High temperature (nano)thermometers based on LiLuF4:Er3+, Yb3+ nano- and microcrystals. Confounded results for core-shell nanocrystals

Recent technological developments require knowledge of temperature down to the micro- or even nanoscale. Lanthanide-doped nanoparticles became a popular tool to achieve this. Their temperature sensitive luminescence enables their application as remote thermometer and for mapping temperature profiles with high spatial resolution. Applicability of luminescence thermometry is, however, often limited at high temperatures. In nanoelectronics or chemical reactors, high temperatures above 500 K are common and new approaches for accurate high temperature sensing need to be developed. In this work, we report three different shapes of upconverting LiLuF₄: 2% Er³⁺, 18% Yb³⁺ nanocrystals both with and without shells and study the influence of the shell on the thermometric properties. We observed peculiar behavior of the core-shell particles suggesting the presence of the dopants within the protective and ‘undoped’ shells.  Coating the nanoparticles with a silica layer extends the operational temperature range. In an upconversion (UC) Yb³⁺-Er³⁺ system temperature sensing relies on thermal coupling between the ⁴S_3/2 and ²H_11/2 energy levels. At sufficiently high temperatures (> 550 K), we observe additional thermal coupling involving the higher ⁴F_7/2 energy levels.  The larger energy gap allows to increase the relative sensitivity at elevated temperatures and to sustain a high temperature precision over a wider temperature range than for a two-level Boltzmann thermometer. The thermal coupling between the ⁴S_3/2 and ²H_11/2 energy levels is used for lower temperature sensing (< 550 K) and the ⁴F_7/2 energy level is crucial for higher temperature sensing (> 550 K).