Intense Hydrochromic Photon Upconversion from Lead-Free 0D Metal Halides For Water Detection and Information Encryption

Hydrochromic materials that change their luminescence color upon exposure to moisture have attracted considerable attention owing to their applications in sensing and information encryption. However, the existing materials lack high hydrochromic response and color tunability. This study reports the development of a new and bright 0D Cs3 GdCl6 metal halide as the host for hydrochromic photon upconversion in the form of polycrystals (PCs) and nanocrystals. Lanthanides co-doped cesium gadolinium chloride metal halides exhibit upconversion luminescence (UCL) in the visible-infrared region upon 980 nm laser excitation. In particular, PCs co-doped with Yb3+ and Er3+ exhibit hydrochromic UCL color change from green to red. These hydrochromic properties are quantitatively confirmed through the sensitive detection of water in tetrahydrofuran solvent via UCL color changes. This water-sensing probe exhibits excellent repeatability and is particularly suitable for real-time and long-term water monitoring. Furthermore, the hydrochromic UCL property is exploited for stimuli-responsive information encryption via cyphertexts. These findings will pave the way for the development of new hydrochromic upconverting materials for emerging applications, such as noncontact sensors, anti-counterfeiting, and information encryption.

Encapsulating lithium at the microscale: selective deposition in carbon-doped graphitic carbon nitride spheres

For stable lithium deposition without dendrites, three-dimensional (3D) porous structure has been intensively investigated. Here, we report the use of carbon-doped graphitic carbon nitride (C-doped g-C3N4) microspheres as a 3D host for lithium to suppress dendrite formation, which is crucial for stable lithium deposition. The C-doped g-C3N4microspheres have a high surface area and porosity, allowing for efficient lithium accommodation with high accessibility. The carbon-doping of the g-C3N4microspheres confers lithiophilic properties, which facilitate the regulation of Li+flux and dense filling of cavities with nucleated lithium, thereby preventing volume expansion and promoting dendrite-free Li deposition. The electrochemical performance was improved with cyclic stability and high Coulombic efficiency over 260 cycles at 1.0 mA cm-2for 1.0 mAh cm-2, and even over 70 cycles at 5.0 mA cm-2for 3.0 mAh cm-2. The use of C-doped g-C3N4microspheres as a 3D Li host shows promising results for stable lithium deposition without dendrite formation.

Utilizing VO2 as a Hole Injection Layer for Efficient Charge Injection in Quantum Dot Light-Emitting Diodes Enables High Device Performance

Quantum dot light-emitting diodes (QLEDs) are promising devices for display applications. Polyethylenedioxythiophene:polystyrene sulfonate (PEDOT:PSS) is a common hole injection layer (HIL) material in optoelectronic devices because of its high conductivity and high work function. Nevertheless, PEDOT:PSS-based QLEDs have a high energy barrier for hole injection, which results in low device efficiency. Therefore, a new strategy is needed to improve the device efficiency. Herein, we have demonstrated a bilayer-HIL using VO₂ and a PEDOT:PSS-based QLED that exhibits an 18% external quantum efficiency (EQE), 78 cd/A current efficiency (CE), and 25,771 cd/m² maximum luminance. In contrast, the PEDOT:PSS-based QLED exhibits an EQE of 13%, CE of 54 cd/A, and maximum luminance of 14,817 cd/m². An increase in EQE was attributed to a reduction in the energy barrier between indium tin oxide (ITO) and PEDOT:PSS, caused by the insertion of a VO₂ HIL. Therefore, our results could demonstrate that using a bilayer-HIL is effective in increasing the EQE in QLEDs.

Synthesis of Thermally Stable and Highly Luminescent Cs5 Cu3 Cl6 I2 Nanocrystals with Nonlinear Optical Response

Low-dimensional Cu(I)-based metal halide materials are gaining attention due to their low toxicity, high stability and unique luminescence mechanism, which is mediated by self-trapped excitons (STEs). Among them, Cs₅ Cu₃ Cl₆ I₂ , which emits blue light, is a promising candidate for applications as a next-generation blue-emitting material. In this article, an optimized colloidal process to synthesize uniform Cs₅ Cu₃ Cl₆ I₂ nanocrystals (NCs) with a superior quantum yield (QY) is proposed. In addition, precise control of the synthesis parameters, enabling anisotropic growth and emission wavelength shifting is demonstrated. The synthesized Cs₅ Cu₃ Cl₆ I₂ NCs have an excellent photoluminescence (PL) retention rate, even at high temperature, and exhibit high stability over multiple heating-cooling cycles under ambient conditions. Moreover, under 850-nm femtosecond laser irradiation, the NCs exhibit three-photon absorption (3PA)-induced PL, highlighting the possibility of utilizing their nonlinear optical properties. Such thermally stable and highly luminescent Cs₅ Cu₃ Cl₆ I₂ NCs with nonlinear optical properties overcome the limitations of conventional blue-emitting nanomaterials. These findings provide insights into the mechanism of the colloidal synthesis of Cs₅ Cu₃ Cl₆ I₂ NCs and a foundation for further research.

Enhanced Phase, Thermal and Aqueous Stability of the Embedded the CsPbI3 Ncs in Cs4PbI6 Ncs

Increasing the stability of lead halide perovskites (LHPs) is required for integrating them into light-emitting devices. To date, most studies toward this direction have primarily concentrated on improving the chemical stability of green-emitting LHPs. In this work, red-emitting CsPbI3–Cs4PbI6 hybrid nanocrystals (NCs) were synthesized with a high photoluminescence (PL) quantum yield of ∼90%. Their hybrid structure was examined via structural (Rietveld) refinement analysis and transmission electron microscopy. Rietveld refinement also revealed that the black polymorph of CsPbI3 NCs is an orthorhombic perovskite rather than a cubic one. The thermodynamic stability of the CsPbI3 NCs in Cs4PbI6 matrices is enhanced in both solutions and films for up to several weeks. The enhanced stability of the embedded CsPbI3 NCs is attributed to the lowering of their Gibbs free energy, as determined on the basis of experimental data. Additionally, the hybrid NCs exhibit unprecedented emission stability—maintaining 65% of their original PL efficiency at 150 °C—and improved aqueous stability.

Multimodal Digital X-ray Scanners with Synchronous Mapping of Tactile Pressure Distributions using Perovskites

Visual and tactile information are the key intuitive perceptions in sensory systems, and the synchronized detection of these two sensory modalities can enhance accuracy of object recognition by providing complementary information between them. Herein, multimodal integration of flexible, high-resolution X-ray detectors with a synchronous mapping of tactile pressure distributions for visualizing internal structures and morphologies of an object simultaneously is reported. As a visual-inspection method, perovskite materials that convert X-rays into charge carriers directly are synthesized. By incorporating pressure-sensitive air-dielectric transistors in the perovskite components, X-ray detectors with dual modalities (i.e., vision and touch) are attained as an active-matrix platform for digital visuotactile examinations. Also, in vivo X-ray imaging and pressure sensing are demonstrated using a live rat. This multiplexed platform has high spatial resolution and good flexibility, thereby providing highly accurate inspection and diagnoses even for the distorted images of nonplanar objects.

Sub-micro droplet reactors for green synthesis of Li3VO4 anode materials in lithium ion batteries

The conventional solid-state reaction suffers from low diffusivity, high energy consumption, and uncontrolled morphology. These limitations are competed by the presence of water in solution route reaction. Herein, based on concept of combining above methods, we report a facile solid-state reaction conducted in water vapor at low temperature along with calcium doping for modifying lithium vanadate as anode material for lithium-ion batteries. The optimized material, delivers a superior specific capacity of 543.1, 477.1, and 337.2 mAh g-1 after 200 and 1000 cycles at current densities of 100, 1000 and 4000 mA g-1, respectively, which is attributed to the contribution of pseudocapacitance. In this work, we also use experimental and theoretical calculation to demonstrate that the enhancement of doped lithium vanadate is attributed to particles confinement of droplets in water vapor along with the surface and structure variation of calcium doping effect.

Multidimensional Perovskite Cesium Lead Halide Nanocrystal for Light Conversion Applications

Zero-dimensional (0D) inorganic perovskites have recently emerged as a new class of material for optoelectronics owing to their outstanding excitonic properties, strong photoluminescence (PL), and high exciton binding energy. These materials have a unique quantum-confined structure, which originates from the presence of fully isolated octahedra exhibiting single-molecule behavior. In this work, we probed the optical behavior of single molecule-like isolated octahedra in 0D Cs4PbX6 (X = Cl, Br/Cl) perovskite through isovalent (Mn2+) doping at Pb sites. The incorporation of Mn2+ stabilizes the Cs4PbX6 phase by lowering the symmetry of PbX6 via enhanced octahedral distortion (preventing the high-symmetry cubic CsPbX3 impurity) and controlling the compositional variation of Cs-Pb salts. This strategy enabled the synthesis of CsPbX3 free Cs4PbX6 nanocrystals. A high PL quantum yield (QY) of Mn2+ emission was obtained in the colloidal (29%) and solid (21%, powder) forms. These performances can be attributed to structure-induced confinement effects, which enhance the energy transfer from localized host exciton states to Mn2+ dopant within the isolated octahedra of 0D perovskite. The present findings could lead to designing of low-dimensional perovskites as efficient light emitters with photo and chemical stability for high-performance optoelectronic devices.

Mechanochemistry as a Green Route: Synthesis, Thermal Stability, and Postsynthetic Reversible Phase Transformation of Highly-Luminescent Cesium Copper Halides

Cesium copper halides (CCHs) show promise for optoelectronic applications, and their syntheses usually involve high-temperatures and hazard solvents. Herein, the synthesis of highly luminescent and phase-pure Cs₃Cu₂X₅ (X = Cl, Br, and I) and CsCu₂I₃ via a solvent-free mechanochemical approach through manual grinding is demonstrated. This cost-effective approach can produce CCHs on a scale of tens to hundreds of grams. Rietveld refinement analysis of the X-ray diffraction patterns of the as-synthesized CCHs reveals their structural details. Notably, the emission characteristics of green-emitting, chloride-based CCHs remain stable even at elevated temperatures-maintaining 80% of initial PL efficiency at 150 °C. Lastly, a postsynthetic reversible transformation between zero- and one-dimensional CCH materials is demonstrated, indicating the labile nature of their crystal structure. The proposed study suggests that mechanochemistry can be an alternative and promising synthetic tool for fabricating high-quality lead-free metal halides.

Narrow-Band SrMgAl10O17:Eu2+, Mn2+ Green Phosphors for Wide-Color-Gamut Backlight for LCD Displays

The strength of the photoluminescence excitation (PLE) spectrum of SrMgAl₁₀O₁₇:Eu²⁺, Mn²⁺ (SAM:Eu²⁺, Mn²⁺) phosphor increased at deep blue (∼430 nm) and red-shifted from violet to deep blue with increasing concentrations of both Eu²⁺ ions Mn²⁺ ions. Eu²⁺-Mn²⁺ energy transfer between Eu²⁺ ions in Sr-O layer and Mn²⁺ ions at Al-O tetrahedral sites was maximized, and the photoluminescence (PL) intensity of the narrow-band Mn²⁺ emission was improved by optimizing the concentrations of Eu²⁺ and Mn²⁺ ions. The PL emission spectrum of the (Sr_0.6Eu_0.4)(Mg_0.4Mn_0.6)Al₁₀O₁₇ (SAM:Eu²⁺, Mn²⁺) phosphor peaks was optimized at 518 nm at a full width at half-maximum (FWHM) of 26 nm under light-emitting diode (LED) excitation at 432 nm LED. The color gamut area of a color-filtered RGB triangle of down-converted white LEDs (DC-WLEDs) incorporated with optimum SAM:Eu²⁺, Mn²⁺ green and K₂SiF₆:Mn⁴⁺ (KSF:Mn⁴⁺) red phosphors is enlarged by 114% relative to that of the NTSC standard system in the CIE 1931 color space. The luminous efficacy of our DC-WLED was measured and found to be ∼92 lm/W at 20 mA. Increased energy transfers between dual activators and red-shifted band-edge and enhanced intensity of PLE spectrum indicate the possibility of developing dual-activated narrow-band green phosphors for wide-color gamut in an LCD backlighting system.

Facile Green Synthesis of Pseudocapacitance-Contributed Ultrahigh Capacity Fe2(MoO4)3 as an Anode for Lithium-Ion Batteries

The investigation into the use of earth-abundant elements as electrode materials for lithium-ion batteries (LIBs) is becoming more urgent because of the high demand for electric vehicles and portable devices. Herein, a new green synthesis strategy, based on a facile solid-state reaction with the assistance of water droplets' vapor, was conducted to prepare Fe₂(MoO₄)₃ nanosheets as anode materials for LIBs. The obtained sample possesses a two-dimensional stacked nanosheet construction with open gaps providing a much higher surface area compared to the bulk sample conventionally synthesized. The nanosheet sample delivers an ultrahigh reversible capacity (1983.6 mA h g^-1) at a current density of 100 mA g^-1 after 400 cycles, which could be related to the contribution of pseudocapacitance. The enhancement in cyclability and rated performance with an interesting increased capacity could be caused by the effect of electrochemical milling and the in situ formation of metallic particles in its lithium-ion storage mechanism.

Robust, Brighter Red Emission from CsPbI3 Perovskite Nanocrystals via Endotaxial Protection

Increasing the stability of lead halide perovskites (LHPs) is required for integrating them into light-emitting devices. To date, most studies toward this direction have primarily concentrated on improving the chemical stability of green-emitting LHPs. In this work, red-emitting CsPbI₃-Cs₄PbI₆ hybrid nanocrystals (NCs) were synthesized with a high photoluminescence (PL) quantum yield of ∼90%. Their hybrid structure was examined via structural (Rietveld) refinement analysis and transmission electron microscopy. Rietveld refinement also revealed that the black polymorph of CsPbI₃ NCs is an orthorhombic perovskite rather than a cubic one. The thermodynamic stability of the CsPbI₃ NCs in Cs₄PbI₆ matrices is enhanced in both solutions and films for up to several weeks. The enhanced stability of the embedded CsPbI₃ NCs is attributed to the lowering of their Gibbs free energy, as determined on the basis of experimental data. Additionally, the hybrid NCs exhibit unprecedented emission stability-maintaining 65% of their original PL efficiency at 150 °C-and improved aqueous stability.

Zero reduction luminescence of aqueous-phase alloy core/shell quantum dots via rapid ambient-condition ligand exchange

Quantum dots (QDs) have been widely studied as promising materials for various applications because of their outstanding photoluminescence (PL). Although ligand exchange methods for QDs have been developed over two decades, the PL quantum yield (QY) of aqueous phase QDs is still lower than that of their organic phase and the mechanism of quenching has not been clearly understood. In this study, we demonstrate for the first time that 3-mercaptopropionic-capped CdZnSeS/ZnS core/shell QDs obtained via ligand exchange in a ternary solvent system containing chloroform/water/dimethyl sulfoxide can enable the fast phase transfer and zero reduction of PL under ambient condition. The new solvent system allows the ligand-exchanged QDs to exhibit enhanced QYs up to 8.1% of that of the organic-phase QDs. Based on both theoretical calculation and experiment, it was found that control over the physical/chemical perturbation between the organic/aqueous phases by choosing appropriate solvents for the ligand exchange process is very important to preserve the optical properties of QDs. We believe that our new technologies and theoretical knowledge offer opportunities for the future design and optimization of highly stable and highly luminescent aqueous-phase QDs for various applications.

Rechargeable Intermetallic Calcium-Lithium-O2 Batteries

The growing demand for rechargeable batteries with high energy density has triggered research on batteries based on polyvalent cations such as Ca²⁺ , Mg²⁺ , Al³⁺ , and Y³⁺ . Ca is, in particular, a promising anode material as an alternative to Li because of its mechanical strength (ρ=1.55 g cm^-3 ), safety in terms of thermal runaway (m.p.=839 °C), earth-abundance (world production of 150 million tons of gypsum in 2012), high specific charge capacity (1.340 mAh g^-1 or 2.077 mAh cm^-3 ), and standard reduction potential (-2.87 V vs. normal hydrogen electrode, NHE) comparable to that of Li. As with Mg, the practical application of Ca in rechargeable batteries with organic liquid electrolytes has been hindered by the passivation layer resulting from undesirable reactions between metallic Ca and electrolytes, which precludes the possibility of reversible plating of any metal cations on Ca electrodes. Here, a battery system based on intermetallic CaLi₂ anodes was developed. Li was used as a host for Ca through the formation of an intermetallic compound, which simultaneously enabled 1) the assembly of a rechargeable battery system with Ca anodes and liquid organic electrolytes and 2) coupling these with an earth-abundant, high-energy-density air cathode without special passivation agents. This strategy is simple and broadly applicable to the other polyvalent cations listed above, opening a new avenue to further engineer the electrode materials required for practical, efficient electrochemical energy-storage systems.

Correction: Highly stable hetero-structured green-emitting cesium lead bromide nanocrystals via ligand-mediated phase control

Correction for 'Highly stable hetero-structured green-emitting cesium lead bromide nanocrystals via ligand-mediated phase control' by G. Krishnamurthy Grandhi et al., Nanoscale, 2019, 11, 21137-21146.

Mechanoluminescent, Air-Dielectric MoS2 Transistors as Active-Matrix Pressure Sensors for Wide Detection Ranges from Footsteps to Cellular Motions

Tactile pressure sensors as flexible bioelectronic devices have been regarded as the key component for recently emerging applications in electronic skins, health-monitoring devices, or human-machine interfaces. However, their narrow range of sensible pressure and their difficulty in forming high integrations represent major limitations for various potential applications. Herein, we report fully integrated, active-matrix arrays of pressure-sensitive MoS₂ transistors with mechanoluminescent layers and air dielectrics for wide detectable range from footsteps to cellular motions. The inclusion of mechanoluminescent materials as well as air spaces can increase the sensitivity significantly over entire pressure regimes. In addition, the high integration capability of these active-matrix sensory circuitries can enhance their spatial resolution to the level sufficient to analyze the pressure distribution in a single cardiomyocyte. We envision that these wide-range pressure sensors will provide a new strategy toward next-generation electronics at biomachine interfaces to monitor various mechanical and biological phenomena at single-cell resolution.

Highly stable hetero-structured green-emitting cesium lead bromide nanocrystals via ligand-mediated phase control

Green-emissive Cs₄PbBr₆ shows promise for light-emitting diode devices superior to that of CsPbBr₃ NCs owing to their stability and high photoluminescence efficiency. Nevertheless, there is still no consensus regarding the basis of their green emission, which decelerates their advance in light-emitting applications. Herein, a systematic investigation on the concentration of capping ligands (oleylamine and oleic acid), which determines the predominant phase between CsPbBr₃ and Cs₄PbBr₆ for a given Cs to Pb feed ratio, is conducted. This study deduces that oleylamine to oleic acid ratio plays a crucial role in obtaining either green-emissive or non-emissive Cs₄PbBr₆ NCs. Scrutiny of Cs₄PbBr₆ microscopic and optical data in addition to their emission quenching study with a hole-withdrawing molecule reveals that the green emission originates from the CsPbBr₃ impurity phase. Furthermore, stable green emission is observed for CsPbBr₃/Cs₄PbBr₆ nanocrystals when CsPbBr₃ particles are well protected by the Cs₄PbBr₆ matrix. These CsPbBr₃/Cs₄PbBr₆ films remained highly luminescent even after UV exposure for hours or annealing at ∼150 °C for days in addition to their long-term stability under an ambient atmosphere, which are the desirable properties for various practical applications.

Molecular Cooperative Assembly-Mediated Synthesis of Ultra-High-Performance Hard Carbon Anodes for Dual-Carbon Sodium Hybrid Capacitors

Although sodium hybrid capacitors (NHCs) have emerged as one of the most promising next-generation energy storage systems, further advancement is delayed primarily by the absence of high-performance battery-type anodes. Herein, we report a nature-inspired synthesis route to prepare hard carbon anodes with high capacity, rate capability, and cycle stability for dual-carbon NHCs. Shape- and size-controllable crystal aggregates of inexpensive triazine molecules are utilized as reactive templates that perform triple duties of structure-directing agent, porogen, and nitrogen source. This enables the fine control of microstructure/morphology/composition and thereby electrochemical reactions toward Na-ion. The resulting hard carbon optimized in terms of lateral size, interlayer spacing, and surface affinity of graphene-like layers achieves a specific capacity of ∼380 mAh/g after 100 cycles at a current density of 250 mA/g mainly via intercalation, the current record of hard carbons. Combined with a commercial microporous carbon fiber cathode, the full cell is able to deliver a volumetric energy density of 2.89 mWh/cm³ and a volumetric power density of 160 mW/cm³, outperforming NHCs based on inorganic Na-ion anode materials. More importantly, such performance could not only be retained for 10000 cycles (4.5 F/cm³ at 10 mA/cm³) with 0.000 028 6% loss per cycle at >97% Coulombic efficiency but also successfully transferred to flexible pouch cells without significant performance loss after 300 bending cycles or during wrapping at a 10R condition. Simple preparation of hard carbon anodes using organic crystal reactive templates, therefore, demonstrates great potential for the manufacture of high-performance flexible NHCs using only carbon electrode materials.

Probing molecule-like isolated octahedra via-phase stabilization of zero-dimensional cesium lead halide nanocrystals

Zero-dimensional (0D) inorganic perovskites have recently emerged as an interesting class of material owing to their intrinsic Pb2+ emission, polaron formation, and large exciton binding energy. They have a unique quantum-confined structure, originating from the complete isolation of octahedra exhibiting single-molecule behavior. Herein, we probe the optical behavior of single-molecule-like isolated octahedra in 0D Cesium lead halide (Cs4PbX6, X = Cl, Br/Cl, Br) nanocrystals through isovalent manganese doping at lead sites. The incorporation of manganese induced phase stabilization of 0D Cs4PbX6 over CsPbX3 by lowering the symmetry of PbX6 via enhanced octahedral distortion. This approach enables the synthesis of CsPbX3 free Cs4PbX6 nanocrystals. A high photoluminescence quantum yield for manganese emission was obtained in colloidal (29%) and solid (21%, powder) forms. These performances can be attributed to structure-induced confinement effects, which enhance the energy transfer from localized host exciton states to Mn2+ dopant within the isolated octahedra.

A Phosphosilicate Compound, NaCa3PSiO8: Structure Solution and Luminescence Properties

NaCa₃PSiO₈ was synthesized in a microwave-assisted solid-state reaction. The crystal structure of the synthesized compound was solved using a least-squares method, followed by simulated annealing. The compound was crystallized in the orthorhombic space group Pna2₁, belonging to Laue class mmm. The structure consisted of two layers of cation planes, each of which contained three cation channels. The cation channels in each of the layers ran antiparallel to that of the adjacent layer. All the major cations together constituted four distinct crystallographic sites. The Rietveld refinement of the powder X-ray diffraction data, followed by the maximum-entropy method analysis, confirmed the obtained structure solutions. The electronic band structure of the compound was analyzed through density function theory calculations. Luminescence properties of the compound, upon activating with Eu²⁺ ions, were analyzed through photoluminescence measurements and decay profile analysis. The compound was found to exhibit green luminescence centered at ∼502 nm, with a typical broadband emission due to the transition from the crystal-field split 4f⁶5d to 4f⁷ levels.

Colloidal Organolead Halide Perovskite with a High Mn Solubility Limit: A Step Toward Pb-Free Luminescent Quantum Dots

Organolead halide perovskites have emerged as a promising optoelectronic material for lighting due to its high quantum yield, color-tunable, and narrow emission. Despite their unique properties, toxicity has intensified the search for ecofriendly alternatives through partial or complete replacement of lead. Herein, we report a room-temperature synthesized Mn²⁺-substituted 3D-organolead perovskite displacing ∼90% of lead, simultaneously retaining its unique excitonic emission, with an additional orange emission of Mn²⁺ via energy transfer. A high Mn solubility limit of 90% was attained for the first time in lead halide perovskites, facilitated by the flexible organic cation (CH₃NH₃)⁺ network, preserving the perovskite structure. The emission intensities of the exciton and Mn were influenced by the halide identity that regulates the energy transfer to Mn. Homogeneous emission and electron spin resonance characteristics of Mn²⁺ indicate a uniform distribution of Mn. These results suggest that low-toxicity 3D-CH₃NH₃Pb_1-xMn_xBr_3-(2x+1)Cl_2x+1 nanocrystals may be exploited as magnetically doped quantum dots with unique optoelectronic properties.

A zero-thermal-quenching phosphor

Phosphor-converted white light-emitting diodes (pc-WLEDs) are efficient light sources used in lighting, high-tech displays, and electronic devices. One of the most significant challenges of pc-WLEDs is the thermal quenching, in which the phosphor suffers from emission loss with increasing temperature during high-power LED operation. Here, we report a blue-emitting Na_3-2xSc₂(PO₄)₃:xEu²⁺ phosphor (λ_em = 453 nm) that does not exhibit thermal quenching even up to 200 °C. This phenomenon of zero thermal quenching originates from the ability of the phosphor to compensate the emission losses and therefore sustain the luminescence with increasing temperature. The findings are explained by polymorphic modification and possible energy transfer from electron-hole pairs at the thermally activated defect levels to the Eu²⁺ 5d-band with increasing temperature. Our results could initiate the exploration of phosphors with zero thermal quenching for high-power LED applications.

Hydrophobic Organic Skin as a Protective Shield for Moisture-Sensitive Phosphor-Based Optoelectronic Devices

A moisture-stable, red-emitting fluoride phosphor with an organic hydrophobic skin is reported. A simple strategy was employed to form a metal-free, organic, passivating skin using oleic acid (OA) as a hydrophobic encapsulant via solvothermal treatment. Unlike other phosphor coatings that suffer from initial efficiency loss, the OA-passivated K₂SiF₆:Mn⁴⁺ (KSF-OA) phosphor exhibited the unique property of stable emission efficiency. Control of thickness and a highly transparent passivating layer helped to retain the emission efficiency of the material after encapsulation. A moisture-stable KSF-OA phosphor could be synthesized because of the exceptionally hydrophobic nature of OA and the formation of hydrogen bonds (F···H) resulting from the strong interactions between the fluorine in KSF and hydrogen in OA. The KSF-OA phosphor exhibited excellent moisture stability and maintained 85% of its emission intensity even after 450 h at high temperature (85 °C) and humidity (85%). As a proof-of-concept, this strategy was used for another moisture-sensitive SrSi₂O₂N₂:Eu²⁺ phosphor which showed enhanced moisture stability, retaining 85% of emission intensity after 500 h under the same conditions. White light-emitting devices were fabricated using surface-passivated KSF and Y₃Al₅O₁₂:Ce³⁺ which exhibited excellent color rendering index of 86, under blue LED excitation.

New melilite (Ca,Sr,Ba)4MgAl2Si3O14:Eu2+ phosphor: structural and spectroscopic analysis for application in white LEDs

Tunable asymmetric broad-band emission ranging from blue to green region of the spectra was obtained.

A morphology, porosity and surface conductive layer optimized MnCo2O4 microsphere for compatible superior Li(+) ion/air rechargeable battery electrode materials

Uniform surface conductive layers with porous morphology-conserved MnCo2O4 microspheres are successfully synthesized, and their electrochemical performances are thoroughly investigated. It is found that the microwave-assisted hydrothermally grown MnCo2O4 using citric acid as the carbon source shows a maximum Li(+) ion lithiation/delithiation capacity of 501 mA h g(-1) at 500 mA g(-1) with stable capacity retention. Besides, the given microsphere compounds are effectively activated as air cathode catalysts in Li-O2 batteries with reduced charge overpotentials and improved cycling performance. We believe that such an affordable enhanced performance results from the appropriate quasi-hollow nature of MnCo2O4 microspheres, which can effectively mitigate the large volume change of electrodes during Li(+) migration and/or enhance the surface transport of the LiOx species in Li-air batteries. Thus, the rationally designed porous media for the improved Li(+) electrochemical reaction highlight the importance of the 3D macropores, the high specific area and uniformly overcoated conductive layer for the promising Li(+) redox reaction platforms.

Effective Heat Dissipation from Color-Converting Plates in High-Power White Light Emitting Diodes by Transparent Graphene Wrapping

We have developed a hybrid phosphor-in-glass plate (PGP) for application in a remote phosphor configuration of high-power white light emitting diodes (WLEDs), in which single-layer graphene was used to modulate the thermal characteristics of the PGP. The degradation of luminescence in the PGP following an increase in temperature could be prevented by applying single-layer graphene. First, it was observed that the emission intensity of the PGP was enhanced by about 20% with graphene wrapping. Notably, the surface temperature of the graphene-wrapped PGP (G-PGP) was found to be higher than that of the bare PGP, implying that the graphene layer effectively acted as a heat dissipation medium on the PGP surface to reduce the thermal quenching of the constituent phosphors. Moreover, these experimental observations were clearly verified through a two-dimensional cellular automata simulation technique and the underlying mechanisms were analyzed. As a result, the proposed G-PGP was found to be efficient in maintaining the luminescence properties of the WLED, and is a promising development in high power WLED applications. This research could be further extended to generate a new class of optical or optoelectronic materials with possible uses in a variety of applications.

Correction: Intrinsically conductive polymer binders for electrochemical capacitor application

Correction for ‘Intrinsically conductive polymer binders for electrochemical capacitor application’ by M. Kang et al., RSC Adv., 2014, 4, 27939–27945.

A complete inorganic colour converter based on quantum-dot-embedded silicate glasses for white light-emitting-diodes

A complete inorganic quantum dot color converter for a white LED is achieved using silicate-based quantum-dot-embedded glasses (QDEGs).

Influence of Ti(4+) on the electrochemical performance of Li-rich layered oxides - high power and long cycle life of Li2Ru1-xTixO3 cathodes

Li-rich layered oxides are the most attractive cathodes for lithium-ion batteries due to their high capacity (>250 mAh g(-1)). However, their application in electric vehicles is hampered by low power density and poor cycle life. To address these, layered Li2Ru0.75Ti0.25O3 (LRTO) was synthesized and the influence of electroinactive Ti(4+) on the electrochemical performance of Li2RuO3 was investigated. LRTO exhibited a reversible capacity of 240 mAh g(-1) under 14.3 mA g(-1) with 0.11 mol of Li loss after 100 cycles compared to 0.22 mol of Li for Li2Ru0.75Sn0.25O3. More Li(+) can be extracted from LRTO (0.96 mol of Li) even after 250 cycles at 143 mA g(-1) than Li2RuO3 (0.79 mol of Li). High reversible Li extraction and long cycle life were attributed to structural stability of the LiM2 layer in the presence of Ti(4+), facilitating the lithium diffusion kinetics. The versatility of the Li2MO3 structure may initiate exploration of Ti-based Li-rich layered oxides for vehicular applications.

Role of co-vapors in vapor deposition polymerization

Polypyrrole (PPy)/cellulose (PPCL) composite papers were fabricated by vapor phase polymerization. Importantly, the vapor-phase deposition of PPy onto cellulose was assisted by employing different co-vapors namely methanol, ethanol, benzene, water, toluene and hexane, in addition to pyrrole. The resulting PPCL papers possessed high mechanical flexibility, large surface-to-volume ratio, and good redox properties. Their main properties were highly influenced by the nature of the co-vaporized solvent. The morphology and oxidation level of deposited PPy were tuned by employing co-vapors during the polymerization, which in turn led to change in the electrochemical properties of the PPCL papers. When methanol and ethanol were used as co-vapors, the conductivities of PPCL papers were found to have improved five times, which was likely due to the enhanced orientation of PPy chain by the polar co-vapors with high dipole moment. The specific capacitance of PPCL papers obtained using benzene, toluene, water and hexane co-vapors was higher than those of the others, which is attributed to the enlarged effective surface area of the electrode material. The results indicate that the judicious choice and combination of co-vapors in vapor-deposition polymerization (VDP) offers the possibility of tuning the morphological, electrical, and electrochemical properties of deposited conducting polymers.

Narsarsukite-structure fluorosilicate as a blue component for white LEDs: structural and optical properties

A new blue-emitting phosphor, K2ScSi4O10F:Eu>2+ (KSSOF:Eu2+), was synthesized through a solid-state reaction. The structural and optical properties of KSSOF:Eu2+ phosphor, in addition to its thermal quenching and fabrication of white LEDs (WLEDs), were investigated for the first time. The phosphor showed broad blue emission, with a maximum at ∼434 nm under near-ultraviolet excitation due to 5d→4f transition of the Eu2+ ion. The critical distance was calculated to be 12 Å using the critical concentration of Eu2+ and Dexter's theory for energy transfer. WLEDs were fabricated by blending KSSOF:Eu2+, commercial Lu3Al5O12:Ce3+, and (Sr,Ca)AlSiN3:Eu2+ phosphors, showed a high color rendering index of 88 at a correlated color temperature of 4134 K under a forward bias current of 100 mA.

Control of chromaticity by phosphor in glasses with low temperature sintered silicate glasses for LED applications

Phosphor-in-glass (PiG) color converters for LED applications were fabricated with a mixture of phosphors, Y₃Al₅O₁₂:Ce³⁺ (yellow) and CaAlSiN₃:Eu²⁺ (red). The low sintering temperature (550°C) of SiO₂-Na₂O-RO (R=Ba, Zn) glass powder enabled the inclusion of CaAlSiN₃:Eu²⁺ (red) phosphor which cannot be embedded with conventional glass powders for PiGs. By simply varying the mixing ratio of glass to phosphors as well as the ratio of yellow to red phosphors, the facile control of the CIE chromaticity coordinates and correlated color temperature of the LED following the Planckian locus has been achieved. Phosphors were well distributed within the glass matrix without noticeable reactions, preserving the enhanced thermal quenching property of the PiG compared to those with silicone resins, for LEDs.

Stacked quantum dot embedded silica film on a phosphor plate for superior performance of white light-emitting diodes

Application of quantum dots as a color converter in white light-emitting diodes (WLEDs) has been highly restrained because of its lower stability under the operating conditions of LEDs. The feasibility of using quantum dots in WLEDs has been studied and demonstrated by developing a non-conventional packing technique. Multiple core shell CuInS2/ZnS quantum dots were coated by silica, and the silica-coated quantum dots were dispersed in ethoxylated trimethylolpropane triacrylate to form a color conversion film. This along with phosphor in a glass plate made of Y3Al5O12:Ce(3+) phosphor was stacked in different configurations, and its effect on color rendering of WLEDs was studied. In addition, the configuration developed here protects the color converter from thermal strain and moisture.

Smart design to resolve spectral overlapping of phosphor-in-glass for high-powered remote-type white light-emitting devices

The white light-emitting diode (WLED) is a state-of-the-art solid state technology, which has replaced conventional lighting systems due to its reduced energy consumption, its reliability, and long life. However, the WLED presents acute challenges in device engineering, due to its lack of color purity, efficacy, and thermal stability of the lighting devices. The prime cause for inadequacies in color purity and luminous efficiency is the spectral overlapping of red components with yellow/green emissions when generating white light by pumping a blue InGaN chip with yellow YAG:Ce³⁺ phosphor, where red phosphor is included, to compensate for deficiencies in the red region. An innovative strategy was formulated to resolve this spectral overlapping by alternatively arranging phosphor-in-glass (PiG) through cutting and reassembling the commercial red CaAlSiN₃:Eu²⁺ and green Lu₃Al₅O₁₂:Ce³⁺ PiG. PiGs were fabricated using glass frits with a low softening temperature of 600°C, which exhibited excellent thermal stability and high transparency, improving life time even at an operating temperature of 200°C. This strategy overcomes the spectral overlapping issue more efficiently than the randomly mixed and patented stacking design of multiple phosphors for a remote-type WLED. The protocol for the current design of PiG possesses excellent thermal and chemical stability with high luminous efficiency and color purity is an attempt to make smarter solid state lighting for high-powered remote-type white light-emitting devices.

Pyro-synthesis of a high rate nano-Li3V2(PO4)3/C cathode with mixed morphology for advanced Li-ion batteries

A monoclinic Li3V2(PO4)3/C (LVP/C) cathode for lithium battery applications was synthesized by a polyol-assisted pyro-synthesis. The polyol in the present synthesis acts not only as a solvent, reducing agent and a carbon source but also as a low-cost fuel that facilitates a combustion process combined with the release of ultrahigh exothermic energy useful for nucleation process. Subsequent annealing of the amorphous particles at 800°C for 5 h is sufficient to produce highly crystalline LVP/C nanoparticles. A combined analysis of X-ray diffraction (XRD) and neutron powder diffraction (NPD) patterns was used to determine the unit cell parameters of the prepared LVP/C. Electron microscopic studies revealed rod-type particles of length ranging from nanometer to micrometers dispersed among spherical particles with average particle-sizes in the range of 20-30 nm. When tested for Li-insertion properties in the potential windows of 3-4.3 and 3-4.8 V, the LVP/C cathode demonstrated initial discharge capacities of 131 and 196 mAh/g (~100% theoretical capacities) at 0.15 and 0.1 C current densities respectively with impressive capacity retentions for 50 cycles. Interestingly, the LVP/C cathode delivered average specific capacities of 125 and 90 mAh/g at current densities of 9.6 C and 15 C respectively within the lower potential window.

Preparation of electrospun pyrochlore-structure KGdTa2O7:Eu3+ phosphor: the optical and structural properties for white light emitting diode applications

Red emitting nanofibers, KGdTa2O7:Eu3+ were synthesized by electrospinning technique followed by heat treatment. As-prepared uniform fiber precursor with diameter ranging from about 700 nm to about 900 nm were calcined after removing organic species by calcination. The fiber surface become rough and diameter decreased to about 250-340 nm range due to decomposition of organic species and formation of inorganic phase. Morphology, structural and photoluminescent properties of fibers were analyzed using thermogravimetric and differential thermal analysis (TG-DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL). TG-DTA analysis indicates that KGdTa2O7:Eu3+ began to crystalize at 520 degrees C. Fibers annealed at 900 degrees C formed well crystallized uniform fibers. Under ultraviolet excitation KGdTa2O7:Eu3+ exhibits red emission due to transitions in 4f states of Eu3+. The excitation band is dominated by the Eu(3+)--O2-charge transfer band peaked at 289 nm. The emission peak is in the region that is ideal for red light emission.

Doped lanthanum nickelates with a layered perovskite structure as bifunctional cathode catalysts for rechargeable metal-air batteries

Rechargeable metal-air batteries have attracted a great interest in recent years because of their high energy density. The critical challenges facing these technologies include the sluggish kinetics of the oxygen reduction-evolution reactions on a cathode (air electrode). Here, we report doped lanthanum nickelates (La2NiO4) with a layered perovskite structure that serve as efficient bifunctional electrocatalysts for oxygen reduction and evolution in an aqueous alkaline electrolyte. Rechargeable lithium-air and zinc-air batteries assembled with these catalysts exhibit remarkably reduced discharge-charge voltage gaps (improved round-trip efficiency) as well as high stability during cycling.

Robust moisture and thermally stable phosphor glass plate for highly unstable sulfide phosphors in high-power white light-emitting diodes

Potential white light-emitting diode (LED) phosphor SrGa2S4, which remains superfluous due to its unstable nature in the presence of moisture, was successfully integrated in a high-power white LED system by developing a glass-based phosphor plate. A glass system with softening temperature at around 600°C, which lies far below the possible decomposition temperature of the sulfide phosphor, provides a stable shield. Physical properties such as thermal stability, transparency, and lower porosity along with chemical stability under operating conditions of the LEDs ensure long-term operability. H2S emission due to the decomposition of sulfide phosphors, which leads to corrosion of LED electrodes, was contained using the developed plate. Higher thermal resistivity of the developed glass system in comparison with conventional resins ensures lower thermal quenching of the luminescence and better color purity.

Facile synthesis of electrospun Li(1.2)Ni(0.17)Co(0.17)Mn(0.5)O2 nanofiber and its enhanced high-rate performance for lithium-ion battery applications

The Li1.2Ni0.17Co0.17Mn0.5O2 nanofibers were synthesized by a simple electrospinning process. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that electrospun nanofibers with small particle size of 10-30 nm were formed. It was found that the electrospinning process leads to the formation of an effective conducting nanofiber, which provides improved intercalation kinetics. The eletrospun Li1.2Ni0.17Co0.17Mn0.5O2 nanofibers showed a high discharge capacity of 256 mA h g(-1) during the first cycle. In particular, the electrospun Li1.2Ni0.17Co0.17Mn0.5O2 nanofiber sample exhibited excellent rate capability when compared to the co-precipitated Li1.2Ni0.17Co0.17Mn0.5O2 particle sample.

Energy transfer in Sr2MgSi2O7:Eu2+ phosphors in nano scale and their application to solid state lighting with excellent color rendering

On the basis of structural information of its host material which shows excellent stability and absorption efficiency in ultra-violet (UV) region, a blue-emitting Sr2MgSi2O7:Eu2+ (SMS:Eu2+) phosphor was synthesized, and its photoluminescence (PL) performance was systematically optimized. In order to enhance its PL properties, Ce3+ was added as a sensitizer based on the energy transfer from the absorption energy of Ce3+ to Eu2+. It was due to the spectral overlap between the photoluminescence excitation spectrum of Ce3+ and the PL spectrum of Eu2+. Moreover, the energy transfer rate from Ce3+ to Eu2+ is generally faster than the emission rate of Ce3+ in the dipole-dipole interaction. Depending upon the amount of Ca2+ substituted into Sr site, their maximum wavelength was varied from -460 to -540 nm in terms of the crystal field effect confirmed by the structural analysis via Rietveld refinement method. Finally, the optimized blue-emitting SMS:Eu2+ and Ca(2+)-substituted yellowish green-emitting SMS:Eu2+ phosphors were applied with Eu(2+)-sensitized red-emitting Ca3Mg3(PO4)4:Mn2+ phosphor introduced in our previous research to UV light emitting diode (LED)-pumped white LEDs. The fabricated white LEDs showed a natural white light with the color coordinate of (0.3298, 0.3280) and the excellent color rendering index of 94.

Tunable emission from blue to white light in single-phase Na(0.34)Ca(0.66-x-y)Al(1.66)Si(2.34)O8:xEu2+,yMn2+ (x = 0.07) phosphor for white-light UV LEDs

A series of single-phased emission-tunable Na(0.34)Ca(0.66)Al(1.66)Si(2.34)O(8):Eu(2+),Mn(2+) phosphors were successfully synthesized by a wet-chemical synthesis method. Photoluminescence excitation (PLE) spectra indicate that the phosphor can be efficiently excited by UV radiation from 250 to 420 nm. Also, NCASO:Eu(2+),Mn(2+) phosphor exhibit a broad blue emission band at 440 nm and an orange emission band at 570 nm, which originate from Eu(2+) and Mn(2+) ions, respectively. Therefore, overall emission color can be tuned from blue to white by increasing the concentration of Mn(2+) ions in the host lattice utilizing energy transfer from Eu(2+) to Mn(2+) ions. This energy transfer phenomenon was demonstrated to be a resonant type through dipole-dipole interaction determined with the help of PL spectra, decay time measurement, and energy transfer efficiency of the phosphor. These results indicate that NCASO:Eu(2+),Mn(2+) can be a promising single-phased white-emitting phosphor for white-light UV LEDs.

Novel blue-emitting Na(x)Ca(1-x)Al(2-x)Si(2+x)O8:Eu2+ (x = 0.34) phosphor with high luminescent efficiency for UV-pumped light-emitting diodes

A novel blue-emitting phosphor, Na(0.34)Ca(0.66)Al(1.66)Si(2.34)O(8):Eu(2+) (NCASO:Eu(2+)), was prepared by a wet chemical synthesis method based on the hydrolysis of tetraorthosiliate (TEOS) and confirmed the formation of NCASO:Eu(2+) from Rietveld analysis. Photoluminescence (PL) results showed that the phosphor can be efficiently excited by UV light from 250 to 420 nm, and emitted bright broad blue emission, which has maximum intensity at around 445 nm. Under 365 nm excitation, the PL emission intensity area of optimized NCASO:Eu(2+) was found to be 99.72% of that of a commercial BaMgAl(10)O(17):Eu(2+) (BAM:Eu(2+)) phosphor. Moreover, the optical absorbance, internal quantum efficiency, and external quantum efficiency of NCASO:Eu(2+) were calculated to be 112%, 94%, and 105% of that of the commercial BAM:Eu(2+) phosphor, respectively. The WLEDs were fabricated using the blue NCASO:Eu(2+) phosphor, a green-emitting β-SiAlON:Eu(2+), and a red-emitting CaAlSiN(3):Eu(2+) phosphors with a near-UV chip. The WLED device exhibited an excellent color-rendering index R(a) of 94 at a correlated color temperature of 5956 K with CIE coordinates of x = 0.323, y = 0.335. These results suggest that NCASO:Eu(2+) is a promising blue-emitting phosphor for UV LED applications.

Phosphor in glasses with Pb-free silicate glass powders as robust color-converting materials for white LED applications

Phosphor-in-glass (PiG) typed robust color converters were fabricated using Pb-free silicate glasses for high-power white LED applications. SiO2-B2O3-RO(R=Ba,Zn) glass powder showed good sintering behavior and high visible transparency under the sintering condition of 750 °C for 30 min without noticeable interaction with phosphors. By simply changing the thickness of the PiG plate or mixing ratio of glass to Y3Al5O12:Ce3+ phosphor, CIE chromaticity coordinates of the LED can be easily controlled. Enhanced thermal quenching property of PiG compared to phosphor with conventional silicone resin suggests its prominent feasibility for high-power/high-brightness white LEDs.

Bredigite-structure orthosilicate phosphor as a green component for white LED: the structural and optical properties

A green-emitting phosphor, Ca₁₄-xEuxMg₂[SiO₄]₈ (CMS:Eu²⁺), has been synthesized as a component of white light emitting diodes (WLEDs). The emission spectrum is broad, with a maximum at about 505 nm under 400 nm excitation due to the transition from the 4f⁶5d excited state to the 4f⁷-ground state of a Eu²⁺ ion. The dipole-dipole interaction was a dominant energy transfer mechanism of the electric multipolar character of CMS:Eu²⁺. The critical distance was calculated as 12.9 Å and 14.9 Å using a critical concentration of Eu²⁺ and Dexter's theory for energy transfer. When CMS:Eu²⁺ and red phosphor are incorporated with an encapsulant on an ultraviolet (λmax = 395 nm) light emitting diodes (LEDs), white light with a color rendering index of 91 under a forward bias current of 20 mA was obtained. The structural and optical characterization of the phosphor is described.