An Advanced Tunable Multimodal Luminescent La4GeO8: Eu2+, Er3+ Phosphor for Multicolor Anticounterfeiting

The development of advanced luminescent materials is of great importance to the anticounterfeiting application and still confronts with lots of challenges. At present, most anticounterfeiting luminescent materials are based on a monotonous photoluminescence model, which is easily faked by substitutes. Therefore, in this work, a multimodal La₄GeO₈: Eu²⁺, Er³⁺ material is reported, which can emit red, purple, baby blue, and green light under the increased excitation wavelength from 250 to 380 nm. Meanwhile, the phosphor also shows green upconversion luminescence under the NIR (980 and 808 nm) laser irradiation. Moreover, the phosphor features excellent stability and humidity resistance against harsh conditions. Based on the integrated feature, a functional anticounterfeiting application is designed. Results demonstrate that the multimodal luminescent feature can be easily detected by using a portable ultraviolet lamp or NIR (808 or 980 nm) laser. The unique characteristic will be complicated to counterfeit and show high-level security in the field of advanced anticounterfeiting.     

Photoluminescence Tuning in Stretchable PDMS Film Grafted Doped Core/Multishell Quantum Dots for Anticounterfeiting

The development of new luminescent materials for anticounterfeiting is of great importance, owing to their unique physical, chemical, and optical properties. The authors report the use of color‐tunable colloidal CdS/ZnS/ZnS:Mn²⁺/ZnS core/multishell quantum dots (QDs)‐functionalized luminescent polydimethylsiloxane film (LPF) for anticounterfeiting applications. Both luminescent QDs and as‐fabricated, stretchable, and transparent LPF show blue and orange emission simultaneously, which are ascribed to CdS band‐edge emission and the ⁴T₁ → ⁶A₁ transition of Mn²⁺, respectively; their emission intensity ratios are dependent on the power‐density of a single‐wavelength excitation source. Additionally, photoluminescence tuning of CdS/ZnS/ZnS:Mn²⁺/ZnS QDs in hexane or embedded in LPF can also be realized under fixed excitation power due to a resonance energy transfer effect. Tunable photoluminescence of these flexible LPF grafted doped core/shell QDs can be finely controlled and easily realized, depending on outer excitation power and intrinsic QD concentration, which is intriguing and inspires the fabrication of many novel applications.     

Organic–Inorganic Hierarchical Self‐Assembly into Robust Luminescent Supramolecular Hydrogel

Luminescent hydrogels are of great potential for many fields, particularly serving as biomaterials ranging from fluorescent sensors to bioimaging agents. Here, robust luminescent hydrogels are reported using lanthanide complexes as emitting sources via a hierarchical organic–inorganic self‐assembling strategy. A new organic ligand is synthesized, consisting of a terpyridine unit and two flexibly linked methylimidazole moieties to coordinate with europium(III) (Eu³⁺) tri‐thenoyltrifluoroacetone (Eu(TTA)₃), leading to a stable amphiphilic Eu³⁺‐containing monomer. Synergistic coordination of TTA and terpyridine units allows the monomer to self‐assemble into spherical micelles in water, thus maintaining the luminescence of Ln complexes in water. The micelles further coassemble with exfoliated Laponite nanosheets coated with sodium polyacrylate into networks based on the electrostatic interactions, resulting in the supramolecular hydrogel possessing strong luminescence, extraordinary mechanical property, as well as self‐healing ability. The results demonstrate that hierarchical organic–inorganic self‐assembly is a versatile and effective strategy to create luminescent hydrogels containing lanthanide complexes, giving rise to great potential applications as a soft material.     

First Evidence of Electron Trapped Ln2+ Promoting Afterglow on Eu2+, Ln3+ Activated Persistent Phosphor-Example of BaZrSi3O9:Eu2+, Sm3+

BaZrSi₃O₉:Eu²⁺, Sm³⁺ (Em:525 nm) is prepared. The role played by the trivalent co-doping ion Sm³⁺ in the afterglow and the type of trap are clarified. BaZrSi₃O₉:Eu²⁺, Sm³⁺ is found to produce Sm²⁺ during the excitation by X-ray absorption near-edge structure (XANES), etc., and it is thus proved that Sm³⁺ exists as an electron trap in the afterglow process. In the field of persistent phosphors activated by Eu²⁺ and Re³⁺ such as Sm³⁺ or Dy³⁺ having been widely utilized as emergency guide lights, clock faces, etc. for > 25 years, for the first time it is successfully observed that after excitation Re²⁺ is formed, transferring its electron to 5d band of Eu²⁺, returning to Re³⁺ by itself, where the decrease in Sm²⁺ coincides with the increase in Sm³⁺, and the two decay time τ₁ and τ₂ of PL (⁵D₀→⁷F₀) of Sm²⁺ coincides with the two evolution time of PL (5d→4f) of Eu²⁺. The behavior of electron transfer from Sm²⁺ to Eu²⁺ as a key of afterglow is detected. The detailed afterglow mechanism is proposed by analysis of thermoluminescence and defect reaction, which is very important for the in-depth investigation of the long afterglow material and the further improvement of the mechanism.     

Efficient Mechanoluminescent Elastomers for Dual‐Responsive Anticounterfeiting Device and Stretching/Strain Sensor with Multimode Sensibility

In this work, an environmentally friendly and novel oxide‐based mechanoluminescent material, Sr₃Al₂O₆: Eu³⁺, which can serve as the alternative for the widely used but environmentally hazardous transition metal–doped sulfides is reported. This oxide could exhibit highly efficient photoluminescence, but even more efficient mechanoluminescence as embedded into polydimethylsiloxane matrix under mechanical stimulation. The emitting color of the resultant Sr₃Al₂O₆: Eu³⁺/polydimethylsiloxane elastomer composites could be further manipulated by adjusting the synthesis atmosphere of the Sr₃Al₂O₆: Eu³⁺ based on its unique self‐reduction characteristic. Moreover, by combining the wavelength selectivity of photoluminescence and dynamic stress response of mechanoluminescence, Sr₃Al₂O₆: Eu³⁺ enables the design of two types of intriguing devices. They are a dual‐responsive anticounterfeiting flexible device activated by either photons or mechanics, and a comprehensive stretching/strain sensor capable of sensing both strain level and stretching states. In comparison to the conventional luminescent materials, with a rare combination of efficient photoluminescence, highly sensitive mechanoluminescence, and facile color tunability, Sr₃Al₂O₆: Eu³⁺ is much more versatile and ideal for various advanced applications.     

Electroluminescence of SrAl2O4:Eu phosphor

The SrAl₂O₄:Eu²⁺ phosphor powders have been synthesized by sol-gel process. Electroluminescent (EL) properties of the SrAl₂O₄:Eu²⁺ phosphor were investigated using a convenient thick film device. Green light emitting at a peak of 508 nm was obtained when driven by sine alternating current (AC). The color coordinate of the emission was x=0.148 and y=0.635. Luminance-voltage and afterglow characteristics of the SrAl₂O₄:Eu²⁺ EL devices were studied. The results show that SrAl₂O₄:Eu²⁺ can be used as green phosphor for EL displays.     

3D Fluorescent Hydrogel Origami for Multistage Data Security Protection

Current fluorescence‐based anti‐counterfeiting strategies primarily encode information onto single 2D planes and underutilize the possibility of encrypting data inside 3D structures to achieve multistage data security. Herein, a fluorescent‐hydrogel‐based 3D anti‐counterfeiting platform is demonstrated, which extends data encryption capability from single 2D planes to complex 3D hydrogel origami geometries. The materials are based on perylene‐tetracarboxylic‐acid‐functionalized gelatin/poly(vinyl alcohol) hydrogels, which simultaneously show Fe³⁺‐responsive fluorescence quenching, borax‐triggered shape memory, and self‐healing properties. By employing an origami technique, various complex 3D hydrogel geometries are facilely fabricated. On the basis of these results, a 3D anti‐counterfeiting platform is demonstrated, in which the data printed by using Fe³⁺ as the ink are safely protected inside complex 3D hydrogel origami structures. In this way, the encrypted data cannot be read until after specially predesigned procedures (both the shape recovery and UV light illumination actions), indicating higher‐level information security than the traditional 2D counterparts. This facile and general strategy opens up the possibility of utilizing 3D fluorescent hydrogel origami for data information encryption and protection.     

Synthesis of Luminescent ZrO2:Eu3+ Nanoparticles and Their Holographic Sub‐Micrometer Patterning in Polymer Composites

Here, the facile synthesis of fluorescent ZrO₂:Eu³⁺ nanoparticles with luminescence quantum yield of up to 8.7% that can be easily dispersed in organic solvents and utilized for the preparation of organic/inorganic volume holographic gratings is presented. The nanoparticles are prepared through a one‐step solvothermal process resulting in spherical particles with a mean size of 4 nm that were highly crystalline directly after the synthesis, without any need for calcination treatment. Detailed luminescence studies of the nanoparticles as a function of Eu³⁺ content demonstrate that the dopant concentration and its site symmetry play an important role in the emissive properties and lifetime of the luminescent centers. It is shown that the luminescence quantum yield of the colloidal ZrO₂:Eu³⁺ nanoparticles increases with dopant concentration up to a critical concentration of 11 mol% while the luminescence lifetime is shortened from 1.8 to 1.4 ms. Holographic photopolymerization of suitable monomer mixtures containing the luminescent nanoparticles demonstrated the ability to inscribe volume Bragg gratings (refractive index contrast n₁ up to 0.011) with light‐emissive properties, evidencing the high suitability of this approach for the fabrication of tailored nanomaterials for elaborate and demanding applications.     

Synthesis and Optical Properties of Europium‐Doped ZnS: Long‐Lasting Phosphorescence from Aligned Nanowires

Quasi‐aligned Eu²⁺‐doped wurtzite ZnS nanowires on Au‐coated Si wafers have been successfully synthesized by a vapor deposition method under a weakly reducing atmosphere. Compared with the undoped counterpart, incorporation of the dopant gives a modulated composition and crystal structure, which leads to a preferred growth of the nanowires along the [01 0] direction and a high density of defects in the nanowire hosts. The ion doping causes intense fluorescence and persistent phosphorescence in ZnS nanowires. The dopant Eu²⁺ ions form an isoelectronic acceptor level and yield a high density of bound excitons, which contribute to the appearance of the radiative recombination emission of the bound excitons and resonant Raman scattering at higher pumping intensity. Co‐dopant Cl^– ions can serve not only as donors, producing a donor–acceptor pair transition with the Eu²⁺ acceptor level, but can also form trap levels together with other defects, capture the photoionization electrons of Eu²⁺, and yield long‐lasting (about 4 min), green phosphorescence. With decreasing synthesis time, the existence of more surface states in the nanowires forms a higher density of trap centers and changes the crystal‐field strength around Eu²⁺. As a result, not only have an enhanced Eu²⁺ 4f⁶5d¹–4f⁷ intra‐ion transition and a prolonged afterglow time been more effectively observed (by decreasing the nanowires' diameters), but also the Eu²⁺ related emissions are shifted to shorter wavelengths.     

Multimode Emission from Lanthanide-Based Metal–Organic Frameworks for Advanced Information Encryption

Although remarkable progress on luminescent materials is made in advanced optical information storage and anti-counterfeiting applications, many challenges still remain in these fields. Currently, most luminescent materials are based on a single photoluminescent model that can be easily imitated by substitutes. In this work, a series of multimodal emission lanthanide-based metal–organic frameworks (MOFs) are developed, where they emit red and green light originating from Eu³⁺ and Tb³⁺ under ultraviolet light irradiation. Meanwhile, under 980 nm near-infrared laser irradiation, these MOFs show cyan upconversion cooperative luminescence derived from Yb³⁺ and characteristic upconversion luminescence from lanthanide activators (Eu³⁺, Tb³⁺, or Ho³⁺), respectively. Based on the integrated optical functionality, the functional information storage applications are successfully designed, which indicates that multimodal emission features can be easily detected under ultraviolet lamps (254 or 393 nm) or 980 nm near-infrared laser. And, the unique optical features show a high level of security in the advanced information storage application, which would be sufficiently complex to be forged.     

An organic hydrogel film with micron-sized pillar array for real-time and indicator-free detection of Zn

A hydrogel sensing film for a real-time and indicator-free detection of Zn²⁺ is developed by embedding a fluorescent indicator 11,16-bis(phenyl)-6,6,21,21-tetramethyl-m-benzi-6,21-porphodimethene in a hydrogel host poly(2-hydroxyethyl methacrylate). The sensing film shows high stability and selectivity to Zn²⁺. The sensitivity of the sensing film is increased by fabricating a micron-sized pillar array on the surface of the sensing film to increase the surface area. For Zn²⁺ concentrations of 10⁻⁴ and 10⁻³ M, the response time is 30 and 3 s, respectively.     

The effect of different lattice sites substitution on photoluminescence characteristics of Eu2+/Ce3+ doped M5(ZO4)3X

The empirical formula of Van Uitert is applied to calculating the emission wavelengths of haloapatite and silicon apatite phosphors doped with Eu²⁺/Ce³⁺. The relationship between emission wavelengths and occupied lattice sites of Eu²⁺/Ce³⁺ is discussed in haloapatite crystal. For phosphors of haloapatite and silicon apatite doped with Eu²⁺, the emission bands of the long-wave region are interpreted reasonably. Phosphors Sr₅(PO₄)₂SiO₄ doped with Eu²⁺/Ce³⁺ are synthesized by high temperature solid state reaction under two different atmospheres, the spectral characteristics of Eu²⁺/Ce³⁺ occupying different lattice sites are studied. The luminescent materials Sr₅(PO₄)₂SiO₄ doped with Eu²⁺/Ce³⁺ are promising blue-green phosphors for application in white-LEDs.     

Patterning of YVO4:Eu3+ Luminescent Films by Soft Lithography

Ordered arrays of luminescent YVO₄:Eu³⁺ films with square (side length 19.17 ± 2.05 μm) and dot (diameter 11.20 ± 1.82 μm) patterns were fabricated by two kinds of soft lithography processes, namely, microtransfer molding (μTM) and microcontact printing (μCP), respectively. Both soft‐lithography processes utilize a PDMS elastomeric mold as the stamp combined with a Pechini‐type sol‐gel process to produce luminescent patterns on quartz plates, in which a YVO₄:Eu³⁺ precursor solution was employed as ink. The ordered luminescent YVO₄:Eu³⁺ patterns are revealed by optical micro­scopy and their microstructure, consisting of nanometer‐scale particles, is unveiled by scanning electronic microscopy (SEM) observations. Additionally, photoluminescence (PL) and cathodoluminescence (CL) were carried out to characterize the patterned YVO₄:Eu³⁺ samples. A strong red emission as a result of ⁵D₀–⁷F₂ transition of Eu³⁺ was observed under UV‐light or electron‐beam excitation, which implies that combining soft lithography with a Pechini‐type sol‐gel route has potential for fabricating rare‐earth luminescent pixels for next‐generation field‐emission display devices.     

Improving spectral response of monocrystalline silicon photovoltaic modules using high efficient luminescent down‐shifting Eu3+ complexes

One way to improve the spectral response of solar cells in the ultraviolet (UV) region is to convert high energy photons into lower energy ones via luminescent down‐shifting (LDS) technique. Eu³⁺ complexes are excellent LDS species because of their high luminescence quantum efficiency and large Stokes‐shift. In this paper, we aim to optimize the LDS property of Eu³⁺ complexes for monocrystalline silicon (c‐Si) photovoltaic (PV) modules by chemical modification of the UV absorbing antenna ligands. Our results show that the LDS performances of Eu³⁺ complexes are strongly dependent on their absorption and emission properties. By carefully modifying the absorption and emission features, the LDS performances of Eu³⁺ complexes can be significantly improved. The spectroscopic features of the Eu³⁺ complex with a bispinene‐containing bipyridyl ligand match well with the requirement of ideal LDS species for the c‐Si PV module. Simple coating of polyvinyl acetate film doped with this complex onto the surface of c‐Si PV module leads to increase of the external quantum efficiency in the UV region and enhancement of the PV module efficiency η (from 16.05% to 16.37%). Copyright © 2012 John Wiley & Sons, Ltd.     

Design and Synthesis of Multifunctional Drug Carriers Based on Luminescent Rattle‐Type Mesoporous Silica Microspheres with a Thermosensitive Hydrogel as a Controlled Switch

A novel approach for the fabrication of multifunctional microspheres integrating several advantages of mesoporous, luminescence, and temperature responses into one single entity is reported. First, the hollow mesoporous silica capsules are fabricated via a sacrificial template route. Then, Gd₂O₃:Eu³⁺ luminescent nanoparticles are incorporated into the internal cavities to form rattle‐type mesoporous silica nanocapsules by an incipient‐wetness impregnation method. Finally, the rattle‐type capsules serve as a nanoreactor for successfully filling temperature‐responsive hydrogel via photoinduced polymerization to form the multifunctional composite microspheres. The organic–inorganic hybrid microspheres show a red emission under UV irradiation due to the luminescent Gd₂O₃:Eu³⁺ core. The in vitro cytotoxicity tests show that the samples have good biocompatibility, which indicates that the nanocomposite could be a promising candidate for drug delivery. In addition, flow cytometry and confocal laser scanning microscopy (CLSM) confirm that the sample can be effectively taken up by SKOV3 cells. For in vitro magnetic resonance imaging (MRI), the sample shows the promising spin‐lattice relaxation time (T₁) weighted effect and could potentially apply as a T₁‐positive contrast agent. This composite drug delivery system (DDS) provides a positive temperature controlled “on‐off”drug release pattern and the drug, indomethacin (IMC), is released fast at 45 °C (on phase) and completely shut off at 20 °C (off phase). Meanwhile Gd₂O₃:Eu³⁺ plays an important role as the luminescent tag for tracking the drug loading and release process by the reversible luminescence quenching and recovery phenomenon. These results indicate that the obtained multifunctional composite has the potential to be used as a smart DDS for biomedical applications.     

Enhanced Photoluminescence and Photoresponsiveness of Eu3+ Ions-Doped CsPbCl3 Perovskite Quantum Dots under High Pressure

Metal halide perovskite quantum dots (QDs) have garnered tremendous attention in optoelectronic devices owing to their excellent optical and electrical properties. However, these perovskite QDs are plagued by pressure-induced photoluminescence (PL) quenching, which greatly restricts their potential applications. Herein, the unique optical and electrical properties of Eu³⁺-doped CsPbCl₃ QDs under high pressure are reported. Intriguingly, the PL of Eu³⁺ ions displays an enhancement with pressure up to 10.1 GPa and still preserves a relatively high intensity at 22 GPa. The optical and structural analysis indicates that the sample experiences an isostructural phase transition at approximately 1.53 GPa, followed by an amorphous state evolution, which is simulated and confirmed through density functional theory calculations. The pressure-induced PL enhancement of Eu³⁺ ions can be associated with the enhanced energy transfer rate from excitonic state to Eu³⁺ ions. The photoelectric performance is enhanced by compression and can be preserved upon the release of pressure, which is attributed to the decreased defect density and increased carrier mobility induced by the high pressure. This work enriches the understanding of the high-pressure behavior of rare-earth-doped luminescent materials and proves that high pressure technique is a promising way to design and realize superior optoelectronic materials.     

An Adaptable Tough Elastomer with Moisture‐Triggered Switchable Mechanical and Fluorescent Properties

Smart materials with coupled optical and mechanical responsiveness to external stimuli, as inspired by nature, are of interest for the biomimetic design of the next generation of soft machines and wearable electronics. A tough polymer that shows adaptable and switchable mechanical and fluorescent properties is designed using a fluorescent lanthanide, europium (Eu). The dynamic Eu‐iminodiacetate (IDA) coordination is incorporated to build up the physical cross‐linking network in the polymer film consisting of two interpenetrated networks. Reversible disruption and reformation of Eu‐IDA complexation endow high stiffness, toughness, and stretchability to the polymer elastomer through energy dissipation of dynamic coordination. Water that binds to Eu³⁺ ions shows an interesting impact simultaneously on the mechanical strength and fluorescent emission of the Eu‐containing polymer elastomer. The mechanical states of the polymer, along with the visually optical response through the emission color change of the polymer film, are reversibly switchable with moisture as a stimulus. The coupled response in the mechanical strength and emissive color in one single material is potentially applicable for smart materials requiring an optical readout of their mechanical properties.     

Hydrothermal Synthesis and Luminescent Properties of BiPO4:Eu3+ Phosphors

In this study, BiPO₄:Eu³⁺ phosphors were synthesized by a facile hydrothermal route at different temperatures. The BiPO₄:Eu³⁺ particles were characterized by x-ray powder diffraction (XRD), infrared spectra, and luminescence spectroscopy. The XRD results reveal that the BiPO₄:Eu³⁺ particles present different phases for different hydrothermal temperatures. It is found that a hexagonal phase is formed at 100°C, which transforms to a low-temperature monoclinic phase (MP) when the hydrothermal temperature is increased to 150°C. This low-temperature MP transforms to high-temperature MP when the temperature is increased beyond 200°C. The luminescent properties of the BiPO₄:Eu³⁺ particles were studied using an excitation wavelength of 270 nm. The emission spectra display the bands associated with the ⁵D₀ → ⁷F _J (J = 1, 2, 3, and 4) electronic transitions of the Eu³⁺ cations. The intensity of the emission spectra increases with increasing hydrothermal temperature. These results demonstrate that BiPO₄:Eu³⁺ with different phases can be obtained through the hydrothermal method, which may enrich the solution chemistry for preparation of advanced materials with tailored functionality.     

New Ternary Europium Aluminate Luminescent Nanoribbons for Advanced Photonics

Developing novel one‐dimensional (1D) luminescent nanostructures (e.g., nanowires and nanoribbons) is highly desired for enabling progress in nanophotonics and other emerging optical technologies. Previous studies on 1D luminescent nanostructures were mostly focused on elemental and binary semiconductor materials, the light emission of which originates from the radiative recombination of electrons and holes via either intrinsic states or extrinsic defect states. Herein, three kinds of ternary europium aluminate nanoribbons are reported that have localized Eu²⁺ luminescent centers and exhibit new compositions, new crystal lattice structures, and new luminescence properties and mechanisms. These three europium aluminate nanoribbons are: blue luminescent EuAl₆O₁₀ with a new composition and a new tetragonal lattice structure, green luminescent EuAl₂O₄ with a monoclinic lattice structure, and orange luminescent EuAl₂O₄ with a new hexagonal lattice structure and extremely large band width and Stokes shift of emission. These materials have promising applications as nanometer‐scale light generators and waveguides in nanophotonics and as light converting phosphors in warm white light‐emitting diodes.     

Sonication-Responsive Organic Afterglow Emulsions

In contrast to their fluorescence counterparts, stimuli-responsive organic afterglow materials display distinct advantages to function normally in the presence of strong fluorescence background. However, most of the stimuli-responsive afterglow materials can only work in their solid states with those in aqueous medium being rarely reported. In addition, the types of these materials are limited to mechanical force-, pH-, ion-, and temperature-responsive ones. Here, a serendipitous finding of sonication-responsive organic afterglow emulsions in aqueous medium is reported. The afterglow emulsions are produced by dopant-matrix strategy and emulsion technique. In-depth studies reveal that the key to achieve the sonication responsiveness is the selection of a specialized organic matrix that can form microcrystalline particles in aqueous medium with the aid of surfactants. In matrix's microcrystalline state, the triplet excited states of luminescent dopant within the matrix can be protected to exhibit room-temperature afterglow. Under sonication, matrix's microcrystalline structure can be disrupted and consequently the triplet excited states of luminescent dopants lose protection to show afterglow quenching. To the best of the authors’ knowledge, this study represents the first report of sonication-responsive afterglow materials in aqueous medium, which show promising biomedical applications.