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 ²H_11/2 and ⁴S_3/2 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.