Tunable Emission of Low-Dimensional Organic Metal Halides by Stoichiometric Ratio and Metal Center.

Low-dimensional (LD) organic metal halides (OMHs) have a bright future due to their excellent photoelectric characteristics and unique structure. However, the synthesis and emission control of LD-OMHs are still unclear. Herein, the different dimensional (zero-dimensional (0D), one-dimensional (1D), and three-dimensional (3D)) of OMHs were obtained by the reaction of 1,4-diazabicyclo (2.2.2) octane with PbBr2 in different stoichiometric ratios. This discovery shows that the structure and properties of OMHs can be regulated while maintaining the functional organic cations of OMHs, which broadens the path for the development of functional LD-OMHs. Among them, 0D-OMH 1 and 1D-OMH 3 have narrow-band (full width at half-maximum (fwhm) = 74 nm) and broad-band (fwhm = 201 nm) emission, respectively. We found that when organic cations have no contribution to the formation of conduction band minimum and valence band maximum, and the distances between polyhedrons are larger than the van der Waals diameter of the halogen atom, the effect of phonons on exciton transitions can be reduced to achieve a narrow-band emission. Further, Cu(I)- and Mn (II)-based 0D-OMHs were synthesized, which have high photoluminescence quantum yield (PLQY) (33.97 and 47.33%, respectively). When the emitting of 0D-OMHs produced by the interaction of the metal-center and halogens, the asymmetric planar metal-halogen structure will result in a higher PLQY.

Ligand Engineering Enables Efficient Pure Red Tin-Based Perovskite Light-Emitting Diodes.

With increasing ecological and environmental concerns, tin (Sn)-based perovskite light-emitting diodes (PeLEDs) are competitive candidates for future displays because of their environmental friendliness, excellent photoelectric properties, and low-cost solution-processed fabrication. Nonetheless, their electroluminescence (EL) performance still lags behind that of lead (Pb)-based PeLEDs due to the fast crystallization rate of Sn-based perovskite films and undesired oxidation from Sn2+ to Sn4+ , leading to poor film morphology and coverage, as well as high density defects. Here, we propose a ligand engineering strategy to construct high-quality phenethylammonium tin iodide (PEA2 SnI4 ) perovskite films by using L-glutathione reduced (GSH) as surface ligands toward efficient pure red PEA2 SnI4 -based PeLEDs. We show that the hydrogen-bond and coordinate interactions between GSH and PEA2 SnI4 effectively reduce the crystallization rate of the perovskites and suppress the oxidation of Sn2+ and formation of defects. The improved pure red perovskite films not only show excellent uniformity, density, and coverage but also exhibit enhanced optical properties and stability. Finally, state-of-the-art pure red PeLEDs achieve a record external quantum efficiency of 9.32 % in the field of PEA2 SnI4 -based devices. This work demonstrates that ligand engineering represents a feasible route to enhance the EL performance of Sn-based PeLEDs.

Organic Sulfonium-Stabilized High-Efficiency Cesium or Methylammonium Lead Bromide Perovskite Nanocrystals.

Herein, we report a new X-type ligand, i.e., organic sulfonium bromide, for high-efficiency CsPbBr3 and MAPbBr3 (MA=methylammonium) perovskite nanocrystals (PNCs). We first confirmed the facile synthesis of the titled ligands in N,N-dimethylformamide at room temperature. By reacting dodecylmethylsulfide with allyl bromide, (3-bromopropyl)trimethoxysilane, and 1,4-dibromobutane, respectively, three representative ligands (named DAM, DSM, and DMM) are acquired. All of them result in CsPbBr3 and MAPbBr3 PNCs with near-unity photoluminescence quantum yield (PLQY) and decent ambient stability (no less than 90 % PLQY after 2 months) using a room-temperature ligand-assisted reprecipitation technique. Among them, DSM and DMM endow CsPbBr3 PNCs with higher thermal/light stability arising from the cross-linkable or bidentate ligand structure. Further, DSM-CsPbBr3 PNCs can be incorporated into polystyrene through in situ thermal polymerization and the composite shows a record-high PLQY of 93.8 %.

Down-Conversion Nitride Materials for Solid State Lighting: Recent Advances and Perspectives.

Advances in solid state white lighting technologies witness the explosive development of phosphor materials (down-conversion luminescent materials). A large amount of evidence has demonstrated the revolutionary role of the emerging nitride phosphors in producing superior white light-emitting diodes for lighting and display applications. The structural and compositional versatility together with the unique local coordination environments enable nitride materials to have compelling luminescent properties such as abundant emission colors, controllable photoluminescence spectra, high conversion efficiency, and small thermal quenching/degradation. Here, we summarize the state-of-art progress on this novel family of luminescent materials and discuss the topics of materials discovery, crystal chemistry, structure-related luminescence, temperature-dependent luminescence, and spectral tailoring. We also overview different types of nitride phosphors and their applications in solid state lighting, including general illumination, backlighting, and laser-driven lighting. Finally, the challenges and outlooks in this type of promising down-conversion materials are highlighted.

Aggregation-Induced Emission Gold Clustoluminogens for Enhanced Low-Dose X-ray-Induced Photodynamic Therapy.

The use of gold nanoparticles as radiosensitizers is an effective way to boost the killing efficacy of radiotherapy while drastically limiting the received dose and reducing the possible damage to normal tissues. Herein, we designed aggregation-induced emission gold clustoluminogens (AIE-Au) to achieve efficient low-dose X-ray-induced photodynamic therapy (X-PDT) with negligible side effects. The aggregates of glutathione-protected gold clusters (GCs) assembled through a cationic polymer enhanced the X-ray-excited luminescence by 5.2-fold. Under low-dose X-ray irradiation, AIE-Au strongly absorbed X-rays and efficiently generated hydroxyl radicals, which enhanced the radiotherapy effect. Additionally, X-ray-induced luminescence excited the conjugated photosensitizers, resulting in a PDT effect. The in vitro and in vivo experiments demonstrated that AIE-Au effectively triggered the generation of reactive oxygen species with an order-of-magnitude reduction in the X-ray dose, enabling highly effective cancer treatment.

Synthesis and Photoluminescence Properties of a Blue-Emitting La3Si8N11O4:Eu2+ Phosphor.

Eu2+-doped La3Si8N11O4 phosphors were synthesized by the high temperature solid-state method, and their photoluminescence properties were investigated in this work. La3Si8N11O4:Eu2+ exhibits a strong broad absorption band centered at 320 nm, spanning the spectral range of 300-600 nm due to 4f7 → 4f65d1 electronic transitions of Eu2+. The emission spectra show a broad and asymmetric band peaking at 481-513 nm depending on the Eu2+ concentration, and the emission color can be tuned in a broad range owing to the energy transfer between Eu2+ ions occupying two independent crystallographic sites. Compared to the Ce3+-doped La3Si8N11O4, the Eu2+-doped one shows a larger thermal quenching, predominantly owing to photoionization. Under 320 nm excitation, the internal and external quantum efficiencies are 44 and 33%, respectively.

Eu2+-Doped Sr2B2-2xSi2+3xAl2-xN8+x: A Boron-Containing Orange-Emitting Nitridosilicate with Interesting Composition-Dependent Photoluminescence Properties.

Novel Sr2-yEuyB2-2xSi2+3xAl2-xN8+x phosphors were investigated as a function of the boron and aluminum over silicon ratio and as a function of the Eu2+ concentration. Samples were prepared via solid-state reaction synthesis by carefully controlling the synthesis conditions and composition. At high boron and aluminum content, that is, x = 0, a Eu2+ 5d-4f emission is observed of which the maximum shifts from 595 nm for low Eu concentrations (y = 0.005) toward 623 nm for high Eu concentrations (y = 0.5). The samples can be excited by UV or blue light up to ∼475 nm. Substitution of [B2Al]9+ units by [Si3N]9+ units, increasing x up to 0.15, greatly improves the luminescence efficiency up to 46% and shows a very large redshift of the excitation bands with ∼100 nm, while the emission band shifts with ∼10 nm. The shifts are attributed to the lowering of the 5d level as a result of the decreased Eu-N distance upon substitution. Temperature-dependent measurements show that the Eu2+ 5d-4f emission is largely thermally quenched at room temperature for x = 0 due to thermal ionization toward the conduction band, explaining the low luminescence efficiency. The lowering of the 5d level at larger values of x reduces the thermal ionization and consequently increases the thermal stability and quantum efficiency, resulting in strongly luminescent blue-to-orange conversion phosphors that are interesting for light-emitting diode applications.

Study on Trap Levels in SrSi2AlO2N3:Eu2+,Ln3+ Persistent Phosphors Based on Host-Referred Binding Energy Scheme and Thermoluminescence Analysis.

We investigated the effect of trivalent lanthanide substitution on a novel oxynitride persistent phosphor SrSi2AlO2N3:Eu2+,Ln3+, which shows green persistent luminescence for more than 2 h. First, an energy level diagram by using the host-referred binding energy (HRBE) scheme was constructed. The location of the energy levels of all divalent and trivalent lanthanides referred to the energy band of the host SrSi2AlO2N3 was estimated. Then, thermoluminescence (TL) measurements in the target persistent phosphors were performed to obtain direct experimental results on the trap depth. We found that the trap levels based on the TL measurements coincided well with the 4f ground states of divalent lanthanide codopants in SrSi2AlO2N3:Eu2+,Ln3+. The result strongly suggests the effective traps for persistent luminescence in SrSi2AlO2N3:Eu2+,Ln3+ could be due to the aliovalent substitution of Ln3+ for Sr2+, which can be controlled by selecting suitable codopant Ln3+. The work shows the HRBE scheme may offer a way to understand the nature of defects in the persistent phosphor as well as a possible guideline to design new persistent phosphors with required trap depths.

Prevention of thermal- and moisture-induced degradation of the photoluminescence properties of the Sr2Si5N8:Eu(2+) red phosphor by thermal post-treatment in N2-H2.

A red phosphor of Sr2Si5N8:Eu(2+) powder was synthesized by a solid state reaction. The synthesized phosphor was thermally post-treated in an inert and reductive N2-H2 mixed-gas atmosphere at 300-1200 °C. The main phase of the resultant phosphor was identified as Sr2Si5N8. A passivation layer of ∼0.2 µm thickness was formed around the phosphor surface via thermal treatment. Moreover, two different luminescence centers of Eu(SrI) and Eu(SrII) in the synthesized Sr2Si5N8:Eu(2+) phosphor were proposed to be responsible for 620 nm and 670 nm emissions, respectively. More interestingly, thermal- and moisture-induced degradation of PL intensity was effectively reduced by the formation of a passivation layer around the phosphor surface, that is, the relative PL intensity recovered 99.8% of the initial intensity even after encountering thermal degradation; both moisture-induced degraded external and internal QEs were merely 1% of the initial QEs. The formed surface layer was concluded to primarily prevent the Eu(2+) activator from being oxidized, based on the systemic analysis of the mechanisms of thermal- and moisture-induced degradation.

Highly efficient narrow-band green and red phosphors enabling wider color-gamut LED backlight for more brilliant displays.

In this contribution, we propose to combine both narrow-band green (ß-sialon:Eu(2+)) and red (K(2)SiF(6):Mn(4+)) phosphors with a blue InGaN chip to achieve white light-emitting diodes (wLEDs) with a large color gamut and a high efficiency for use as the liquid crystal display (LCD) backlighting. ß-sialon:Eu(2+), prepared by a gas-pressure sinteing technique, has a peak emission at 535 nm, a full width at half maximum (FWHM) of 54 nm, and an external quantum efficiency of 54.0% under the 450 nm excitation. K(2)SiF(6):Mn(4+) was synthesized by a twe-step co-precipitation methods, and exhibits a sharp line emission spectrum with the most intensified peak at 631 nm, a FWHM of ~3 nm, and an external quantum efficiency of 54.5%. The prepared three-band wLEDs have a high color temperature of 11,184 - 13,769 K (i.e., 7,828 - 8,611 K for LCD displays), and a luminous efficacy of 91 - 96 lm/W, measured under an applied current of 120 mA. The color gamut defined in the CIE 1931 and CIE 1976 color spaces are 85.5 - 85.9% and 94.3 - 96.2% of the NTSC stanadard, respectively. These optical properties are better than those phosphor-cpnverted wLED backlights using wide-band green or red phosphoprs, suggesting that the two narrow-band phosphors investigated are the most suitable luminescent materials for achieving more bright and vivid displays.

Strong Energy-Transfer-Induced Enhancement of Luminescence Efficiency of Eu(2+)- and Mn(2+)-Codoped Gamma-AlON for Near-UV-LED-Pumped Solid State Lighting.

A series of Eu(2+)- and Mn(2+)-codoped γ-AlON (Al1.7O2.1N0.3) phosphors was synthesized at 1800 °C under 0.5 MPa N2 by using the gas-pressure sintering method (GPS). Eu(2+) and Mn(2+) ions were proved to enter into γ-AlON host lattice by means of XRD, CL, and EDS measurements. Under 365 nm excitation, two emission peaks located at 472 and 517 nm, resulting from 4f(6)5d(1) → 4f(7) and (4)T1(4G) → (6)A1 electron transitions of Eu(2+) and Mn(2+), respectively, can be observed. Energy transfer from Eu(2+) to Mn(2+) was evidenced by directly observing appreciable overlap between the excitation spectrum of Mn(2+) and the emission spectrum of Eu(2+) as well as by the decreased decay time of Eu(2+) with increasing Mn(2+) concentration. The critical energy-transfer distance between Eu(2+) and Mn(2+) and the energy-transfer efficiency were also calculated. The mechanism of energy transfer was identified as a resonant type via a dipole-dipole mechanism. The external quantum efficiency was increased 7 times (from 7% for γ-AlON:Mn(2+) to 49% for γ-AlON:Mn(2+),Eu(2+) under 365 nm excitation), and color-tunable emissions from blue-green to green-yellow were also realized with the Eu(2+) → Mn(2+) energy transfer in γ-AlON.

Europium(ii)-activated oxonitridosilicate yellow phosphor with excellent quantum efficiency and thermal stability - a robust spectral conversion material for highly efficient and reliable white LEDs.

Knowing the physicochemical properties of a material is of great importance to design and utilize it in a suitable way. In this paper, we conduct a comprehensive survey of photoluminescence spectra, localized cathodoluminescence, temperature-dependent luminescence efficiency, and applications of Eu(2+)-doped Sr0.5Ba0.5Si2O2N2 in solid-state lighting. This phosphor exhibits a broad emission band with a maximum at 560-580 nm and a full-width at half maximum of 92-103 nm upon blue light excitation, whereas a dual-band emission (i.e., 470 nm and 550 nm) is observed under electron beam irradiation due to perhaps the intergrowth of BaSi2O2N2:Eu(2+) and Sr0.5+σBa0.5-σSi2O2N2:Eu(2+) in each phosphor particle. Under 450 nm blue light irradiation, this yellow phosphor exhibits excellent luminescence properties with absorption, internal and external efficiencies of 83.2, 87.7 and 72.6%, respectively. Furthermore, it also possesses high thermal stability, with the quantum efficiency being decreased by only 4.2% at 150 °C and a high quenching temperature of 450 °C. High-efficiency white LEDs using the title phosphor have a luminous efficacy, color temperature and color rendition of ∼120 lm W(-1), 6000 K and 61, respectively, validating its suitability for use in solid-state white lighting.

Water-Soluble Quantum Dots for Inkjet Printing Color Conversion Films with Simultaneous High Efficiency and Stability.

Water-soluble quantum dots (QDs) are necessary to prepare patterned pixels or films for high-resolution displays with less environmental burden but are very limited by the trade-off between photoluminescence and stability of QDs. In this work, we proposed synthesizing water-soluble QDs with simultaneous excellent luminescence properties and high stability by coating the amphiphilic poly(maleic anhydride-alt-1-octadecene)-ethanol amine (PMAO-EA) polymer on the surface of silane-treated QDs. These coated QDs show a photoluminescence quantum yield (PLQY) as high as 94%, and they have good photoluminescence stability against light irradiation and thermal attacks, owing to the suppression of the nonradiative recombination by the polymer layer and the isolation of oxygen and water by the silica layer. The water-soluble QDs, mixed with ethylene glycol, enable inkjet printing of QD color conversion films (QD-CCFs) with an average diameter of 68 µm for each pixel and a high PLQY of 91%. The QD-CCFs are demonstrated to fabricate red-emitting mini-LEDs by combining with blue mini-LED chips, which have an external quantum efficiency as high as 25.86% and a luminance of 2.44 × 107 cd/m2. We believe that the proposed strategy is applicable to other water-soluble QDs and paves an avenue for inkjet printing environmentally friendly QD-CCFs for mini/micro-LED displays.

Enabling Visible-Light-Charged Near-Infrared Persistent Luminescence in Organics by Intermolecular Charge Transfer.

Visible light is a universal and user-friendly excitation source; however, its use to generate persistent luminescence (PersL) in materials remains a huge challenge. Herein, the concept of intermolecular charge transfer (xCT) is applied in typical host-guest molecular systems, which allows for a much lower energy requirement for charge separation, thus enabling efficient charging of near-infrared (NIR) PersL in organics by visible light (425-700 nm). Importantly, NIR PersL in organics occurs via the trapping of electrons from charge-transfer aggregates (CTAs) into constructed trap states with trap depths of 0.63-1.17 eV, followed by the detrapping of these electrons by thermal stimulation, resulting in a unique light-storage effect and long-lasting emission up to 4.6 h at room temperature. The xCT absorption range is modulated by changing the electron-donating ability of a series of acenaphtho[1,2-b]pyrazine-8,9-dicarbonitrile-based CTAs, and the organic PersL is tuned from 681 to 722 nm. This study on xCT interaction-induced NIR PersL in organic materials provides a major step forward in understanding the underlying luminescence mechanism of organic semiconductors and these findings are expected to promote their applications in optoelectronics, energy storage, and medical diagnosis.

Quantifying the interfacial triboelectricity in inorganic-organic composite mechanoluminescent materials.

Mechanoluminescence (ML) sensing technologies open up new opportunities for intelligent sensors, self-powered displays and wearable devices. However, the emission efficiency of ML materials reported so far still fails to meet the growing application requirements due to the insufficiently understood mechano-to-photon conversion mechanism. Herein, we propose to quantify the ability of different phases to gain or lose electrons under friction (defined as triboelectric series), and reveal that the inorganic-organic interfacial triboelectricity is a key factor in determining the ML in inorganic-organic composites. A positive correlation between the difference in triboelectric series and the ML intensity is established in a series of composites, and a 20-fold increase in ML intensity is finally obtained by selecting an appropriate inorganic-organic combination. The interfacial triboelectricity-regulated ML is further demonstrated in multi-interface systems that include an inorganic phosphor-organic matrix and organic matrix-force applicator interfaces, and again confirmed by self-oxidization and reduction of emission centers under continuous mechanical stimulus. This work not only gives direct experimental evidences for the underlying mechanism of ML, but also provides guidelines for rationally designing high-efficiency ML materials.

Local analysis of Eu2+ emission in CaAlSiN3.

We have investigated the local luminescence properties of Eu-doped CaAlSiN3 by using low-energy electron beam (e-beam) techniques. The particles yield broad emission centered at 655 nm with a shoulder at higher wavelength under light excitation, and a broad band around 643 nm with a tail at 540 nm under e-beam excitation. Using cathodoluminescence (CL) in a scanning electron microscope (SEM), we have observed small and large particles, which, although with different compositions, exhibit Eu2+-related emissions at 645 and 635 nm, respectively. Local CL measurements reveal that the Eu2+ emission may actually consist of several bands. In addition to the red broad band, regularly spaced sharp peaks have been occasionally observed. These luminescence variations may originate from a variation in the composition inside CaAlSiN3.

A super-high brightness and excellent colour quality laser-driven white light source enables miniaturized endoscopy.

A laser-driven white light source promises intrinsic advantages for miniaturized endoscopic illumination. However, it remains a great challenge to simultaneously achieve high brightness and excellent colour rendition due to the shortage of highly efficient and thermally robust red-emitting laser phosphor converters. Here, we designed CaAlSiN3:Eu@Al (CASN@Al) converters with neglectable efficiency loss by tightly bonding all-inorganic phosphor films on an aluminium substrate. A layer-by-layer phosphor converter (LuAG/CASN@Al), i.e., stacking a green-emitting Lu3Al5O12:Ce (LuAG) layer on CASN@Al, was constructed to enhance light conversion efficiency and reduce reabsorption loss under blue laser excitation, which thus produces an excellent white light source with a luminous efficacy of 258 lm W-1 and a colour rendering index of 91. A miniaturized endoscopy with a coupling efficiency twice that of the commercial white LEDs was demonstrated by using the laser-driven white light and showed a central illuminance as high as 52 730 lx, more vivid images and long-term reliability.

Correction: A super-high brightness and excellent colour quality laser-driven white light source enables miniaturized endoscopy.

Correction for 'A super-high brightness and excellent colour quality laser-driven white light source enables miniaturized endoscopy' by Shuxing Li et al., Mater. Horiz., 2023, 10, 4581-4588, https://doi.org/10.1039/D3MH01170D.

Perovskite Light-Emitting Diodes with an External Quantum Efficiency Exceeding 30.

Perovskite light-emitting diodes (PeLEDs) are strong candidates for next-generation display and lighting technologies due to their high color purity and low-cost solution-processed fabrication. However, PeLEDs are not superior to commercial organic light-emitting diodes (OLEDs) in efficiency, as some key parameters affecting their efficiency, such as the charge carrier transport and light outcoupling efficiency, are usually overlooked and not well optimized. Here, ultrahigh-efficiency green PeLEDs are reported with quantum efficiencies surpassing a milestone of 30% by regulating the charge carrier transport and near-field light distribution to reduce electron leakage and achieve a high light outcoupling efficiency of 41.82%. Ni0.9 Mg0.1 Ox films are applied with a high refractive index and increased hole carrier mobility as the hole injection layer to balance the charge carrier injection and insert the polyethylene glycol layer between the hole transport layer and the perovskite emissive layer to block the electron leakage and reduce the photon loss. Therefore, with the modified structure, the state-of-the-art green PeLEDs achieve a world record external quantum efficiency of 30.84% (average =  29.05 ± 0.77%) at a luminance of 6514 cd m-2 . This study provides an interesting idea to construct super high-efficiency PeLEDs by balancing the electron-hole recombination and enhancing the light outcoupling.

Constructing Perovskite/Polymer Core/Shell Nanocrystals with Simultaneous High Efficiency and Stability for Mini-LED Backlights.

Lead halide perovskite nanocrystals (NCs) have been the star material in lighting and displays owing to their excellent photoelectrical properties, but they have not simultaneously realized high photoluminescence quantum yield (PLQY) and high stability. To solve this problem, we propose a perovskite/linear low-density polyethylene (perovskite/LLDPE) core/shell NC by the synergistic role of the pressure effect and steric effect. Green CsPbBr3/LLDPE core/shell NCs with near-unity PLQY and nonblinking behavior were synthesized through an in situ hot-injection process. The mechanism of the improved photoluminescence (PL) properties is the enhanced pressure effect resulting in increased radiative recombination and interaction between the ligand and perovskite crystals, as confirmed by the PL spectra and finite element calculations. Meanwhile, the NCs show high stability under ambient conditions (with a PLQY of 92.5% after 166 days) and against 365 nm UV light (maintaining 61.74% of the initial PL intensity after continuous radiation for 1000 min). This strategy also works well in the blue and red perovskite/LLDPE NCs and red InP/ZnSeS/ZnS/LLDPE NCs. Finally, white-emitting Mini-LEDs were fabricated by combining the green CsPbBr3/LLDPE and red CsPbBr1.2I1.8/LLDPE core/shell NCs with blue Mini-LED chips. The white-emitting Mini-LEDs exhibit super wide color gamut (∼129% of the National Television Standards Committee or 97% of the Rec. 2020 standards).