Understanding the thermometric action of LiLuF4:Tm3+, Yb3+

Abstract

While the classic Er3+, Yb3+ upconversion couple with its background-free green emission from the two thermally coupled 2H11/2 and 4S3/2 levels of Er3+ is a “working horse” example for luminescent Boltzmann thermometry, the Tm3+, Yb3+ couple is far from well established. The Tm3+, Yb3+ system offers the intense 3H4 → 3H6 electronic transition (800 nm) as a possible reference emission for UC thermometry in the NIR-I. However, so far the Tm3+, Yb3+ thermometry system is not fully understood, hindering the application in medicinal/biological applications, 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+ fed by energy transfer via Yb3+ giving rise to two emission lines at 680 nm and 800 nm. Taking LiLuF4: 1%Tm, 30%Yb as a representative example compound, we have analyzed both micro- and nanocrystalline samples to elucidate the impact of surface-attached ligands with higher vibrational energies than the low cutoff phonon energies (~ 500 cm-1) of the fluoride matrix itself. We show that Tm3+ can only work as a wide-range Boltzmann thermometer in microcrystalline, ligand-free samples with sufficiently low phonon energy host compounds and its action is limited in nanocrystalline systems. A combination of experimental studies and kinetic modelling helps elucidate clear-cut guidelines and compare this UC system to the well-established working horse of Er3+, Yb3+ in the performance as a luminescent thermometer.

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