Highly efficient Mn-doped CsPb(Cl/Br)3 quantum dots for white light-emitting diodes.

White light-emitting diodes (WLEDs) based on all-inorganic perovskite CsPbX3 (X = Cl, Br, I) quantum dots (QDs) have attracted much attention and rely on mixing several colors of perovskites. However, this inevitably leads to a non-uniform light distribution and serious light loss. Here, a novel strategy was demonstrated to obtain white emission by combining the orange and blue emission from CsPb/Mn(Cl/Br)3 QDs. Notably, highly efficient white emission with a photoluminescence quantum yield of 94% was achieved by an anion exchange surface engineering (AESE) strategy. After AESE treatment the surface traps can be eliminated, resulting in improved exciton and Mn2+ emission. A prototype WLED device was fabricated and exhibited excellent optical stability, demonstrating great potential for perovskite QDs in the field of optoelectronics.

Shape-controlled synthesis of Ag/Cs4PbBr6 Janus nanoparticles.

The poor light absorption of visible light for Cs4PbBr6 nanocrystals (NCs) has severely impeded their practical applications. Although the semiconductor/perovskite heterostructure holds great promise for enhancement in absorption, it has remained a serious challenge for synthesizing a semiconductor/perovskite heterostructure. In this work, monodispersed Janus heterostructures composed of Cs4PbBr6 decorated with either multiple Ag or single Ag on its surface (named as mAg/Cs4PbBr6 and sAg/Cs4PbBr6 respectively), are successfully prepared. The size of Ag seeds has an important effect on the shape of the products. Small-sized Ag seeds lead to the formation of mAg/Cs4PbBr6 Janus NCs, while relatively large-sized Ag seeds produce sAg/Cs4PbBr6 Janus NCs. It is noted that this work not only provides a novel method for the modification of individual Cs4PbBr6 NCs, but also enhances the absorption of the Cs4PbBr6 in the visible region, indicating great potential for optoelectronic applications, such as photocatalysis and solar cells.

High-performance nanoporous-GaN metal-insulator-semiconductor ultraviolet photodetectors with a thermal oxidized ß-Ga2O3 layer.

We report on the high-performance nanoporous (NP) GaN-based metal-insulator-semiconductor (MIS) ultraviolet (UV) photodetectors (PDs) with a thermal oxidized ß-Ga2O3insulating layer. The devices show a high responsivity of 4.5×105 A/W and maximum external quantum efficiency of 1.55×108% at 360 nm under a 10 V applied bias, which are attributed to the trap-assisted tunneling induced internal gain mechanism. Correspondingly, a specific detectivity of 8.27×1015 Jones and excellent optical switching repeatability are also observed in our fabricated PDs. The NP-GaN/ß-Ga2O3 MIS UV PD may act as an excellent candidate for the application in UV photodetection due to the high performance and simple fabrication process.

White light-emitting diodes based on carbon dots and Mn-doped CsPbMnCl3 nanocrystals.

CsPbX3 perovskite nanocrystals (NCs) are becoming a promising material for optoelectronic devices that possess an optically tunable bandgap, and bright photoluminescence. However, the toxic Pb is not environmentally friendly and the quantum yield (QY) of blue emitting NCs is relatively low. In addition, the red emitting perovskite containing iodine is not stable under light illumination. In this paper, high QY, blue emitting, non-toxic fluorescent nanomaterial carbon dots and orange-emitting CsPb0.81Mn0.19Cl3 NCs with partial Pb replacement are combined to fabricate white light-emitting diodes (WLEDs). A WLED with color coordinates of (0.337, 0.324) and a correlated color temperature of 4804 K is fabricated. Compared to red emitting perovskite containing iodine, the CsPb0.81Mn0.19Cl3 NCs are stable no matter whether they are stored in the air or exposed under ultraviolet light. Therefore, the as-fabricated WLED shows good color stability against increasing currents and long-term working stability.

Controlled Growth of CH3 NH3 PbBr3 Perovskite Nanocrystals via a Water-Oil Interfacial Synthesis Method.

Fundamental insights into the reaction kinetics of organic-inorganic lead halide perovskite nanocrystals (LHP NCs) are still limited due to their ultrafast formation rate. Herein, we develop a water-oil interfacial synthesis of MAPbBr3 NCs (MA=CH3 NH3 + ), which prolongs the reaction time to tens of minutes. This method makes it possible to monitor in situ the formation process of MAPbBr3 NCs and observe successive spectral evolutions from 438 to 534 nm in a single reaction by extending reaction time. The implementation of this method depends on reducing the formation rate of PbBr6 4- octahedra and the diffusion rate of MA. The formation of PbBr6 4- is a rate-determining step, and the biphasic system offers a favorable reaction condition to control the mass transfer of MA. The effects of temperature and concentration of precursor and ligand are investigated in detail.

Establishment of the relationship between the electron energy and the electron injection for AlGaN based ultraviolet light-emitting diodes.

This work establishes the relationship between the electron energy and the electron concentration within the multiple quantum wells (MQWs) for AlGaN based deep ultraviolet light-emitting diodes (DUV LEDs). The electron energy of different values can be obtained by modulating the Si doping concentration in the n-AlGaN layer and/or engineering the polarization induced interface charges. The modulated Si doping concentration in the n-AlGaN layer will cause the interface depletion region within which the electric field can be generated and then tunes the electron energy. The polarization induced charges and the polarization induced electric field can be obtained by stepwisely reducing the AlN composition for the n-AlGaN layer along the [0001] orientation. We find that the electron concentration in the MQWs can be increased once the electron energy is reduced to a proper level, which correspondingly improves the external quantum efficiency (EQE) for DUV LEDs. According to our investigations, it is more advisable to adopt the n-AlGaN layer with the stepwise AlN composition, which can make both the EQE and the wall plug efficiency high.

Aqueous Synthesis of Methylammonium Lead Halide Perovskite Nanocrystals.

Methylammonium lead halide perovskite nanocrystals offer attractive optoelectronic properties but suffer from fast degradation in the presence of water. In contradiction to this observation, we demonstrate the possibility of a direct aqueous synthesis of CH3 NH3 PbX3 (X=Br or Cl/Br) nanocrystals through the reaction between the lead halide complex and methylamine when the pH is maintained in the range of 0-5. Under these synthetic conditions, the positively charged surface of the perovskite nanocrystals and the proper ionic balance help to prevent their decomposition in water. Additional surface capping with organic amine ligands further improves the photoluminescence quantum yield of the perovskite nanocrystals to values close to 40 %, ensures their stability under ambient conditions for several months, and their photoluminescence performance under continuous 0.1 W mm-2 405 nm light irradiation for over 250 hours.

On the electric-field reservoir for III-nitride based deep ultraviolet light-emitting diodes.

The drift velocity for holes is strongly influenced by the electric field in the p-type hole injection layer for III-nitride based deep ultraviolet light-emitting diodes (DUV LEDs). In this work, we propose an electric-field reservoir (EFR) consisting of a p-AlxGa1-xN/p-GaN architecture to facilitate the hole injection and improve the internal quantum efficiency (IQE). The p-AlxGa1-xN layer in the EFR can well reserve the electric field that can moderately adjust the drift velocity and the kinetic energy for holes. As a result, we are able to enhance the thermionic emission for holes to cross over the p-EBL with a high Al composition provided that the composition in the p-AlxGa1-xN layer is properly optimized to avoid a complete hole depletion therein.

UVA light-emitting diode grown on Si substrate with enhanced electron and hole injections.

In this work, III-nitride based ∼370 nm UVA light-emitting diodes (LEDs) grown on Si substrates are demonstrated. We also reveal the impact of the AlN composition in the AlGaN quantum barrier on the carrier injection for the studied LEDs. We find that, by properly increasing the AlN composition, both the electron and hole concentrations in the multiple quantum wells (MQWs) are enhanced. We attribute the increased electron concentration to the better electron confinement within the MQW region when increasing the AlN composition for the AlGaN barrier. The improved hole concentration in the MQW region is ascribed to the reduced hole blocking effect by the p-type electron blocking layer (p-EBL). This is enabled by the reduced density of the polarization-induced positive charges at the AlGaN last quantum barrier (LB)/p-EBL interface, which correspondingly suppresses the hole depletion at the AlGaN LB/p-EBL interface and decreases the valence band barrier height for the p-EBL. As a result, the optical power is improved.

Efficient Homojunction/Heterojunction Photocatalyst via Integrating CsPbBr3 Quantum Dot Homojunction with TiO2 for Degradation of Organic Dyes.

A novel TiO2-CsPbBr3(Q) photocatalyst is proposed and rationally constructed, where CsPbBr3 perovskite quantum dots (QDs) of various sizes inside mesopore TiO2 (M-TiO2) are integrated. These perovskite QDs, generated in situ within M-TiO2, establish a type-II homojunction. Interestingly, a Z-scheme heterojunction is simultaneously formed at the interface between CsPbBr3 and TiO2. Due to the coexistence of the type-II homojunction and the Z-scheme heterojunction, photogenerated electrons are effectively transferred from TiO2 to CsPbBr3, thereby suppressing carrier recombination and thus enhancing the degradation of rhodamine B (RhB). Compared with pure CsPbBr3 and TiO2, TiO2-CsPbBr3(Q) shows significantly enhanced photocatalytic performance for RhB degradation. The degradation efficiency of RhB in the presence of the TiO2-CsPbBr3(Q) attains 97.7% in 5 min under light illumination, representing the highest efficiency observed among photocatalysts based on TiO2. This study will facilitate the development of superior semiconductor catalysts for photocatalytic applications.

Facile synthesis of Mn2+ doped ultrathin (n = 2) NPLs and their application in anti-counterfeiting.

Ultrathin 2D perovskite nanoplatelets (NPLs) have many excellent optical properties including narrow absorption and emission spectra, and large exciton binding energies. Doping Mn2+ into perovskite NPLs also introduces strong orange luminescence due to the d-d de-excitation recombination. However, it is very challenging to synthesize Mn2+ doped ultrathin perovskite NPLs. Here, we report the successful development of a room temperature method for Mn ion doped perovskite NPLs. The impact of the Mn2+ concentration on their optical properties has been systematically investigated. The highest PLQY is up to 71% when the Mn2+ doping level is 38.6%. Furthermore, we have observed a self-purification effect of these n = 2 NPLs, where the Mn ions were ejected from the n = 2 NPLs and injected into the n = 3 NPLs. An efficient energy transfer from the n = 2 host to the n = 3 NPLs has also been found. Additionally, we have used this fast ejecting and injecting property to fabricate an anti-counterfeiting film. The film shows weak blue and strong orange color under room temperature and high temperature, respectively. Most importantly, the process can be repeated several cycles without damage, which shows great potential for anti-counterfeiting applications.

Perovskite energy funnels for efficient white emission.

Doping Mn2+ into CsPbCl3 nanocrystals (NCs) yields strong orange emission, while the related emission in Mn2+ doped CsPbBr3 NCs is impaired seriously. This is mainly ascribed to back energy transfer from the Mn2+ dopant to the host. Doping Mn2+ into perovskites with multiple-quantum-well (MQW) structures may address this issue, where the energy funnels ensure a rapid energy transfer process, and thus resulting in a high photoluminescence quantum yield (PLQY). Here, we have developed an Ag+ assisted Mn2+ doping method in which Mn2+ can be easily doped into Br-based MQW perovskites. In this MQW perovskites, both nanoplatelets (NPLs) and NCs were formed simultaneously, where efficient energy transfer occurred from the NPLs with a higher energy bandgap to the NCs with a smaller energy bandgap, and then to the Mn2+ dopants. White lighting solution with a PLQY up to 98% has been acquired by altering the experimental parameters, such as reaction time and the Pb-to-Mn feed ratio. The successful doping of Mn2+ into CsPbBr3 host has great significance and shows promising application for next-generation white lighting.

Mn-Doped Multiple Quantum Well Perovskites for Efficient Large-Area Luminescent Solar Concentrators.

Luminescent solar concentrators (LSCs) can be used as large-area sunlight collectors, which show great potential in the application of building-integrated photovoltaic areas. Achieving highly efficient LSCs requires the suppression of reabsorption losses while maintaining a high photoluminescence quantum yield (PLQY) and broad absorption. Perovskites as the superstar fluorophores have recently emerged as candidates for large-area LSCs. However, highly emissive perovskites with a large Stokes shift and broad absorption have not been obtained up to now. Here, we devised a facile synthetic route to obtain Mn-doped multiple quantum well (MQW) Br-based perovskites. The Br-based perovskite host ensures broad absorption. Efficient energy transfer from the exciton to the Mn dopant produces a large Stokes shift and high PLQY simultaneously. By further coating the perovskites with Al2O3, the stability and PLQY are greatly elevated. A large area of liquid LSC (40 cm × 40 cm × 0.5 cm) is fabricated, which possesses an internal quantum efficiency (ηint) of 47% and an optical conversion efficiency (ηopt) reaching 11 ± 1%, which shows the highest value for large-area LSCs.

Solvent- and initiator-free fabrication of efficient and stable perovskite-polystyrene surface-patterned thin films for LED backlights.

We report a one-pot route for the synthesis of CsPbBr3 perovskite nanocrystals (PNCs) in styrene to form a glue-like polystyrene (PS) pre-polymer incorporating mono-dispersed PNCs. The pre-polymer enables solvent- and initiator-free fabricating and patterning PNC-PS light down-conversion films for liquid crystal display application. The mechanistic study reveals that the styrene molecules adsorbed on the PNC surface undergo self-initiated polymerization in the pre-polymerization process, forming stable surface capsulation over the PNCs. The PNC-PS pre-polymer and composite film display high photoluminescent quantum yield (PLQY) and resistance to air, light irradiation and water. The micropatterned PNC-PS film with a period of 1000 nm was fabricated through imprinting of the pre-polymer. The micropatterned thin film displays an enlarged viewing angle, improved light distribution and PLQY of >90%. The backlight employing the PNC-PS film displays bright green color and a wide color gamut of >120% NTSC. This solvent-free and one-pot strategy could find promising potential in the development of diverse luminescent nanocomposites to meet the requirements of micro/nano-manufacturing and high performance display application.

Low-temperature synthesis of tetrapod CdSe/CdS quantum dots through a microfluidic reactor.

Tetrapod CdSe/CdS quantum dots (QDs) have attracted extensive research interest in light-emitting applications due to their anisotropic optical properties and large absorption cross-section. Traditional synthesis methods for tetrapod CdSe/CdS QDs usually employ fatty phosphonic acid ligands to induce the growth of wurtzite CdS arms on cubic CdSe QDs at high temperatures (350-380 °C). Here, a low temperature (120 °C) route was developed for the synthesis of tetrapod CdSe/CdS QDs using mixed amine ligands instead of phosphonic acid ligands. A study of the growth mechanism reveals that the amine ligands induce the orientation growth of cubic CdS arms on wurtzite CdSe QDs through a pyramid-shaped intermediate structure. The low reaction temperature facilitates the growth control of the tetrapod CdSe/CdS QDs through a microfluidic reactor. This study substantially simplifies the synthetic chemistry for the anisotropic growth of CdS on CdSe QDs, paving the way for green and economic production of tetrapod CdSe/CdS QDs towards efficient light-emitting applications.

Lead-free, stable orange-red-emitting hybrid copper based organic-inorganic compounds.

Metal halide perovskites have been extensively studied recently by virtue of their extraordinary luminescence characteristics. However, they still suffer from severe stability issues, and contain a toxic metal lead. Here, single crystals of (PEA)4Cu4I4, a lead-free orange-red-emitting organic-inorganic copper-halide compound with a photoluminescence quantum yield (PLQY) of 68%, were synthesized via a simple solvent vapor diffusion process with commercially-available phenylethylamine (PEA) as a ligand. The crystals show superior stability to perovskites with retaining 60% of their initial photoluminescence (PL) intensity after 60 days in water, which is due to the hydrophobic nature of PEA and the stable Cu-N bonds. Phase transition is found to take place upon lowering the temperature, which causes a redshift of the PL peak. The emission band is identified to be associated with triplet cluster-centered (CC) excited states because of their shortened Cu-Cu distances, excitation-independent PL and long PL lifetime. In addition, micron-sized oleic acid capped (PEA)4Cu4I4 particles were developed by a hot-injection method, and they possess similar stability to that of bulk crystals. A monochrome LED was further fabricated by employing the as-prepared micron-sized particles as phosphors, demonstrating their potential for optoelectronic applications.

Eco-Friendly and Efficient Luminescent Solar Concentrators Based on a Copper(I)-Halide Composite.

Luminescent solar concentrators (LSCs) show great promise in reducing the cost of silicon solar cells due to their potential use for high-efficiency energy harvesting. Compared to narrow absorption organic dyes, quantum dots (QDs) are a favorable approach to acquire stable LSCs. However, the use of toxic heavy metals in QDs and the small Stokes shift largely restrict their development. Here, a toxic metal-free, highly luminescent ink based on a copper(I)-halide hybrid cluster is reported, whose quantum yield (QY) exceeds 68%. Under the interaction with halohydrocarbon, CuI and phenethylamine (PEA) can be easily dissolved and the ink can be facilely acquired. The obtained film exhibits strong orange light emission with a large Stokes shift. As a proof-of-concept experiment, (PEA)4Cu4I4 has been used to fabricate LSCs. The as-prepared LSC (4 cm × 4 cm × 0.3 cm) exhibits an internal quantum efficiency (ηint) as high as 44.1%. After coupling to a solar cell, an optical conversion efficiency (ηopt) of 6.85% is acquired from this LSC. In addition, the LSC possesses high stability such as air stability, water stability, and photostability. These results demonstrate that the (PEA)4Cu4I4 film can be employed as a promising candidate for large-area and high-efficiency LSCs.

Magnetic perovskite nanoparticles for latent fingerprint detection.

Fingerprints form when fingers touch a solid surfaceand are considered the best way for individual identification. However, the current latent fingerprint (LFP) developing methods cannot meet the demand for high sensitivity and being convenient and healthy. Herein, bifunctional Fe3O4@SiO2-CsPbBr3 powders have been designed and fabricated and exhibit good magnetic and strong fluorescent properties. The magnetism of Fe3O4 can avoid dust flying, while the fluorescence of CsPbBr3 ensures the high definition of LFPs. Clear fingerprints have been detected on various solid substrates using the Fe3O4@SiO2-CsPbBr3 powders instead of eikonogen. Detailed characterization studies suggest that the ammonium cationic groups on the surface of nanoparticles (NPs) have strong adhesive interactions with the residues of fingerprints because of the electrostatic attraction between them. Therefore, the convenient operation and excellent resolution offer great opportunity in the practical application of fingerprint detection and other areas.

Study on the Color Compensation Effect of Composite Orange-Red Quantum Dots in WLED Application.

Quantum dots (QDs) as emerging light-converting materials show the advantage of enhancing color quality of white light-emitting diode (WLED). However, WLEDs employing narrow-emitting monochromic QDs usually present unsatisfactory color rendering in the orange region. Herein, composite orange-red QDs (composite-QDs) are developed through mixing CdSe/ZnS-based orange QDs (O-QDs) and red QDs (R-QDs) to compensate the orange-red light for WLEDs. We investigated the effect of self-absorption and fluorescence resonance energy transfer (FRET) process in composite-QDs on the spectral controllability and fluorescent quenching in WLEDs. The concentration and donor/acceptor ratios were also taken into account to analyze the FRET efficiency and help identify suitable composite-QDs for color compensation in the orange-red light region. As the result, the optimized composite-QDs effectively improve the color rendering index of the WLED compared with monochromatic QDs.

Highly Efficient and Thermally Stable QD-LEDs Based on Quantum Dots-SiO2-BN Nanoplate Assemblies.

Silica encapsulation effectively elevates the resistance of quantum dots (QDs) against water and oxygen. However, QDs-SiO2 composites present low thermal conductivity and strong thermal accumulation, leading to considerable fluorescence quenching of QDs in optoelectronic devices at high power. Here, a sandwich structural QDs-SiO2-BN nanoplate assembly material (QDs-SiO2-BNAs) is developed to reduce the thermal quenching and enhance the stability of QDs in LEDs. The QDs-SiO2-BNAs is fabricated by embedding QDs-SiO2 into the interlayer of layer-by-layer assembled BN nanoplates, and the BN nanoplates are pretreated by SiO2 encapsulation to strengthen the interaction with QDs-SiO2. This assembly structure endows the QDs with fast heat dissipation and double surface protection against air. The medium power QDs-converted LEDs (QD-LEDs) fabricated by direct on-chip packaging of the QDs-SiO2-BNAs gain 44.2 °C temperature reduction at 0.5 W in comparison with conventional QD-LEDs. After aging, the resulting QD-LEDs present degradation of only 1.2% under sustained driving for 250 h. The QD-LEDs also pass the 1 week reliability test at 85 °C/85% RH with <±0.01 shift of the color coordinates, demonstrating the profound potential of the QDs-SiO2-BNAs in LED lighting and display applications.