Abstract
Ratiometric luminescence thermometry with trivalent lanthanide ions and their 4fn energy levels is an emerging technique for non-invasive remote temperature sensing with high spatial and temporal resolution. Conventional ratiometric luminescence thermometry often relies on thermal coupling between two closely lying energy levels governed by Boltzmann’s law. Despite its simplicity, Boltzmann thermometry with two excited levels allows precise temperature sensing, but only within a limited temperature range. While low temperatures slow down the nonradiative transitions required to generate a measurable population in the higher excitation level, temperatures that are too high favour equalized populations of the two excited levels, at the expense of low relative thermal sensitivity. In this work, we extend the concept of Boltzmann thermometry to more than two excited levels and provide quantitative guidelines that link the choice of energy gaps between multiple excited states to the performance in different temperature windows. By this approach, it is possible to retain the high relative sensitivity and precision of the temperature measurement over a wide temperature range within the same system. We demonstrate this concept using YAl3(BO3)4 (YAB):Pr3+, Gd3+ with an excited 6PJ crystal field and spin-orbit split levels of Gd3+ in the UV range to avoid a thermal black body background even at the highest temperatures. This phosphor is easily excitable with inexpensive and powerful blue LEDs at 450 nm. Zero-background luminescence thermometry is realized by using blue-to-UV energy transfer upconversion with the Pr3+−Gd3+ couple upon excitation in the visible range. This method allows us to cover a temperature window between 30 and 800 K.
Highlights
Chemical reactions, biological functionalities, and various physical phenomena are all strongly determined by the local temperature
An alternative method of remote temperature sensing is luminescence thermometry, which exploits the fact that luminescence properties such as the emission intensity or the luminescence decay time are highly dependent on the local temperature of the surroundings of the luminescent species[4,5,6]
Any ratiometric luminescent thermometer devised for high temperature sensing best relies on emission in the ultraviolet range, which is clearly unaffected by black-body background at temperatures below 1200 K. We present such a designed multilevel luminescent thermometer that uses the three excited 6PJ (J = 7/2, 5/2, 3/2) crystal fields and spin-orbit levels of the UV B-emitting lanthanide ion Gd3+; we were motivated by thermodynamic considerations and demonstrated how a single luminescent phosphor can be optimized for precise thermometry from cryogenic (30 K) to high temperatures (800 K) with constantly high relative sensitivities above 0.5% K−1
Summary
Biological functionalities, and various physical phenomena are all strongly determined by the local temperature. The measurement of local temperatures on successively smaller scales requires new methods of temperature sensing that rely on a remote detection principle. An alternative method of remote temperature sensing is luminescence thermometry, which exploits the fact that luminescence properties such as the emission intensity or the luminescence decay time are highly dependent on the local temperature of the surroundings of the luminescent species[4,5,6]. It requires a simple setup consisting of a laser excitation source, luminescent micro- or nanocrystals or
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