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Luminescence (nano)thermometry is an emerging and promising field for remote temperature sensing and thermal imaging of both the surface and interior of objects. While the field is dominated by trivalent lanthanide ions (Ln3+) the transition metal ions have a so far underestimated role in that regard, although their luminescence properties are far more sensitive to changes in the embedding host matrix than in the case of the Ln3+ ions. In this work, we demonstrate the potential of the abundant Fe3+ ion as a candidate for sensitive luminescence thermometry in the aluminate hosts MAl4O7 (M = Ca, Sr, Ba). Although this ion is usually considered an efficient luminescence quencher, it is shown that a targeted control of its luminescence properties makes this activator an interesting and only scarcely investigated alternative to the Ln3+ ions. We investigated the possibility for different modes of luminescence thermometry with Fe3+ (eventually co-doped with Tb3+ as internal thermally insensitive intensity reference), which include absolute intensity, ratiometric or lifetime thermometry. By selective changes in the Fe–O bond distances, it is possible to modulate the relative sensitivity towards values as high as around 2% K−1 at room temperature in CaAl4O7:Fe3+,Tb3+ and connected relative temperature uncertainties of around 0.3%.

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 2kBT0 and 3.41kBT0. 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 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 4F3/2 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 4F5/2 and 4F3/2 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 4F3/2 and 4F5/2 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 4F5/2 and 4F3/2 spin-orbit levels of Nd3+ makes it possible to tailor the thermometric performance of Nd3+ to enable physiological temperature sensing.

The Eu2+-doped mixed alkaline metal strontium borohydride chlorides ASr(BH4)3–xClx (A = K, Rb, Cs) and Eu2+-doped strontium borohydride chloride Sr(BH4)2–xClx have been prepared by mechanochemical synthesis. Intense blue photoluminescence for Sr(BH4)2–xClx (λem= 457 nm) and cyan photoluminescence for the perovskite-type mixed alkaline metal strontium borohydride chlorides ASr(BH4)3–xClx (A = K, Rb, Cs) (λem = 490 nm) is already observable after short milling times. Temperature dependent luminescence measurements reveal an appreciable blue shift with increasing temperature for all ASr(BH4)3–xClx (A = K, Rb, Cs) until 500 K. This extremely large shift, caused by structural relaxation, as well as the vibrationally induced emission band broadening can serve as a sensitive response signal for temperature sensing, and this unique behavior has, to the best of our knowledge, not been reported in any Eu2+ doped phosphor so far. Additionally, bright luminescence, high quantum efficiencies, and very low thermal quenching of these Eu2+-doped borohydrides show that such host materials could serve in solid state lighting applications., This project has received funding from the European Union's Horizon 2020 FET Open programme under grant agreement No 801305 (NanoTBTech).

The Eu2+-doped mixed alkaline metal strontium borohydride chlorides ASr(BH4)3–xClx (A = K, Rb, Cs) and Eu2+-doped strontium borohydride chloride Sr(BH4)2–xClx have been prepared by mechanochemical synthesis. Intense blue photoluminescence for Sr(BH4)2–xClx (λem= 457 nm) and cyan photoluminescence for the perovskite-type mixed alkaline metal strontium borohydride chlorides ASr(BH4)3–xClx (A = K, Rb, Cs) (λem = 490 nm) is already observable after short milling times. Temperature dependent luminescence measurements reveal an appreciable blue shift with increasing temperature for all ASr(BH4)3–xClx (A = K, Rb, Cs) until 500 K. This extremely large shift, caused by structural relaxation, as well as the vibrationally induced emission band broadening can serve as a sensitive response signal for temperature sensing, and this unique behavior has, to the best of our knowledge, not been reported in any Eu2+ doped phosphor so far. Additionally, bright luminescence, high quantum efficiencies, and very low thermal quenching of these Eu2+-doped borohydrides show that such host materials could serve in solid state lighting applications.

Luminescence (nano)thermometry is an emerging and promising field for remote temperature sensing and thermal imaging of both the surface and interior of objects. While the field is dominated by trivalent lanthanide ions (Ln3+) the transition metal ions have a so far underestimated role in that regard, although their luminescence properties are far more sensitive to changes in the embedding host matrix than in the case of the Ln3+ ions. In this work, we demonstrate the potential of the abundant Fe3+ ion as a candidate for sensitive luminescence thermometry in the aluminate hosts MAl4O7 (M = Ca, Sr, Ba). Although this ion is usually considered an efficient luminescence quencher, it is shown that a targeted control of its luminescence properties makes this activator an interesting and only scarcely investigated alternative to the Ln3+ ions. We investigated the possibility for different modes of luminescence thermometry with Fe3+ (eventually co-doped with Tb3+ as internal thermally insensitive intensity reference), which include absolute intensity, ratiometric or lifetime thermometry. By selective changes in the Fe–O bond distances, it is possible to modulate the relative sensitivity towards values as high as around 2% K−1 at room temperature in CaAl4O7:Fe3+,Tb3+ and connected relative temperature uncertainties of around 0.3%., This project has received funding from the European Union's Horizon 2020 FET Open programme under grant agreement No 801305 (NanoTBTech).

Lanthanide-doped luminescent nanoparticles are an appealing system for nanothermometry with biomedical applications due to their sensitivity, reliability and minimally invasive thermal sensing properties. Here, we propose four unique hybrid organic-inorganic materials prepared by combining β-NaGdF4 and PMOs (Periodic Mesoporous Organosilica) or mSiO2 (mesoporous silica). PMO/mSiO2 materials are excellent candidates for biological/biomedical applications as they show high biocompatibility with the human body. On the other hand, the β-NaGdF4 matrix is an excellent host for doping lanthanide ions, even at very low concentrations with yet very efficient luminescence properties. We propose a new type of Er3+-Yb3+ upconversion luminescence nanothermometers operating both in the visible and near infrared regime. Both spectral ranges permit promising thermometry performance even in aqueous environment. It is additionally confirmed that these hybrid materials are non-toxic to cells, which makes them very promising candidates for real biomedical thermometry applications. In several of these materials the presence of additional voids leaves space for future theranostic or combined thermometry and drug delivery applications in the hybrid nanostructures., This project has received funding from the European Union's Horizon 2020 FET Open programme under grant agreement No 801305 (NanoTBTech).

Lanthanide-doped luminescent nanoparticles are an appealing system for nanothermometry with biomedical applications due to their sensitivity, reliability and minimally invasive thermal sensing properties. Here, we propose four unique hybrid organic-inorganic materials prepared by combining β-NaGdF4 and PMOs (Periodic Mesoporous Organosilica) or mSiO2 (mesoporous silica). PMO/mSiO2 materials are excellent candidates for biological/biomedical applications as they show high biocompatibility with the human body. On the other hand, the β-NaGdF4 matrix is an excellent host for doping lanthanide ions, even at very low concentrations with yet very efficient luminescence properties. We propose a new type of Er3+-Yb3+ upconversion luminescence nanothermometers operating both in the visible and near infrared regime. Both spectral ranges permit promising thermometry performance even in aqueous environment. It is additionally confirmed that these hybrid materials are non-toxic to cells, which makes them very promising candidates for real biomedical thermometry applications. In several of these materials the presence of additional voids leaves space for future theranostic or combined thermometry and drug delivery applications in the hybrid nanostructures.

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 4S3/2 and 2H11/2 energy levels. At sufficiently high temperatures (> 550 K), we observe additional thermal coupling involving the higher 4F7/2 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 4S3/2 and 2H11/2 energy levels is used for lower temperature sensing (< 550 K) and the 4F7/2 energy level is crucial for higher temperature sensing (> 550 K)., This project has received funding from the European Union's Horizon 2020 FET Open programme under grant agreement No 801305 (NanoTBTech).

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 4S3/2 and 2H11/2 energy levels. At sufficiently high temperatures (> 550 K), we observe additional thermal coupling involving the higher 4F7/2 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 4S3/2 and 2H11/2 energy levels is used for lower temperature sensing (< 550 K) and the 4F7/2 energy level is crucial for higher temperature sensing (> 550 K).

Ratiometric luminescence thermometry for remote temperature sensing has been constantly gaining an increasing popularity due to its simplicity with yet high precision. Transition metal ions still have an underestimated potential in that area as their luminescence properties can be tuned by targeted modification of the host compound. In this work, we demonstrate that concept in the phosphor LaScO3:Cr3+ by co-doping with various lanthanide ions that partially substitute the La3+ ions of the host. It is demonstrated that the incorporated lanthanide ions affect the bond lengths of the [CrO6]9- octahedra in the structure and thus, have a subtle impact on the local crystal field around the Cr3+ ions. The changes in the crystal field strength lead to a variation in the energy gap between the thermally coupled 2E(g) and 4T2(g) states of Cr3+, which affects the thermal quenching behavior of the related 3d3-3d3 luminescence. This has an immediate consequence for the potential future use of Cr3+ together with lanthanide ions in a ratiometric luminescence thermometry and allows to selectively tune the performance of this structurally sensitive ion.