Luminescence (nano)thermometry is an increasingly important field for remote temperature sensing with high spatial resolution. Most typically, ratiometric sensing of the luminescence emission intensities of two thermally coupled emissive states based on a Boltzmann equilibrium is used to detect the local temperature. Dependent on the temperature range and preferred spectral window, various choices for potential candidates appear possible. Despite extensive experimental research in the field, a universal theory covering the basics of luminescence thermometry is virtually nonexistent. In this manuscript, a general theoretical framework of single ion luminescent thermometers is presented that offers simple, user‐friendly guidelines for both the choice of an appropriate emitter and respective embedding host material for optimum temperature sensing. The results show that the optimum performance (thermal response and sensitivity) around T₀ is realized for an energy gap ∆E₂₁ between thermally coupled levels between 2k_BT₀ and 3.41k_BT₀. Analysis of the temperature‐dependent excited state kinetics shows that host lattices in which ∆E₂₁ can be bridged by one or two phonons are preferred over hosts in which higher order phonon processes are required. Such a framework is relevant for both a fundamental understanding of luminescent thermometers but also the targeted design of novel and superior luminescent (nano)thermometers.

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 ⁶P_J 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.

Ratiometric luminescence thermometry employing luminescence within the biological transparency windows provides high potential for biothermal imaging. Nd³⁺ is a promising candidate for that purpose due to its intense radiative transitions within biological windows (BWs) I and II and the simultaneous efficient excitability within BW I. This makes Nd³⁺ almost unique among all lanthanides. Typically, emission from the two ⁴F_3/2 crystal field levels is used for thermometry but the small ~100 cm⁻¹ energy separation limits the sensitivity. A higher sensitivity for physiological temperatures is possible using the luminescence intensity ratio (LIR) of the emissive transitions from the ⁴F_5/2 and ⁴F_3/2 excited spin-orbit levels. Herein, we demonstrate and discuss various pitfalls that can occur in Boltzmann thermometry if this particular LIR is used for physiological temperature sensing. Both microcrystalline, dilute (0.1%) Nd³⁺-doped LaPO₄ and LaPO₄: x% Nd³⁺ (x = 2, 5, 10, 25, 100) nanocrystals serve as an illustrative example. Besides structural and optical characterization of those luminescent thermometers, the impact and consequences of the Nd³⁺ concentration on their luminescence and performance as Boltzmann-based thermometers are analyzed. For low Nd³⁺ concentrations, Boltzmann equilibrium starts just around 300 K. At higher Nd³⁺ concentrations, cross-relaxation processes enhance the decay rates of the ⁴F_3/2 and ⁴F_5/2 levels making the decay faster than the equilibration rates between the levels. It is shown that the onset of the useful temperature sensing range shifts to higher temperatures, even above ~ 450 K for Nd concentrations over 5%. A microscopic explanation for pitfalls in Boltzmann thermometry with Nd³⁺ is finally given and guidelines for the usability of this lanthanide ion in the field of physiological temperature sensing are elaborated. Insight in competition between thermal coupling through non-radiative transitions and population decay through cross-relaxation of the ⁴F_5/2 and ⁴F_3/2 spin-orbit levels of Nd³⁺ makes it possible to tailor the thermometric performance of Nd³⁺ to enable physiological temperature sensing.

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.