Research Progress and Prospects of Inorganic Rare Earth Luminescence Thermometry Technology

Abstract

Temperature is a physical quantity that represents the degree of heat or cold of an object and has significant application value across various fields. Traditional contact temperature measurement technologies, such as thermocouples and infrared thermometers, suffer from limitations like poor environmental adaptability and low spatial resolution, which makes it difficult to meet the temperature measurement requirements for micro-/nano-devices and extreme environments. In recent years, non-contact optical temperature measurement technology based on the luminescence characteristics of rare earth ions has garnered widespread attention due to its high sensitivity, strong interference resistance, and good environmental adaptability. In addition to inorganic luminescent materials, lanthanide-based molecular and coordination-complex thermometers have also become an important branch of this field; however, this paper focuses on inorganic rare earth luminescence thermometry. This paper provides a systematic review of the mechanisms of temperature measurement using rare earth ion luminescence, including single-energy-level luminescence intensity measurement and luminescence intensity ratio measurement based on thermally coupled levels (TCLs) and non-thermally coupled levels (NTCLs). It analyzes the principles of various technologies, performance parameters (such as absolute sensitivity S_a, relative sensitivity S_r, and temperature resolution δT), and their application progress in fields such as biomedical imaging, high-temperature aerospace environments, and the integration of micro-/nano-devices. Special attention is paid to emerging research directions, including Stark sublevel engineering for enhanced sensitivity, negative thermal expansion (NTE) host design for anti-thermal quenching, multi-modal collaborative thermometry, and artificial intelligence (AI)-assisted material design and data processing. The article also discusses the challenges currently faced by the technology, such as high-temperature fluorescence quenching and signal interference, and looks forward to future development directions, including artificial intelligence-assisted material design and multi-modal cooperative temperature measurement, aiming to provide a reference for the research and application of rare earth luminescence temperature sensing technology.

1. Introduction

Temperature is a physical quantity in nature that indicates the degree of heat or cold of an object, reflecting the intensity of molecular thermal motion. It plays an irreplaceable role in material preparation, biomedicine, industrial production, aerospace, and microelectronics [1,2,3]. With the development of science and technology, accurate, fast, and high spatial resolution temperature measurement methods have become increasingly important in various fields [4]. Traditional contact temperature measurement methods, such as thermocouples and thermistors, have the advantages of high accuracy and stability and are widely used in large-scale temperature measurements. However, they have significant limitations in extreme environment adaptability, dynamic response speed, spatial resolution, and localized detection, making it difficult to meet the temperature detection needs of modern high-integration systems, small-scale devices, and complex environments [5,6,7,8,9,10].

To address the challenges and issues faced by traditional contact temperature measurement technologies, it is crucial to develop non-contact temperature measurement technologies with high temperature sensitivity, strong interference resistance, sensitive response, and strong environmental adaptability. This field includes two major branches, inorganic rare earth luminescent materials and lanthanide-based molecular/coordination-complex thermometers, and the present review focuses on inorganic systems. In recent years, researchers have developed non-contact optical temperature measurement technologies based on the unique energy level structure and luminescence characteristics of rare earth ions. Interestingly, once developed, this luminescence temperature measurement technology quickly became a hot topic of research in the field of temperature sensing due to its high sensitivity, strong interference resistance, and wide applicability in harsh environments [11,12]. These temperature measurement materials have unique electronic structures and luminescence characteristics, enabling rapid response to environmental temperature over a wide spectral range (ultra-high frequency-visible near-infrared) [13]. It is worth noting that the 4f orbitals of rare earth ions are shielded by the abundant outer 5s², 5p⁶ electrons, resulting in sharp emission lines, long lifetimes, and minimal influence from crystal fields, providing good photo-thermal stability and reliability [14].

However, current rare earth luminescence thermometry still faces several key bottlenecks: excitation power dependence and laser-induced heating seriously degrade measurement accuracy, high-temperature thermal quenching limits the upper operating temperature, single-modal readout lacks self-calibration ability and anti-interference performance, and the rational design of high-performance materials relies heavily on trial-and-error with insufficient artificial intelligence (AI) assistance. To overcome these limitations, emerging strategies including Stark sublevel engineering, negative thermal expansion (NTE) host design for anti-quenching, multi-modal collaborative thermometry, and AI-assisted material design and data processing have attracted extensive attention, providing new avenues to further improve sensitivity, stability, and practical applicability.

In current research, rare earth optical temperature measurement strategies are mainly divided into three categories: single-energy-level luminescence intensity temperature measurement, temperature measurement based on the thermally coupled levels (TCL) model, and temperature measurement based on the non-thermally coupled levels (NTCL) model with dual-emission centers [15,16,17,18]. This paper will discuss rare earth luminescence temperature measurement technology based on luminescence intensity, systematically organize recent research results and reports, analyze the current limitations and challenges faced by rare earth luminescence temperature measurement technology, and provide an outlook and prediction for future development trends, offering a reference for the continuous development and application of the field of rare earth luminescence temperature sensing.

2. Fundamentals of Rare Earth Ion Luminescence Thermometry Mechanisms

2.1. Rare Earth Energy Level Structure and Characteristics of 4f Transitions

Rare earth refers to scandium and yttrium, the two elements of the III B group in the periodic table, and the fifteen lanthanide elements numbered 57–71, which are also known as rare earths due to their scarce distribution in nature [19]. Rare earths generally exist most commonly as trivalent ions (Re³⁺), and their unique electronic structure and optical properties make them irreplaceable in fields such as luminescent materials, energy, information technology, and biomedicine, earning them the reputation of being the “vitamins” of modern industry [20].

The electronic configuration of trivalent rare earth ions is [Xe] 4fⁿ (where n = 0~14), with the inner 4f orbitals partially filled and surrounded by the outer 5s² and 5p⁶ electron clouds, forming a natural “shielding layer” that makes the 4f electrons less affected by the external crystal field environment, keeping their energy levels stable across different matrices [21]. This shielding effect from the outer electron cloud endows rare earth ions with spectral stability, which is a potential advantage for their use as luminescent activators.

It is worth noting that the 4f orbitals of trivalent rare earth ions Y³⁺, Sc³⁺, and La³⁺ are empty and the 4f shell of Lu³⁺ is completely filled, and these two special cases greatly limit the electronic transitions between energy levels, making these rare earth ions exhibit insurmountable inertia in luminescence. In contrast, other rare earth ions with partially filled 4f orbitals have a rich array of transition energy levels [14,22,23].

Due to spin–orbit coupling and crystal field splitting, the 4f orbital energy levels split into multiple sublevels, forming a rich and complex energy level system. When rare earth-doped materials are excited by light (such as ultraviolet or visible light), these 4f electrons absorb photons and transition between different energy levels within the 4f orbital (direct transition) or transition to higher energy 5d orbitals (indirect transition) [24]. Among them, the 4f-4f transitions (parity forbidden) have long energy level lifetimes (up to milliseconds), and the resulting emission spectra are characterized by narrow, sharp lines with very high color purity, covering the ultraviolet and near-infrared bands, such as the red light of Eu³⁺ and the green light of Tb³⁺ [13]. The 4f-5d transitions have a large absorption cross-section, and their energy levels are easily adjustable, thus exhibiting broad spectral characteristics and high temperature sensitivity (such as the blue light emission of Ce³⁺) [25,26].

As shown in Figure 1 [22,27], the strong correlation between the optical performance of luminescent materials and temperature is mainly reflected in emission wavelength, peak shape (fluorescence intensity ratio), polarization anisotropy, bandwidth, single peak intensity, fluorescence lifetime, etc., depending on the trend of the luminescent performance of rare earth ion-doped materials over time to measure the environmental temperature. Figure 2a is a schematic diagram of temperature measurement based on the luminescence characteristics of rare earth, showing the principle of temperature measurement based on rare earth luminescence. Under the illumination of an excitation light source at a specific wavelength, rare earth ions are excited and emit characteristic fluorescence due to electron transitions. These fluorescence signals are captured by a fluorescence collection system, along with a filter component to eliminate the interference of background light sources, ensuring that the collected signal is solely the emission light of the rare earth ions. The emission light signal is transmitted to a spectrometer via a fluorescence detection system to obtain detailed information about the fluorescence, including key parameters such as luminescence intensity and fluorescence lifetime. Subsequently, these parameters are processed and analyzed by a data system to establish a quantitative relationship between fluorescence characteristics and temperature, and corresponding images are plotted. On this basis, further curve fitting and temperature calibration are carried out to accurately measure the temperature. In addition to rare earth ions, transition metal ions (e.g., Mn⁴⁺, Cr³⁺, Ti³⁺) and main group ions (e.g., Bi³⁺, Eu²⁺) have also gained rapid momentum in optical thermometry in recent years, owing to their high crystal field sensitivity, low cost, and simple preparation processes.

Table 1. Comparison of rare earth and transition metal thermometric materials.

Type	Advantages	Disadvantages	Applicable Scenarios
Rare Earth	Narrow emission linewidth, long lifetime, good thermal stability, biocompatible	Relatively high cost, low sensitivity in some systems	Bioimaging, high-temperature sensing, long-term monitoring
Transition Metal	Low cost, crystal field-sensitive, high sensitivity	Broad emission band, poor thermal stability, susceptible to environmental interference	Industrial temperature measurement, low-cost rapid detection

provides a detailed comparison of the key differences between rare earth ion- and transition metal ion-based thermometric materials, offering a clear reference for material selection.

2.2. Single-Energy-Level Luminescence Intensity Thermometry Technology

Single-level luminescence intensity thermometry is a straightforward and direct temperature measurement method, which is based on monitoring the variation of the intensity of a specific emission peak with temperature to reflect the temperature level. This approach can clearly reveal the quantitative relationship between the fluorescence intensity of the emission peak and temperature, as described by Equation (1) [28]:

$$I(T)=I_0\mathrm{e}\mathrm{x}\mathrm{p}(-{\displaystyle \frac{E_a}{K_{B}T}})$$

(1)

where I(T) is the luminescence intensity at temperature T; I₀ is the luminescence intensity at a reference temperature; E_a is the activation energy, related to the energy level structure of rare earth ions; K_B is the Boltzmann constant; and T is the absolute temperature. Generally, when the luminescence intensity and concentration are determined, the luminescence intensity is often most influenced by the energy transfer efficiency. The energy transfer efficiency, in turn, depends on the energy transfer efficiency between the host material and the rare earth luminescent centers, as well as the probability of non-radiative transitions of electrons. Notably, these parameters are constrained by temperature. The effect of temperature changes on the luminescence intensity of fluorescent materials exhibits non-monotonic characteristics, primarily due to two competing microscopic mechanisms. First, changes in temperature directly affect the coupling state between the luminescent centers and the host lattice field, thereby modulating phonon-involved physical processes and enhancing the energy transfer efficiency from the host to the luminescent centers. Second, as the temperature increases, lattice vibrations intensify, increasing the probability of non-radiative relaxation of excited rare earth ions, leading to a decline in luminescence intensity. Although both mechanisms influence luminescence intensity, their dependence on temperature differs, and their dominance varies across different temperature ranges. In the lower temperature range, the enhancement of energy transfer efficiency predominates, while in the higher temperature range, the increase in non-radiative transition probability becomes the dominant factor. Consequently, a distinct turning point emerges in the relationship between fluorescence intensity I and temperature T. When the temperature is below this point, the promoting effect of temperature on the energy transfer process outweighs the inhibitory effect of non-radiative transitions, causing the fluorescence intensity to increase with rising temperature. Beyond this point, the increased number of phonons and enhanced electron–phonon interactions lead to a significant rise in non-radiative transition probability, causing the fluorescence intensity to gradually decline until thermal quenching occurs. Based on this non-monotonic dependence, temperature monitoring can be achieved by observing changes in fluorescence intensity. Additionally, the selection of host materials and luminescent centers plays a crucial role in energy transfer efficiency and non-radiative transition probability. By combining and matching different host materials and luminescent centers, the competitive balance between the two mechanisms can be significantly altered, enabling the regulation of the temperature-sensitive properties of the material.

Zhang et al. [29] prepared SrB₄O₇:Tm²⁺ phosphor via the conventional high-temperature solid-state method and achieved temperature measurement based on the temperature dependence of the ⁴f₁₂⁵d → ⁴f₁₃ (²F_7/2) transition of Tm²⁺ ions. The material exhibited a temperature sensitivity S_r of 3.55% K⁻¹, reaching its maximum at 363 K.

Figure 2b illustrates the principle diagram of single-energy-level luminescence intensity thermometry. When the temperature exceeds the turning point, the probability of non-radiative transitions within the rare earth ion luminescence centers becomes significant, surpassing the energy transfer between the host material and the luminescent centers. As the temperature continues to rise, typical thermal quenching occurs, leading to a decline in fluorescence intensity. Based on this variation, the temperature-dependent spectra corresponding to a single energy level can be obtained. By analyzing and fitting the acquired data, the relationship between luminescence intensity and temperature is derived, thereby enabling temperature determination based on the detected luminescence intensity.

Although single-energy-level luminescence intensity thermometry offers the advantage of a simple principle, it still exhibits several limitations in practical applications. Notably, it strongly depends on the excitation light intensity; even minor fluctuations in excitation light can directly affect the measurement results. Simultaneously, the generated fluorescence signal is susceptible to attenuation due to absorption and scattering, which, to some extent, reduces measurement accuracy. Furthermore, this method shows significant thermal quenching of fluorescence at high temperatures, resulting in markedly weakened signals and imposing high sensitivity requirements on detection equipment. Due to these factors, this technique faces application limitations in complex environments or scenarios requiring high precision, making it difficult to meet the diverse demands of contemporary optical device development.

2.3. Luminescence Intensity Ratio Thermometry Based on Thermally Coupled Energy Levels (TCLs)

2.3.1. Traditional Luminescence Intensity Ratio Thermometry Based on Thermally Coupled Levels

The traditional single-energy-level luminescence intensity thermometry faces significant challenges in practical applications due to various influencing factors such as environmental interference and material stability. In recent years, luminescence intensity ratio thermometry based on thermally coupled energy levels (TCLs) has been proposed and extensively studied [30,31]. This technique enables temperature measurement by monitoring the variation with temperature in the ratio of fluorescence intensities corresponding to transitions from two thermally coupled energy levels within the same luminescent center [32]. As early as 1976, Kusama et al. [33] first proposed the fluorescence intensity ratio (FIR) technique based on the Boltzmann distribution of thermally coupled energy levels (TCLs) for temperature detection. Figure 3 provides a schematic diagram illustrating the mechanism of TCL-based thermometry. In 1990, Berthou and colleagues [34] first observed thermally coupled energy level phenomena in Er³⁺ ions within an Er³⁺/Yb³⁺ co-doped fluoride system and conducted temperature sensing research based on this observation. With the growing interest in TCL-based FIR thermometry, researchers have identified several rare earth ions possessing thermally coupled energy levels suitable for FIR-based temperature measurement, including Eu³⁺, Er³⁺, Sm³⁺, Dy³⁺, Pr³⁺, Yb³⁺, Tm³⁺, and Ho³⁺. Notably, Yb³⁺ is frequently employed as a sensitizer due to its large absorption cross-section at 980 nm, enhancing the luminescence intensity of other rare earth ions such as Er³⁺, Nd³⁺, Tm³⁺, and Ho³⁺ [35].

Table 2. Common thermal coupling energy levels and energy level spacings of rare earth ions.

Rare Earth Ion	Transitions	ΔE/cm−1	Ref.
Er3+	4S3/2, 2H11/2	800	[36]
Dy3+	4I15/2, 4F9/2	1100	[37]
Pr3+	3P0, 3P1/1I6	650	[38]
Ho3+	5G6/5F1, 5F2,3/3K8	1500	[39]
Tm3+	3F2,3, 3H4	1600	[40]
Nd3+	4F7/2, 4F3/2	1700	[41]
Eu3+	5D1, 5D0	1700	[42]

provides a detailed list of the common thermally coupled levels and their corresponding energy level spacings for the aforementioned rare earth ions.

Generally, the energy level difference of thermally coupled energy level pairs is relatively small, typically in the range of 200–2000 cm⁻¹ [33]. If the energy level spacing between TCLs is too small (<200 cm⁻¹), it becomes difficult to separate the emission peaks from different energy levels. Conversely, if the spacing is too large (>2000 cm⁻¹), low-temperature decoupling may occur [43,44,45]. As the temperature increases, electrons in the lower energy level are thermally excited to the higher energy level. The distribution of electrons between these two energy levels follows Boltzmann statistics. In this scenario, the electron population of the two levels can be inferred using the FIR of the TCLs, thereby enabling temperature calculation. Since the luminescence intensity of thermally coupled levels is proportional to the population, the FIR formula for TCLs can be expressed as follows [6,46,47]:

$$\mathrm{FIR} = {\displaystyle \frac{I_2}{I_1}}={\displaystyle \frac{N_2}{N_1}}={\displaystyle \frac{g_2{\sigma }_{20}{\omega }_2}{g_1{\sigma }_{10}{\omega }_1}}\exp (-{\displaystyle \frac{∆E}{k_{B}T}})=A\mathrm{e}\mathrm{x}\mathrm{p}(-{\displaystyle \frac{∆E}{K_{B}T}})$$

(2)

I₁, N₁, g₁, σ₁₀, and ω₁ represent the luminescence intensity, population, degeneracy, emission cross-section, and fluorescence transition angular frequency of the upper energy level (J = 2) and lower energy level (J = 1), respectively; A = g₂σ₂₀ω₂/g₁σ₁₀ω₁; ΔE is the energy difference between the thermally coupled levels; K_B is the Boltzmann constant; and T is the absolute temperature. The formula indicates that the FIR increases monotonically with temperature, and for a given ΔE, the FIR depends solely on temperature. Therefore, temperature measurement can be achieved based on the FIR of thermally coupled levels. The thermally coupled level pair ²H_11/2–⁴S_3/2 of Er³⁺ (energy gap ΔE ≈ 800 cm⁻¹), known for its high transition intensity and moderate energy spacing, has become a classic system for FIR-based optical thermometry.

Tong et al. [48] prepared Na₂YMg₂(VO₄)₃:Er³⁺/Yb³⁺ phosphors via a high-temperature solid-state method. Utilizing the fluorescence intensity ratio (FIR) behavior of the dual green emissions from the ²H_11/2, ⁴S_3/2, and ⁴F_9/2 → ⁴I_15/2 transitions, a relative sensitivity of 1.104% K⁻¹ was achieved at 303 K. Meanwhile, the total green emission intensity surged by 254 times with increasing temperature, demonstrating excellent signal distinguishability.

The team led by Chen [49] synthesized Yb³⁺/Er³⁺/Cr³⁺ co-doped aluminosilicate glass (SiO₂–Al₂O₃–YF₃–Ga₂O₃–NaF–LiF) using melt-quenching and crystallization techniques. Figure 4a illustrates the energy level distribution of Yb³⁺/Er³⁺ in the YF₃ phase and Cr³⁺ in the Ga₂O₃ phase. The thermally coupled energy levels realize temperature measurement via fluorescence intensity ratio (FIR) and lifetime technology, respectively. Meanwhile, spatial isolation suppresses the detrimental energy transfer between ions. As shown in Figure 4b–d, by leveraging the thermally coupled characteristics of Er³⁺ and the H_11/2/⁴S_3/2 energy level thermometry, this glass achieves a maximum relative sensitivity of 0.25% K⁻¹ at 514 K. It exhibits excellent high-temperature sensing performance and provides new insights for related research.

Furthermore, Rao et al. [50] introduced multiple sets of TCLs in the double perovskite CsNaInCl₆:Er³⁺, including ²H_11/2/⁴S_3/2 (ΔE ≈ 865 cm⁻¹) and ⁴G_11/2/²H_9/2 (ΔE ≈ 1160 cm⁻¹). As shown in Figure 4e,f, this system achieved an extremely broad temperature measurement window from 100 to 880 K, with S_r values consistently above 0.3% K⁻¹ throughout the entire range. This work not only verified the role of energy level engineering in expanding the temperature measurement range but also provided new ideas for multi-mode optical thermometry in extreme environments.

The team led by Gao [51] successfully achieved co-doping of Er³⁺/Yb³⁺ in CaLaAl₃O₇. Utilizing the FIR effect of the ²H_11/2/⁴S_3/2 energy levels, a maximum sensitivity of 0.00345 K⁻¹ was attained at 453 K.

In addition to Er³⁺-based oxide thermometric materials, Eu³⁺/Tb³⁺ dual-emission systems also demonstrate outstanding ratiometric thermometric performance in molecular-based materials. Yin et al. [52] fabricated thermally stable metal–organic complexes using Brønsted acidic ionic liquids as functional ligands. Utilizing the thermally coupled energy level response of Eu³⁺/Tb³⁺, the system achieved a maximum relative sensitivity of 3.29% K⁻¹ in the physiological to moderate high-temperature range (303–403 K), accompanied by color changes visible to the naked eye. This provides a new molecular-based material platform for high-sensitivity visual thermometry in mild temperature regions.

Current research on temperature measurement technology based on the luminescence intensity ratio of thermally coupled energy levels has been extensively conducted, covering not only various rare earth ions but also extending to different types of host materials. A brief summary of selected ions is provided in this paper, as shown in

Table 3. Research on temperature sensing based on the fluorescence intensity ratio of thermally coupled energy levels of different rare earths.

Host Lattice: Luminescent Ln3+	Transitions	ΔT/K	Sr, Max/(%·K−1)	Ref.
Ca2.5Hf2.5Ga3O12:Dy3+	4I15/2, 4F9/2	298–523	2.120 (523 K)	[53]
Mg2YVO6:Sm3+	I563/I654	298–448	0.5700 (298 K)	[54]
Ca2MgWO6:Dy3+	4F9/2/6H15/2, 4F9/2/6H13/2	303–483	/	[55]
Er/O-doped crystalline silicon	4I13/2, 4I15/2	4–200	1.170 (50 K)	[56]
NaGdF4:Er3+	2H11/2, 4S3/2	298–573	2.130 (416 K)	[57]
CaLa3(SiO4)3O:Dy3+	4I15/2, 4F9/2	298–548	0.1673 (298 K)	[58]
GdPO4:Dy3+	4I15/2, 4F9/2	290–530	1.550 (290 K)	[59]
YAG:Dy3+	4I15/2, 4F9/2	540–1542	0.633 (540 K)	[60]
CaWO4:Er3+	2H11/2, 4S3/2	98–773	1.080 (298 K)	[61]
CaWO4:Dy3+	4I15/2, 4F9/2	300–475	1.700 (300 K)	[62]

. In summary, through a synergistic strategy of host selection, energy level design, and process optimization, the Er^{3+ 2}H_11/2/⁴S_3/2 system continues to set new records in key indicators such as sensitivity, linearity, and temperature measurement range. This lays a solid material foundation for non-contact temperature monitoring in biological in vivo applications, micro-/nano-electronic devices, and high-temperature extreme scenarios.

2.3.2. Advanced Level Engineering

Beyond conventional thermally coupled levels (TCLs) and non-thermally coupled levels (NTCLs), advanced level engineering has emerged as a core strategy to break the traditional sensitivity limit (S_r ≤ ΔE_eff/(k_BT²)) and significantly improve thermometric performance. This approach precisely manipulates the energy level structure of rare earth ions via crystal field modulation, host engineering, and ion doping, enabling novel thermometric pathways with enhanced sensitivity, extended temperature ranges, and improved signal discriminability. Stark sublevel engineering leverages crystal field-induced splitting of 4f levels, enabling high sensitivity from subtle temperature-dependent population redistribution [63,64]. Three-level cascaded transitions construct sequential energy transfer pathways, improving sensitivity while suppressing excitation power dependence [65]. Rational regulation of energy gaps via host design and co-doping further optimizes thermometric performance.

For instance, Ding et al. [66] demonstrated that the fine splitting of Er³⁺ energy levels into multiple Stark sublevels enables ultrasensitive thermometry in β-NaYF₄:Yb³⁺/Er³⁺@NaGdF₄ core–shell nanoparticles. By constructing luminescence intensity ratios from thermally coupled Stark sublevels (²H_11/2(1)(2) and ⁴S_3/2(1)(2)(3)), the maximum relative sensitivity was enhanced from 10.9 × 10⁻³ K⁻¹ (conventional LIR) to 17.8 × 10⁻³ K⁻¹ (Stark-LIR), highlighting the great potential of Stark-level engineering for high-sensitivity optical thermometry.

Furthermore, Xiang et al. [65] proposed a novel three-level thermometry strategy based on the Er³⁺:⁴F_7/2/²H_11/2/⁴S_3/2 system in YNbO₄:Yb³⁺/Er³⁺ phosphors. Benefiting from the large energy gap (~2000 cm⁻¹) between ⁴F_7/2 and ⁴S_3/2, and the intermediate ²H_11/2 level that effectively suppresses thermal decoupling, an ultra-high relative sensitivity of 2.67% K⁻¹ was achieved, which is significantly superior to most conventional TCL-based thermometers.

In addition, customized energy gap regulation via host lattice design and co-doping further optimizes thermometric performance. By tuning the crystal field strength, phonon energy, and local coordination environment, the energy difference between coupled levels can be precisely controlled within the optimal range (typically 500–2000 cm⁻¹), balancing high sensitivity and good signal separation. Combined with defect engineering and energy transfer modulation, advanced-level engineering not only addresses the limitations of traditional thermometric mechanisms but also provides a solid theoretical basis for the development of next-generation high-performance rare earth luminescence thermometers.

2.4. Luminescence Intensity Ratio Thermometry Technology Based on Non-Thermally Coupled Levels

Generally, a smaller ΔE may cause significant overlap between the two emission peaks, while a larger ΔE can lead to an insufficient population of electrons in the higher-energy state within a given temperature range, resulting in decoupling [43,44,45]. Theoretically, the relative sensitivity S_r is proportional to ΔE, meaning that a larger energy level spacing generally leads to better temperature sensing performance. However, in practice, due to the constraints of ΔE between TCLs, improving relative sensitivity becomes a contradictory issue. Compared to the FIR technique based on TCLs, the FIR technique based on non-thermally coupled levels (NTCLs) is no longer limited by the energy gap, enabling high signal discriminability, as emission peaks can be chosen from distinct spectral regions. NTCLs feature dual-emission centers, and the luminescence from these two centers exhibits different temperature-dependent behaviors, allowing the construction of a non-coupled energy level thermometric model [67]. Although the physical mechanism behind it has not yet been fully clarified, this model scheme has been experimentally verified and widely adopted for temperature detection. The following formula can be approximately used for fitting [46]:

$$\mathrm{FIR} = A\exp \left(-{\displaystyle \frac{B}{T}}\right)+C$$

(3)

Since 2021, to overcome the limitations of traditional thermally coupled level (TCL) thermometry in terms of sensitivity or temperature range, researchers have turned their attention to “non-TCL” strategies involving dual-emission centers or multi-center synergy. Nozha Ben Amar et al. [68] synthesized Y₂Mo₃O₁₂:2% Pr³⁺/15% Yb³⁺ nanostructures using the sol–gel method. By employing the emissions of Pr³⁺ (³P₀ → ³F₂, 705 nm) and Yb³⁺ (²F_5/2 → ²F_7/2, 980 nm) as non-coupled channels for temperature sensing, this system achieved a maximum relative sensitivity of 2.00% K⁻¹ over the temperature range of 298–648 K, significantly surpassing most TCL-based materials in the same temperature interval.

The team led by Ding [69] prepared Yb³⁺/Ho³⁺ co-doped GYTO single crystals via the Czochralski method and realized temperature sensing based on the fluorescence intensity ratio of non-thermally coupled levels of Ho³⁺. As shown in Figure 5a,b, the crystal exhibits two-photon upconversion luminescence under 980 nm excitation, reaching a maximum relative sensitivity of 0.0037 K⁻¹ within the range of 330–660 K, which outperforms most phosphor-based matrices and demonstrates promising prospects for optical thermometry.

Zhou et al. [70] constructed LiYF₄:Ho@LiYF₄:Yb@LiYF₄ nanoparticles with a multilayer core–shell structure. By spatially separating the sensitizer Yb³⁺ and the activator Ho³⁺, they suppressed back-energy transfer and enhanced upconversion luminescence through interfacial energy transfer. As illustrated in Figure 5c,d, this system achieved red-to-green ratio thermometry based on non-thermally coupled levels of Ho³⁺, attaining a relative sensitivity of 15.1% K⁻¹ at 50 K. Furthermore, the introduction of a Tm³⁺ shell extended the temperature measurement range to 443 K.

Zhang et al. [71] constructed Nd³⁺-based double perovskite Cs₂NaNdCl₆ co-doped with Tm³⁺, Er³⁺, and Yb³⁺. Benefiting from its large unit cell and low phonon energy, concentration quenching under high doping levels was effectively suppressed. As shown in Figure 5e,f, the material realized multi-mode fluorescence intensity ratio thermometry on non-thermally coupled levels (e.g., I₅₆₀/I₅₅₅ and I₅₄₇/I₄₅₅), achieving a maximum relative sensitivity of 9.5% K⁻¹ in the range of 80–340 K, demonstrating significant advantages for wide-range, high-sensitivity optical temperature sensing.

For non-thermally coupled level thermometry, Zhang et al. [72] effectively enhanced the upconversion luminescence efficiency of YOF:Er/Tm/Yb nanocrystals by doping with metal ions, such as K⁺, and markedly improved their non-thermally coupled level fluorescence intensity ratio thermometric performance. When the K⁺ doping concentration reached 40 mol%, the material exhibited an absolute sensitivity of 116.41 × 10⁻³ K⁻¹ under the NTCL strategy—more than ten times higher than that achieved with the thermal-coupling strategy—highlighting its superior sensitivity and application potential in the field of non-contact optical thermometry.

In line with the development of non-thermally coupled level thermometry in coordination systems, Zhernakov et al. [73] systematically explored Dy³⁺/Eu³⁺ and Tb³⁺/Sm³⁺ dual-center coordination compounds as novel thermometric platforms. Benefiting from the favorable energy level structure of these new ion pairs, the materials achieved a maximum relative sensitivity of 4.11% K⁻¹ within the 253–353 K range. This work provides new, high-performance alternatives to conventional Tb³⁺/Eu³⁺ systems, advancing the development of molecular-based luminescence thermometers.

In addition to inorganic hosts, molecular and coordination compounds represent another major and indispensable category for lanthanide luminescent thermometers, especially owing to their structural tunability, easy functionalization, and favorable biocompatibility. A representative summary of the optical thermometric parameters for typical molecular-/coordination-based lanthanide thermometers operating via the non-thermally coupled intensity ratio (NFIR) mechanism is presented in

Table 4. Partial optical thermometric parameters of molecular- and coordination-based lanthanide thermometers.

Material System	Sr, Max/(%·K−1)	ΔT/K	λex	λem	Ref.
Eu4L4	2.04	250–320	380	497, 615	[74]
Ln-CPs (Sm, Eu, Gd, Tb, Dy)	8.41	305–340	334	545, 612	[75]
Tb1−xEux-TPDB	7.32	291–321	338	542, 615	[76]
Tb0.96Eu0.04-HS	16.8	273–333	296	546, 616	[77]
Tb0.99Eu0.01-BDC-F4	0.76	50–300	303	544, 619	[78]
La1−xLnx(BDC)1−γ(ABDC)γCl(DMF) (Ln = Eu, Tb, Dy, Sm)	11.1 (10 K); 2.2 (150 K)	10–330	300	390–600 (ligand); 545 (Tb3+); 613 (Eu3+)	[79]

.

2.5. Luminescence Temperature Sensitivity Parameters: S_a, S_r, and δT

The performance of temperature sensing devices is typically characterized by absolute sensitivity (S_a), relative sensitivity (S_r), and temperature resolution (δT). The formula for absolute sensitivity is given below [80]:

$$S_{\mathrm{a}}={\displaystyle \frac{\mathrm{dLIR}}{\mathrm{d}T}}$$

(4)

S_a is defined as the absolute change in the measured LIR per 1 K change in temperature, with units of K⁻¹. It can be used to compare the performance of temperature sensors based on the same measurement principle. For temperature sensing devices employing different measurement principles, relative sensitivity is generally used for comparison. It is defined as the relative change in the measured LIR per 1 K change in temperature, with units of % K⁻¹. Its formula is shown below [32]:

$$S_{\mathrm{r}}={\displaystyle \frac{1}{\mathrm{LIR}}}{\displaystyle \frac{\mathrm{dLIR}}{\mathrm{d}T}}$$

(5)

Furthermore, temperature resolution (δT) indicates the minimum detectable temperature change by the temperature sensor, with units of K.

$$\delta T={\displaystyle \frac{1}{S_{\mathrm{r}}}}{\displaystyle \frac{\mathrm{dLIR}}{\mathrm{LIR}}}$$

(6)

In this formula, δLIR/LIR represents the relative uncertainty of the LIR, which is primarily influenced by the detection setup and the luminescent material. The signal-to-noise ratio of the detector and the luminescence efficiency of the material are key factors affecting the δT value. In addition, the stability of the sample and the data acquisition conditions can also introduce errors into δLIR/LIR. Therefore, achieving an excellent δT value relies mainly on relative sensitivity (S_r value), optimal measurement conditions, and high-efficiency luminescent performance of the material.

However, it is crucial to clarify that the relative uncertainty δLIR/LIR (and consequently δT) is not merely determined by the detector’s SNR, excitation stability, or material photostability. Recent explicit error propagation models demonstrate that δLIR/LIR is often heavily dominated by systematic spectral and environmental artifacts, which are frequently overlooked in idealized descriptions of thermometric performance. These effects introduce temperature-independent biases that severely degrade the accuracy and reproducibility of luminescence thermometry, even when high-sensitivity materials and optimized instruments are used.

For instance, Rafael Vieira Perrella et al. [81] demonstrated that background superposition between the host matrix’s broad emission and rare earth signals severely distorts the measured LIR in YVO₄:Eu³⁺ systems. Uncorrected overlap with the VO₄^3- host emission inflated temperature uncertainties by nearly an order of magnitude and underestimated relative sensitivity by more than 60%, revealing that matrix luminescence is a critical yet often ignored source of error.

High-power excitation artifacts present another major challenge, as shown by Allison R. Pessoa et al. [82] in single-particle upconversion studies of Y₂O₃:Yb³⁺/Er³⁺. Under high irradiance, higher-order non-thermally coupled upconversion bands spectrally overlap with thermally coupled levels, introducing significant power-dependent readout uncertainties that cannot be eliminated by simple improvements in detector SNR.

Furthermore, photonic environmental artifacts were systematically investigated by Freddy T. Rabouw et al. [83] in NaYF₄:Er³⁺ and Ho³⁺ systems. They found that modulation of the local density of optical states (LDOS) by scattering or reflecting interfaces can distort emission spectra independent of the thermometer’s intrinsic properties, leading to massive ratiometric readout errors of up to hundreds of degrees.

These examples collectively illustrate that achieving reliable, high-precision luminescence thermometry requires not only high-performance materials and instrumentation but also rigorous consideration of systematic artifacts. Moving forward, the development of robust correction strategies—including advanced spectral deconvolution, power-dependent calibration protocols, and environmentally insensitive ratiometric designs—will be essential to translate laboratory demonstrations into practical, real-world applications. Addressing these challenges will be key to unlocking the full potential of luminescence thermometry in complex biological, industrial, and photonic environments.

2.6. Multi-Modal Collaborative and Self-Calibrating Thermometry Strategies

To overcome the limitations of single-modal FIR thermometry, multi-modal and self-calibrating strategies have been widely explored. Integrating FIR with luminescence lifetime measurements eliminates excitation power dependence and concentration interference. Time-gated detection effectively rejects autofluorescence in biological media, enabling deep tissue thermometry. Additionally, combining photoluminescence with Raman scattering or colorimetric analysis broadens the temperature range and improves sensitivity, while NIR-II window emission further enhances biocompatibility and detection depth.

Table 5. Comparison of key thermometric modalities.

Modality	Principle	Advantages	LimitationSe	δT Range
FIR (TCLs)	Boltzmann distribution of TCLs	Self-referenced, simple	Sensitivity ceiling, limited ΔE	0.1–1 K
FIR (NTCLs)	Dual-center independent temperature response	Ultra-high sensitivity, large spectral separation	Complex mechanism, limited material design	0.01–0.5 K
Luminescence Lifetime	Temperature-dependent decay rate	Immune to concentration/scattering, self-calibrating	Requires lifetime detection equipment	0.05–0.5 K
Multi-Modal (FIR + Lifetime)	Combined FIR and lifetime readouts	High accuracy, strong anti-interference	Complex data processing	0.01–0.2 K

presents a comparison of key temperature measurement methods.

2.6.1. Fluorescence Lifetime Thermometry

Luminescence lifetime (τ) of RE³⁺ ions is intrinsically immune to concentration fluctuations, scattering, and excitation power variations, making it a robust self-calibrating readout [84]. The temperature dependence of (τ) arises from enhanced non-radiative decay at higher temperatures.

A representative example is the Er³⁺/Tm³⁺ co-doped NaYF₄ core–shell nanoparticle system reported by Raza et al. [85], where a luminescence lifetime ratio (LLR) strategy was proposed to further boost performance in biological media. As shown in Figure 6a, unlike conventional single-lifetime thermometry, this approach leverages the opposite temperature responses of Tm³⁺ (800 nm emission, decreasing lifetime with temperature) and Er³⁺ (1530 nm emission, increasing lifetime with temperature) within the biological window, yielding a high relative sensitivity of 0.36% K⁻¹. Notably, using the thermometric method and experimental results presented in Figure 6b,c, the LLR method maintained stable calibration curves in ex vivo experiments with 1–3 mm thick chicken tissue, whereas the traditional luminescence intensity ratio (LIR) method suffered severe signal distortion and measurement drift, demonstrating the superior anti-interference capability of lifetime-based readouts in complex biological environments.

2.6.2. Multi-Modal Synergistic Thermometry

Multi-modal synergistic thermometry integrates multiple optical parameters, such as luminescence intensity, lifetime, and Raman scattering, to overcome the limitations of single-mode thermometry. This strategy effectively mitigates environmental interference, extends the temperature sensing range, and significantly enhances detection sensitivity by combining the complementary advantages of different readout mechanisms [86].

A typical example is the Raman–photoluminescence intensity ratio (RPIR) method proposed by De et al. [87], which synergistically uses the temperature-dependent responses of photoluminescence and Raman signals in Eu³⁺-doped BaTiO₃ phosphors. The PL intensity decreases with temperature, while the Raman intensity increases, forming a highly sensitive RPIR parameter. This system achieves an ultra-high relative sensitivity of 2.4% K⁻¹ at room temperature and maintains excellent performance over a wide temperature range from 10 K to 573 K, demonstrating the great potential of multi-modal strategies for high-precision optical thermometry.

2.6.3. Time-Gated Detection for Biological Applications

Time-gated detection suppresses autofluorescence from biological tissues by collecting RE³⁺ luminescence (long lifetime, µs–ms) after a delay, rejecting short-lived autofluorescence (ns). This technique enables precise deep tissue thermometry (>5 mm) in vivo, as it effectively isolates the probe signal from background noise that plagues conventional steady-state measurements.

A representative demonstration of this advantage was reported by Kurahashi et al. [88], who integrated time-gated imaging with lifetime-based thermometry using Nd³⁺/Yb³⁺ co-doped β-NaYF₄ nanoparticles. By applying time-gated detection, they successfully eliminated tissue autofluorescence and achieved clear, high-contrast temperature mapping in deep biological tissues, validating the critical role of this technique in enabling robust, high-precision in vivo optical thermometry.

In summary, multi-modal collaborative and self-calibrating strategies significantly enhance the reliability and practicality of rare earth luminescence thermometry, addressing key issues such as susceptibility to environmental interference and lack of self-calibration capability in single-modal thermometry. In the future, by further integrating emerging mechanisms, such as NIR-II/NIR-III band luminescence, defect energy levels, and charge transfer bands, and combining artificial intelligence algorithms to optimize multi-parameter data processing, it is expected to construct a rare earth luminescence thermometry system with higher precision, better stability, and closer alignment with practical application scenarios, providing new technical support for biomedical applications, high-temperature aerospace environments, and micro-/nano-device thermal management.

2.7. Emerging Mechanisms: Defects and Charge Transfer

2.7.1. Defect Energy Levels

Beyond the conventional thermally coupled level (TCL) strategy, defect-related luminescence has emerged as a novel and promising mechanism for optical thermometry. Defects such as oxygen vacancies, interstitial ions, or surface traps often exhibit distinct thermal quenching behavior compared to lanthanide f–f transitions [89]. By combining defect emission with lanthanide luminescence, self-calibrated ratiometric thermometers can be constructed, offering high sensitivity and anti-interference capability against excitation power fluctuations and environmental noise.

For instance, Drabik et al. [90] demonstrated this concept in Tb³⁺-doped Y₂O₃ and Lu₂O₃ nanocrystals, where defect-related luminescence and Tb³⁺ f–f transitions exhibited opposite thermal quenching behaviors. By combining these two signals, they constructed a self-calibrated ratiometric thermometer with a high relative sensitivity of up to 4.92% K⁻¹, effectively eliminating interference from excitation power fluctuations.

2.7.2. Charge Transfer Bands (CTBs)

Charge transfer bands (CTBs), including ligand-to-metal charge transfer (LMCT) and metal-to-metal charge transfer (MMCT), represent another emerging pathway for luminescence thermometry. Unlike lanthanide f–f transitions, which are typically weakly dependent on temperature, CTBs often show strong and even anomalous thermal quenching responses. By leveraging the opposite or complementary temperature dependencies between CTB and f–f emission, highly sensitive ratiometric thermometers can be realized, breaking the sensitivity bottleneck of traditional TCL-based methods [91].

For example, Zhou et al. [92] developed an optical thermometer by exploiting the abnormal thermal quenching of LMCT bands in rare earth-doped phosphors. The LMCT band showed a remarkable thermal enhancement with increasing temperature, while the Eu³⁺/Nd³⁺ f–f emissions underwent conventional thermal quenching. By establishing the intensity ratio of CTB to f–f transition, an ultra-high relative sensitivity was achieved, breaking the limitation of traditional thermally coupled level-based sensors.

Overall, defect- and CTB-based luminescence thermometry offer complementary advantages: defect levels provide robust anti-interference performance, while CTBs enable ultra-high sensitivity. Both mechanisms advance the development of next-generation optical temperature sensors beyond conventional TCL strategies.

3. Applications of Multi-Rare Earth Luminescence Intensity Thermometry Technology

The primary applications of rare earth luminescence intensity thermometry are concentrated in two key temperature domains: the physiological temperature range (approximately 298–350 K) and ultra-high-temperature environments (usually above 900 K). These two fields impose distinctly different requirements on the temperature sensing performance of materials.

In contrast, for ultra-high-temperature thermometry, the foremost and most stringent requirement is that the material itself can withstand extremely high-temperature environments while maintaining stable luminescence performance. For example, fluorescent temperature-sensitive coatings applied to aerospace engines often need to operate above 1000 K. This demands that the material not only possesses excellent thermal stability but also maintains a sufficiently strong luminescence intensity at elevated temperatures, posing a dual challenge for material design.

In summary, the practical application of this technology clearly leads to differentiated material performance requirements. Future research urgently needs to take these specific application needs as the fundamental starting point for targeted material design and development, thereby promoting the translation of scientific value into practical significance. Figure 7 summarizes the core requirements for temperature measurement performance in the two temperature intervals mentioned above.

3.1. Temperature Imaging and Localization in Microenvironments of Biological Tissues

Temperature plays a fundamental regulatory role in biomedical processes. It not only profoundly modulates the metabolic rate and gene expression patterns of organisms, but even minor fluctuations can trigger significant changes in physiological functions. Clinically, an increase in body temperature is often regarded as a key biomarker of infection, inflammation, and various diseases [93]. Therefore, developing precise and sensitive temperature monitoring technology holds important practical significance for early disease warning and health management.

In recent years, with advancements in nanomaterials and micro-/nano-fabrication technologies, researchers have been dedicated to developing functional temperature sensing units at the micrometer or nanometer scale. Such thermometers typically leverage the dependence of optical responses—such as variations in fluorescence intensity, luminescence lifetime, or spectral shift—on temperature to accomplish non-contact temperature measurement in microenvironments [94]. Related technologies are continuously expanding toward high spatial resolution, real-time monitoring, and biocompatibility, providing innovative tools for cutting-edge applications such as in vivo cell thermometry and tumor hyperthermia monitoring.

In 2022, Meng et al. [95] successfully prepared core–shell structured nanoprobes NaYF₄:Yb³⁺/Tm³⁺@NaYF₄:Yb³⁺/Er³⁺. Under 980 nm excitation within the biological window, the upconversion luminescence of these probes exhibited a penetration depth exceeding 3 cm. Based on the Tm³⁺ 700/646 nm energy level pair, a relative sensitivity S_r of 2.155% K⁻¹ was achieved, with a resolution δT lower than 0.0139 K. Figure 8 demonstrates the potential of this sample for in vivo temperature measurement via fiber-optic sensors.

Pratikshya Parajuli et al. [96] fabricated multilayer PDMS-La₂O₂S:Eu³⁺ phosphor composite films using spin-coating technology, enabling temperature sensing from −40 to 75 °C. The films exhibit excellent thermal responsiveness, mechanical flexibility, tunable thermal conductivity, and biocompatibility.

Tian et al. [97] employed the Sc₂(MoO₄)₃ matrix, which exhibits low phonon energy and excellent chemical stability. Through tri-doping with Nd³⁺/Yb³⁺/Er³⁺ and excitation by an 808 nm laser, localized heating with low tissue damage was achieved. In this system, the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ produces an emission at 1538 nm, located within the NIR-II window. As the temperature increased from 303 K to 403 K, the emission intensity was enhanced by approximately 127.5 times, while the lifetime extended from tens of microseconds to 204 μs. The temperature sensing sensitivity S_r based on the lifetime temperature dependence reached as high as 6.03% K⁻¹ at 323 K, significantly surpassing most rare earth-doped systems. Furthermore, by utilizing a hygroscopicity control strategy of Nd³⁺, the doping concentration could be precisely regulated to further balance excitation efficiency and thermal response, offering a new material platform for real-time, non-invasive temperature mapping of living tissues.

Building on these advances in NIR-II luminescence thermometry, recent research has further expanded into the NIR-III biological window (1500–1850 nm), where drastically reduced light scattering and tissue absorption enable deep tissue temperature measurements beyond 5 mm. This spectral range is particularly attractive for in vivo applications, as it minimizes photodamage and improves the signal-to-noise ratio, opening new avenues for high-performance, non-invasive temperature sensing in living systems [98].

Specifically, Mengmeng Dai et al. [99] reported that, benefiting from the prominent negative thermal expansion of ScF₃ nanocrystals in the Yb³⁺–Ho³⁺–Tm³⁺ co-doped system, excitation energy is efficiently guided to the target emitting states, enabling intense NIR-III emission at ~1625 nm (Tm³⁺: ³F₄ → ³H₆). This work first unlocked the high-performance NIR-III luminescence thermometry based on the ScF₃ host, which significantly expands the detection range into the deep near-infrared region, provides a low-background and deep tissue sensing channel for biological applications, and lays a solid foundation for developing multi-modal ratiometric nanothermometers with ultra-high sensitivity.

Collectively, biological luminescence thermometry has advanced significantly toward deeper tissue penetration, shifting focus from conventional optical windows to NIR-II and NIR-III bands. However, several key challenges remain to be addressed. Improving luminescence quantum yields in longer-wavelength regions, validating the long-term biocompatibility of nano-scale probes for in vivo use and developing high-sensitivity, real-time imaging systems compatible with deep tissue measurement. Looking ahead, future work should prioritize optimizing host matrices and energy transfer pathways to enhance signal brightness, conducting systematic safety evaluations, and integrating advanced optical detection techniques to translate these technologies into reliable tools for biological and clinical applications.

3.2. Thermal Management and Non-Contact Monitoring in High-Temperature Aerospace Environments

In extreme environments, such as high temperatures and strong radiation, the performance requirements for temperature sensing materials become exceptionally stringent. These materials must not only withstand extremely high temperatures but also possess excellent radiation resistance to maintain stable and accurate temperature measurement functions under complex working conditions. Identifying materials with outstanding chemical stability, thermal stability, and radiation resistance has become an ideal solution to meet these demanding needs.

Mikołaj Łukaszewicz et al. [100] synthesized Y₂O₃:Er³⁺ and Yb³⁺ with an extremely wide temperature range via the nitrate decomposition method. By utilizing the ²H_11/2/⁴S_3/2 TCL-FIR mode of Er³⁺, temperature sensing was achieved over 175–1100 K. Combined with the intrinsic thermal radiation signal of the matrix, the monitoring range was further extended to 1300–2100 K, enabling continuous temperature detection from 400 to 2100 K. The FIR sensitivity reached 1.4% K⁻¹ at 300 K, making this material an ideal candidate for real-time temperature mapping of aerospace thermal protection layers.

Wang [101] discovered that KGaGeO₄:Yb/Nd exhibits an anomalous negative thermal quenching phenomenon (intensity increased by 10²–10³ times) within the temperature range of 298–653 K, with a thermoluminescence sensitivity (S_r) as high as 4.7% K⁻¹, making it suitable for extreme temperature environments.

Li et al. [102] reported SrGdLiTeO₆:Mn⁴⁺/Tb³⁺ phosphors as thermal quenching warning materials. By leveraging the intrinsic difference between the easily quenched red emission of Mn⁴⁺ and the stable green emission of Tb³⁺, the material exhibits a visually distinguishable red-to-green color change warning above 500 K. Simultaneously, dual-mode thermometry combining FIR and lifetime achieved an S_r value of 1.88% K⁻¹ at 573 K, realizing an integrated system of “visual warning + quantitative temperature measurement.” This provides a simple and reliable optical label for safety monitoring in high-temperature work environments.

Overall, these examples demonstrate three key advances in high-temperature aerospace luminescence thermometry: ultra-wide-range temperature monitoring, negative thermal quenching (NTQ) materials, and dual-mode integrated sensing. However, critical challenges remain, including maintaining long-term stability under combined high-temperature and radiation conditions, balancing ultra-high sensitivity with anti-interference performance, and scaling these lab-level materials into practical, deployable aerospace monitoring systems.

3.3. Exploration of Applications in Optoelectronic Systems and Micro-/Nano-Device Integration

As devices such as chips and micro-LEDs progressively advance toward sub-millimeter or even micrometer scales, traditional thermocouples or infrared thermal imagers can no longer simultaneously meet the demands for spatial resolution, response speed, and system compatibility. Luminescent thermometric materials, owing to their capabilities of being fabricated into thin films, patterned, and read out in multiple modes, are gradually becoming core candidate solutions for micro-/nano-scale thermal management and the integration of functional optoelectronic systems.

Chen et al. [103] synthesized Sr₉In(PO₄)₇:Yb³⁺/Er³⁺ phosphors, which exhibit high-color-purity green emission (>90%). Upon temperature increase, the green emission continuously shifts toward blue-green, a change discernible to the naked eye. The research team directly inkjet-printed these phosphors into dynamic QR codes, achieving both FIR-based temperature measurement over 298–473 K (S_r = 1.13% K⁻¹) and imparting an anti-counterfeiting feature of temperature-dependent color change to the product. This provides an integrated label with “one-print, dual-function” capability for cold chain logistics and high-end electronic products.

Mikhail A. Kurochkin [104] synthesized nano-sized Gd₂O₃:Tb³⁺/Eu³⁺ dual-center phosphors via the Pechini foam method, successfully realizing remote temperature measurement of micro-electronic components in the range of 303.15–363.15 K and verifying their application on resistors and microcontrollers.

Xia et al. [105] synthesized SrLaLiTeO₆:Eu³⁺ phosphors by a high-temperature solid-state method. Based on a dual-excitation single-emission-band ratio (SBR) strategy, they achieved both temperature measurement and latent fingerprint development, capable of resolving Level I–III details.

Chen et al. [106] synthesized Cs₂NaErCl₆:Yb³⁺ via a solvothermal method and fabricated flexible films using PDMS, developing a self-calibrating triple-mode optical thermometer (TCLs/NTCLs/luminescence lifetime) with a thermal resolution of 0.07 K, suitable for precise monitoring of hotspots in electronic components.

Wang et al. [107] synthesized KBT:Er/In transparent ceramics using the conventional high-temperature solid-state method. These ceramics maintain an ultra-high sensitivity of S_r = 8.73% K⁻¹ even in the low-temperature region (<200 K). Simultaneously, Er/In co-doping induced photochromic centers, enabling reversible modulation of luminescence intensity by 76.59% under 405 nm laser irradiation, which can be used for rewritable optical storage. The ceramics themselves can be co-sintered with silicon-based optoelectronic arrays, providing an on-chip integrated solution of “temperature sensing—storage” for scenarios such as deep space exploration and low-temperature packaging of quantum devices.

T. H. Q. Vu et al. [108] prepared Ba₂MgWO₆:Er³⁺ phosphors via a co-precipitation method. Owing to their high sensitivity at low temperatures (S_r = 2.78% K⁻¹ @ 198 K) and low-temperature uncertainty (δT < 0.1 K), these phosphors have been verified as suitable for non-contact temperature measurement applications in low-temperature micro-/nano-devices.

In summary, luminescent thermometric materials have evolved from “laboratory spectroscopic tools” into “integratable, programmable, and multifunctional” micro-/nano-photonic components. Through thin-film fabrication processes, patterned printing, and heterogeneous integration technologies, it is envisioned that thermal imaging, anti-counterfeiting traceability, optical storage, and real-time temperature control closed loops could be realized on a single chip in the future, thereby truly bridging the entire chain from “materials to devices to systems.”

4. Technical Challenges and Future Development Directions

4.1. High-Temperature Quenching and Luminescent Stability Issues

Under high-temperature operating conditions, lattice vibrations within the material intensify significantly, leading to a drastic enhancement of multi-phonon non-radiative transition processes. This, in turn, severely weakens the efficiency of radiative recombination. As a result, when the temperature reaches the 400–500 K range, the luminescence intensity of the vast majority of fluorescent thermometric materials exhibits a sharp, “plummeting” decline. This effect fundamentally limits the luminescence performance of materials in high-temperature environments, directly caps the upper limit of their effective temperature measurement capability, and severely constrains the practical application scope of fluorescent thermometry under high-temperature conditions.

In the Ca₂Sb₂O₇ host matrix, Shi et al. [109] introduced Eu³⁺ as a single dopant into the lattice via the conventional high-temperature solid-state method, pioneering a self-referenced thermometry strategy based on a “dual-emission-center” approach. The study indicates that efficient energy transfer from the host to Eu³⁺ not only endows the material with remarkable resistance to thermal quenching—exhibiting a 240% enhancement in emission intensity with increasing temperature—but also achieves an excellent relative sensitivity of 3.369% K⁻¹.

Wang et al. [110] effectively suppressed IVCT-induced non-radiative transitions at high temperatures by introducing Yb³⁺ to regulate the electron population of Pr³⁺. This enabled La₂Mo₃O₁₂:Yb³⁺ and Pr³⁺ to maintain measurement repeatability above 99% even under conditions exceeding 700 K. The temperature cycling tests shown in Figure 9a,b visually demonstrate the stable performance of the material at elevated temperatures.

Building on these anti-quenching strategies, recent advances have introduced a transformative approach using negative thermal expansion (NTE) hosts, which turn thermal quenching into thermal enhancement. Unlike conventional hosts that suffer from luminescence degradation at high temperatures, NTE materials undergo lattice contraction upon heating, shortening interionic distances and boosting energy transfer efficiency, thereby enabling significant upconversion luminescence (UCL) enhancement and promising optical thermometry applications.

Wang et al. [111] developed (KMg)³⁺-doped Sc₂W₃O₁₂:Er³⁺/Yb³⁺ phosphors with exceptional anti-quenching UCL enhancement. Benefiting from the synergistic effect of NTE characteristics and impurity doping, the upconversion intensity at 573 K was enhanced 6000-fold compared with room temperature, and the constructed all-fiber temperature sensor enabled accurate real-time monitoring of CPU chips.

Dai et al. [112] synthesized NTE ScF₃:Yb³⁺/Tm³⁺ nanorods with thermally boosted UCL. The NTE-induced lattice contraction strengthened phonon-assisted energy transfer, leading to a remarkable enhancement of the 698 nm emission. The luminescence intensity ratio (LIR) thermometry achieved a high relative sensitivity of 1.60% K⁻¹, demonstrating great potential for high-sensitivity optical temperature sensing.

While current anti-quenching strategies, including low-phonon-energy hosts and ion co-doping, have raised the temperature ceiling of luminescent thermometry to ~700 K, key challenges persist. Long-term thermal stability, high-temperature measurement repeatability, and scalable material synthesis remain major obstacles to real-world deployment. Furthermore, the precise manipulation of non-radiative transitions and energy transfer pathways under extreme thermal conditions requires deeper mechanistic insights. Future research will likely focus on transformative approaches, such as negative thermal expansion (NTE) hosts, which leverage lattice contraction to reverse thermal quenching, as well as multi-modal sensing designs combining optical signals with structural stability engineering. These innovations are poised to extend the practical temperature range to over 700 K, enabling robust optical thermometry in harsh scenarios such as aerospace thermal management and deep earth resource exploration.

4.2. Signal Accuracy and Suppression of Complex Background Interference

Background noise in complex environments—such as scattering in biological tissues, obscuration by industrial dust, and excitation power drift—has become a critical bottleneck limiting the accuracy of optical thermometry. Taking biological tissue as an example, its strong scattering of visible–near-infrared light significantly attenuates the green emission signal of Er³⁺, while fluctuations in the excitation source power directly amplify errors in single-intensity readings. To break through this dual “environment-instrument” interference, on the one hand, a multi-parameter collaborative strategy is used to collect multi-dimensional information such as FIR, lifetime, and color coordinates simultaneously for cross-validation. For instance, the Ba₃In(PO₄)₃:Er³⁺/Yb³⁺ system studied by Lei et al. [113] employs a quadruple readout combining the FIR, chromaticity coordinate shift, and lifetime variation of Er³⁺, effectively suppressing systematic errors caused by excitation power drift. On the other hand, self-calibrating systems achieve internal reference correction by constructing “sensitive-robust” dual-emission centers within the same particle. A typical example is the Ce³⁺/Tb³⁺ co-doped β-NaYF₄:Ce³⁺ system [114], in which the Ce³⁺ signal is highly sensitive to environmental perturbations while the Tb³⁺ signal remains almost unaffected. The ratio of the two signals can serve as a real-time calibration factor, maintaining a temperature measurement error of <3% in biological scattering or dusty environments without requiring an external reference. Additionally, by utilizing Ln³⁺-stabilized fluorescence as a reference signal, leveraging differential decay mechanisms in organic semiconductor composite systems, and regulating MLCT/IVCT charge transfer, external environmental and cross-sensitivity interferences are effectively suppressed. This ultimately enables temperature measurements with both high sensitivity (up to 22% K⁻¹) and strong anti-interference capability. In addition to these approaches, novel error optimization methods have also been proposed. For example, Quan et al. [60] introduced a wavelength and bandwidth optimization method based on error theory to improve the temperature measurement accuracy of YAG:Dy and YSZ:Dy under high-temperature conditions (540–1542 K). Figure 9c displays the variation of systematic error with temperature for the YAG:Dy material before and after optimization. The blue curve (reference group, i.e., the traditional thermally coupled levels method) shows a systematic error as high as approximately 2.68% (close to 2.5%) at high temperature (1542 K), whereas the red curve (optimized group) maintains a systematic error below 0.5% across the entire temperature range, with a significantly reduced maximum error of merely 0.37% (close to 0.36%) at high temperatures.

In addition, excitation power dependence and laser-induced heating are critical practical limitations of conventional FIR thermometry. High-power continuous wave excitation (>10–50 mW) often causes significant temperature readout drift (10–50 K), severely compromising accuracy. To address this, power-independent calibration protocols, square-wave/pulsed excitation to suppress thermal loading, and laser heating correction algorithms have been developed, effectively reducing measurement errors and improving reliability in practical applications [115,116,117].

For instance, Li et al. [118] systematically investigated the laser power dependence of ratiometric thermometry in CaWO₄:Yb³⁺/Ho³⁺ phosphors, revealing that different non-thermally coupled levels exhibit distinct power-dependent behaviors, which can lead to large temperature errors when excitation conditions change. To address this, power-independent calibration protocols, square-wave/pulsed excitation to suppress thermal loading, and laser heating correction algorithms have been developed, effectively reducing measurement errors and improving reliability in practical applications.

In summary, excitation power drift, laser-induced heating, and complex environmental interferences, such as scattering and dust, have become major bottlenecks limiting the practical accuracy of conventional luminescence thermometry. To address these challenges, multi-parameter collaborative strategies, self-calibrating dual-emission centers, and error optimization methods have been developed. These approaches effectively suppress systematic errors, significantly improve anti-interference capability, and maintain high sensitivity across diverse conditions. Together, these advances highlight the importance of both mechanism optimization and calibration design in achieving reliable, high-precision optical thermometry for real-world applications.

4.3. AI-Assisted Prediction

With the advancement of artificial intelligence (AI) technology, AI-assisted prediction is increasingly becoming a core driver for the rational design of materials and breakthroughs in performance. Traditionally, rare earth luminescent materials, leveraging their unique electronic transition properties, have demonstrated advantages of high sensitivity and multi-modal response in multi-parameter sensing of temperature, stress, chemical environment, etc. However, their performance optimization relied solely on extensive trial-and-error experimentation, resulting in low efficiency. Today, drawing on the successful paradigm of machine learning in the discovery of LED phosphors, research has shifted towards constructing a new pathway of “data-driven → feature engineering → intelligent prediction → experimental closed-loop [119].”

By integrating material databases and high-throughput calculations, research can extract key descriptors and symbols from composition, structure, and local coordination environments. Algorithms such as XGBoost and Support Vector Machines (SVMs) can then be utilized to accurately predict the excitation/emission wavelengths, quantum efficiency, and thermal stability of rare earth ions (e.g., Eu²⁺, Ce³⁺). This enables the targeted screening of materials for specific sensing needs, thereby reducing trial-and-error costs. Active learning and Bayesian optimization methods further allow for efficient exploration within complex multi-component spaces, enabling the rapid identification of high-performance candidate materials with minimal experimental iterations. This approach will significantly accelerate the development of novel rare earth luminescent sensing materials.

Beyond material innovation, advanced data analytics have emerged as a critical direction to address the limitations of conventional ratiometric thermometry, such as subjective parameter selection and low precision [120,121]. Ximendes et al. [122] pioneered the use of dimensionality reduction (DR) techniques (PCA and t-SNE) to process high-dimensional luminescence spectra of rare earth nanoparticles. By extracting core thermometric features from complex spectral data, they achieved a thermal resolution as low as 0.09 °C, outperforming traditional intensity ratio methods. Building on this, Santos et al. [123] further introduced automated machine learning (AutoML) into Nd³⁺:YAG luminescence thermometry. The AutoML model automatically optimized regression pipelines without manual feature screening, reducing temperature uncertainty from 9.8 °C to 1.2 °C and enabling real-time, robust measurements.

In summary, these works highlight the transformative impact of AI-driven techniques. While dimensionality reduction and AutoML address the subjectivity and low-precision issues of traditional methods, convolutional neural networks (CNNs) further advance the field by directly extracting complex thermal features from raw spectra. Despite the remarkable progress, key challenges remain. Current models often lack generalization to real-world environments, and the interpretability of complex algorithms needs to be improved. Future efforts should focus on enhancing model robustness and developing interpretable AI frameworks for practical applications.

5. Conclusions

Rare earth luminescence thermometry, featuring non-contact operation, high sensitivity, superior spatial resolution, and strong anti-interference capability, effectively overcomes the limitations of traditional contact-based techniques, such as thermocouples and infrared thermometers, in micro-/nano-scale, extreme environments, and in vivo biological applications, making it a cutting-edge research hotspot in temperature sensing. This paper systematically reviews the fundamental mechanisms of rare earth luminescence thermometry, including single-level intensity thermometry, thermally coupled level (TCL) ratiometric thermometry, and non-thermally coupled level (NTCL) ratiometric thermometry. The physical meanings and optimization principles of key parameters such as absolute sensitivity (S_a), relative sensitivity (S_r), and temperature resolution (δT) are clarified. The progress of this technology in biomedical imaging, high-temperature aerospace, and micro-/nano-device integration is also summarized, highlighting its irreplaceable value in precision sensing and extreme environment monitoring.

Despite remarkable progress, several challenges remain for broader and more demanding applications: (1) severe thermal quenching in the moderate-to-high temperature range (400–700 K), which limits long-term stability; (2) environmental interference and calibration issues caused by excitation power drift, tissue scattering, and background fluorescence; (3) insufficient adaptability for biological applications, including limited penetration depth of visible/NIR-I signals and poor long-term biocompatibility; and (4) low efficiency in material design, relying heavily on trial and error rather than theoretical guidance.

Looking ahead, several emerging directions are driving further development.

NIR-II thermometry represents a key breakthrough in biomedical research. With weak scattering, deep penetration, and low autofluorescence, it enables non-invasive deep tissue temperature measurement for tumor hyperthermia and in vivo imaging. Intracellular thermometry facilitates micro-scale life science research. Rare earth nanoprobes, with small size, low toxicity, and stable luminescence, allow precise temperature sensing at the single-cell and subcellular levels, supporting studies on metabolism and thermal stress. Self-calibrating probes effectively suppress interference from excitation power, concentration, and matrix background, significantly improving measurement reliability in complex conditions. Multi-modal sensing integrates multiple optical signals to achieve high precision, a wide temperature range, and strong anti-interference performance. Single-particle thermometry meets micro-/nano-scale requirements, offering nanometer spatial resolution for chip hotspot monitoring and quantum material research.

In the long term, rare earth luminescence thermometry will develop toward high performance, practicality, and intelligence. Thermal quenching will be addressed via negative thermal expansion matrices and energy level engineering. Artificial intelligence and machine learning will accelerate rational material design. The integration of NIR-II probes, self-calibrating systems, and multi-modal sensing will promote real-world applications. With interdisciplinary integration, rare earth luminescence thermometry will become an indispensable core technology in precision temperature sensing.