model. Additionally, Tm,Yb:LiLuF4@LiYF4 nanocrystals were prepared and a possible energy transfer to incorporated Gd3+ ions was investigated.