Ratiometric Luminescent Thermometers with a Customized Phase-Transition-Driven Fingerprint in Perovskite Oxides

A brief review of characteristic luminescence properties of Eu3+ in mixed-anion compounds

Trivalent europium (Eu3+) ions show red luminescence with sharp spectral lines owing to the intraconfigurational 4f-4f transitions. Because of their characteristic luminescence properties, various Eu3+-doped inorganic compounds have been developed to meet the demands of optoelectronic devices. Regardless of shielding by the outer 5s and 5p orbitals, the properties of the Eu3+:4f-4f transition depend on the local environment, such as the shapes of the coordination polyhedra, site symmetry, nephelauxetic effects, crystal field effects, and bonding character. Mixed-anion coordination, where multiple types of anions surround a single Eu3+ ion, can directly affect the optical properties of Eu3+. We review the luminescence properties of Eu3+ ions in mixed-anion compounds of the oxynitride YSiO2N and oxyhalides YOX (X = Cl or Br). Oxynitride and oxyhalide coordination results in characteristic transition probabilities and branching ratios of the 5D0 → 7F0-6 transitions due to distorted structural environments and red-shifted charge transfer excitation bands due to an upward shift of the valence band. The expected and experimentally observed features of Eu3+ luminescence in mixed-anion compounds are outlined based on band and Judd-Ofelt theories. Future applications of the intense red luminescence at ∼620 nm under near-ultraviolet light illumination in Eu3+-doped mixed-anion compounds are introduced, and material design guidelines for new functional Eu3+-doped phosphors are presented.

Spotlight on Luminescence Thermometry: Basics, Challenges, and Cutting-Edge Applications

Luminescence (nano)thermometry is a remote sensing technique that relies on the temperature dependency of the luminescence features (e.g., bandshape, peak energy or intensity, and excited state lifetimes and risetimes) of a phosphor to measure temperature. This technique provides precise thermal readouts with superior spatial resolution in short acquisition times. Although luminescence thermometry is just starting to become a more mature subject, it exhibits enormous potential in several areas, e.g., optoelectronics, photonics, micro- and nanofluidics, and nanomedicine. This work reviews the latest trends in the field, including the establishment of a comprehensive theoretical background and standardized practices. The reliability, repeatability, and reproducibility of the technique are also discussed, along with the use of multiparametric analysis and artificial-intelligence algorithms to enhance thermal readouts. In addition, examples are provided to underscore the challenges that luminescence thermometry faces, alongside the need for a continuous search and design of new materials, experimental techniques, and analysis procedures to improve the competitiveness, accessibility, and popularity of the technology.

Investigation of thermal control in phase-changing ABO3 perovskites via first-principles predictions: general mechanism of solar absorptivity

The fundamental mechanism of solar absorbance during the phase-change process is investigated in ABO₃ perovskites based on first-principles predictions. A Gaussian-like relationship between the solar absorbance and band gaps is established, which follows the Shockley-Queisser limiting efficiency. For ABO₃ perovskites with bandgaps of E_g > 3.5 eV, a low solar absorbance is obtained, whereas a high solar absorbance is obtained for ABO₃ perovskites, with band gaps ranging from 0.25 to 2.2 eV. The relationship between the orbital character of the density of states (DOS) and the absorption spectra reveals that ABO₃ perovskites with magnetic (strongly interacting) and distorted crystal structures always exhibit a higher solar absorptivity. In contrast, non-magnetic and cubic ABO₃ perovskites always exhibit a lower solar absorptivity. Moreover, the tunable solar absorptivity always undergoes a phase change from cubic to large distorted crystal structures in ABO₃ perovskites with strong interactions. These results can be attributed to a rich structural, electronic, and magnetic phase diagram resulting from the strong interplay between the lattice, spin, and orbital degrees of freedom, which induce highly tunable optical characteristics in the phase-change process. The findings presented in this study are critical for the development of ABO₃ perovskite-based smart thermal control materials in the spacecraft field.

Effect of Sr2+ ions on the structure, up-conversion emission and thermal sensing of Er3+, Yb3+ co-doped double perovskite Ba(2-x)SrxMgWO6 phosphors

Investigating the effect of different phases on the optical performance is crucial for thermal sensing phosphor materials. Ba_(2-x)Sr_xMgWO₆:Er³⁺, Yb³⁺, K⁺ double perovskite phosphors were successfully prepared using a high-temperature solid-phase method. The dominant up-conversion luminescent (UCL) mechanism was deduced by analyzing the power-dependence spectra and energy level diagrams. By X-ray diffraction tests and tolerance factor calculations, it was demonstrated that the substitution of Sr²⁺ ions for Ba²⁺ ions led to the phase changing from cubic to tetragonal. The phase transition led to a decrease in the crystallographic symmetry of the compounds and changes in the optical thermometric properties. The optical temperature sensing properties were investigated using the fluorescence intensity ratio of thermally coupled energy levels (²H_11/2 and ⁴S_3/2 to the ground state energy level ⁴I_15/2) of Er³⁺ ions in Ba₂MgWO₆, BaSrMgWO₆ and Sr₂MgWO₆. The maximum absolute sensitivities obtained for Ba₂MgWO₆, BaSrMgWO₆ and Sr₂MgWO₆ doped with 7% Er³⁺, 2% Yb³⁺ and 9% K⁺ were 6.77 × 10^-4 K^-1, 10.09 × 10^-4 K^-1 and 23.4 × 10^-4 K^-1, respectively. The comparison revealed that the phase transition caused an increase in the luminescence intensity and absolute sensitivity. This provides a useful pathway for modulating the subsequent thermometric performance.

Luminescent Properties of Phosphonate Ester-Supported Neodymium(III) Nitrate and Chloride Complexes

This study examines the synthesis of two geminal bisphosphonate ester-supported Ln³⁺ complexes [Ln(L3)₂(NO₃)₃] (Ln = Nd³⁺ (5), La³⁺ (6)) and optical properties of the neodymium(III) complex. These results are compared to known mono-phosphonate ester-based Nd³⁺ complexes [Nd(L1/L2)₃X₃]_n (X = NO₃^-, n = 1; Cl^-, n = 2) (1-4). The optical properties of Nd³⁺ compounds are determined by micro-photoluminescence (µ-PL) spectroscopy which reveals three characteristic metal-centered emission bands in the NIR region related to transitions from ⁴F_3/2 excited state. Additionally, two emission bands from ⁴F_5/2, ²H_9/2 → ⁴I_J (J = 11/2, 13/2) transitions were observed. PL spectroscopy of equimolar complex solutions in dry dichloromethane (DCM) revealed remarkably higher emission intensity of the mono-phosphonate ester-based complexes in comparison to their bisphosphonate ester congener. The temperature-dependent PL measurements enable assignment of the emission lines of the ⁴F_3/2 → ⁴I_9/2 transition. Furthermore, low-temperature polarization-dependent measurements of the transitions from R₁ and R₂ Stark sublevel of ⁴F_3/2 state to the ⁴I_9/2 state for crystals of [Nd(L3)₂(NO₃)₃] (5) are discussed.

An Er3+ doped Ba2MgWO6 double perovskite: a phosphor for low-temperature thermometry

A bifunctional luminescent material is one of the most intriguing topics in recent years with significant growth in the number of investigations. Herein, we report the potential of Ba₂MgWO₆ doped with Er³⁺ as a candidate for white-light emitting phosphor and noncontact luminescent thermometry. The synthesis of the samples was carried out by the co-precipitation method. The influence of the dopant concentration on the emission intensity, as well as the capability of temperature readout, was investigated for the first time. The highest emission intensity exhibits a sample comprising 4% Er³⁺; above it, the concentration quenching process by the dipole-dipole interaction occurs. However, high quality white light generates Ba₂MgWO₆ with 0.5% of Er³⁺ due to the coexistence of the host and erbium ion emission with a CIE of (0.30, 0.35). To construct a non-contact luminescent thermometer based on Er³⁺, the ratio of the emission from ⁴I_11/2 → ⁴I_15/2 to the host emission was examined. The highest sensitivity S_r of the obtained luminescent thermometers was 2.78% K^-1 at 198 K. The repeatability of the calculated results and the uncertainty δT of the temperature readout were investigated.

Fluorescence Thermometer: Intermediation of the Fontal Temperature and Light

The rapid advance of thermal materials and fluorescence spectroscopy has extensively promoted micro-scale fluorescence thermometry development in recent years. Based on the advantages of fast response, high sensitivity, simple operation,...

Enhancing the Upconversion Luminescence and Sensitivity of Nanothermometry through Advanced Design of Dumbbell-Shaped Structured Nanoparticles

The core-shell engineering of lanthanide-doped nanoparticles has captured considerable attention because it can safeguard the luminescence intensity of the core by reducing surface defects. However, the limited surface area of the traditional spherical core-shell structure hinders the further breakthrough of the brightness. Herein, a unique NaYF₄:Yb³⁺/RE³⁺@NaYF₄:Yb³⁺/RE³⁺@NaNdF₄:Yb³⁺ (RE³⁺ = Ho³⁺ or Er³⁺) dumbbell-shaped multilayer nanoparticle featuring a high surface area is reported. Its upconversion luminescence intensity is higher than that of the conventional spherical core-shell structure. A thorough investigation is performed on the luminescence and thermometric mechanisms of Ho³⁺/Er³⁺ distributed in the core and the first shell. Remarkably, when Ho³⁺/Er³⁺ ions are distributed in the first shell, the relative sensitivity of the biological luminescence nanothermometer composed of downshifting near-infrared emissions is increased to 2.543% K^-1 (328 K), which considerably exceeds most reported values. The increased value is attributed to the more thermal-sensitive phonon-assisted energy transfer. For potential biological applications, dumbbell-shaped nanoparticles (DSNPs) with hydrophilic modification show excellent thermometric performance and high tissue penetration depth. Overall, the insights provided by this work will broaden the scope of novel DSNPs in the fields of luminescence and nanothermometry.

High-Pressure Photoluminescence Properties of Cr3+-Doped LaGaO3 Perovskites Modulated by Pressure-Induced Phase Transition

The photoluminescence properties of Cr³⁺-doped LaGaO₃ perovskites are investigated by high-pressure spectroscopy. The pressure-induced phase transition from orthorhombic (Pbnm) to rhombohedral (R3̅c) at around 2 GPa is confirmed by Raman spectroscopy. Cr³⁺-doped LaGaO₃ shows deep-red emission peaks around 730 nm due to the zero-phonon line (R-line) and the phonon sidebands, which correspond to Cr³⁺: ²E_g → ⁴A₂g transitions in the ideal octahedral site and the Cr-Cr pair luminescence (N-line) under ambient condition. Under a high pressure, the R-line shifts to a lower energy at a rate of -13 cm^-1/GPa. From the pressure dependence of photoluminescence excitation (PLE) spectra, it is suggested that the redshift of the R-line is caused by the decrease of Racah parameters B and C. Moreover, the N-line luminescence becomes stronger relative to the R-line with increasing pressure and the N-line/R-line can be used to monitor the phase transition pressure. Under a high pressure, the tilt angle of the GaO₆ octahedral unit becomes smaller. It implies that the enhanced N-line luminescence is caused by the stronger superexchange interaction between Cr³⁺ ions due to the increased Cr-O-Cr bond angle closer to 180°.

Investigation on the upconversion luminescence and ratiometric thermal sensing of SrWO4:Yb3+/RE3+ (RE = Ho/Er) phosphors

SrWO₄ phosphors doped with Ho³⁺(Er³⁺)/Yb³⁺ are successfully prepared by a high temperature solid-state reaction method. The upconversion (UC) luminescence properties of all the samples have been investigated under 980 nm excitation. Strong green emissions are obtained in the SrWO₄:Yb³⁺/Ho³⁺ and SrWO₄:Yb³⁺/Er³⁺ samples with the naked eyes. In a temperature range going from 303 K to 573 K, the UC emission spectra of the phosphors have been measured. Then the temperature sensing properties also have been discussed via fluorescence intensity ratio (FIR) technology. For the SrWO₄:Yb³⁺/Ho³⁺ phosphor, the FIR technologies based on thermal coupling levels (TCLs)(⁵F₄,⁵F₅) and non-thermal coupling levels (non-TCLs)(⁵S₂, ⁵F₄/⁵F₅) are used for investigating the sensitivity. The results show that the maximum absolute sensitivity reaches 0.0158 K⁻¹ with non-TCLs. As for Yb³⁺/Er³⁺ codoped SrWO₄ phosphor, the maximum absolute sensitivity reaches 0.013 K⁻¹ with TCLs (²H_11/2,⁴S_5/2) at a temperature of 513 K. These significant results demonstrate that the SrWO₄:Ho³⁺(Er³⁺)/Yb³⁺ phosphors are robust for optical temperature sensors.

SrWO₄ phosphors doped with Ho³⁺(Er³⁺)/Yb³⁺ are successfully prepared by a high temperature solid-state reaction method.

High-sensitivity and wide-temperature-range dual-mode optical thermometry under dual-wavelength excitation in a novel double perovskite tellurate oxide

Novel double perovskite SrLaLiTeO₆ (abbreviated as SLLT):Mn⁴⁺,Dy³⁺ phosphors synthesized using a solid-state reaction strategy exhibit distinct dual-emission of Mn⁴⁺ and Dy³⁺. High-sensitivity and wide-temperature-range dual-mode optical thermometry was exploited taking advantage of the diverse thermal quenching between Mn⁴⁺ and Dy³⁺ and the decay lifetime of Mn⁴⁺. The thermometric properties in the range of 298-673 K were investigated by utilizing the fluorescence intensity ratio (FIR) of Dy³⁺ (⁴F_9/2→⁶H_13/2)/Mn⁴⁺ (²E_g→⁴A_2g) and the Mn⁴⁺ (²E_g→⁴A_2g) lifetime under 351 nm and 453 nm excitation, respectively. The maximum relative sensitivities (S_R) of the resultant SLLT:1.2%Mn⁴⁺,7%Dy³⁺ phosphor under 351 nm and 453 nm excitation employing the FIR technology were determined to be 1.60% K^-1 at 673 K and 1.44% K^-1 at 673 K, respectively. Additionally, the maximum S_R values based on the lifetime-mode were 1.59% K^-1 at 673 K and 2.18% K^-1 at 673 K, respectively. It is noteworthy that the S_R values can be manipulated by different excitation wavelengths and multi-modal optical thermometry. These results suggest that the SLLT:Mn⁴⁺,Dy³⁺ phosphor has prospective potential in optical thermometry and provide conducive guidance for designing high-sensitivity multi-modal optical thermometers.

Multimodal Non-Contact Luminescence Thermometry with Cr-Doped Oxides

Luminescence methods for non-contact temperature monitoring have evolved through improvements of hardware and sensor materials. Future advances in this field rely on the development of multimodal sensing capabilities of temperature probes and extend the temperature range across which they operate. The family of Cr-doped oxides appears particularly promising and we review their luminescence characteristics in light of their application in non-contact measurements of temperature over the 5-300 K range. Multimodal sensing utilizes the intensity ratio of emission lines, their wavelength shift, and the scintillation decay time constant. We carried out systematic studies of the temperature-induced changes in the luminescence of the Cr3+-doped oxides Al2O3, Ga2O3, Y3Al5O12, and YAlO3. The mechanism responsible for the temperature-dependent luminescence characteristic is discussed in terms of relevant models. It is shown that the thermally-induced processes of particle exchange, governing the dynamics of Cr3+ ion excited state populations, require low activation energy. This then translates into tangible changes of a luminescence parameter with temperature. We compare different schemes of temperature sensing and demonstrate that Ga2O3-Cr is a promising material for non-contact measurements at cryogenic temperatures. A temperature resolution better than ±1 K can be achieved by monitoring the luminescence intensity ratio (40-140 K) and decay time constant (80-300 K range).

Robust liquid-core nanocapsules as biocompatible and precise ratiometric fluorescent thermometers for living cells

Intracellular thermometry with favorable biocompatibility and precision is essential for insight into temperature-related cellular events. Here, liquid-core nanocapsule ratiometric fluorescent thermometers (LCN-RFTs) were prepared by encapsulating thermosensitive organic fluorophores (N,N'-di(2-ethylhexyl)-3,4,9,10-perylene tetracarboxylic diimide, DEH-PDI) with hydrophobic solvent (2,2,4-trimethylpentane, TMP) into polystyrene/silica hybrid nanoshells. As the fluorescent thermosensitive unit of the LCN-RFT, the TMP solution of DEH-PDI was responsible for the fluorescence response to temperature. Benefitting from the hydrophilic nanoshells, the LCN-RFTs exhibited favorable anti-interference and biocompatibility. Furthermore, the LCN-RFTs showed an excellent precision of 0.02 °C-0.10 °C in a simulated physiological environment from 10.00 °C to 90.00 °C, and were employed to realize intracellular thermometry with an outstanding precision of 0.06 °C-0.14 °C. This work provides a feasible method of using hydrophobic organic fluorophores for intracellular thermometry by encapsulating them into nanocapsules.

Pushing the Limit of Boltzmann Distribution in Cr3+-Doped CaHfO3 for Cryogenic Thermometry

Luminescence Boltzmann thermometry is one of the most reliable techniques used to locally probe temperature in a contactless mode. However, to date, there is no report on cryogenic thermometers based on the highly sensitive and reliable Boltzmann-based ⁴T₂ → ⁴A₂/²E → ⁴A₂ emission ratio of Cr³⁺. On the basis of structural information of the local HfO₆ octahedral site we demonstrated the potential of the CaHfO₃:Cr³⁺ system by combining deep theoretical and experimental investigation. The material exhibits simultaneous emission from both the ²E and ⁴T₂ excited states, following the Boltzmann law in a cryogenic temperature range of 40-150 K. The promising thermometric performance corroborates the potential of CaHfO₃:Cr³⁺ as a Boltzmann cryothermometer, being characterized by a high relative sensitivity (∼ 2%·K^-1 at 40 K) and exceptional thermal resolution (0.045-0.77 K in the 40-150 K range). Moreover, by exploiting the flexibility of the ⁴T₂-²E energy gap controlled by the crystal field of the local octahedral site, the design proposed herein could be expanded to develop new Cr³⁺-doped cryogenic thermometers.

Eu3+-based luminescence ratiometric thermometry

Recently, luminescence ratiometric thermometry has gained ever-increasing attention due to its merits of rapid response, non-invasiveness, high spatial resolution, and so forth. For research fields relying on temperature measurements, achieving a higher relative sensitivity of this measurement is still an important task. In this work, we developed a strategy for achieving a more sensitive temperature measurement, one merely depending on the photoluminescence of Eu³⁺. We showed that using the ⁵D₁–⁷F₁ transition and the hypersensitive ⁵D₀–⁷F₂ transition of Eu³⁺ can boost the relative sensitivity compared with the method relying on the ⁵D₁–⁷F₁ and ⁵D₀–⁷F₁ transitions of Eu³⁺. The difference between these two strategies was studied and was explained by the hypersensitive ⁵D₀–⁷F₂ transition more steeply decreasing than the ⁵D₀–⁷F₁ transition with a rise in temperature. Our work is expected to help researchers design sensitive optical thermometers via proper use of this hypersensitive transition.

We show that more sensitive luminescence ratiometric thermometry can be achieved using a hypersensitive Eu³⁺ transition.