A Ho3+-Based Luminescent Thermometer for Sensitive Sensing over a Wide Temperature Range

Abstract

Luminescence thermometry is used in a variety of research fields for noninvasive temperature sensing. Lanthanide-doped micro-/nanocrystals are exceptionally suitable for this. The popular concept of luminescence-intensity-ratio thermometry is based on emission from thermally coupled levels in a single lanthanide ion, following Boltzmann's law. These thermometers can measure temperature with low uncertainty, but only in a limited temperature range. In this work, a Ho3+-based thermometer is presented and quantitatively modeled with sustained low temperature uncertainty from room temperature up to 873 K. The thermometer shows bright green and red luminescence with a strong and opposite dependence on temperature and Ho3+ concentration. This is the result of temperature-dependent competition between multi-phonon relaxation and energy transfer, feeding the green- and red-emitting levels, respectively, following excitation with blue light. This simple and quantitative model of this competition predicts the output spectrum over a wide range of temperatures (300–873 K) and Ho3+ concentrations (0.1–30%). The optimum Ho3+ concentration can thus be determined for reliable measurements over any temperature range of interest. Quantitative modeling as presented here is crucial to optimally benefit from the potential of energy-transfer thermometers to achieve low measurement uncertainties over a wide temperature range.