Cs4PbBr6/CsPbBr3 Perovskite Composites with Near-Unity Luminescence Quantum Yield: Large-Scale Synthesis, Luminescence and Formation Mechanism, and White Light-Emitting Diode Application

Stable, Scalable, and Free-Standing Perovskite Quantum Dots Composite Reinforced by Cellulose Fibers

Perovskite quantum dots (PQDs) have attracted emerging attention as fluorescent and light-absorbing materials for next-generation optoelectronics due to their outstanding properties and cost-efficiency. However, PQD thin film suffers significant instability due to structure and material failures, which hinders their application in flexible and reliable PQD-based advanced wearable devices. Herein, we use commercial cellulose fiber-based filter paper as a substrate to synthesize PQDs in situ and fabricate PQD-paper free-standing flexible composite film. The abundant hydroxy capping ligands of cellulose fibers and the unique dense network structure of the filter paper can facilitate confined crystallization, forming strong interactions between the PQDs and substrate, the unpackaged PQD composite film showed extraordinary stability (>30 days) in the air with high humidity (90%). Meanwhile, the strong interaction between PQDs and paper enables an ultrasimple drop-cast synthesis process with excellent process tolerance, making it customizable and easy to scale up (10 cm in diameter). Due to the uniformly dispersed PQDs on cellulose fibers of the substrate, the composite demonstrates impressive photo-responsive properties. Photodetector (PD) arrays were designed on free-standing PQD paper and flexible graphitic electrodes, and circuits were fabricated by drawing. The PD arrays can work as optical and electrical dual-mode image sensors with incredible bending robustness, enduring up to 100,000 cycles at 180°.

One-Step Fabrication of 0D Cs4PbBr6 Perovskite with Nonlinear Optical Properties for Ultrafast Pulse Generation

Low-dimensional lead halide perovskites demonstrate remarkable nonlinear optical characteristics attributed to their distinctive physical structures and electronic properties. Nevertheless, the investigation into their nonlinear optical properties remains in its incipient stages. This study addresses this gap by precisely controlling solvent volumes to synthesize both 0D Cs4PbBr6 and Cs4PbBr6/CsPbBr3 perovskites. Remarkably, as saturable absorbers, both pure Cs4PbBr6 and Cs4PbBr6/CsPbBr3 composites exhibit favorable nonlinear optical properties within the C-band, showcasing modulation depths of 9.22% and 16.83%, respectively. Moreover, for the first time, Cs4PbBr6 and Cs4PbBr6/CsPbBr3 composites have been successfully integrated into erbium-doped fiber lasers to realize the mode-locking operations. The utilization of the Cs4PbBr6/CsPbBr3 composites as a saturable absorber that enables the generation of conventional soliton mode-locked laser pulses with a pulse duration of 688 fs, and a repetition frequency of 10.947 MHz at a central wavelength of 1557 nm. Cs4PbBr6 is instrumental in generating laser pulses at a frequency of 10.899 MHz, producing pulse widths of 642 fs at the central wavelength of 1531.2 nm and 1.02 ps at the central wavelength of 1565.3 nm, respectively. The findings of this investigation underscore the potential utility of 0D Cs4PbBr6 and Cs4PbBr6/CsPbBr3 composites as promising materials for optical modulation within fiber laser applications.

Cold-Sintered All-Inorganic Perovskite Bulk Composite Scintillators for Efficient X-ray Imaging

Cost-effective bulk scintillators with high density, large-area, and long-term stability are desirable for high-energy radiation detections. Conventional bulk polycrystalline or single-crystal scintillators are generally synthesized by high-temperature approaches, and it is challenging to realize simultaneously high detectivity/responsivity, spatial resolution, and rapid time response. Here, we report the cold sintering of bulk scintillators (at 90 °C) based on an "emitter-in-matrix" principle, in which emissive CsPbBr3 nanocrystals are embedded in a durable and transparent Cs4PbBr6 matrix. These bulk scintillators exhibit high light yield (33,800 photons MeV-1), low detection limit (79 nGyair s-1), fast decay time (9.8 ns), and outstanding spatial resolution of 8.9 lp mm-1 to X-ray radiation and an energy resolution of 19.3% for γ-ray (59.6 keV) detection. The composite scintillator also shows exceptional stability against environmental degradation and cyclic X-ray radiation. Our results demonstrate a cost-effective strategy for developing perovskite-based bulk transparent scintillators with exceptional performance and high radioluminescence stability for high-energy radiation detection and imaging.

Promoted Growth and Multiband Emission in Heterostructured Perovskites Through Cs+ -Sublattice Interaction

Precise control of exciton confinement in metal halide perovskites is critical to the development of high-performance, stable optoelectronic devices. A significant hurdle is the swift completion of ionic metathesis reactions, often within seconds, making consistent control challenging. Herein, the introduction of different steric hindrances in a Cs+ sublattice within CsYb2 F7 is reported, which effectively modulates the reaction rate of Cs+ with lead (Pb2+ ) and halide ions in solution, extending the synthesis time for perovskite nanostructures to tens of minutes. Importantly, the Cs+ sublattice provides a crystal facet-dependent preference for perovskite growth and thus exciton confinement, allowing the simultaneous occurrence of up to six emission bands of CsPbBr3 . Moreover, the rigid CsYb2 F7 nano template offers high activation energy and enhances the stability of the resulting perovskite nanostructures. This methodology provides a versatile approach to synthesizing functional heterostructures. Its robustness is demonstrated by in-situ growth of perovskite nanostructures on Cs+ -mediated metal-organic frameworks.

High-Efficiency CsPbBr3 Light-Emitting Diodes using One-Step Spin-Coating In Situ Dynamic Thermal Crystallization

All-inorganic perovskite materials (such as CsPbBr₃) have received widespread attention because of their better stability than hybrid counterparts, but their poor film morphology and crystalline quality limit their application in perovskite light-emitting devices (PeLEDs). Some previous studies have attempted to improve the morphology and crystalline quality of perovskite films by heating the substrate, but there are still some problems such as inaccurate temperature control, excessive temperature is not conducive to flexible applications, and the mechanism of action is not clear. In this work, we used a one-step spin-coating, low-temperature in situ thermally assisted crystallization process, in which the temperature was accurately monitored using a thermocouple in the range of 23-80 °C, and explored the effect of the in situ thermally assisted crystallization temperature on the crystallization of the all-inorganic perovskite material CsPbBr₃ and the performance of PeLEDs. In addition, we focused on the influence mechanism for the in situ thermally assisted crystallization process on the surface morphology and phase composition of the perovskite films and promote its possible application in inkjet printing and scratch coating methods.

Highly Emissive Zero-Dimensional Cesium Lead Iodide Perovskite Nanocrystals with Thermally Activated Delayed Photoluminescence

We utilized a modified reverse-microemulsion method to develop highly emissive and photostable zero-dimensional (0D) Cs₄Pb(Br_1-xI_x)₆ perovskite nanocrystals (PNCs). We employed single-particle photoluminescence (PL) spectroscopy to explore blinking statistics and demonstrate single-photon emission from individual PNCs. Low-temperature blinking and photon correlation studies revealed a transition from single- to multiphoton emission with progressively longer "delayed" PL components, reaching ∼70 ns at room temperature and representing a distinctive behavior to previously known iodide PNCs. Such thermally activated PL emission is explained by the existence of defect-related "reservoir" states, feeding back into the PNC's emissive state and providing multiple photons within a single excitation cycle. This work establishes a new member in the 0D class of perovskite materials, studies its photophysical properties, and reveals its potential for future optoelectronic applications.

Multiple Stimuli-Response Polychromatic Carbon Dots for Advanced Information Encryption and Safety

Optical information encryption and safety have aroused great attention since they are closely correlated to data protection and information safety. The development of multiple stimuli-response optical materials for constructing large-capacity information encryption and safety is very important for practical applications. Carbon dots (CDs) have many gratifying merits, such as polychromatic emission, diverse luminous categories, and stable physicochemical properties, and are considered as one of the most ideal candidates for information protection. Herein, carbon core, functional groups, solvents, and other crucial factors are reviewed for outputting polychromatic emission of multiple luminous categories. In particular, substrate engineering strategies have been emphasized for their critical role in yielding excellent optical features of multiple luminous categories. High-capacity information encryption and safety strategies are reviewed by relying on the rich optical properties of CDs, such as polychromatic emission, multiple luminous categories of fluorescence, afterglow, and upconversion, as well as external-stimuli-assisted optical changes. Some perspectives for preparing excellent CDs and further developing information security strategies are proposed. This review provides a good reference for the manipulation of polychromatic CDs and the development of next-generation information encryption and safety.

Solvent Evaporation Induced Large-Scale Synthesis of Cs4PbBr6 and CsPbBr3 Microcrystals: Optical Properties and Backlight Application for LEDs

The contemporary work focuses on embossing the emissive nature of lead halide perovskite materials, specifically Cs₄PbBr₆ microcrystal powder prepared via single step bulk recrystallization method followed by the solvent evaporation route from gram to kilogram scale. The X-ray diffraction pattern confirms the formation of phase pure Cs₄PbBr₆ with a goodness of fit value of 1.51 calculated from Rietveld refinement and the fluorophore powder manifesting an intrinsic band gap of 3.76 eV. The experimental yield of 99.4% indicates the absence of any unreacted precursors. The fabricated flexible, free-standing Cs₄PbBr₆@PMMA film encompassed better moisture stability without undergoing phase transitions for 400 days. The temperature-dependent photoluminescence spectra denote that 51% of the intensity was retained when cooled back to room temperature after heating it till 180 °C. Moisture studies at two extreme humidity conditions also reveal the appreciable stability of the fluorophore film against moisture. The stability studies with respect to UV irradiation substantiate that the film retained its stability even after exposing it continuously to UV radiation for seven days. The outstanding optical properties of these microcrystals, owing to the higher exciton binding energy, make them a promising candidate as excellent fluorophores for color conversion, backlight, and light-emitting applications. The Cs₄PbBr₆@PMMA film was employed as the top cover of a commercial blue LED, producing a robust green emission which revealed its possible application as a phosphor material.

Highly Stable and Photoluminescent CsPbBr3/Cs4PbBr6 Composites for White-Light-Emitting Diodes and Visible Light Communication

Inorganic lead halide perovskite is one of the most excellent fluorescent materials, and it plays an essential role in high-definition display and visible light communication (VLC). Its photochromic properties and stability determine the final performance of light-emitting devices. However, efficiently synthesizing perovskite with high quality and stability remains a significant challenge. Here, we develop a facile and environmentally friendly method for preparing high-stability and strong-emission CsPbBr₃/Cs₄PbBr₆ composites using ultrasonication and liquid paraffin. Tuning the contents of liquid paraffin, bright-emission CsPbBr₃/Cs₄PbBr₆ composite powders with a maximum PLQY of 74% were achieved. Thanks to the protection of the Cs₄PbBr₆ matrix and liquid paraffin, the photostability, thermostability, and polar solvent stability of CsPbBr₃/Cs₄PbBr₆-LP are significantly improved compared to CsPbBr₃ quantum dots and CsPbBr₃/Cs₄PbBr₆ composites that were prepared without liquid paraffin. Moreover, the fabricated CsPbBr₃/Cs₄PbBr₆-LP-based WLEDs show excellent luminescent performance with a power efficiency of 129.5 lm/W and a wide color gamut, with 121% of the NTSC and 94% of the Rec. 2020, demonstrating a promising candidate for displays. In addition, the CsPbBr₃/Cs₄PbBr₆-LP-based WLEDs were also demonstrated in a VLC system. The results suggested the great potential of these high-performance WLEDs as an excitation light source to achieve VLC.

Highly emissive green CsPbBr3/Cs4PbBr6 composites: formation kinetics, excellent heat, light, and polar solvent resistance, and flexible light-emitting application

Benefit from their near-unity photoluminescence quantum yield (PL QY), narrow emission band, and widely tunable bandgap, metal halide perovskites have shown promising in light-emitting applications. Despite such promise, how to facile, environmentally-friendly, and large-scale prepare solid metal halide perovskite with high emission and stability remains a challenging. Herein, we demonstrate a convenient and environmentally-friendly method for the mass synthesis of solid CsPbBr₃/Cs₄PbBr₆ composites using high-power ultrasonication. Adjusting key experimental parameters, bright emitting CsPbBr₃/Cs₄PbBr₆ solids with a maximum PL QY of 71% were obtained within 30 min. XRD, SEM, TEM, Abs/PL, XPS, and lifetime characterizations provide solid evidence for forming CsPbBr₃/Cs₄PbBr₆ composites. Taking advantage of these composites, the photostability, thermostability, and polar solvent stability of CsPbBr₃/Cs₄PbBr₆ are much improved compared to CsPbBr₃. We further demonstrated CsPbBr₃/Cs₄PbBr₆ use in flexible/stretchable film and high-power WLEDs. After being subjected to bending, folding, and twisting, the film retains its bright emission and exhibits good resistance to mechanical deformation. Additionally, our WLEDs display a superior, durable high-power-driving capability, operating currents up to 300 mA and maintaining high luminous intensity for 50 hours. Such highly emissive and stable metal halide perovskites make them promising for solid-state lighting, lasing, and flexible/stretchable display device applications.

Mechanochemistry-driven engineering of 0D/3D heterostructure for designing highly luminescent Cs–Pb–Br perovskites

Embedding metal-halide perovskite particles within an insulating host matrix has proven to be an effective strategy for revealing the outstanding luminescence properties of perovskites as an emerging class of light emitters. Particularly, unexpected bright green emission observed in a nominally pure zero-dimensional cesium–lead–bromide perovskite (Cs₄PbBr₆) has triggered intensive research in better understanding the serendipitous incorporation of emissive guest species within the Cs₄PbBr₆ host. However, a limited controllability over such heterostructural configurations in conventional solution-based synthesis methods has limited the degree of freedom in designing synthesis routes for accessing different structural and compositional configurations of these host–guest species. In this study, we provide means of enhancing the luminescence properties in the nominal Cs₄PbBr₆ powder through a guided heterostructural configuration engineering enabled by solid-state mechanochemical synthesis. Realized by an in-depth study on time-dependent evaluation of optical and structural properties during the synthesis of Cs₄PbBr₆, our target-designed synthesis protocol to promote the endotaxial formation of Cs₄PbBr₆/CsPbBr₃ heterostructures provides key insights for understanding and designing kinetics-guided syntheses of highly luminescent perovskite emitters for light-emitting applications.

While emission and stability of metal–halide perovskites can be enhanced through heterostructural encapsulation, a controlled synthesis route to such structures is not trivial to realize. Here, the authors design a mechanochemistry-driven protocol for synthesizing highly luminescent CsPbBr₃/Cs₄PbBr₆ heterostructures.

Formation mechanisms of CsPbBr3/Cs4PbBr6 microscale composites assisted with imidazolium cations and their device applications

The instability of all-inorganic perovskite nanocrystals (NCs) remains a major challenge to their practical applications in displays and lighting. In order to improve their environmental resistance, highly luminescent CsPbBr₃ NC embedded Cs₄PbBr₆ microcrystals (MCs) have been fabricated by an anti-solvent reprecipitation method with the assistance of 1-alkyl-3-methyl-imidazolium bromides. The heterostructure of the MCs is investigated in detail, and their formation mechanism is discussed in terms of the dissolution-precipitation equilibria and the total energy of variously scaled nanoparticles via density functional theory (DFT) calculations. Although the imidazolium ligands are not directly capping with the inner CsPbBr₃ NCs, they are important to the formation of these MCs. The MCs exhibit better thermal resistance compared to conventional CsPbBr₃ NCs prepared by the hot-injection method. Additionally, a prototype white light-emitting diode (WLED) was fabricated to demonstrate its practical application prospects. Thanks to the narrow emission bands and enhanced stability of the MCs, the WLED shows greater performance compared to bare perovskite NCs, indicating its great potential as a green phosphor in lighting and display applications.

Progress of Backlight Devices: Emergence of Halide Perovskite Quantum Dots/Nanomaterials

The technology behind the display is becoming ever more prevalent in our daily lives. It has many applications, including smartphones, tablets, desktop monitors, TVs, and augmented reality/virtual reality devices. The display technology has progressed drastically over the past decade, from the bulky cathode ray tube to the flat panel displays. In the flat panel displays, the liquid crystal display (LCD) and organic light-emitting diodes (OLEDs) are the two dominant technologies. Nevertheless, due to low stability and color tunability, OLEDs remain behind the LCDs. The LCD screen has a backlight, usually a white LED, which comprises a blue LED covered with a red and green enhanced layer (color-converting layers). Although InP/CdSe QDs attracted more attention due to their solution processability and better color gamut than the previous technologies, the complexity of their synthesis was still an obstacle to their commercialization. Later, the emergence of perovskite with highly intense and tunable PL emission, high color purity, and low-cost synthesis route attracted the attention of display researchers. Owing to the relatively higher performance of perovskite quantum dots (PQDs) than that of bulk (3D) perovskite in backlit display devices, these PQDs are being used for high color contrast and bright display devices. Furthermore, the color gamut for PQDs was observed as 140% of the NTSC standard, that is, close to that of the commercial OLED devices. In this review, we have discussed the progress of display technologies with a clear classification of the pros and cons of each technology. Also, the application of perovskite QD/nanomaterials in LCD backlit devices has been discussed, and the future direction of further improvement in their stability and performance has been listed.

Highly Emissive Blue Quantum Dots with Superior Thermal Stability via In Situ Surface Reconstruction of Mixed CsPbBr3 -Cs4 PbBr6 Nanocrystals

Although metal halide perovskites are candidate high-performance light-emitting diode (LED) materials, blue perovskite LEDs are problematic: mixed-halide materials are susceptible to phase segregation and bromide-based perovskite quantum dots (QDs) have low stability. Herein, a novel strategy for highly efficient, stable cesium lead bromide (CsPbBr₃ ) QDs via in situ surface reconstruction of CsPbBr₃ -Cs₄ PbBr₆ nanocrystals (NCs) is reported. By controlling precursor reactivity, the ratio of CsPbBr₃ to Cs₄ PbBr₆ NCs is successfully modulated. A high photoluminescence quantum yield (PLQY) of >90% at 470 nm is obtained because octahedron CsPbBr₃ QD surface defects are removed by the Cs₄ PbBr₆ NCs. The defect-engineered QDs exhibit high colloidal stability, retaining >90% of their initial PLQY after >120 days of ambient storage. Furthermore, thermal stability is demonstrated by a lack of heat-induced aggregation at 120 °C. Blue LEDs fabricated from CsPbBr₃ QDs with reconstructed surfaces exhibit a maximum external quantum efficiency of 4.65% at 480 nm and excellent spectral stability.

Reversible Transformation between Cs3Cu2I5 and CsCu2I3 Perovskite Derivatives and Its Anticounterfeiting Application

Lead halide perovskites have promising values in photoelectronic and photovoltaic applications, but the toxicity of lead is a hard barrier. Copper halide perovskite derivatives (CHPDs), as a lead-free substitution of lead halide perovskites, also exhibit excellent photoelectric properties. Here, we present a facile one-step route for the synthesis of blue-emissive Cs₃Cu₂I₅ (emission at 440 nm) and yellow-emissive CsCu₂I₃ (emission at 552 nm) CHPDs in ethanol at room temperature. Triggered by ethanol or CsI, a reversible chemical transformation accompanied by emissive color change between Cs₃Cu₂I₅ and CsCu₂I₃ CHPDs was achieved. The reversible transformation mechanism was discussed, and this transformation was employed for effective anticounterfeiting.

Revealing photoluminescence mechanisms of single CsPbBr3/Cs4PbBr6 core/shell perovskite nanocrystals

CsPbBr3 nanocrystals (NCs) encapsulated by Cs4PbBr6 has attracted extensive attention due to good stability and high photoluminescence (PL) emission efficiency. However, the origin of photoluminescence (PL) emission from CsPbBr3/Cs4PbBr6 composite materials has been controversial. In this work, we prepare CsPbBr3/Cs4PbBr6 core/shell nanoparticles and firstly study the mechanism of its photoluminescence (PL) at the single-particle level. Based on photoluminescence (PL) intensity trajectories and photon antibunching measurements, we have found that photoluminescence (PL) intensity trajectories of individual CsPbBr3/Cs4PbBr6 core/shell NCs vary from the uniform longer periods to multiple-step intensity behaviors with increasing excitation level. Meanwhile, second-order photon correlation functions exhibit single photon emission behaviors especially at lower excitation levels. However, the PL intensity trajectories of individual Cs4PbBr6 NCs demonstrate apparent "burst-like" behaviors with very high values of g 2(0) at any excitation power. Therefore, the distinguishable emission statistics help us to elucidate whether the photoluminescence (PL) emission of CsPbBr3/Cs4PbBr6 core/shell NCs stems from band-edge exciton recombination of CsPbBr3 NCs or intrinsic Br vacancy states of Cs4PbBr6 NCs. These findings provide key information about the origin of emission in CsPbBr3/Cs4PbBr6 core/shell nanoparticles, which improves their utilization in the further optoelectronic applications.

High-Efficiency Down-Conversion Radiation Fluorescence and Ultrafast Photoluminescence (1.2 ns) at the Interface of Hybrid Cs4PbBr6-CsI Nanocrystals

The research of fast scintillators in positron emission tomography and other applications based on time-of-flight technology promotes the development of radiation detection. However, because of the current lack of efficient and fast carrier radiation recombination pathways, the research on scintillator radioluminescence (RL) still faces severe challenges. Here, we propose an effective interface carrier transport mechanism: CsI:Na crystal and Cs₄PbBr₆ nanocrystals (NCs) interface to form a new phase and a continuous heterostructure, providing an effective channel for X-ray excited carrier transfer to Cs₄PbBr₆. Then, the excited carriers realize efficient recombination luminescence through the self-trapped excitons inside Cs₄PbBr₆. On the basis of this mechanism, the heterostructure composite scintillator composed of CsI:Na/Cs₄PbBr₆ exhibits high-efficiency radiant fluorescence and an ultrafast photoluminescence (PL) decay time of 1.22 ns. The effective interface carrier transport shown in this work provides an optimization idea that can be used for reference in the research of fast scintillators.

Erasable Cs4PbBr6 quantum dots glass with switchable photoluminescence

Encapsulating quantum dots (QDs) into a transparent glass matrix can not only increase their stability but also explore new luminescence properties. Herein, erasable Cs₄PbBr₆ QDs with switchable photoluminescence are successfully obtained by a heat-treatment method in a borosilicate glass matrix. First, QDs can self-crystalize in borosilicate glass without the heat treatment. Second, it is found that when the QDs glass is heat treated below T_g, QDs can be destroyed. Further, when the QDs glass is reheat treated above T_g, atom rearrangement can occur and QDs can reprecipitate in the glass matrix. Consequently, the switchable photoluminescence of QDs can be achieved. The special erasable QDs glass and switchable photoluminescence in this work can resolve the technological challenge of recovering the luminescence performance of QDs after they have been destroyed.

Formation and Near-Infrared Emission of CsPbI3 Nanoparticles Embedded in Cs4PbI6 Crystals

Cs₄PbI₆, as a rarely investigated member of the Cs₄PbX₆ (X is a halogen element) family, has been successfully synthesized at low temperatures, and the synthetic conditions have been optimized. Metal iodides such as LiI, KI, NiI₂, CoI₂, and ZnI₂, as additives, play an important role in enhancing the formation of the Cs₄PbI₆ microcrystals. ZnI₂ with the lowest dissociation energy is the most efficient additive to supply iodide ions, and its amount of addition has also been optimized. Strong red to near-infrared (NIR) emission properties have been detected, and its optical emission centers have been identified to be numerous embedded perovskite-type α-CsPbI₃ nanocrystallites (∼5 nm in diameter) based on investigations of temperature- and pressure-dependent photoluminescent properties. High-resolution transmission electron microscopy was used to detect these hidden nanoparticles, although the material was highly beam-sensitive and confirmed a "raisin bread"-like structure of the Cs₄PbI₆ crystals. A NIR mini-LED for the biological application has been successfully fabricated using as-synthesized Cs₄PbI₆ crystals. This work provides information for the future development of infrared fluorescent nanoscale perovskite materials.

On the Role of Cs4PbBr6 Phase in the Luminescence Performance of Bright CsPbBr3 Nanocrystals

CsPbBr3 nanocrystals have been identified as a highly promising material for various optoelectronic applications. However, they tend to coexist with Cs4PbBr6 phase when the reaction conditions are not controlled carefully. It is therefore imperative to understand how the presence of this phase affects the luminescence performance of CsPbBr3 nanocrystals. We synthesized a mixed CsPbBr3-Cs4PbBr6 sample, and compared its photo- and radioluminescence properties, including timing characteristics, to the performance of pure CsPbBr3 nanocrystals. The possibility of energy transfer between the two phases was also explored. We demonstrated that the presence of Cs4PbBr6 causes significant drop in radioluminescence intensity of CsPbBr3 nanocrystals, which can limit possible future applications of Cs4PbBr6-CsPbBr3 mixtures or composites as scintillation detectors.

Recent progress of zero-dimensional luminescent metal halides

This review provides in-depth insight into the structure–luminescence–application relationship of 0D all-inorganic/organic–inorganic hybrid metal halide luminescent materials.

Highly Luminescent CsPbBr3@Cs4PbBr6 Nanocrystals and Their Application in Electroluminescent Emitters

Zero-dimensional perovskite nanocrystals (NCs) are becoming the most attractive material due to their excellent optical performance and better stability compared with high-dimensional perovskite. However, their application in electroluminescent (EL) emitters for high-quality displays is still limited. In this work, we successfully achieved CsPbBr₃@Cs₄PbBr₆ NCs around 13.9 ± 0.2 nm by using the hot-injection method. Additional SnBr₂ was mixed in the PbBr₂ precursor to provide extra Br^- ions and reduce the excessive amount of Pb²⁺ ions to promote the formation of CsPbBr₃@Cs₄PbBr₆. Time resolution photoluminescence analysis indicated that the green emission of our CsPbBr₃@Cs₄PbBr₆ NCs originated from the embedded CsPbBr₃ NCs, which corresponds to our previous research. The Cs₄PbBr₆ crystals passivated the surface of CsPbBr₃ NCs, resulting in the absence of trions for the high photoluminescence quantum yield. The as-synthesized CsPbBr₃@Cs₄PbBr₆ NCs were used to fabricate quantum dot light-emitting diode (QLED) devices with the highest current efficiency of 4.89 cd/A. This is the best performance of the CsPbBr₃@Cs₄PbBr₆-system QLED device, which reveals the great potential of CsPbBr₃@Cs₄PbBr₆ NCs and will inspire further study of zero-dimensional perovskite composite NCs for EL emitters.

Fluorescence-enhanced Cs4 PbBr6 /CsPbBr3 composites films synthesized by double-films solid phase reaction method

Due to indispensable ligands, polluted organic solution, or complex vapour deposition, stable CsPbBr₃ film is hard to be prepared directly using a simple and environmentally friendly method. To improve the stability of CsPbBr₃ film and its synthesis methods, the double-films solid phase reaction was developed, and Cs₄ PbBr₆ /CsPbBr₃ composites were designed. Although the synthesized particle had a size of 2-5 μm, much larger than that of quantum dots, in ambient conditions the composites films still showed good photoluminescence properties, with the highest photoluminescence quantum yield of 80%. It had good stability against air, temperature and humidity, and even had interesting fluorescence-enhanced phenomenon after about 4 days.

Highly Thermotolerant Metal Halide Perovskite Solids

By virtue of their narrow emission bands, near-unity quantum yield, and low fabrication cost, metal halide perovskites hold great promise in numerous aspects of optoelectronic applications, including solid-state lighting, lasing, and displays. Despite such promise, the poor temperature tolerance and suboptimal quantum yield of the existing metal halide perovskites in their solid state have severely limited their practical applications. Here, a straightforward heterogeneous interfacial method to develop superior thermotolerant and highly emissive solid-state metal halide perovskites is reported and their use as long-lasting high-temperature and high-input-power durable solid-state light-emitting diodes is illustrated. It is found that the resultant materials can well maintain their superior quantum efficiency after heating at a temperature over 150 °C for up to 22 h. A white light-emitting diode (w-LED) constructed from the metal halide perovskite solid exhibits superior temperature sustainable lifetime over 1100 h. The w-LED also displays a highly durable high-power-driving capability, and its working current can go up to 300 mA. It is believed that such highly thermotolerant metal halide perovskites will unleash the possibility of a wide variety of high-power and high-temperature solid-state lighting, lasing, and display devices that have been limited by existing methods.

Efficient Flexible Inorganic Perovskite Light-Emitting Diodes Fabricated with CsPbBr3 Emitters Prepared via Low-Temperature in Situ Dynamic Thermal Crystallization

The present study systematically investigates the morphology and crystallization process of inorganic CsPbBr₃ perovskite layer films fabricated by thermal coevaporation in conjunction with continuous low-temperature thermal annealing to promote in situ dynamic thermal crystallization. The results confirm for the first time that both the crystal grain size and the compactness of the CsPbBr₃ films can be tuned during the thermal coevaporation fabrication process via in situ dynamic thermal crystallization. The performance of the PeLEDs employing the CsPbBr₃ films as the emitter layer is investigated in detail with respect to the substrate temperature and deposition rate employed during deposition of the CsPbBr₃ film. This study provides guidelines for developing suitable film production processes and highlights future challenges that must be addressed to facilitate the commercial development of large-area, uniform, and flexible perovskite-based optoelectronic devices.

Curing the fundamental issue of impurity phases in two-step solution-processed CsPbBr3 perovskite films

Inorganic lead halide perovskite CsPbBr₃ offers attractive photophysical properties and phase stability for high-performance optoelectronic devices. However, CsPbBr₃ films produced by the classic solution-based two-step method are always accompanied with impurity phases of CsPb₂Br₅ and Cs₄PbBr₆, which represents a major efficiency-limiting factor for future advances of CsPbBr₃-based devices. The challenge lies in the complexity of the Cs-Pb-Br phase system, requiring both spatially and temporally precise control of the precursor stoichiometry during solution-phase growth of CsPbBr₃ films. By adopting 2-methoxyethanol as the solution conversion medium instead of commonly applied methanol, the reaction between CsBr and PbBr₂ can be finely controlled to yield single phase CsPbBr₃ films within a few minutes; extending the solution-conversion step to 24 h does not alter the phase purity of resulting CsPbBr₃ films. The present work paves the way to regulate the crystal growth behaviors of two-step solution-processed CsPbBr₃ films by simple solvent engineering.

Synthesis and dimensional control of CsPbBr3 perovskite nanocrystals using phosphorous based ligands

Nanocrystals of the inorganic perovskite, CsPbBr₃, display outstanding photo-physical properties, making them ideal for next generation optical devices. However, the typical combination of oleic acid and oleylamine ligands employed in their synthesis is easily displaced, leading to poor stability that can hinder their applicability. In this work, we look toward the replacement of the oleic acid and amine with phosphorous-based ligands. We synthesize CsPbBr₃ nanocrystals using an oleylamine/alkylphosphonic acid combination with near perfect monodispersity with the ability to tune the bandgap by varying the alkyl chain length. We further investigate the replacement of the oleylamine giving a ligand combination of alkylphosphonic acid/trioctylphosphine oxide for perovskite nanocrystal nucleation and growth. This combination is typical for the widely studied metal chalcogenide synthesis and in our study with CsPbBr₃ yields a pure phase perovskite.

Antithermal Quenching of Luminescence in Zero-Dimensional Hybrid Metal Halide Solids

Zero-dimensional (0D) hybrid metal halides have emerged as a new generation of luminescent phosphors owing to their high radiative recombination rates, which, akin to their three-dimensional cousins, commonly demonstrate thermal quenching of luminescence. Here, we report on the finding of antithermal quenching of luminescence in 0D hybrid metal halides. Using (C₉NH₂₀)₂SnBr₄ single crystals as an example system, we show that 0D metal halides can demonstrate antithermal quenching of luminescence. A combination of experimental characterizations and first-principles calculations suggests that antithermal quenching of luminescence is associated with trap states introduced by structural defects in (C₉NH₂₀)₂SnBr₄. Importantly, we find that antithermal quenching of luminescence is not only limited to (C₉NH₂₀)₂SnBr₄ but also exists in other 0D metal halides. Our work highlights the important role of defects in impacting photophysical properties of hybrid metal halides and may stimulate new efforts to explore metal halides exhibiting antithermal quenching of luminescence at higher temperatures.

Laser-Assisted Fabrication for Metal Halide Perovskite-2D Nanoconjugates: Control on the Nanocrystal Density and Morphology

We report on a facile and rapid photo-induced process to conjugate graphene-based materials with metal-halide perovskite nanocrystals. We show that a small number of laser pulses is sufficient to decorate the 2-dimensional (2D) flakes with metal-halide nanocrystals without affecting their primary morphology. At the same time, the density of anchored nanocrystals could be finely tuned by the number of irradiation pulses. This facile and rapid room temperature method provides unique opportunities for the design and development of perovskite-2D nanoconjugates, exhibiting synergetic functionality by combining nanocrystals of different morphologies and chemical phases with various 2D materials.

Light-Driven Halide Exchange Facilitates Complete Crystal Transformation in Nanostructured Perovskites

Colloidal perovskite nanocrystals (NCs) have shown promise in visible and near-infrared light emission compared with conventional quantum dots because of their sharp and highly efficient emission. Light-driven halide exchange offers high degrees of tunability of perovskite NC composition and photoluminescence properties. A remaining challenge is the incomplete change between bromide and chloride due to the presence of radical quenchers. Herein, an optimized in situ halide exchange method of perovskite NCs is reported. Chloroform is used as the solvent and the precursor to effectively generate halide radicals and ions under ultraviolet light irradiation in an inert atmosphere. This strategy achieves complete crystal transformation in colloidal perovskite NCs such as CsPbBr₃-to-CsPbCl₃ and Cs₄PbBr₆-to-CsPbCl₃. The exchange reaction rate can be further tuned by the presence of B-site dopants such as Zn²⁺ and Ce³⁺. The dopant cations are partially remained in the exchanged perovskite NCs, indicating the predominance of thermodynamic control in the crystal transformation.

Negative photoconductivity in Cs4PbBr6 single crystal

Negative photoconductivity is firstly observed in large size Cs₄PbBr₆ single crystal that grown from Cs-rich solution. The Br vacancy and free excitons are responsible for this novel phenomena.

A halide perovskite as a catalyst to simultaneously achieve efficient photocatalytic CO2 reduction and methanol oxidation

We have firstly demonstrated the photocatalytic utilization of a halide perovskite for combining reduction of CO₂ with selective oxidation of methanol.

CsPbBr3-Cs4PbBr6 composite nanocrystals for highly efficient pure green light emission

All-inorganic perovskite CsPbBr₃-Cs₄PbBr₆ composite nanocrystals (NCs) were synthesized via a convenient solution process without inert gas protection and systematically studied as green phosphors for light emitting diode (LED) applications. While colloidal composite NCs emit green color with an emission peak at around 515 nm, their thin films on top of blue GaN chips exhibit a redshift of ∼15-20 nm due to subsequential aggregation in solid state. The Commission Internationale De I'eclairage (CIE) chromaticity coordinates of the green LEDs assembled by composite NCs reached (0.201, 0.746) with nearly 100% green ratio. Moreover, the pure green LED displayed a luminous efficiency of 45 lm W^-1 under 10 mA driving current. The colloidal CsPbBr₃-Cs₄PbBr₆ composite NCs have the photoluminescence quantum yield (PLQY) as high as 74%. The high PLQY originates from cubic CsPbBr₃ NCs well passivated by the zero-dimensional Cs₄PbBr₆ matrix, substantially suppressing the nonradiative recombination. The comparison between the green LEDs fabricated with pure and composite perovskites imply that the CsPbBr₃-Cs₄PbBr₆ composites with higher quantum yield could be an effective way to get the brightness and the stability of pure green LEDs.

Disentangling the Luminescent Mechanism of Cs4PbBr6 Single Crystals from an Ultrafast Dynamics Perspective

New all-inorganic perovskites like Cs₄PbBr₆ provide rich luminescent tools and particularly novel physical insights, including their zero-dimensional structure and controversial emitting mechanism. The ensuing debate over the origin of the luminescence of Cs₄PbBr₆ inspired us to tackle the issue through fabricating high-quality Cs₄PbBr₆ single crystals and employing ultrafast dynamics study. Upon photoexcitation, Cs₄PbBr₆ underwent dynamics steps distinct from that of CsPbBr₃, including exciton migration to the defect level on a time scale of several hundred femtoseconds, exciton relaxation within the defect states on the picosecond time scale, and exciton recombination from the subnanosecond to nanosecond time scale. The observation disclosed that crystal defects of Cs₄PbBr₆ induced green emission while CsPbBr₃ mainly relied on quantum confinement to emit at room temperature. The study provides an in-depth understanding of the photoinduced multistep dynamics steps of Cs₄PbBr₆ associated with display and photovoltaic applications, establishing Cs₄PbBr₆ as a new candidate for uses associated with the perovskite family of materials.

CsPbX3/SiOx (X = Cl, Br, I) monoliths prepared via a novel sol-gel route starting from Cs4PbX6 nanocrystals

We developed a facile synthesis of nanocomposite powders of CsPbX₃ nanocrystals (NCs) embedded in silica. The synthesis starts from colloidal Cs₄PbX₆ NCs that are mixed with tetraethyl orthosilicate in the presence of nitric acid, which triggers the sol-gel reaction yielding the formation of SiO_x and the conversion of starting NCs into CsPbX₃ ones. The overall reaction delivers CsPbX₃ NCs encased in a silica matrix. The resulting CsPbX₃/SiO_x nano-composite powders exhibited enhanced moisture and thermal stability in air. Also, when mixing different CsPbX₃/SiO_x samples having diverse anion compositions, no interparticle anion exchange processes were observed, which is a further indication that the silica matrix acts as a robust barrier surrounding the NCs. Finallly, we used these composites as down-converter phosphors on top of a blue light-emitting diode (LED), delivering nearly ideal white light emission with the Commission Internationale de l'Eclairage (CIE) color coordinates (0.32, 0.33).

The optical properties of Cs4PbBr6-CsPbBr3 perovskite composites

Although the optoelectronic applications of metal halide perovskites have been intensively investigated in recent years, the fundamental carrier dynamics of zero-dimensional (0D) Cs4PbBr6 perovskites has been relatively underexplored; in particular, the nature of the green fluorescence is highly debated. Nevertheless, the unique photophysical properties are of immense interest for a variety of potential applications. In this work, the green emission of the CsPbBr3-Cs4PbBr6 perovskite composites is studied using temperature dependent photoluminescence (PL). The PL spectra at different temperatures simultaneously contain two sub-peaks (520 nm and 550 nm), which are ascribed to the emissions of the band-edge and the defect trapped exciton of CsPbBr3. This finding will help to understand the controversial photoluminescence currently observed in different 0D Cs4PbBr6 perovskites.

From Nonluminescent to Blue-Emitting Cs4 PbBr6 Nanocrystals: Tailoring the Insulator Bandgap of 0D Perovskite through Sn Cation Doping

All-inorganic cesium lead halide perovskite nanocrystals (NCs) with different dimensionalities have recently fascinated the research community due to their extraordinary optoelectronic performance such as tunable bandgaps over the entire visible spectral region. However, compared to well-developed 3D CsPbX₃ perovskites (X = Cl, Br, and I), the bandgap tuning in 0D Cs₄ PbX₆ perovskite NCs remains an arduous task. Herein, a simple but valid strategy is proposed to tailor the insulator bandgap (≈3.96 eV) of Cs₄ PbBr₆ NCs to the blue spectral region by changing the local coordination environment of isolated [PbBr₆ ]^4- octahedra in the Cs₄ PbBr₆ crystal through Sn cation doping. Benefitting from the unique Pb²⁺ -poor and Br^- -rich reaction environment, the Sn cation is successfully introduced into the Cs₄ PbBr₆ NCs, forming coexisting point defects comprising substitutional Sn_Pb and interstitial Br_i , thereby endowing these theoretically nonluminescent Cs₄ PbBr₆ NCs with an ultranarrow blue emission at ≈437 nm (full width at half maximum, ≈12 nm). By combining the experimental results with first-principles calculations, an unusual electronic dual-bandgap structure, comprising the newly emerged semiconducting bandgap of ≈2.87 eV and original insulator bandgap of ≈3.96 eV, is found to be the underlying fundamental reason for the ultranarrow blue emission.

Optical Characterization of Cesium Lead Bromide Perovskites

CsPbBr3 and Cs4PbBr6 perovskite powders have been synthesized through a relatively simple low-temperature and low-cost method. Nanocrystalline films have also been deposited from solutions with four different molar compositions of binary salt precursors. Optical absorption, emission and excitation spectra have been performed in the UV-visible spectral range while X-ray diffraction (XRD) has been recorded to characterize the nanocrystal morphology for the different molar compositions. A preferential orientation of crystallites along the (024) crystalline plane has been observed as a function of the different deposition conditions in films growth. All the crystals show an absorption edge around 530 nm; Tauc plots of the absorption returned bandgaps ranging from 2.29 to 2.35 eV characteristic of CsPbBr3 phase. We attribute the UV absorption band peaked at 324 nm to the fundamental band-to-band transition for Cs4PbBr6. It was observed that the samples with the most ordered Cs4PbBr6 crystals exhibited the most intense emission of light, with a bright green emission at 520 nm, which are however due to the luminescence of the inclusion of CsPbBr3 nanoclusters into the Cs4PbBr6. The latter shows instead an intense UV emission. Differently, the pure CsPbBr3 powder did not show any intense fluorescent emission. The excitation spectra of the green fluorescent emission in all samples closely resemble the CsPbBr3 absorption with the peculiar dip around 324 nm as expected from density of state calculations reported in the literature.

Integration of Environmental Friendly Perovskites for High-efficiency White Light-emitting Diodes

Perovskite quantum dots (QDs) have been widely used in white light-emitting diodes (WLEDs), due to their high quantum yield (QY), tunable bandgap, and simple preparation. However, the red-emitting perovskite QDs are usually containing iodine (I), which is not stable under continuous light irradiation. Herein, perovskite-based WLED is fabricated by lead-free bismuth (Bi)-doped inorganic perovskites Cs₂SnCl₆ and less-lead Mn-doped CsPbCl₃ QDs, which emits white light with color coordinates of (0.334, 0.297). The Bi-doped Cs₂SnCl₆ and Mn-doped CsPbCl₃ QDs both show excellent stability when kept in the ambient air. As benefits from this desired characteristic, the as-prepared WLED shows excellent stability along with operating time. These results can promote the application of inorganic perovskite QDs in the field of WLEDs.

Metal Halide Perovskite Nanocrystals: Synthesis, Post-Synthesis Modifications, and Their Optical Properties

Metal halide perovskites represent a flourishing area of research, which is driven by both their potential application in photovoltaics and optoelectronics and by the fundamental science behind their unique optoelectronic properties. The emergence of new colloidal methods for the synthesis of halide perovskite nanocrystals, as well as the interesting characteristics of this new type of material, has attracted the attention of many researchers. This review aims to provide an up-to-date survey of this fast-moving field and will mainly focus on the different colloidal synthesis approaches that have been developed. We will examine the chemistry and the capability of different colloidal synthetic routes with regard to controlling the shape, size, and optical properties of the resulting nanocrystals. We will also provide an up-to-date overview of their postsynthesis transformations, and summarize the various solution processes that are aimed at fabricating halide perovskite-based nanocomposites. Furthermore, we will review the fundamental optical properties of halide perovskite nanocrystals by focusing on their linear optical properties, on the effects of quantum confinement, and on the current knowledge of their exciton binding energies. We will also discuss the emergence of nonlinear phenomena such as multiphoton absorption, biexcitons, and carrier multiplication. Finally, we will discuss open questions and possible future directions.

Consecutive Interfacial Transformation of Cesium Lead Halide Nanocubes to Ultrathin Nanowires with Improved Stability

Although all-inorganic CsPbX₃ (X = Cl, Br, I) nanocrystals (NCs) have been considered as a promising material for photoelectronic devices, their applications are still limited because of their poor stability and the lack of in-depth understanding. Here, we demonstrate a post-treatment method for the preparation of ultrathin CsPbX₃ nanowires (NWs) by treating CsPbBr₃ nanocubes with thiourea solution. A systematic study showed a consecutive interfacial transformation process, in which CsPbBr₃ nanocubes were first converted to Cs₄PbBr₆ NCs in the presence of thiourea, followed by a further transformation to CsPbBr₃ NCs through an interfacial CsX-stripping process. To reduce the surface energy, an oriented attachment process has been realized and CsPbBr₃ NCs aggregated to form ultrathin NWs. The ultrathin CsPbBr₃ NWs exhibited high photoluminescence quantum yield (up to 60%) and high resistance to water treatment, which can be attributed to the surface passivation by thiourea. In addition to thiourea, cysteine and thioacetamide that contain the thiol group can also be used to trigger this transformation. This work can not only offer a facile method for the synthesis of efficient and stable ultrathin CsPbBr₃ NWs but also help to reveal the in-depth mechanisms which may be very useful in the field of metal halide perovskite NCs.

An overview on enhancing the stability of lead halide perovskite quantum dots and their applications in phosphor-converted LEDs

Beyond the unprecedented success achieved in photovoltaics (PVs), lead halide perovskites (LHPs) have shown great potential in other optoelectronic devices. Among them, nanometer-scale perovskite quantum dots (PQDs) with fascinating optical properties including high brightness, tunable emission wavelength, high color purity, and high defect tolerance have been regarded as promising alternative down-conversion materials in phosphor-converted light-emitting diodes (pc-LEDs) for lighting and next-generation of display technology. Despite the promising applications of perovskite materials in various fields, they have received strong criticism for the lack of stability. The poor stability has also attracted much attention. Within a few years, numerous strategies towards enhancing the stability have been developed. This review summarizes the mechanisms of intrinsic- and extrinsic-environment-induced decomposition of PQDs. Simultaneously, the strategies for improving the stability of PQDs are reviewed in detail, which can be classified into four types: (1) compositional engineering; (2) surface engineering; (3) matrix encapsulation; (4) device encapsulation. Finally, the challenges for applying PQDs in pc-LEDs are highlighted, and some possible solutions to improve the stability of PQDs together with suggestions for further improving the performance of pc-LEDs as well as the device lifetime are provided.

Cs4PbBr6/CsPbBr3 perovskite composites for WLEDs: pure white, high luminous efficiency and tunable color temperature

Cs₄PbBr₆/CsPbBr₃ perovskite composites are fabricated by room-temperature one-pot mixing synthesis, which is short in time, free from inert gases and delivers a high product yield. Temperature-dependent photoluminescence shows that a larger exciton binding energy of 291.1 meV exhibits better thermal stability compared with that of pure Cs₄PbBr₆ and CsPbBr₃ materials. The CIE chromaticity coordinates (0.1380, 0.7236) of green LEDs designed with Cs₄PbBr₆/CsPbBr₃ perovskite composites show almost no variation under driving current changing from 5 to 30 mA. Furthermore, the ground Cs₄PbBr₆/CsPbBr₃ perovskite composites mixed with red emitting K₂SiF₆:Mn⁴⁺ phosphor are dropped and casted on a blue-emitting InGaN chip. The white light emitting diodes (WLEDs) are presented, which have good luminous efficiency of 65.33 lm W⁻¹ at 20 mA, a correlated color temperature of 5190 K, and the white gamut with chromaticity coordinate of (0.3392, 0.3336). According to the state of art, these excellent characteristics observed are much superior to the reported results of conventional perovskite-based WLEDs, which demonstrate the immense potential and great prospect of Cs₄PbBr₆/CsPbBr₃ perovskite composites to replace conventional phosphors in lighting devices.

WLED devices are designed with high luminous efficiency of 65.33 lm W⁻¹ and excellent CIE chromaticity coordinates of (0.3392, 0.3336). The properties of material and the luminous performance of device are calculated and discussed comprehensively.

Highly fluorescent dual-emission red carbon dots and their applications in optoelectronic devices and water detection

Synthesis of dual-emission nitrogen and sulfur co-doped (N,S)-CDs with a quantum yield up to 29.7% and their applications in white LEDs and water detection.