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Lead halide perovskites (LHPs) are suitable as the emissive layers in light-emitting diodes (LEDs). The external quantum efficiency of green LEDs based on LHPs is now over 20%. Nevertheless, the blue LHP LEDs lag behind the green ones in terms of efficiency. Photoluminescence (PL) quantum yield (QY) and stability of the NCs under various operating conditions are two major factors that influence the LED performance. Therefore, to promote the efforts towards achieving improved LED efficiencies, herein, we summarize several synthetic methods that produce blue-emitting LHP NC, followed by several approaches devised to boost their PL QYs up to near unity. Light-induced anion segregation is one of the limitations of using blue-emitting mixed-halide LHPs, which triggers the attention to single halide, quantum-confined LHP nanoplatelets (NPLs). Syntheses, structure, and luminescent properties of organic–inorganic and all-inorganic blue-emitting LHP NPLs are discussed elaborately. In the last portion, the luminescent properties of lead-free metal halides, which are of current interest, are discussed, followed by an outlook and future directions. In conclusion, our review discusses various literature attempts to obtain stable blue-emitting LHP NCs, which can be helpful in a better design of the blue-emitting LHP NCs towards various light-emitting applications.

A new mechanoluminescent phosphor was developed from a sodium (Na) superionic conductor (NASICON)-structured Na3Sc2(PO4)3:Eu2+ phosphor that is known for its self-healing properties. The compound that crystalized assuming monoclinic C2/c symmetry was found to be blue-emitting, with no noticeable persistent luminescence. Thermoluminescence analysis showed that the phosphor had two prominent distinct thermal emission bands corresponding to the high-temperature β- and γ-phases of the composition. The mechanoluminescence properties of the material that does not have any persistent luminescence at room temperature were investigated by imparting impulsive strain. The compound on charging with 365 nm radiation was found to have significant mechanoluminescent emission originating from shallow defects present in the β-phase of composition that formed by the stress-induced phase transition process. Its emission characteristics and temporal behavior were investigated by varying the impact velocity.

In this study, we report on the development of a new garnet phosphor with enhanced optical properties and cost reduction. Samples were prepared using the solid-solution method, in which the chemica...

Cracked tooth syndrome (CTS) is an incomplete fracture of a human tooth that commonly arises from chewing hard foods. Although it is a very common syndrome, CTS is often difficult to diagnose owing to the common small size of tooth cracks. Conventional techniques for the detection of cracks, such as transillumination and radiographic methods, are inaccurate and have poor imaging resolution. In this study, we devise a novel method for the in vivo detection of tooth microcracks by exploiting the mechanoluminescence (ML) phenomenon. ZrO2:Ti4+ (ZRT) phosphor particles are pasted onto suspected regions of tooth cracks and emit cyan-colored light as a result of masticatory forces. Then, a stretchable and self-healable photodetector (PD) array laminated on top of the phosphor particles converts the emitted photons into a photocurrent, which facilitates the two-dimensional mapping of the tooth cracks. Because of the high photosensitivity of the PD, intense ML and small size of ZRT phosphor particles, it is possible to image submicron- to micron-sized cracks with high resolution. Furthermore, the uniqueness of this technique over the conventional techniques stems from the application of a simple optical phenomenon, i.e., ML, for obtaining precise information regarding the locations, depth, and length of tooth cracks.

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.

A new strategy for developing solid-solution compounds from three constituent compounds was analyzed. Using this strategy, a series of yellow-light-emitting phosphors derived from ternary solid-solution host compositions were prepared using solid-state synthesis. Three vertex members, Sr3SiO5 (space group P4/ncc), Sr3AlO4F (space group I4/mcm), and LaSr2AlO5 (space group I4/mcm), were selected from similar crystal structures with slightly different symmetries. The crystallization space group of the composition was found to be dependent on the end members, and from the vertex of the ternary plot to the centroid, the composition assumed the space group that was more tolerant toward substitutions. All the compositions, except those lying at the periphery of the ternary plot, formed a single phase and emitted yellow light when activated with Ce3+ ions. The luminescence properties of the 28 selected compositions on the ternary plot were analyzed. The compound that was formed from a 1:2:1 ratio of end members assumed a I4/mcm structure and showed the highest-intensity emission. Phosphor derived from the developed host lattice was analyzed for application in solid-state lighting devices by preparing a white-light emitting device.

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 (Sr0.6Eu0.4)(Mg0.4Mn0.6)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.

Europium-doped UCr4C4 cuboid phosphors with narrow and tunable emission show promise for application in designing light-emitting diodes with high color purity and thermal stability. Nevertheless, their poor chemical stability bottlenecks their use in lighting applications. Herein, a dual narrow-band blue-emitting Ce3+-doped CsNa2K(Li3SiO4)4 (CNKLSO) cuboid phosphor with a quantum yield of 80% is synthesized. The dual-band emission is attributed to the occupation of Ce3+ at two distinct sites of CNKLSO, which is consistent with the density functional theory outcomes. Ce3+ doping produces stable emission characteristics, in contrast to Eu2+ incorporation, because of the improved site stability, as realized from bond valence sum calculations. Remarkably, no drop in the emission intensity is observed even at 200 °C, making CNKLSO:Ce3+ the first Ce3+-based zero-thermal-quenching phosphor. A CNKLSO:Ce3+-based white light-emitting diode displays an excellent color rendering index (~95) at a high flux current of 1000 mA. The proposed study indicates that Ce3+ doping in other UCr4C4- type oxide phosphors may improve the site stability, which in turn the chemical stability of the phosphor materials can be boosted.

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.

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 MoS

A series of new Eu2+-activated melilite-structured phosphor compounds was developed through solid-state reactions. The structural and spectroscopic properties of the phosphors were analyzed; all phosphors showed emissions in the blue to green regions of visible light. All developed compounds showed asymmetric broad-bands with shoulders on the lower-energy sides. The spectroscopic parameters of Eu2+ emission in the host compound were estimated and their correlation with the chemical composition of the phosphor was verified. The luminescence mechanism in the phosphors was analyzed through luminescence decay measurements. The broad emission band in the developed compound, due to transitions in the 4f65d–4f7 levels of Eu2+, was found to be ideal for application in solid-state lighting devices. The feasibility of the compound as a potential white LED phosphor was demonstrated by fabricating a white LED with excellent emission properties.

Spinel-based LiMn 2 O 4 is the most attractive cathode for Li-ion battery due to high voltage, low cost, and non-toxicity. The cycle life of the spinel cathodes could be improved by replacing Mn 4+ with Ti 4+ leading to the formation of new spinel cathode, LiMnTiO 4 . However, its application is restricted due to the associated loss in the specific capacity. In this work, spinel-layered Li 1+ z MnTiO 4+ δ ( z = 0, 0.5, and 1.0; δ is the value to reflect the composite character of the material) cathodes were fabricated to achieve long cycle life, without compromising on the specific capacity. Cathodes with excess Li ( z = 0.5 and 1.0) formed a spinel-layered composite structure with notation (1- a )LiMn 2- x Ti x O 4 . a Li 2 Mn y Ti 1- y O 3 [ y = 0.5–((1/ a − 1)(1 − x ))]. These cathodes exhibited an enhanced specific capacity of ∼218 mAh g −1 (20% higher), with a capacity retention of 94% after 60 cycles. The structural and electrochemical properties of these cathodes were investigated using X-ray diffraction, galvanostatic cycling, cyclic voltammetry, and the galvanostatic intermittent titration technique to understand the mechanisms underlying the enhanced capacity and cycle stability. The effect of the Li-rich layered phase on the electrochemical performance of the Li 1+ z MnTiO 4+ δ cathodes was also investigated.

Two series of the phosphors Na(Sr,Ba)PO4 and Ba2Mg(BO3)2 codoped with different concentrations of Eu2+ and Mn2+ were synthesized using a solid-state reaction method. The energy transfer between Eu2+ and Mn2+ in both host crystals was investigated by steady-state and time-resolved photoluminescence measurements. For Na(Sr,Ba)PO4, Eu2+ has an emission centered at 460 nm which overlaps with multiple excitation transition of Mn2+. In contrast, for Ba2Mg(BO3)2, Eu2+ emission is red-shifted to 612 nm, which causes a single spectral overlap between Eu2+ and Mn2+ energy levels. As a result, the energy transfer efficiency is improved between Eu2+ and Mn2+ in Ba2Mg(BO3)2 with a maximum energy transfer efficiency of 91% compared to that in Na(Sr,Ba)PO4 with that of 57%.

In recent times, non-Eu2+ based persistent phosphor materials show promise for self-sustained night vision surveillance and bio-imaging owing to their ability to luminesce overnight, high chemical stability, and broad-band absorption. While green and yellow wavelengths are widely explored, the research and development of persistent phosphors in the blue region are still lacking, owing to the limited understanding of the luminescence persistence mechanism. Herein, it is to be reported a new blue-emitting Na3ScSi3O9:Ce3+, Dy3+ phosphor, which exhibits a strong emission at 450 nm that persists for nearly 17 h above the recognizable intensity level (0.32 mcd/m2), which is the longest persistent phosphor in the blue region. Thermoluminescence experiments reveal a fifth-order-of-magnitude rise in the trap density by Dy3+ incorporation into Na3ScSi3O9:Ce3+, which is responsible for the long persistence of the emission. Furthermore, by employing Mn2+, Tb3+, and Dy3+ as co-activators, it is realized that the trap density is proportional to the persistence time. Finally, an underlying mechanism for this blue emission persistence is proposed. These results might push forward the design of new blue-emitting phosphors with higher persistent luminescence efficiencies.

NaCa

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...

A series of britholite compounds were synthesized by simultaneous introduction of trivalent La3+ and Si4+ ions into an apatite structure. The variations in the average structure, electronic band structure, and microstructural properties resulting from the introduction of cation pairs were analyzed as a function of their concentration. The effects of the structural variance and microstructural properties on the broad-band-emitting activator ions were studied by introducing Eu2+ ions as activators. For the resulting compound, which had dual emission bands in the blue and yellow regions of the spectrum, the emission peak position and strength were dependent upon the concentration of La3+−Si4+ pairs. By engineering the relative sizes of the two possible activator sites in the structure, 4f and 6h, through the introduction of a combination of trivalent La3+ and a polyanion, the preferential site occupancy of the activator ions was favorably altered. Additionally, the activator ions responsible for the lower-St...

Layered metal oxide nanosheets have been the most promising class of two-dimensional material in the post-graphene area due to their diversity in composition, structure, and functionality. Devising new exfoliation pathways has been an essential requirement in the fabrication of functional devices employing inorganic nanosheets. Here, we report on the exfoliation of a double-layered perovskite, NaGdMgWO6:Eu3+ (NGMWO:Eu3+), into nanosheets via a proton exchange process and intercalation of ethylamine ions. A colloidal suspension of nanosheets exhibited red emission arising from the 5D0 → 7FJ transitions of Eu3+ under host-mediated excitation and direct excitation of Eu3+. The exfoliated nanosheets exhibited intense Eu3+ emission compared with the bulk form, largely arising from host-mediated excitation rather than direct Eu3+ excitation. The enhanced nanosheet emission is attributed to the increased energy transfer efficiency from the host to Eu3+, high surface to volume ratio, and lower dimensionality of nanosheets. H2O2 sensing in the synthesized NGMWO:Eu3+ nanosheets was demonstrated via PL quenching, which followed a combined static and dynamic quenching mechanism.

Extremely long, electrically conductive, ductile, free-standing nanosheets of water-stabilized KxCoO2·yH2O are synthesized using the sol-gel and electric-field induced kinetic-demixing (SGKD) process. Room temperature in-plane resistivity of the KxCoO2·yH2O nanosheets is less than ~4.7 mΩ·cm, which corresponds to one of the lowest resistivity values reported for metal oxide nanosheets. The synthesis produces tens of thousands of very high aspect ratio (50,000:50,000:1 = length/width/thickness), millimeter length nanosheets stacked into a macro-scale pellet. Free-standing nanosheets up to ~50 μm long are readily delaminated from the stacked nanosheets. High-resolution transmission electron microscopy (HR-TEM) studies of the free-standing nanosheets indicate that the delaminated pieces consist of individual nanosheet crystals that are turbostratically stacked. X-ray diffraction (XRD) studies confirm that the nanosheets are stacked in perfect registry along their c-axis. Scanning electron microscopy (SEM) based statistical analysis show that the average thickness of the nanosheets is ~13 nm. The nanosheets show ductility with a bending radius as small as ~5 nm.

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.

Materials with different frameworks and topologies have an important place in various scientific applications, and several such compounds have been developed over the past few years. The insights offered by such developments have altered the thought process behind material formation and their potential topologies. Materials with certain particular frameworks are often channelized toward specific applications, based on the understandings then; however, these materials may also offer a considerable number of topologies and are finding new applications that were considered unviable a few decades ago. Among the most prominent and well-understood materials that form such frameworks are aluminosilicates. Although most of these classes of compounds are natural in origin, specific applications based on these materials have only recently been developed, because of limitations in understanding their precise chemical and structural details. In this work, we synthesized a new phosphosilicate compound, using a microwave-assisted solid-state synthesis method. The lattice parameters of the crystalline compound were estimated from first-principle calculations, and the space group was identified through a least-squares method. The detailed structure solution was carried out through a simulated annealing method, which estimated the agreement between the assumed model and the observed diffraction pattern and chemical restraints. Therefore, the crystal structure solved was verified through the Rietveld refinement and maximum-entropy method (MEM) analysis. High-resolution transmission electron microscopy (HRTEM) was also used to confirm the lattice parameters of the solved crystal structure. Photoluminescence properties of the structure were also studied by activating it with the broadband-emitting activator (Eu2+), and the structural and luminescence correlation was also verified. Figure 1. Rietveld refinement of the powder X-ray diffraction profile of NaCa3PSiO8. Data (point) and fit (lines), different profile, and expected reflection positions are displayed. The inset shows the coordination environment of the major cations in the structure. Figure 1

There is a critical need for new earth-abundant phosphors to enable next-generation, highly efficient solid-state lighting. Here, we report the discovery of Sr2LiAlO4, the first known Sr-Li-Al-O quaternary crystal, via a carefully-targeted data-driven structure prediction and screening effort using density functional theory calculations. Sr2LiAlO4 is predicted and experimentally confirmed to be a thermodynamically and thermally stable phosphor host that can be excited with near-UV/blue sources. The Eu2+ and Ce3+-activated Sr2LiAlO4 phosphors exhibit broad emissions at lmax ~ 512 nm (green-yellow) and lmax ~ 434 nm (blue), respectively, with excellent thermal quenching resistance of > 88% intensity at 150°C. A prototype phosphor-converted white LED utilizing Sr2LiAlO4-based phosphors yields an excellent color rendering index exceeding 90. Sr2LiAlO4 therefore exhibits great potential for industrial applications in low-cost, high-color-quality WLEDs. Figure 1

Solid-state LED lighting has been in the spotlight due to its high efficient, energy savings, and eco-friendly nature. Conventional blue LED based white lighting in combination with yellow phosphors showed high luminous efficiency (>30 lm/W) but have serious issue of poor color rendering index (65) due to weak red emission. As an alternative, UV based white LEDs has been under intensive research due to its high color rendering index (99), excellent conversion efficiency and a wide emission band ranging 350 to 700 nm. In this study, a blue-emitting melilite phosphor was developed with wide absorption in the near-UV by solid-solution. The synthesized blue phosphor has wide absorption band with a maxima at 420 nm. The emission wavelength was tunable in the window of 470 to 505 nm. The structural and optical characteristics were investigated using X-ray diffraction, Rietveld refinement, photoluminescence, thermal quenching and decay time. Finally white LED was fabricated using the optimized melilite blue phosphor in combination with commercial green and red phosphor under 420 nm LED chip. The results indicate melilite blue phosphor could be promising blue component for UV based white LEDs. Fig1. Normalized excitation and emission spectra of the Sr4-x-y Ca ­x Ba y MgAl2Si3O14:Eu2+ Figure 1