Toward Ultra-High Sensitivity Optical Thermometers and Bright Yellow LEDs Based on Phonon-Assisted Energy Transfer in Rare Earth-Doped La2ZnTiO6 Double Perovskite.

Double perovskites, a class of ceramic oxides with unique crystal structures and diverse physical properties, show promise for various technological applications including solar cells, photodetectors, and light-emitting diodes (LEDs). Despite limited research on rare earth-doped double perovskites, leveraging their ultrahigh luminous efficiency to achieve bright yellow LED emission and addressing energy transfer challenges between Yb3+ and Nd3+ ions in double perovskite La2ZnTiO6 with moderate phonon energy are explored in this work. Through phonon-assisted energy transfer, an ultrasensitive optical thermometer covering a wide temperature range is developed by utilizing the different temperature responses of Er3+ emission in the visible light region and Nd3+ emission in the near-infrared region based on the luminescence intensity ratio (LIR). All the results demonstrate that the rare earth (Yb-Er, Yb-Nd, and Yb-Nd-Er)-doped La2ZnTiO6 phosphors can be effectively utilized for ultrabright LED illumination and ultrahigh sensitivity self-calibrated temperature sensing. This research underscores the significance of phonon-assisted energy transfer in improving material properties and provides valuable insights for the advancement of multifunctional materials.

Engineering Visible to Near-Infrared Luminescence through a Selective Doping Strategy for High-Performance Temperature Sensing.

Luminescence nanothermometers have garnered considerable attention due to their noncontact measurement, high spatial resolution, and rapid response. However, many nanothermometers employing single-mode measurement encounter challenges regarding their relative sensitivity. Herein, a unique class of tunable upconversion (UC) and downshifting (DS) luminescence covering the visible to near-infrared range (400-1700 nm) is reported, characterized by the superior Tm3+, Ho3+, and Er3+ emissions induced by efficient energy transfer. The outstanding negative thermal expansion characteristic of ScF3 nanocrystals has been found to guide excitation energy toward the relevant emitting states in the Yb3+-Ho3+-Tm3+-codoped system, consequently resulting in remarkable near-infrared III (NIR-III) luminescence at ∼1625 nm (Tm3+:3F4 → 3H6 transition), which in turn presents numerous opportunities for designing multimode ratiometric luminescence thermometry. Furthermore, by facilitating phonon-assisted energy transfer in Er3+-Ho3+-codoped systems, the luminescence intensity ratio (LIR) of 4I13/2 of Er3+ and 5I6 of Ho3+ in ScF3:Yb3+/Ho3+/Er3+ exhibits a strong temperature dependence, enabling NIR-II/III luminescence thermometry with superior thermal sensitivity and resolution (Sr = 0.78% K-1, δT = 0.64 K). These findings not only underscore the distinctive and ubiquitous attributes of lanthanide ion-doped nanomaterials but also hold significant implications for crafting luminescence thermometers with unparalleled sensitivity.

Towards core-shell engineering for efficient luminescence and temperature sensing.

Research on the core-shell design of rare earth-doped nanoparticles has recently gained significant attention, particularly in exploring the synergistic effects of combining active and inert shell layers. In this study, we successfully synthesized 8 types of spherical core-shell Na-based nanoparticles to enhance the efficiency of core-shell design in upconversion luminescence and temperature sensing through the strategic arrangement of inert and active layers. The most effective upconversion luminescence was observed under 980 nm and 808 nm laser excitation using NaYF4 inert shell NaYF4:Yb3+, Er3+@ NaYF4 and NaYF4@ NaYF4:Yb3+, Nd3+ core-shell nanostructures. Moreover, the incorporation of the NaYbF4 active shell structure led to a significant increase in relative sensitivity in ratio luminescence thermometry. Notably, the NaYF4:Yb3+, Nd3+, Er3+@ NaYbF4 core-shell structure demonstrated the highest relative sensitivity of 1.12 %K-1. This research underscores the crucial role of inert shell layers in enhancing upconversion luminescence in core-shell structure design, while active layers play a key role in achieving high-sensitivity temperature detection capabilities.

The Art of Nanoparticle Design: Unconventional Morphologies for Advancing Luminescent Technologies.

The advanced design of rare-earth-doped (RE-doped) fluoride nanoparticles has expanded their applications ranging from anticounterfeiting luminescence and contactless temperature measurement to photodynamic therapy. Several recent studies have focused on developing rare morphologies of RE-doped nanoparticles. Distinct physical morphologies of RE-doped fluoride materials set them apart from contemporary nanoparticles. Every unusual structure holds the potential to dramatically improve the physical performance of nanoparticles, resulting in a remarkable revolution and a wide range of applications. This comprehensive review serves as a guide offering insights into various uniquely structured nanoparticles, including hollow, dumbbell-shaped, and peasecod-like forms. It aims to cater to both novices and experts interested in exploring the morphological transformations of nanoparticles. Discovering new energy transfer pathways and enhancing the optical application performance have been long-term challenges for which new solutions can be found in old papers. In the future, nanoparticle morphology design is expected to involve more refined microphysical methods and chemically-induced syntheses. Targeted modification of nanoparticle morphology and the aggregation of nanoparticles of various shapes can provide the advantages of different structures and enhance the universality of nanoparticles.

Optimization of deliquescence-proof perovskite-like Cs3ErF6 phosphor and dual-mode luminescent intensity ratio thermometry.

The susceptibility of Cs-based fluorides to deliquescence has led to the fact that lanthanide-doped Cs-based fluorides and their related applications have hardly been reported. Herein, the method to solve the deliquescence of Cs3ErF6 and its excellent temperature measurement performance were discussed in this work. Initially, the soaking experiment of Cs3ErF6 found that water had irreversible damage to the crystallinity of Cs3ErF6. Subsequently, the luminescent intensity was ensured by the successful isolation of Cs3ErF6 from the deliquescence of vapor by the silicon rubber sheet encapsulation at room temperature. In addition, we also removed moisture by heating samples to obtain temperature-dependent spectra. According to spectral results, two luminescent intensity ratio (LIR) temperature sensing modes were designed. The LIR mode which can quickly respond to temperature parameters by monitoring single band Stark level emission named as "rapid mode". The maximum sensitivity of 7.362%K-1 can be obtained in another "ultra-sensitive mode" thermometer based on the non-thermal coupling energy levels. This work will focus on the deliquescence effect of Cs3ErF6 and the feasibility of silicone rubber encapsulation. At the same time, a dual-mode LIR thermometer is designed for different situations.

A novel differential display material: K3LuSi2O7: Tb3+/Bi3+ phosphor with thermal response, time resolution and luminescence color for optical anti-counterfeiting.

Optical anti-counterfeiting and encryption have become a hotspot in information security. However, the advanced optical anti-counterfeiting technology still suffers from low security by single-luminescent mode. Herein, we present a novel multi-mode anti-counterfeiting strategy based on K3LuSi2O7: Tb3+/Bi3+ (KLSO: Tb3+/Bi3+) phosphors for the first time. KLSO not only provides various lattice sites for Bi3+ ions occupying to achieve tunable luminescence but can also be non-equivalently substituted by Tb3+ ions to produce persistent or thermo-luminescence. Furthermore, in the pattern "8888" constructed by the mixture of polyacrylic acid (PAA) with KLSO: Tb3+/Bi3+ phosphors, we selectively trigger the three luminescent modes of Bi3+ and Tb3+ ions to realize the design of differential display in the fields of thermal response, time resolution, and luminescence color for optical anti-counterfeiting. The differentiated display can only be presented under specific multi-stimuli response, which further improves the security of information. Our work provides a new insight for designing advanced materials and can be expected to inspire future studies to explore optical anti-counterfeiting technology.

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.

The Charge Transfer Band as a Key to Study the Site Selection Preference of Eu3+ in Inorganic Crystals.

On account of the strong oxidizing property of the europium(III) ion, its charge transfer band (CTB) can be easily formed in many inorganic compounds. In this work, the Eu3+ ions were singly doped into the K3LuSi2O7 compound with a hexagonal structure, and two kinds of Eu3+-O2- CTBs were detected by monitoring at specific wavelengths. The qualitative analysis of Eu3+ ion site occupation was illuminated by combining Eu3+-O2- CTBs with the corresponding cell volume. Furthermore, the two kinds of Eu3+ sites are eventually assigned to the K(2) and Lu sites, which means that Eu3+ ions selectively occupy the site with a low coordination number, according to the calculated CT energy by the dielectric theory of complex crystals and the magnitude of CT energy in the excitation spectra. Meanwhile, at high temperatures, the CTB does not show the traditional thermal quenching like f-f transitions but demonstrates thermal enhancement; thus, by using this opposite variation in excitation spectra, a noninvasive optical thermometer is presented, and this opposite variation tendency is thought to be the difference of thermal stability of disparate excited energy states. When new luminescent phosphors are designed with interesting spectral properties, this work will give us an alternative approach to determine the site occupation preference of Eu3+, especially when there are more than two different sites in the compound.

Trimodal Ratiometric Luminescent Thermometer Covering Three Near-Infrared Transparency Windows.

Near-infrared (NIR) transparency windows have evoked considerable interest in biomedical thermal imaging owing to the superior tissue penetration and the high signal-to-noise ratio, allowing in vivo real-time temperature reading with nanometric spatial resolution. Here, we develop a multimode nonintrusive luminescent thermometer based on the Y3Al5O12 (YAG):Cr3+/Ln3+ (Ln = Ho, Er, Yb) phosphor, which covers three NIR biological transparency windows, enabling cross-checking readings with high sensitivity and a high penetration depth. Utilizing the energy transfer between lanthanide ions and transition-metal ions, the Cr3+/Ln3+-activated upconversion emissions provide ideal signals for ratiometric luminescent thermometry of the NIR-I mode. The phonon-assisted downshifting emissions of Er3+/Ho3+ are used to construct the NIR-III/II mode, and the NIR-III mode is based on the thermal coupling between stark levels of 4I13/2 (Er3+). Three independent modes show distinct thermometric performance in different NIR transparency windows and temperature ranges, and the combination of the three modes is conducive to obtain more accurate temperature readings in a broad temperature range, which paves the way toward versatile luminescent thermometers.

Investigation of high-concentration doping performance based on Er3+-ion-doped Ba6Gd2Ti4O17.

Recently, various strategies have been explored during research into the use of lanthanide-doped luminescent materials to mitigate energy loss at elevated dopant concentrations. Herein we report Yb3+/Er3+ co-doped Ba6Gd2Ti4O17 (BGTO) phosphors with a laminated lattice structure, which can allow the high-concentration doping of Er3+ ions into the oxide. Detailed investigations into the luminescence properties and crystal structures of Yb3+/Er3+ co-doped BGTO reveal that an increase in the dopant concentration is associated with the dimensional limitation of energy transfer in the crystal lattice. This finding may provide a novel avenue for the construction of high-dopant-concentration UC luminescent materials.

Upconversion luminescence and temperature sensing characteristics of Yb3+/Tm3+:KLa(MoO4)2 phosphors.

Yb3+/Tm3+ codoped KLa(MoO4)2 phosphors are synthesized by a hydrothermal method. Under 980 nm excitation, the upconversion (UC) emission spectra of the phosphors are observed. The temperature sensing characteristic based on the fluorescence intensity ratio is studied. The maximum sensitivity reaches 2.93% K-1 at 453 K. The sensitivity value of non-thermally coupled levels is higher than that of thermally coupled levels. The results indicate that the KLa(MoO4)2:Yb3+/Tm3+ phosphor could be used in temperature sensors.

What determines the performance of lanthanide-based ratiometric nanothermometers?

Luminescence intensity ratio (LIR) nanothermometers are ideally suited for noninvasive temperature detection of microelectronic devices and living cells, and the painstaking pursuit of new nanothermometers with higher absolute temperature sensitivity (Sa) or relative temperature sensitivity (Sr) has dominated recent research. However, whether higher Sa and Sr values can intrinsically improve the performance of LIR nanothermometers and what factors essentially determine their accuracy have rarely been considered; these considerations are instructive for their design and application while reducing time and costs. Here, we clarify that the accuracy of lanthanide-based LIR nanothermometers is essentially determined by Sr and the relative error of the luminescence intensity (σI/I) but not Sa based on lanthanide-doped NaYF4, YPO4, YVO4, CaF2, YF3, Y2O3, BaTiO3, LaAlO3 and Y3Al5O12 temperature sensors, meaning that our previous pursuit of higher Sa does not contribute to the accuracy of lanthanide-based LIR nanothermometers. Further research reveals that σI/I is primarily influenced by energy level splitting, which can deteriorate the temperature uncertainty. For actual temperature detection of biological tissues, in addition to the above intrinsic factors, we shed light on the effects of probe self-heating, excitation power density, emission intensity and penetration depth on temperature readouts via a polyethyleneimine-modified NaYF4:Er3+/Yb3+@NaYF4-PEI aqueous solution, implying that we will continue to optimize nanothermometers and calibrate readouts according to the local environment. This work unifies the metrics of lanthanide-based LIR nanothermometers, corrects the previous misunderstanding of Sa to mitigate invalid work, and provides careful guidance for their development.

Superior temperature sensing of small-sized upconversion nanocrystals for simultaneous bioimaging and enhanced synergetic therapy.

The upconversion nanoparticles (UCNPs) exhibit versatility applications aiming at biological domains for decades on account of superior optical characteristics. Nevertheless, the UCNPs are confronted with tremendous difficulties in biological field owing to large grain size, low fluorescence efficiency, and single function. Herein, the small-sized CaF2: Yb3+/Er3+ UCNPs coated with NaGdF4 shells (activator and inert, UCNPs-RBHA-Pt-PEG) not only burst out strong fluorescence, but also provide prominent diagnosability by taking advantage of magnetic resonance (MR) imaging as well as temperature sensing and inhibiting capability for CT26 tumor tissues based on synergetic therapy modality of photodynamic therapy (PDT) and chemotherapy. Ultimately, the tumor sizes decrease visibly after injected with UCNPs-RBHA-Pt-PEG and simultaneously irradiated with near infrared (NIR) light at low power density (0.35 W/cm2, 6 min). In summary, the small-sized and strong-fluorescent single nanoparticles with multi-functions may provide a valuable enlightenment for diagnosis and treatment of cancer in the future.

Prediction of Thermal-Coupled Thermometric Performance of Er3.

Predicting the thermometric performance of diverse materials will facilitate the selection and design of nanothermometers to suit complex environments and specific signal outputs while saving much time and expense. Herein we explore and unveil the thermal-coupled thermometric performance of Er3+/Yb3+ codoped in a set of host lattices via the chemical bond theory of complex crystals. The unknown B and ΔE values of the thermometry are accurately estimated by the chemical bond parameters, further deepening our cognition of the correlation between the luminescence properties of Er3+ ions and the microscopic crystal structure. This allows us to precisely forecast the thermal-coupled thermometric performance of Er3+ for varying host lattices in advance.

One-pot synthesis of SiO2-coated Gd2(WO4)3:Yb3+/Ho3+ nanoparticles for simultaneous multi-imaging, temperature sensing and tumor inhibition.

Rare earth ion-doped fluoride upconversion nanoparticles (UCNPs), emerging as a novel class of probes and drug carriers, exhibit superior promise for bio-applications in diagnostics and treatment on account of their strong luminescence, fine biocompatibility, and high drug loading. However, the fine control and manipulation of particle size and the distribution of rare earth ion-doped oxides has remained an insurmountable challenge to date. In this work, we construct and synthesize silica-coated Gd2(WO4)3:Yb3+/Ho3+ nanoparticles by one-pot co-precipitation, with uniform distribution (∼130 nm) and enhanced yellow fluorescence. Particularly, the nanoparticles not only possess outstanding temperature sensing performance at biological temperatures in water by utilizing the fluorescence intensity ratio (FIR) method, but also allow a further serviceable contrast effect in vitro and in vivo based on the prominent T1-weighted magnetic resonance (MR) signal of Gd3+. Compared with cisplatin and platinum(iv) (DSP), the Gd2(WO4)3@SiO2 nanoparticles functionalized with DSP (Gd2(WO4)3@SiO2-Pt-PEG) exert higher lethality against CT26 cells and significantly inhibit the growth of tumors at the same concentration of Pt. This effect occurs through the greater level of cell endocytosis. The lethality value of the latter is 10 times higher than the former after the same length of time according to inductively coupled plasma-mass spectrometry (ICP-MS) results. In short, the monodisperse and strongly fluorescent Gd2(WO4)3@SiO2-Pt-PEG nanoparticles are endowed with dual-mode imaging, temperature sensing and anticancer functions, which provide a significant guide for synthesis and bio-application of lanthanide ion-doped oxides.

Multifunctional Optical Thermometry Based on the Rare-Earth-Ions-Doped Up-/Down-Conversion Ba2TiGe2O8:Ln (Ln = Eu3+/ Er3+/ Ho3+/ Yb3+) Phosphors.

The fabrication of a multifunctional sensor together with a widening temperature-sensing range is an essential challenge in optical thermometers especially for trivalent lanthanide-doped materials. Herein, we design a wide range, highly sensitive, and multifunctional thermometer by exploiting the emission spectrum of Eu3+ ions, and further detailed discussion has been made on the new temperature-sensing mechanism. The sensor can be operated between 358 and 548 K with a maximum relative sensitivity ( Sr) of 0.93% K-1 at 358 K, which is higher than that of most temperature-sensing materials. A paramount superiority is that the calibration parameter can be directly calculated from the single Eu3+ emission spectrum, avoiding the demand of other calibrations, which realizes the coexistence of a simple structure and high precision. Furthermore, other up-conversion thermometers based on Er3+/Ho3+/Yb3+ co-doped Ba2TiGe2O8 (BTG) phosphors as well as the down-conversion thermometer based on Eu3+-doped Ba2TiGe2O8 (BTG:Eu3+) phosphor have been synthesized for comparison, and the results show that the novel thermometer (BTG:Eu3+) has a much higher sensitivity than that of the traditional thermometers (BTG:Er3+/Ho3+/Yb3+). In addition, the versatility of the phosphor (BTG:Eu3+) is simultaneously reflected in its applications to red phosphor for white-light emitting diodes (W-LEDs) and plant growth lamps. All of the results suggest that BTG:Eu3+ could be a good candidate with its highly sensitive Sr value for optical thermometry and as a safety sign in high-temperature environments.

Investigation on the Fluorescence Intensity Ratio Sensing Thermometry Based on Nonthermally Coupled Levels.

Fluorescence intensity ratio (FIR) of rare earth ions has been widely used in real-time and accurate temperature sensing because of its superiority of rapid response, self-reference, and noncontact in recent years. However, the energy gap (ΔE) restriction of thermally coupled levels (TCLs) has hindered the sensitivity and practical use of such detectors. Herein, we investigate the FIR thermometry based on nonthermally coupled levels (NTCLs) of rare earth ions for fabricating a sensitive, precise temperature detector. Compared with the traditional FIR thermometry based on TCLs (TCL-FIR), the designed NTCL-FIR sensing thermometry exhibits a series of excellent performance including extremely low temperature uncertainty (∼0.27 K), an ultrahigh temperature sensitivity (>10% K-1), and satisfactory signal recognition ability. The rise of sensitivity and recognition is attributed to breaking the ΔE restriction of TCLs by using an Arrhenius equation. The proposed ideas and methods of NTCL-FIR sensing thermometry can not only improve the performance of temperature sensing devices but also more importantly contribute to the practical development of rare earth ions.

Investigating the Luminescence Behaviors and Temperature Sensing Properties of Rare-Earth-Doped Ba2In2O5 Phosphors.

We present a strategy for selecting an optimal material in a particular temperature range by investigating the relationship between the absolute sensitivity ( Sa) and energy gap (Δ E), as well as the relationship between Sa and temperature on the basis of Yb3+/Ln3+ (Ln = Er3+, Ho3+)-codoped Ba2In2O5 phosphors. Through an investigation of optical performance, the phosphors exhibit near-infrared (NIR) downshifting and visible upconversion (UC) emissions under 980 nm excitation. The NIR spectral range from 700 to 1800 nm is referred to as the "biological window". The NIR emission peaks of Er3+ and Ho3+ are located at 1550 nm of the third biological window and 1192 nm of the second biological window, respectively. The temperature sensing behaviors based on the UC luminescence in Yb3+/Ln3+-codoped Ba2In2O5 phosphors are recorded by the fluorescence intensity ratio (FIR) technique in the temperature range from 303 to 573 K. The Ba2In2O5:Er3+/Yb3+ sample is usable at temperatures above 350 K, and the Ho3+/Yb3+-codoped Ba2In2O5 phosphor is suitable at temperatures below 350 K in our experimental region. The above results show that the Ba2In2O5:Ln3+/Yb3+ phosphors could be promising candidates for optical temperature sensors and applications in the biological imaging field.

Investigation on Two Forms of Temperature-Sensing Parameters for Fluorescence Intensity Ratio Thermometry Based on Thermal Coupled Theory.

Absolute temperature sensitivity (Sa) reflects the precision of sensors that belong to the same mechanism, whereas relative temperature sensitivity (Sr) is used to compare sensors from different mechanisms. For the fluorescence intensity ratio (FIR) thermometry based on two thermally coupled energy levels of one rare earth (RE) ion, we define a new ratio as the temperature-sensing parameter that can vary greatly with temperature in some circumstances, which can obtain higher Sa without changing Sr. Further discussion is made on the conditions under which these two forms of temperature-sensing parameters can be used to achieve higher Sa for biomedical temperature sensing. Based on the new ratio as the temperature-sensing parameter, the Sa and Sr of the BaTiO3: 0.01%Pr3+, 8%Yb3+ nanoparticles at 313 K reach as high as 0.1380 K-1 and 1.23% K-1, respectively. Similarly, the Sa and Sr of the BaTiO3: 1%Er3+, 3%Yb3+ nanoparticles at 313 K are as high as 0.0413 K-1 and 1.05% K-1, respectively. By flexibly choosing the two ratios as the temperature-sensing parameter, higher Sa can be obtained at the target temperature, which means higher precision for the FIR thermometers.

Inhomogeneous-Broadening-Induced Intense Upconversion Luminescence in Tm3+ and Yb3+ Codoped Lu2O3-ZrO2 Disordered Crystals.

Near-infrared (980 nm) to near-infrared (800 nm) and blue (490 nm) upconversion has been studied in 0.2% Tm3+ and 10% Yb3+ codoped Lu2O3-ZrO2 solid solutions as a function of the ZrO2 content in the range of 0-50%, prepared by a high-temperature solid-state reaction. The continuous enhancement of upconversion luminescence is observed with increasing ZrO2 content up to 30%. Analyses of the Yb3+ emission intensity and lifetime indicate enlarged absorption of a 980 nm excitation laser and enhanced energy transfer from Yb3+ to Tm3+ with the addition of ZrO2. The spectrally inhomogeneous broadening of the dopants in this disordered solid solution is considered to play the main role in the enhancement by providing better matches with the excitation laser line and increasing the spectral overlap for efficient energy transfer from Yb3+ to Tm3+. In addition, the inhomogeneous broadening is also validated to improve energy migration among Yb3+ ions and energy back transfer from Tm3+ to Yb3+. Hence, it is understandable that a drop in the upconversion luminescence intensity occurs as the concentration of ZrO2 is further increased from 30% to 50%. This work indicates the possibility of disordered crystals to achieve intense upconversion luminescence for biological and optoelectronic applications.