Understanding the thermometric behaviour of LiLuF4:Tm3+,Yb3.

The Er3+, Yb3+ upconversion couple, with its background-free green emission arising from the two thermally coupled 2H11/2 and 4S3/2 levels of Er3+, is a classic "workhorse" example of luminescent Boltzmann thermometry. In contrast, the Tm3+, Yb3+ couple remains far less established. The Tm3+, Yb3+ system offers an intense 3H4 → 3H6 electronic transition (800 nm) as a possible reference emission for UC thermometry in the NIR-I window. However, the Tm3+, Yb3+ thermometry system has not yet been fully understood, limiting its potential application in medical and biological contexts, as well as in nanoelectronics, nanophotonics and in general industrial settings. In this work, we demonstrate how to exploit the temperature-dependent multiphonon relaxation from the 3F3 to the 3H4 level of Tm3+. This relaxation is fed by energy transfer from Yb3+ and gives rise to two emission lines at 680 nm and 800 nm. Taking LiLuF4:1% Tm, 30% Yb as a representative example, we analyzed both micro- and nanocrystalline samples to elucidate how surface-attached ligands with higher vibrational energies than the low cutoff phonon energies (∼500 cm-1) of the fluoride matrix itself have on the Boltzmann thermometry behaviour. We show that Tm3+ can only work as a wide-range Boltzmann thermometer in microcrystalline, ligand-free samples with host compounds of sufficiently low cutoff phonon energies and that its action is limited in nanocrystalline systems. A combination of experimental studies and kinetic modelling helps us to elucidate clear-cut guidelines for optimizing the performance of the Tm3+, Yb3+ system as a luminescent thermometer and to compare this UC system to the well-established "workhorse" of the Er3+, Yb3+ UC couple.