Rare Earth Luminescent Materials Research Articles

Tailoring luminescent patterns with rare-earth photonic materials for anti-counterfeiting applications: A lightkey

Anti-counterfeiting strategies are competitively developed against fast-growing counterfeit markets as a cornerstone for the next generation of luminescent security inks, so it will be more difficult to mimic by ever increasing sophisticated counterfeiters. In particular, rare-earth doped up-conversion luminescent materials present important advantages such as invisibility in ambient light, excitation by low cost NIR irradiation, lack in background noise and negligible auto-fluorescence from the surface. Here we present colour tuneable up-conversion emissions in two families of rare-earth doped photonic materials (sol-gel derived nano-glass-ceramics and glasses prepared by melt quenching technique) for tailoring the overall emitted luminescence, as a sort of “lightkey”. In particular, intensity ratios among UV and VIS up-conversion emission bands can be tuned by modifying doping concentration level and excitation power density, as an additional security feature itself, giving rise to a multi-digit security code based on light-responsive encrypted patterns.

CombinedDFTand wave function theory approach to excited states of lanthanide luminescent materials: A case study ofLaF3:Ce3+

AbstractLanthanide luminescent materials play key roles in modern society, but their first‐principles treatment remains a great challenge due to complex manifold of electronic excited states and the difficulty in performing excited state structural relaxations that is necessary to model luminescent properties. Herein, we propose a practical approach that combines embedded cluster model (ECM) based multi‐configurational wave function theory (WFT) and occupancy constrained density‐functional theory plus the HubbardUcorrection (OC‐DFT + U) to treat lanthanide doped luminescent materials, using LaF3:Ce3+, a typical scintillator with low symmetry, as a case study. We show that the combined approach yields accurate absorption energies with an error on the order of 200 cm−1, but the emission energies are significantly underestimated, the origin of which is further clarified by vibrationally resolved absorption and emission spectra calculation. This work demonstrates the possibility of combining ECM‐based wave function theory and periodic DFT into a comprehensive computational scheme for lanthanide luminescent materials and highlights the limitations of the current implementation of OC‐DFT + Ufor excited state structural optimization.

Ultrastable Photoluminescence Enabled by 1D Rare-Earth Metal-Organic Frameworks Based on Double Thiacalix[4]arene-Capped Nodes.

Luminescent stability is a vital factor that dictates the application of lanthanide luminescent materials. Designing luminescent lanthanide cluster nodes that form an extended framework with predictable linking patterns may help enhance the structural stability of the lanthanide complexes and hence lead to improved luminescent stability. Herein, we report a series of one-dimensional (1D) rare-earth metal-organic framework compounds, {Ln4(μ4-OH)(TC4A)2(H2O)2(CH3O)(HCOO)2(HCOOH)}·xCH3OH (Ln = Sm (1), Eu (2), Tb (3), Dy (4); x = 1-5), based on double thiacalix[4]arene-capped Ln4(μ4-OH)(TC4A)2 nodes. The axially capped Ln4(μ4-OH)(TC4A)2 nodes are connected equatorially by formate bridges to form zigzag 1D-metal-organic framework (MOF) chains, which further assemble into a quasi-two-dimensional (2D) structure via hydrogen bonding. These unique features result in a stable structure and therefore superior luminescent stability. For example, the Tb-based 1D-MOF (3) exhibits intensive green photoluminescence with a quantum yield of 53% and an average decay time of 1.33 × 106 ns. It maintains its integrated emission intensity at 96.5, 94.5, and 89.4% of the original value after being exposed to moisture (soaking in water for 10 days), elevated temperature (150 °C), and UV (15 days of continuous radiation), respectively, demonstrating excellent luminescent stability. We adopt the Tb-based 1D-MOF (3) as the green phosphor and successfully fabricate a prototype white-light-emitting diode (LED) with stable emission under long-term operation. Our synthetic strategy allows control over the linking pattern of lanthanide nodes, providing a predictive route to obtain lanthanide MOFs with improved luminescent stability.

Enhancing conversion efficiency of crystalline silicon photovoltaic modules through luminescent down-shifting by using Eu3+-Zn2+complexes

Currently commercial silicon-based solar cells are usually absorbing only a narrow band of sunlight within the visible light range; thus, their conversion efficiency is low. This challenge has been partially improved by using up-converting or down-converting luminescent materials. Amongst those materials, rare earth luminescent materials doped with other rare earth elements have been developed and demonstrated significant improvement of conversion efficiency. This work reports a novel complex Zn0.4Eu0.6(TTA)2phenCl0.6 (named Eu–Zn in short) that have been synthesized through divalent Zn2+ doping into a trivalent Eu3+ luminescent Eu(TTA)3phen matrix. The properties of the Eu–Zn compound have been characterized by using Inductively Coupled Plasma (ICP)-Atomic Emission spectrum, Fourier Transform Infrared (FTIR), Ultraviolet–Visible (UV–Vis) and Photoluminescence (PL) Spectra. The Eu–Zn complex is further integrated in poly(methyl methacrylate) (PMMA) thin films through solution casting and the composite thin film has lower luminescent quantum yield than that of the Eu–Zn complex alone. The Eu–Zn/PMMA composite film as a down-converting luminescent coating layer is constructed in monocrystalline silicon (MCSi, active area of 156 cm2) and polycrystalline silicon (PCSi, active area of 100 cm2) solar cell modules. The external quantum efficiency (EQE) of both MCSi and PCSi modules shows a decrease in the range of 370–400 nm with the increase of Eu–Zn concentration. The conversion efficiency of MCSi cell is improved by 0.37% (from 14.37% to 14.74%) with the concentration of Eu–Zn at 1%.

Mechanisms of Luminescence in Lanthanide Complexes: A Crucial Role of Metal–Ligand Covalency

A current understanding of the luminescence of lanthanide complexes is based on the phenomenological Judd-Ofelt (JO) theory. However, the mechanisms of electric-dipole transitions lying at its basis were never subjected to a rigorous analysis. Here, we investigate the contributions to the electric-dipole transitions in the Er3+ 4S3/2 → 4I15/2 band of an erbium trensal complex using state-of-the-art ab initio calculations. We find that the conventional JO mechanism based on the electrostatic crystal field yields only a quarter of the integral intensity of this band. Accordingly, three quarters of it is contributed by covalent binding of erbium and ligand orbitals via three major mechanisms, the 4f ligand and ligand-ligand electric-dipole transitions and covalent enhancement of the hybridization of 4f and even empty orbitals of erbium. We expect that these findings will inspire the design of efficient rare-earth luminescent materials.

Sunlike White Light-Emitting Diodes Based on Rare-Earth-Free Luminescent Materials.

Solid-state lighting (SSL) sources based on light-emitting diodes represent the new generation of highly efficient illumination systems that significantly impact energy-saving. The development of white light-emitting diodes (WLEDs) with a combination of high color rendering index (CRI) and high deep-red color rendering R9 is an important challenge in the field of solid-state lighting. On the other hand, most WLEDs use rare-earth inorganic luminescent materials. The annual demand for rare-earth metals has doubled to 125,000 tons in 15 years, and the demand is projected to reach 315,000 tons in 2030. The explosion in demand for these materials, combined with a monopolistic supply source, represents a real risk for the development of WLEDs in the next few years. Luminescent organic materials are a relevant and promising alternative. Here, we report a WLED with a very high CRI of 95.7 and R9 of 78.7, obtained using a combination of a blue LED chip (excitation source) and two organic luminescent dyes (Coumarin 6 and Lumogen Red) acting as spectral converters in a multilayer remote phosphor configuration. To the best of our knowledge, this is the first rare-earth-free WLED with such high values of CRI and R9.

Modular Design Using Sustainable Luminescent Materials in Energy-Saving Reflective Cat’s Eyes for Public Road Safety Planning

Recently, there has been a global movement toward environmental protection and energy conservation through the design and development of new products in accordance with sustainable utilisation. In this study, rare earth luminescent materials were used owing to their active light emission and reusability. Additionally, solar light-emitting diode lights and car-light reflection were utilised to increase the recognition and reliability of reflective cat eyes. Along with carbon reduction, this can save energy and enhance road safety. This study considered the Theory of Inventive Problem Solving and a literature review to analyse the issues in existing products. Then, expert interviews were conducted to screen projects and develop product design policies. Finally, the ratio of light-storage materials was experimentally determined and the prototypes implemented. This cat’s eye addresses the shortcomings identified in previous analyses of existing products. We applied energy storage environmental protection materials, together with material proportioning (which balanced warning efficiency against cost-effectiveness) to develop diversified modular kits; these were flexible in terms of quantity and easily assembled. This study achieved four key objectives: (1) reducing the research and development costs of the manufacturer; (2) offering buyers a diverse suite of products; (3) responding to a need to improve diverse road user safety; and (4) reducing government procurement costs for safety warning products. The results provide a reference for the creative modular design of energy-saving products for public road safety planning in various industries.

Progress on Application of Monte Carlo Simulation in Studying Energy Transfer Mechanisms for Rare-earth Luminescent Materials

Progress on Application of Monte Carlo Simulation in Studying Energy Transfer Mechanisms for Rare-earth Luminescent Materials

Reversible Fluorescence Modulation and Applications Based on Chromic Effect in Inorganic Rare-earth Luminescent Materials

Reversible Fluorescence Modulation and Applications Based on Chromic Effect in Inorganic Rare-earth Luminescent Materials

Recent advance of lanthanide luminescent materials in biological imaging

Recent advance of lanthanide luminescent materials in biological imaging

Enhancement of red upconversion emission intensity of Ho3+ ions in NaLuF4:Yb3+/Ho3+/Ce3+@NaLuF4 core–shell nanoparticles

The red upconversion emission of Ho3+ ions, in the optical window of biological tissue, exhibits excellent prospects in biological applications. This study aims to enhance the red upconversion emission intensity of Ho3+ ions in NaLuF4:20%Yb3+/2%Ho3+/12%Ce3+ nanoparticles through building different core–shell structures with different excitation wavelengths. A significantly enhanced red upconversion emission with a higher red-to-green ratio was successfully obtained in NaLuF4:20%Yb3+/2%Ho3+/12%Ce3+@NaLuF4 core–shell nanoparticles by introducing the Yb3+ and Yb3+/Nd3+ ions into the NaLuF4 shell, with enhancement of the red emission occurring when Yb3+ and Nd3+ ions in the shell transfer more excitation energy to the Ho3+ ions. Investigation of the red emission enhancement mechanism is based on spectral characteristics and lifetimes. We examined the synergistic effect of dual-wavelength co-excitation NaLuF4:20%Yb3+/2%Ho3+/12%Ce3+ @NaLuF4:10%Yb3+/15%Nd3+ core–shell nanoparticles to establish optimal excitation conditions. It is hoped that this method, using red upconversion emission core–shell nanoparticles with multi-mode excitation, can provide new ways to expand the applications of rare-earth luminescent materials in biomedicine and anti-counterfeiting.

Concentration effects of fluorescence quenching and optical transition properties of Dy3+ doped NaYF4 phosphor

Doping concentration is an important factor influencing the optical and spectroscopic properties of rare earth doped luminescence materials. In this work, the influences of Dy3+ concentration on fluorescence intensity and optical transition properties were studied. Huang’s and Uitert’s models were compared and used in discovering the fluorescence intensity quenching mechanism of Dy3+ in NaYF4 phosphors, and it was found that from these two models the fluorescence intensity quenching mechanism can be well deduced though the application manners of the models are different. The dependence of optical transition properties on Dy3+ doping concentration was studied in depth. It was found that with increasing Dy3+ concentration the Ω2 value increases and the Ω4 value decreases monotonically, nevertheless the Ω6 value changes slightly. Moreover, the magnitude sequence of the Judd-Ofelt parameters changes from Ω4>Ω6>Ω2 to Ω2>Ω6>Ω4 when the concentrations of Dy3+ increases. Furthermore, it was found that the internal quantum efficiency of 4F9/2 decreases with increasing Dy3+ concentration.

BiPO4:Ln3+ (Ln = Eu, Tb, Eu/Tb) nanorods: Room-temperature synthesis, reaction mechanism, and color-tunable emission

Ln3+ (LnEu, Tb, Eu/Tb) doping hexagonal BiPO4 nanorods were synthesized by a room-temperature co-precipitation method using ethylene glycol as solvent. Powder X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared spectroscopy (IR), thermal gravimetric (TG), and photoluminescence spectroscopy (PL) were utilized to investigate the structure, morphology, surface chemistry, and fluorescence performance of the obtained phosphors, respectively. The results showed that hexagonal phase of BiPO4 formed at room temperature, which consisted almost entirely of nanorods with an average diameter of 70 nm and an average length of 292 nm. The chemical reactions mechanism was well elucidated. Under short-wavelength UV light excitation, the as-obtained BiPO4:Eu3+ and BiPO4:Tb3+ samples exhibited strong orange-red and green emissions, respectively. Moreover, the photoluminescence color could be tuned from red, orange, yellow, and green-yellow, to green by simply adjusting the relative doping concentrations of Tb3+ and Eu3+ ions. The room-temperature synthesis reported in the present study could be extended to prepare other rare-earth doped inorganic luminescent materials.

A potential temperature sensing material: Studies of structure and temperature-sensitive photoluminescent properties of Nb5+, Sm3+ doped YTaO4

Non-contact luminescence temperature measurement technology shows a broad application prospect in medical, high-temperature environment and other fields, which has attracted people's attention for its high-precision, real-time response and so on. In this work, Nb5+, Sm3+ single-doped and co-doped YTaO4 phosphor was designed. YTaO4 with M-type and M′-type structures can be obtained by adjusting the synthesis conditions. The structure and composition of the samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and Scanning electron microscopy (SEM). The photoluminescent properties of NbO43− group and Sm3+ ions in M-type and M′-type YTaO4 matrix were studied. Due to the different thermal responses of Nb5+ and Sm3+ ions, the fluorescence intensity ratio between them can be used as temperature detecting signal. In the range of 77–500 K, the maximum relative sensitivity of the YTaO4: Nb5+, Sm3+ reaches 1.15 %K−1 at 450 K, indicating that the Nb5+ and Sm3+ co-doped M-type YTaO4 phosphors can be used as optical temperature sensor. This research may provide guidance for the design of other rare earth doped luminescent materials for temperature detection.

Combustion synthesis of luminescent Eu-doped single phase Mayenite

Rare earth luminescent materials based on alkali metal oxides such as calcium aluminates have grabbed the attention due to their high luminescent efficiency, non-toxic and eco-friendly nature. Calcium aluminates as host materials for luminescent applications are extremely stable, suppressing the release of non-radiative energy resulting in the characteristic emission of rare earth luminescent centre. Moreover, calcium aluminates are inexpensive for their use as host material for luminescent applications. With texture and color giving the aesthetic contribution, Europium (Eu) doped single phase Mayenite may find potential applications in the design of energy efficient buildings when used as a coating material. Eu doped single phase Mayenite powders in varying concentrations (0%, 0.5%, 1%, and 1.5%) were prepared using self-propagating combustion synthesis method using metal nitrates and glycine as the fuel. The X-ray diffractograms confirmed the formation of single phase Mayenite with some of the Ca2+ ions getting replaced by Eu3+ ions. The photoluminescence emission spectra revealed 5D0 → 7F2 transition due to the crystal field effect and was characterized by the bright red luminescence. The doping of Eu resulted in a decrease of band gap from 4.8 ​eV for undoped Mayenite to 3.5 ​eV for 1.5% Eu doped Mayenite. The samples were found to exhibit excellent fluorescent properties under UV irradiation.

Novel rare earth activator ions-doped perovskite-type La4Ti3O12 phosphors: Facile synthesis, structure, multicolor emissions, and potential applications

A series of novel rare earth activator ions-doped perovskite-type lanthanum titanate La4Ti3O12 phosphors have been synthesized via the Pechini sol-gel method. The influence of calcination temperature and time on crystal structure of the La4Ti3O12 matrix is investigated, and the result indicates that higher temperature and longer calcination time is beneficial for better crystallinity. Upon ultraviolet excitation, the as-synthesized activator ions (Dy3+, Eu3+, Sm3+, and Tb3+) ions doped La4Ti3O12 phosphors display the characteristic quasi-white, reddish orange, orange, and green emissions in visible region. The higher calcination temperature and longer time can induce stronger luminescence due to enhanced crystallinity of La4Ti3O12 host. The optimal doping concentrations of Dy3+, Eu3+, Sm3+, and Tb3+ ions are determined to be 2, 12, 2, and 10 mol%, respectively. The corresponding LED devices fabricated by the phosphors and UV LED chips can exhibit bright characteristic multicolor emissions of activator ions, indicating their potential in LEDs and optoelectronic devices. Moreover, the up-conversion luminescence properties of the Yb3+/Er3+, Yb3+/Tm3+, and Yb3+/Ho3+ doped samples are also studied. The result shows that the excellent up-conversion performances can be also realized by doping appropriate rare earth ions into the host matrix by employing the NIR excitation. It is anticipated that this work may be of significance for the design and preparation of perovskite-type rare earth luminescent materials.

Thermochromic luminescent fiber based on yellow thermochromic microcapsules: preparation, properties, and potential application areas

Luminescent fibers are widely used in textile fields because of their excellent luminescence performance. However, the lack of its sensing property limits the fiber’s application. In this work, two rare earth luminescent materials, SrAl2O4:Eu2+, Dy3+ (SAOED) and Sr2ZnSi2O4:Eu2+, Dy3+ (SZSO), were used as luminous sources, and a yellow thermochromic microcapsule (YTM) was used as a light conversion agent to prepare a thermochromic luminescent fiber (TLF) through wet spinning. The micro-morphology of the TLF was analyzed using scanning electron microscopy (SEM). Three particles were uniformly incorporated in the cellulose fibers, which provide the fibers with a rough surface. Interestingly, the YTM content had no negative effect on the breaking strength of the TLF. Nevertheless, as the concentration of YTM increased from 3 to 10 %, the elongation at break tended to decrease. X-ray diffraction (XRD) results indicate that the cellulose matrix and the wet spinning process did not destroy the phase of rare earth luminescent materials in the fiber. The thermodynamic stability and dynamic phase structure were investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The TLF was stable below 100 °C. The fluorescence spectra and absorption spectra measurements show that TLF exhibits good thermochromic luminescence performance. The TLF appeared white at room temperature (25 °C) and emitted green light, but when the temperature increased (50 °C), the fiber faded and emitted indigo light. Herein, the potential application areas of the TLF can be used in optical materials and thermal sensors.

Blue light-induced rare-earth free phosphors for the highly sensitive and selective imaging of latent fingerprints based on enhanced hydrophobic interaction

The demand for in-situ detection of latent fingerprints (LFPs) in ways of high sensitivity, high selectivity, high contrast, low cost and user-friendly is still urgent. To overcome this challenge, a moisture-stable, red-emitting fluoride phosphor K3AlF6:Mn4+ (KAF:Mn4+) with an organic hydrophobic skin was prepared. The phosphor has a uniform and superfine morphology with excellent luminescence properties. More importantly, this non-ultraviolet (UV) or non-near infrared (NIR) induced phosphor was proved to be an ideal fluorescent label for LFP imaging, which is both friendly for touch DNA analysis and compatible to forensic light sources. The well-defined ridge details with little background interference on various surfaces were presented by the oleic acid (OA) modified KAF:Mn4+ (KAF:Mn4+-OA) phosphor in few seconds using the powder dusting method. To confirm the high selectivity of KAF:Mn4+-OA for LFP imaging, an efficient quantitative evaluation method is proposed with the aid of ImageJ & Origin software. Due to the superiority of the Mn4+-doped fluoride for the rapid imaging of LFPs in terms of low-cost, high compatibility and good availability, it is expected to be a promising candidate for forensic science as well as fluorescence imaging in other fields instead of rare earth luminescent materials.

Preparation and luminescence performance of thermochromic luminescent fiber based on reversible thermochromic red pigment

SrAl2O4: Eu2+, Dy3+ (SAOED) and Y2O2S: Eu3+, Mg2+, Ti4+ (YOS), as two kinds of important rare earth luminescent materials, are widely used in fibers due to their excellent luminescent properties. However, the lack of sensing property limits the application areas of the fibers in intelligent materials. In this work, the thermochromic luminescent fiber (TLF) was prepared by wet spinning from the cellulose acetate (CA)/dimethyl formamide (DMF) spinning solution. The rare earth luminescent materials serviced as the luminescent sources and thermochromic pigments (TP) acted as the thermochromic agent in this color changing system. The absorptivity curves and fluorescence spectra were recorded at 25 and 45 °C to investigate the thermosensitive characteristics of the fiber. The scanning electron microscope (SEM) observation presents that the TLFs were cylindrical with the diameters of 300–400 µm and three kinds of particles were evenly distributed in the cellulose matrix. X-ray diffraction (XRD) results confirm that cellulose matrix and the wet spinning process did not destroy the phase of rare earth luminescent materials in the fiber. Thermal stability of TLF was illustrated by differential scanning calorimetry (DSC) and thermal gravity analysis (TGA). Absorptivity and emission spectra measurement indicates that the TLF had perspicuous thermochromic luminescence properties. Decay curves showed that the TLF glowed continuously. This work exploits a new type of intelligent luminescent fiber that can respond to ambient temperature, which broadens its application in sensors.

Luminescent polyurethane composite with real-time thermal response via visible signal

Visible signals are vital to human communication. Thermal signals are often difficult to observe unless they are in physical contact with the subject or an infrared thermal imaging system is used. In this study, we prepared a luminescent polyurethane composite (LPC), which can intelligently alter its colour and luminescence based on temperature. The LPC is fabricated from rare-earth luminescent materials (Sr2ZnSi2O7: Eu2+, Dy3+ and Y2O2S: Eu3+, Mg2+, Ti4+), a heat-sensitive rose-red TF-R1 thermochromic pigment (TP), polyurethane (PU), and polyester fabric. Microstructure and crystal structure analyses were carried out to determine the basic material properties of the LPC. The reflectivity and K S−1 value under different temperatures indicate that blue and green light were absorbed by LPC at 25 °C and the absorption decreased at 45 °C. This results in the LPC turning white. The emission spectra demonstrate that the quinoid structure of TP blocked the light of Sr2ZnSi2O7: Eu2+, Dy3+. Meanwhile, the lactone structure of TP allowed the light emission of Sr2ZnSi2O7: Eu2+, Dy3+ and Y2O2S: Eu3+, Mg2+, Ti4+. As a result, LPC emits red light at low temperatures and blue light at high temperatures. The advantage of the fabricated LPC is that the output can be easily manipulated into patterns. The uncured PU could form various graphics to cooperate with visible signals. Therefore, the LPC has significant potential as a functional material in smart clothing, flexible electronics, and wearable devices.