From Nonluminescent Cs4PbX6 (X = Cl, Br, I) Nanocrystals to Highly Luminescent CsPbX3 Nanocrystals: Water-Triggered Transformation through a CsX-Stripping Mechanism

What Happens When Halide Perovskites Meet with Water?

Halide perovskites are considered to be next-generation semiconductor materials with bright prospects to advance the technology of photonics and optoelectronics. Because of the intrinsic ionic feature, the interactions between perovskites and water induce serious stability issues, which has been one of the fundamental problems hindering the practical application of perovskites. The degradation of halide perovskites upon water exposure has been intensively studied, resulting in chemical insights into key processes, including hydration, phase transformation, decomposition, and dissolution. In this Perspective, we try to illustrate what happens when halide perovskites meet with water. We summarize the research progress regarding the understanding of these processes and discuss the principle of strategy design toward improved stability against water. In addition to the instability-related interactions, we also discuss the aqueous solution of perovskite precursors for fabricating perovskite-based functional materials. Hopefully, this Perspective can inspire more fundamental studies on the interactions between perovskites and water, such as spectroscopy and simulation, crystal structure and material characterizations, and solution chemistry and crystallization.

Improving the Stability of Ball-Milled Lead Halide Perovskites via Ethanol/Water-Induced Phase Transition

Recently, lead halide perovskite nanocrystals have been considered as potential light-emitting materials because of their narrow full width at half-maximum (FWHM) and high photoluminescence quantum yield (PLQY). In addition, they have various emission spectra because the bandgap can be easily tuned by changing the size of the nanocrystals and their chemical composition. However, these perovskite materials have poor long-term stability due to their sensitivity to moisture. Thus far, various approaches have been attempted to enhance the stability of the perovskite nanocrystals. However, the required level of stability in the mass production process of perovskite nanocrystals under ambient conditions has not been secured. In this work, we developed a facile two-step ball-milling and ethanol/water-induced phase transition method to synthesize stable CsPbBr3 perovskite materials. We obtained pure CsPbBr3 perovskite solutions with stability retention of 86% for 30 days under ambient conditions. Our materials show a high PLQY of 35% in solid films, and excellent thermal stability up to 80 °C. We believe that our new synthetic method could be applicable for the mass production of light-emitting perovskite materials.

Integration of Highly Luminescent Lead Halide Perovskite Nanocrystals on Transparent Lead Halide Nanowire Waveguides through Morphological Transformation and Spontaneous Growth in Water

The integration of highly luminescent CsPbBr₃ quantum dots on nanowire waveguides has enormous potential applications in nanophotonics, optical sensing, and quantum communications. On the other hand, CsPb₂ Br₅ nanowires have also attracted a lot of attention due to their unique water stability and controversial luminescent property. Here, the growth of CsPbBr₃ nanocrystals on CsPb₂ Br₅ nanowires is reported first by simply immersing CsPbBr₃ powder into pure water, CsPbBr_{3- γ}  X_γ (X = Cl, I) nanocrystals on CsPb₂ Br_{5 -γ}  X_γ nanowires are then synthesized for tunable light sources. Systematic structure and morphology studies, including in situ monitoring, reveal that CsPbBr₃ powder is first converted to CsPb₂ Br₅ microplatelets in water, followed by morphological transformation from CsPb₂ Br₅ microplatelets to nanowires, which is a kinetic dissolution-recrystallization process controlled by electrolytic dissociation and supersaturation of CsPb₂ Br₅ . CsPbBr₃ nanocrystals are spontaneously formed on CsPb₂ Br₅ nanowires when nanowires are collected from the aqueous solution. Raman spectroscopy, combined photoluminescence, and SEM imaging confirm that the bright emission originates from CsPbBr_{3 -γ}  X_γ nanocrystals while CsPb₂ Br_{5 -γ}  X_γ nanowires are transparent waveguides. The intimate integration of nanoscale light sources with a nanowire waveguide is demonstrated through the observation of the wave guiding of light from nanocrystals and Fabry-Perot interference modes of the nanowire cavity.

Stokes Shift in Inorganic Lead Halide Perovskites: Current Status and Perspective

Inorganic metal halide perovskite system is considered as a promising candidate for applications from display to biomedical industry. Intrinsic inorganic lead halides possess small Stokes shift or self-absorption, providing negative impact for both photo voltaic and biomedical applications. Therefore, the development of an inorganic halide perovskite system with large Stokes shift is a significant venture. This review aims to provide an updated survey of the Stokes shift phenomena in the inorganic lead halide perovskites. The first section focuses about the mechanism, the second section gives different approaches in preparing inorganic perovskites with distinct Stokes shift, while the third section highlights the potential applications in both photovoltaic and biomedical areas. This review provides deep insight about the importance and usefulness of such phenomena in inorganic lead halides, essential for various applications.

Embedded growth of colorful CsPbX3(X = Cl, Br, I) nanocrystals in metal-organic frameworks at Room Temperature

Herein, we develop a novel strategy for preparing all-inorganic cesium lead halide (CsPbX₃, X = Cl, Br, I) perovskite nanocrystals (NCs)@Zn-based metal-organic framework (MOF) composites through interfacial synthesis. The successful embedding of fluorescent perovskite NCs in Zn-MOFs is due to thein situconfined growth, which is attributed to the re-nucleation of water-triggered phase transformation from Cs₄PbBr₆to CsPbBr₃. The controllable synthesis of mixed-halide based composites with various emission wavelength can be achieved by adding the desired amount of halide (Cl or I) salts in the re-nucleation process. More importantly, the anion exchange reaction is inhibited among various composites with different halogen atoms by being trapped in MOFs. Besides, a white light-emitting diode (WLED) is produced using a blue LED chip with the green-emitting and red-emitting composites, which has a color coordinate of (0.3291, 0.3272) and a wide color gamut. This work provides a novel route to achieving perovskite NCs growth in MOFs, which also can be extended to the other NCs embedded in frames as well.

Phase Control of Cs-Pb-Br Derivatives to Suppress 0D Cs4 PbBr6 for High-Efficiency and Stable All-Inorganic CsPbBr3 Perovskite Solar Cells

The precise phase control of Cs-Pb-Br derivatives from 3D CsPbBr₃ to 0D Cs₄ PbBr₆ highly determines the photovoltaic performance of all-inorganic CsPbBr₃ perovskite solar cells (PSCs). Herein, the preferred phase conversion from precursor to Cs-Pb-Br derivatives is revealed by theoretically calculating the Gibbs free energies (∆G) of various phase conversion processes, allowing for a simplified multi-step solution-processable spin-coating method to hinder the formation of detrimental 0D Cs₄ PbBr₆ phase and enhance the photovoltaic performance of a PSC because of its large exciton binding energy, which is regarded as a recombination center. By further accelerating the interfacial charge extraction with a novel 2D transition metal dichalcogenide ReSe₂ , the hole-free CsPbBr₃ PSC achieves a champion efficiency of 10.67% with an impressive open-circuit voltage of 1.622 V and an excellent long-term stability. This work provides an in-depth understanding on the precise Cs-Pb-Br perovskite phase control and the effect of derivatives on photovoltaic performance of advanced CsPbBr₃ PSCs.

Fabry-Perot Mode-Limited High-Purcell-Enhanced Spontaneous Emission from In Situ Laser-Induced CsPbBr3 Quantum Dots in CsPb2Br5 Microcavities

The patterned metal halide perovskites exhibit novel photophysical properties and high performance in photonic applications. Here, we show that a UV continuous wave laser can induce in situ crystallization of individual and patterned CsPbBr₃ quantum dots (QDs) inside the CsPb₂Br₅ microplatelets. The microplatelet acts as a natural Fabry-Perot cavity and causes the high-Purcell-effect-enhanced (by 287 times) cavity mode spontaneous emission of the embedded CsPbBr₃ QDs. The luminescence exhibits a superlinear emission intensity-excitation intensity relation I(p) ∝ p^2.83, and the exponent is much bigger than that of the free-space exciton spontaneous emission, suggesting arising of stimulated emission at higher photon concentrations. These laser-driven crystallized and patterned cavity mode luminescent perovskite QDs in a waterproof wider-bandgap perovskite microcavity act as an ideal platform for studying the cavity quantum electrodynamics phenomena and for applications in information storage and encryption, anticounterfeiting, and low-threshold lasers.

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.

Equilibriums in Formation of Lead Halide Perovskite Nanocrystals

Physical insights related to ion equilibrium involved in the synthesis of lead halide perovskite nanocrystals remain key parameters for regulating the phase stability and luminescence intensity of these emerging materials. These have been extensively studied since the development of these nanocrystals, and different reaction processes controlling the formation of CsPbX₃ nanocrystals are largely understood. However, growth kinetics related to the formation of these nanocrystals have not been established yet. Hence, more fundamental understanding of the formation processes of these nanocrystals is urgently required. Keeping these in mind and emphasizing the most widely studied nanocrystals of CsPbBr₃, different equilibrium processes involved in their synthesis for phase and composition variations are summarized and discussed in this Perspective. In addition, implementations of these findings for shape modulations by growth are discussed, and several new directions of research for understanding more fundamental insights are also presented.

Enhancing the stability of CsPbX3 (X = Br, I) through combination with Y-zeolites for WLED application

The stability of perovskite quantum dots (PQDs) plays a vital role in practical devices. Besides silica coating, embedding PQDs in zeolites is another strategy to improve their stability significantly. Although the zeolite rigid framework has been reported to protect PQDs, there are few reports on the in situ synthesis of PQDs in zeolites. In this work, cubic phase CsPbX₃ (X = Br, I) nanocrystals were successfully prepared by the ion exchange method combined with a non-polar organic trigger. Dropping a certain amount of ZnM₂ (M = Br, I) solution into the intermediate product Cs₄PbCl₆ nanocrystals resulted in the formation of the final CsPbX₃ nanocrystals. The ZnM₂ solutions were prepared in non-polar solvents (hexane, octane, or chloroform). The highest photoluminescence quantum yield (PLQY) of the synthesized CsPbX₃@zeolite composites can reach 83%, with a lifetime of 1.37 μs. The stability of the CsPbX₃@zeolite composites thin film against damage from air and light is significantly improved. We fabricated white light-emitting diodes (WLEDs) using CsPbBr₃@zeolite as the green light source and CsPbI₃@zeolite as the red light source to further emphasize the practical application effect of the CsPbX₃@zeolite composites. This work not only provides a new method for the synthesis of CsPbX₃ nanocrystals but also involves the in situ synthesis of high stability CsPbX₃@zeolite composites within the zeolite.

Spatially Ordered Arrays of Colloidal Inorganic Metal Halide Perovskite Nanocrystals via Controlled Droplet Evaporation in a Confined Geometry

Inorganic metal halide perovskite nanocrystals, such as quantum dots (QDs), have emerged as intriguing building blocks for miniaturized light-emitting and optoelectronic devices. Although conventional lithographic approaches and printing techniques allow for discrete patterning at the micro/nanoscale, it is still important to utilize intrinsic QDs with the concomitant retaining of physical and chemical stability during the fabrication process. Here, we report a simple strategy for the evaporative self-assembly to produce highly ordered structures of CsPbBr₃ and CsPbI₃ QDs on a substrate in a precisely controllable manner by using a capillary-bridged restrict geometry. Quantum confined CsPbBr₃ and CsPbI₃ nanocrystals, synthesized via a modified hot-injection method with excess halide ions condition, were readily adapted to prepare colloidal QD solutions. Subsequently, the spatially patterned arrays of the perovskite QD rings were crafted in a confirmed geometry with high fidelity by spontaneous solvent evaporation. These self-organized concentric rings were systemically characterized regarding the center-to-center distance, width, and height of the patterns. Our results not only facilitate a fundamental understanding of assembly in the perovskite QDs to enable the solution-printing process but also provide a simple route for offering promising practical applications in optoelectronics.

Current status on synthesis, properties and applications of CsPbX3(X = Cl, Br, I) perovskite quantum dots/nanocrystals

The quantum confinement effect and interesting optical properties of cesium lead halide (CsPbX₃; X = Cl, Br, I) perovskite quantum dots (QDs) and nanocrystals (NCs) have given a new horizon to lighting and photonic applications. Given the exponential rate at which scientific results on CsPbX₃NCs are published in the last few years, it can be expected that the research in CsPbX₃NCs will further receive increasing scientific interests in the near future and possibly lead to great commercial opportunities to realize these materials based practical applications. With the rapid progress in the single-photon emitting CsPbX₃QDs and NCs, practical applications of the quantum technologies such as single-photon emitting light-emitting diode, quantum lasers, quantum computing might soon be possible. But to reach at cutting edge of stable perovskite QDs/NCs, the study of fundamental insight and theoretical aspects of crystal design is yet insufficient. Even more, it has aroused many unanswered questions related to the stability, optical and electronic properties of the CsPbX₃QDs. Aim of the present review is to illustrate didactically a precise study of recent progress in the synthesis, properties and applications of CsPbX₃QDs and NCs. Critical issues that currently restrict the applicability of these QDs will be identified and advanced methodologies currently in the developing queue, to overcome the roadblock, will be presented. And finally, the prospects for future directions will be provided.

Reversible transformation of all-inorganic copper halide perovskite nanocrystals for anti-counterfeiting

This work reports a CsI stripping/insertion process that enables the reversible transformation between blue-emissive Cs₃Cu₂I₅ and yellow-emissive CsCu₂I₃ upon moisture/evaporation treatment. The successful transformation can be ascribed to the unique space confinement of the SiO₂ matrix and ligand-free feature of perovskite nanocrystals, which can be a good candidate for anti-counterfeiting.

Water-Dispersible CsPbBr3 Perovskite Nanocrystals with Ultra-Stability and its Application in Electrochemical CO2 Reduction

Thanks to the excellent optoelectronic properties, lead halide perovskites (LHPs) have been widely employed in high-performance optoelectronic devices such as solar cells and light-emitting diodes. However, overcoming their poor stability against water has been one of the biggest challenges for most applications. Herein, we report a novel hot-injection method in a Pb-poor environment combined with a well-designed purification process to synthesize water-dispersible CsPbBr₃ nanocrystals (NCs). The as-prepared NCs sustain their superior photoluminescence (91% quantum yield in water) for more than 200 days in an aqueous environment, which is attributed to a passivation effect induced by excess CsBr salts. Thanks to the ultra-stability of these LHP NCs, for the first time, we report a new application of LHP NCs, in which they are applied to electrocatalysis of CO₂ reduction reaction. Noticeably, they show significant electrocatalytic activity (faradaic yield: 32% for CH₄, 40% for CO) and operation stability (> 350 h).

State of the Art and Prospects for Halide Perovskite Nanocrystals

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

Size-tunable and stable cesium lead-bromide perovskite nanocubes with near-unity photoluminescence quantum yield

Stable cesium lead bromide perovskite nanocrystals (NCs) showing a near-unity photoluminescence quantum yield (PLQY), narrow emission profile, and tunable fluorescence peak in the green region can be considered the ideal class of nanomaterials for optoelectronic applications. However, a general route for ensuring the desired features of the perovskite NCs is still missing. In this paper, we propose a synthetic protocol for obtaining near-unity PLQY perovskite nanocubes, ensuring their size control and, consequently, a narrow and intense emission through the modification of the reaction temperature and the suitable combination ratio of the perovskite constituting elements. The peculiarity of this protocol is represented by the dissolution of the lead precursor (PbBr₂) as a consequence of the exclusive complexation with the bromide anions released by the in situ SN₂ reaction between oleylamine (the only surfactant introduced in the reaction mixture) and 1-bromohexane. The obtained CsPbBr₃ nanocubes exhibit variable size (ranging from 6.7 ± 0.7 nm to 15.2 ± 1.2 nm), PL maxima between 505 and 517 nm, and near-unity PLQY with a narrow emission profile (fwhm of 17-19 nm). Additionally, the NCs synthesized with this approach preserve their high PLQYs even after 90 days of storage under ambient conditions, thus displaying a remarkable optical stability. Through the rationalization of the obtained results, the proposed synthetic protocol provides a new ground for the direct preparation of differently structured perovskite NCs without resorting to any additional post-synthetic treatment for improving their emission efficiency and stability.

Water-Triggered Transformation of Ligand-Free Lead Halide Perovskite Nanocrystal-Embedded Pb(OH)Br with Ultrahigh Stability

Lead halide perovskite (LHP) nanomaterials have attracted tremendous attention owing to their remarkable optoelectronic properties. However, they are extremely unstable under moist environments, high temperatures, and light illumination due to their intrinsic structural lability, which has been the critical unsolved problem for practical applications. To address this issue, we propose a facile and environmentally friendly ligand-free approach to design and synthesize rod-like CsPb₂Br₅-embedded Pb(OH)Br with excellent stability under various harsh environments such as soaking in water, heating, and ultraviolet (UV) illumination. Plate-like CsPbBr₃- and Cs₄PbBr₆-embedded Pb(OH)Br powders are first formed by evaporating the solvent in a dispersion of ethanol (or methanol, isopropanol), Cs₂CO₃, and PbBr₂. Upon soaking in water, the plate-like sample undergoes phase transformation from CsPbBr₃ and Cs₄PbBr₆ to CsPb₂Br₅ and shape conversion from nanoplate to a microrod, leading to the formation of rod-like CsPb₂Br₅-embedded Pb(OH)Br. The stable Pb(OH)Br coating effectively prevents the luminescent CsPb₂Br₅ nanocrystals from reacting with water, leading to extremely high aqueous stability of the CsPb₂Br₅-embedded Pb(OH)Br. The photoluminescence (PL) intensity of the representative CsPb₂Br₅-embedded Pb(OH)Br sample can maintain 92.2% of the initial PL intensity value even after soaking in room-temperature water for 165 days; in the meantime, the phase and shape are preserved. The typical sample also shows outstanding stability under hot water, UV illumination, and annealing conditions. The ultrahigh aqueous stability, thermal stability, and photostability of the CsPb₂Br₅-embedded Pb(OH)Br nanomaterials suggest an effective, facile, and environmentally friendly technique to grow perovskite-based nanomaterials for promising practical applications in the optoelectronic field.

Dual-Protected Metal Halide Perovskite Nanosheets with an Enhanced Set of Stabilities

Approaches to achieve stable perovskite nanocrystals (PNCs) of interest, in particular those with large structural anisotropy, through protective coating of the inorganic shell at a single-nanocrystal (NC) level are comparatively few and limited in scope. Reported here is a robust amphiphilic-diblock-copolymer-enabled strategy for crafting highly-stable anisotropic CsPbBr₃ nanosheets (NSs) by in situ formation of a uniform inorganic shell (1st shielding) that is intimately ligated with hydrophobic polymers (2nd shielding). The dual-protected NSs display an array of remarkable stabilities (i.e., thermal, photostability, moisture, polar solvent, aliphatic amine, etc.) and find application in white-light-emitting diodes. In principle, by anchoring other multidentate amphiphilic polymer ligands on the surface of PNCs, followed by templated-growth of shell materials of interest, a rich variety of dual-shelled, multifunctional PNCs with markedly improved stabilities can be created for use in optics, optoelectronics, and sensory devices.

Effect of Surface Ligands in Perovskite Nanocrystals: Extending in and Reaching out

The rediscovery of the halide perovskite class of compounds and, in particular, the organic and inorganic lead halide perovskite (LHP) materials and lead-free derivatives has reached remarkable landmarks in numerous applications. First among these is the field of photovoltaics, which is at the core of today’s environmental sustainability efforts. Indeed, these efforts have born fruit, reaching to date a remarkable power conversion efficiency of 25.2% for a double-cation Cs, FA lead halide thin film device. Other applications include light and particle detectors as well as lighting. However, chemical and thermal degradation issues prevent perovskite-based devices and particularly photovoltaic modules from reaching the market. The soft ionic nature of LHPs makes these materials susceptible to delicate changes in the chemical environment. Therefore, control over their interface properties plays a critical role in maintaining their stability. Here we focus on LHP nanocrystals, where surface termination by ligands determines not only the stability of the material but also the crystallographic phase and crystal habit. A surface analysis of nanocrystal interfaces revealed the involvement of Brønsted type acid–base equilibrium in the modification of the ligand moieties present, which in turn can invoke dissolution and recrystallization into the more favorable phase in terms of minimization of the surface energy. A large library of surface ligands has already been developed showing both good chemical stability and good electronic surface passivation, resulting in near-unity emission quantum yields for some materials, particularly CsPbBr₃. However, most of those ligands have a large organic tail hampering charge carrier transport and extraction in nanocrystal-based solid films.

The unique perovskite structure that allows ligand substitution in the surface A (cation) sites and the soft ionic nature is expected to allow the accommodation of large dipoles across the perovskite crystal. This was shown to facilitate electron transfer across a molecular linked single-particle junction, creating a large built-in field across the junction nanodomains. This strategy could be useful for implementing LHP NCs in a p–n junction photovoltaic configuration as well as for a variety of electronic devices. A better understanding of the surface propeties of LHP nanocrystals will also enable better control of their growth on surfaces and in confined volumes, such as those afforded by metal–organic frameworks, zeolites, or chemically patterened surfaces such as anodic alumina, which have already been shown to significantly alter the properties of in-situ-grown LHP materials.

Manipulation of Interfacial Diffusion for Controlling Nanoscale Transformation

The unprecedented development of inorganic nanostructure synthesis has paved the way toward their broad applications in areas such as food science, agroforestry, energy conversion, and biomedicine. The precise manipulation of the nucleation and subsequent growth has been recognized as the central guiding principle for controlling the size and morphology of the nanostructures. However, conventional colloid syntheses based on direct precipitation reactions still have limitations in their versatility and extendibility. The crystal structure of a material determines the limited number of possible morphologies that its nanostructures can adopt. Further, as nucleation and growth kinetics are sensitive to not only the nature of the precipitation reactions but also ligands and ripening effect, rigorous control of reaction conditions must be established for every specific synthesis. In addition, multiple experimental parameters are entangled with each other, thereby requiring rigorous control of all reaction conditions. As a result, it is usually challenging to extend a synthetic recipe from one material to another. As an alternative method, the direct transformation of existing nanostructures into target ones has become an effective and robust approach capable of creating various complex nanostructures that are otherwise challenging to obtain using conventional methods. To this end, an in-depth understanding of nanoscale transformation toward the synthesis of inorganic nanostructures with diverse properties and applications is highly desirable.In this Account, we aim to reveal the critical effect of the interfacial diffusion on controlled nanoscale transformation. We first discuss how the interdiffusion rates determine the morphology and properties of bimetallic nanostructures. While equal interdiffusion rates lead to perfect mixing and generate fully alloyed nanostructures, interdiffusion at unequal rates creates vacancies in the fast diffusion side, which may cause dramatic morphological transformation to the nanostructures. Then, we introduce interfacial reactions, including the Kirkendall cavitation process, elimination reaction, and solid-state reaction, to promote the unbalanced interdiffusion and generalize nanoscale transformations in materials of various compositions, morphologies, and crystal structures. Finally, we discuss the use of capping ligands to inhibit the diffusion of atoms on one side of the interface in order to enable selective etching or transformation of the nanostructures. By modifying the nanostructured surface with specific capping ligands, the diffusion of surface atoms is restricted. When nanoparticles undergo chemical reactions (such as etching or heating), the outward diffusion of substances dominates, thereby successfully achieving chemical and morphological transformations. We believe that controlled interfacial diffusion can effectively manipulate nanoscale transformations, thus providing new strategies for the custom synthesis of multifunctional nanomaterials for various specific applications.

Exploiting the Lability of Metal Halide Perovskites for Doping Semiconductor Nanocomposites

Cesium lead halides have intrinsically unstable crystal lattices and easily transform within perovskite and nonperovskite structures. In this work, we explore the conversion of the perovskite CsPbBr₃ into Cs₄PbBr₆ in the presence of PbS at 450 °C to produce doped nanocrystal-based composites with embedded Cs₄PbBr₆ nanoprecipitates. We show that PbBr₂ is extracted from CsPbBr₃ and diffuses into the PbS lattice with a consequent increase in the concentration of free charge carriers. This new doping strategy enables the adjustment of the density of charge carriers between 10¹⁹ and 10²⁰ cm^-3, and it may serve as a general strategy for doping other nanocrystal-based semiconductors.

Detection of Pb2+ traces in dispersion of Cs4PbBr6 nanocrystals by in situ liquid cell transmission electron microscopy

The Cs4PbBr6 nanocrystals are often used as a starting material for the preparation of green-emitting CsPbBr3 perovskite nanocrystals by means of chemical and physical transformations. Herein, we probe the Cs4PbBr6 nanocrystals dispersed in a solvent by liquid cell transmission electron microscopy (LCTEM). The nanocrystal dispersion in toluene is placed between two electron-transparent membranes separated by a gold spacer in a liquid cell and studied in a high angular annular dark-field scanning TEM mode with a fixed electron dose rate. We observe the spontaneous nucleation and growth of round and dendrite-shaped nanoparticles under electron beam illumination in the areas of solution where no Cs4PbBr6 nanocrystals are seen. These newly-formed nanoparticles show high contrast and contain Pb as the only heavy element, suggesting that they are made from metallic lead and indicating Pb2+-containing species in solution as their precursor. Also, a small amount of Au0 nanoparticles are formed, most likely due to the dissolution of the gold spacer by free Br-containing species in the nanocrystal dispersion and a subsequent reduction of the leached species under the electron beam. The analysis of the UV-Vis absorption spectra of Cs4PbBr6 nanocrystals and the supernatant isolated from the synthesis points to mixed lead(ii) oleate/bromide species as the likely residue, corroborating LCTEM results. The identification of the residual precursors in Cs4PbBr6 nanocrystal samples after the post-synthetic isolation is an important task because the residues may alter the subsequent reactivity of the nanocrystals.

Synthesis of Au or Pt@Perovskite Nanocrystals via Interfacial Photoreduction

The surface modification of perovskite nanocrystals (NCs) (i.e., their decoration with noble metals) holds great promise with respect to the tailoring of their properties but has remained a challenge because perovskite NCs are extremely sensitive to water and alcohols. In this study, Au or Pt@CsPbBr3 NCs were successfully synthesized by photoreduction at the water/hexane interface. First, Cs4PbBr6 NCs were synthesized through the hot-injection method. Then, Cs4PbBr6 was transformed into CsPbBr3 and subjected to noble metal modification, both at the interface. The synthesized CsPbBr3 NCs exhibited a cubic perovskite phase and had an average size of approximately 13.5 nm. The deposited Au and Pt nanoparticles were crystalline, with a face-centered cubic lattice and average diameters of approximately 3.9 and 4.4 nm, respectively. The noble metal modification process had almost no effect on the steady-state photoluminescence (PL) emission wavelength but affected the charge-recombination kinetics of the CsPbBr3 NCs. Time-resolved PL decay spectral analysis indicated that the fluorescence lifetimes of the Au and Pt@CsPbBr3 NCs were shorter than those of the pure CsPbBr3 NCs, probably owing to the quenching of the free charges because of electron transfer from the perovskite to the noble metal nanoparticles.

Water Triggered Synthesis of Highly Stable and Biocompatible 1D Nanowire, 2D Nanoplatelet, and 3D Nanocube CsPbBr3 Perovskites for Multicolor Two-Photon Cell Imaging

Two-photon imaging in the near-infrared window holds huge promise for real life biological imaging due to the increased penetration depth. All-inorganic CsPbX3 nanocrystals with bright luminescence and broad spectral tunability are excellent smart probes for two-photon bioimaging. But, the poor stability in water is a well-documented issue for limiting their practical use. Herein, we present the development of specific antibody attached water-resistant one-dimensional (1D) CsPbBr3 nanowires, two-dimensional (2D) CsPbBr3 nanoplatelets, and three-dimensional (3D) CsPbBr3 nanocubes which can be used for selective and simultaneous two-photon imaging of heterogeneous breast cancer cells in the near IR biological window. The current manuscript reports the design of excellent photoluminescence quantum yield (PLQY), biocompatible and photostable 1D CsPbBr3 nanowires, 2D CsPbBr3 nanoplatelets, and 3D CsPbBr3 nanocubes through an interfacial conversion from zero-dimensional (0D) Cs4PbBr6 nanocrystals via a water triggered strategy. Reported data show that just by varying the amount of water, one can control the dimension of CsPbBr3 perovskite crystals. Time-dependent transition electron microscopy and emission spectra have been reported to find the possible pathway for the formation of 1D, 2D, and 3D CsPbBr3 nanocrystals from 0D Cs4PbBr6 nanocrystals. Biocompatible 1D, 2D, and 3D CsPbBr3 nanocrystals were developed by coating with amine-poly(ethylene glycol)-propionic acid. Experimental data show the water-driven design of 1D, 2D, and 3D CsPbBr3 nanocrystals exhibits strong single-photon PLQY of ∼66-88% as well as excellent two-photon absorption properties (σ2) of ∼8.3 × 105-7.1 × 104 GM. Furthermore, reported data show more than 86% of PL intensity remains for 1D, 2D, and 3D CsPbBr3 nanocrystals after 35 days under water, and they exhibit excellent photostability of keeping 99% PL intensity after 3 h under UV light. The current report demonstrates for the first time that antibody attached 1D and 2D perovskites have capability for simultaneous two-photon imaging of triple negative breast cancer cells and human epidermal growth factor receptor 2 positive breast cancer cells. CsPbBr3 nanocrystals exhibit very high two-photon absorption cross-section and good photostability in water, which are superior to those of commonly used organic probes (σ2 = 11 GM for fluorescein), and therefore, they have capability to be a better probe for bioimaging applications.

Water-assisted synthesis of highly stable CsPbX3 perovskite quantum dots embedded in zeolite-Y

All-inorganic perovskite materials have emerged as highly promising materials for solar cells and photoelectronic applications. However, the poor stability of perovskites in ambient conditions significantly hampers their practical applications. In this work, we report a three-step synthesis of size tunable CsPbX₃ (X = Br, Cl, or I) quantum dots (QDs) embedded in zeolite-Y (CsPbX₃-Y), which involves efficient chemical transformation of non-luminescent Cs₄PbX₆ to highly luminescent CsPbX₃ by stripping CsX through an interfacial reaction with water. We show that the size and the emission of CsPbX₃ in CsPbX₃-Y can be tuned by the amount of water added as well as the halide composition. More importantly, the as-prepared CsPbX₃-Y show significantly enhanced stability against moisture upon protection by zeolite-Y. This work not only reports a new pathway for the preparation of highly luminescent CsPbX₃ but also provided new insights into the chemical transformation behavior and stabilization mechanism of these emerging perovskites.

Embedding CsPbX₃ in zeolite-Y results in significantly enhanced stability and tunable emission upon exposure to water.

Tailoring Optical Properties of Luminescent Semiconducting Nanocrystals through Hydrostatic, Anisotropic Static, and Dynamic Pressures

Luminescent semiconductor nanocrystals are a fascinating class of materials because of their size-dependent emissions. Numerous past studies have demonstrated that semiconductor nanoparticles with radii smaller than their Bohr radius experience quantum confinement and thus size-dependent emissions. Exerting pressure on these nanoparticles represents an additional, more dynamic, strategy to alter their size and shift their emission. The application of pressure results in the lattices becoming strained and the electronic structure altered. In this Minireview, colloidal semiconductor nanocrystals are first introduced. The effects of uniform hydrostatic pressure on the optical properties of metal halide perovskite (ABX₃ ), II-VI, III-V, and IV-VI semiconductor nanocrystals are then examined. The optical properties of semiconductor nanocrystals under static and dynamic anisotropic pressure are then summarized. Finally, future research directions and applications utilizing the pressure-dependent optical properties of semiconductor nanocrystals are discussed.

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.

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.

Nanoscale Encapsulation of Perovskite Nanocrystal Luminescent Films via Plasma-Enhanced SiO2 Atomic Layer Deposition

Photoluminescence perovskite nanocrystals (NCs) have shown significant potential in optoelectronic applications in view of their narrow band emission with high photoluminescence quantum yields and color tunability. The main obstacle for practical applications is to obtain high durability against an external environment. In this work, a low temperature (50 °C) plasma-enhanced atomic layer deposition (PE-ALD) protection strategy was developed to stabilize CsPbBr₃ NCs. Silica was employed as the encapsulation layer because of its excellent light transmission performance and water corrosion resistance. The growth mechanism of inorganic SiO₂ via PE-ALD was investigated in detail. The Si precursor bis(diethylamino)silane (BDEAS) reacted with the hydroxyl groups (-OH) and thereby initiated the subsequent silica growth while having minimal influence to the organic ligands and did not cause PL quenching. Subsequently, O₂ plasma with high reactivity was used to oxidize the amine ligands of the BDEAS precursor while did not etch the NCs. The obtained CsPbBr₃ NCs/SiO₂ film exhibited exceptional stability in water, light, and heat as compared to the pristine NC film. Based on this method, a white light-emitting diode with improved operational stability was successfully fabricated, which exhibited a wide color gamut (∼126% of the National Television Standard Committee). Our work successfully demonstrates an efficient protection scheme via the PE-ALD method, which extends the applied range of other materials for stabilization of perovskite NCs through this approach.

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.

Metal Halide Perovskite Nanorods: Shape Matters

Quasi-1D metal halide perovskite nanorods (NRs) are emerging as a type of materials with remarkable optical and electronic properties. Research into this field is rapidly expanding and growing in the past several years, with significant advances in both mechanistic studies of their growth and widespread possible applications. Here, the recent advances in 1D metal halide perovskite nanocrystals (NCs) are reviewed, with a particular emphasis on NRs. At first, the crystal structures of perovskites are elaborated, which is followed by a review of the major synthetic approaches toward perovskite NRs, such as wet-chemical synthesis, substrate-assisted growth, and anion exchange reactions, and discussion of the growth mechanisms associated with each synthetic method. Then, thermal and aqueous stability and the linear polarized luminescence of perovskite NRs are considered, followed by highlighting their applications in solar cells, light-emitting diodes, photodetectors/phototransistors, and lasers. Finally, challenges and future opportunities in this rapidly developing research area are summarized.

Unveiling the Excited-State Dynamics of Mn2+ in 0D Cs4PbCl6 Perovskite Nanocrystals

Doping is an effective strategy for tailoring the optical properties of 0D Cs4PbX6 (X = Cl, Br, and I) perovskite nanocrystals (NCs) and expanding their applications. Herein, a unique approach is reported for the controlled synthesis of pure-phase Mn2+-doped Cs4PbCl6 perovskite NCs and the excited-state dynamics of Mn2+ is unveiled through temperature-dependent steady-state and transient photoluminescence (PL) spectroscopy. Because of the spatially confined 0D structure of Cs4PbCl6 perovskite, the NCs exhibit drastically different PL properties of Mn2+ in comparison with their 3D CsPbCl3 analogues, including significantly improved PL quantum yield in solid form (25.8%), unusually long PL lifetime (26.2 ms), large exciton binding energy, strong electron-phonon coupling strength, and an anomalous temperature evolution of Mn2+-PL decay from a dominant slow decay (in tens of ms scale) at 300 K to a fast decay (in 1 ms scale) at 10 K. These findings provide fundamental insights into the excited-state dynamics of Mn2+ in 0D Cs4PbCl6 NCs, thus laying a foundation for future design of 0D perovskite NCs through metal ion doping toward versatile applications.

Aqueous Precursor Induced Morphological Change and Improved Water Stability of CsPbBr3 Nanocrystals

In the literature, lead halide perovskites are very notable for their degradation in the presence of polar solvents, such as water. In contrast, in this research, it is observed that adding a minor amount of water into the precursor solution can improve the stability and photoluminescence quantum yield of CsPbBr₃ nanocrystals through a ligand-assisted reprecipitation (LARP) method. In this way, the shape and phase transformation from CsPbBr₃ nanoplates to CsPbBr₃ /Cs₄ PbBr₆ nanorods and Cs₄ PbBr₆ nanowires can be controlled with increasing water content in the precursor solution. Upon adding water up to an ideal amount, CsPbBr₃ maintains its phase and nanoplate morphology. The key role of water amount for tuning the crystallinity, stability, morphology, optical properties, and phase transformation of cesium lead halide perovskite nanocrystals will be beneficial in the future commercialization of optoelectronics.

Metal-Halide Perovskite Design for Next-Generation Memories: First-Principles Screening and Experimental Verification

Memory devices have been advanced so much, but still it is highly required to find stable and reliable materials with low-power consumption. Halide perovskites (HPs) have been recently adopted for memory application since they have advantages of fast switching based on ionic motion in crystal structure. However, HPs also suffer from poor stability, so it is necessary to improve the stability of HPs. In this regard, combined first-principles screening and experimental verification are performed to design HPs that have high environmental stability and low-operating voltage for memory devices. First-principles screening identifies 2D layered AB2X5 structure as the best candidate switching layer for memory devices, because it has lower formation energy and defect formation energy than 3D ABX3 or other layered structures (A3B2X7, A2BX4). To verify results, all-inorganic 2D layered CsPb2Br5 is synthesized and used in memory devices. The memory devices that use CsPb2Br5 show much better stability and lower operating voltages than devices that use CsPbBr3. These findings are expected to provide new opportunity to design materials for reliable device applications based on calculation, screening, and experimental verification.

Long-armed hexapod nanocrystals of cesium lead bromide

Colloidal lead halide perovskite nanocrystals (LHP NCs) assume a variety of morphologies (e.g. cubes, sheets, and wires). Their labile structural and surface characters allow them to undergo post-synthetic evolution of shape and crystallographic characters. Such transformations can be advantageous or deleterious, and it is therefore vital to both understand and exert control over these processes. In this study, we report novel long-armed hexapod structures of cesium lead bromide nanocrystals. These branched structures evolve from quantum-confined CsPbBr3 nanosheets to Cs4PbBr6 hexapods over a period of 24 hours. Time-resolved optical and structural characterization reveals a post-synthesis mechanism of phase transformation, oriented attachment and branch elongation. More generally, the study reveals important processes associated with LHP NC aging and demonstrates the utility of slow reaction kinetics in obtaining complex morphologies.

The Stability of Metal Halide Perovskite Nanocrystals-A Key Issue for the Application on Quantum-Dot-Based Micro Light-Emitting Diodes Display

The metal halide perovskite nanocrystal (MHP-NC), an easy-to-fabricate and low cost fluorescent material, is recognized to be among the promising candidates of the color conversion material in the micro light-emitting diode (micro-LED) display, providing that the stability can be further enhanced. It is found that the water steam, oxygen, thermal radiation and light irradiation-four typical external factors in the ambient environment related to micro-LED display-can gradually alter and destroy the crystal lattice. Despite the similar phenomena of photoluminescence quenching, the respective encroaching processes related to these four factors are found to be different from one another. The encroaching mechanisms are collected and introduced in separate categories with respect to each external factor. Thereafter, a combined effect of these four factors in an environment mimicking real working conditions of micro-LED display are also introduced. Finally, recent progress on the full-color application of MHP-NC is also reviewed in brief.

Hydrochromic CsPbBr3 Nanocrystals for Anti-Counterfeiting

Hydrochromic materials that can reversibly change color upon water treatment have attracted much attention owing to their potential applications in diverse fields. Herein, for the first time, we report that space-confined CsPbBr₃ nanocrystals (NCs) are hydrochromic. When CsPbBr₃ NCs are loaded into a porous matrix, reversible transition between luminescent CsPbBr₃ and non-luminescent CsPb₂ Br₅ can be achieved upon the exposure/removal of water. The potential applications of hydrochromic CsPbBr₃ NCs in anti-counterfeiting are demonstrated by using CsPbBr₃ NCs@mesoporous silica nanospheres (around 100 nm) as the starting material. Owing to the small particle size and negatively charged surface, the as-prepared particles can be laser-jet printed with high precision and high speed. We demonstrate the excellent stability over repeated transformation cycles without color fade. This new discovery may not only deepen the understanding of CsPbX₃ , but also open a new way to design CsPbX₃ materials for new applications.

Improving the Quality and Luminescence Performance of All-Inorganic Perovskite Nanomaterials for Light-Emitting Devices by Surface Engineering

Lead halide perovskites and their applications in the optoelectronic field have garnered intensive interest over the years. Inorganic perovskites (IHP), though a novel class of material, are considered as one of the most promising optoelectronic materials. These materials are widely used in detectors, solar cells, and other devices, owing to their excellent charge-transport properties, high defect tolerance, composition- and size-dependent luminescence, narrow emission, and high photoluminescence quantum yield. In recent years, numerous encouraging achievements have been realized, especially in the research of CsPbX₃ (X = Cl, Br, I) nanocrystals (NCs) and surface engineering. Therefore, it is necessary to summarize the principles and effects of these surface engineering optimization methods. It is also important to scientifically guide the applications and promote the development of perovskites more efficiently. Herein, the principles of surface ligands are reviewed, and various surface treatment methods used in CsPbX₃ NCs as well as quantum-dot light-emitting diodes are presented. Finally, a brief outlook on CsPbX₃ NC surface engineering is offered, illustrating the present challenges and the direction in which future investigations are intended to obtain high-quality CsPbX₃ NCs that can be utilized in more applications.

Cation-doping matters in caesium lead halide perovskite nanocrystals: from physicochemical fundamentals to optoelectronic applications

All-inorganic caesium lead halide perovskite nanocrystals (PeNCs) with different dimensionalities have recently fascinated the research community due to their extraordinary optoelectronic properties including tunable bandgaps over the entire visible spectral region, high photoluminescence quantum yields (PLQYs) close to unity and narrow emission line widths down to 10-20 nm, making them particularly suitable as promising candidates for numerous applications ranging from light-emitting diodes (LEDs), solar cells to scintillators. Despite the considerable progress made in the past six years, the real-world applications of caesium lead halide PeNCs themselves especially in the category of CsPbX₃ (X = Cl, Br and I) are still restricted by their labile crystal lattices and downgraded luminescence when exposed to ambient air conditions. Recent experimental and theoretical studies on cation doping have proven to be an effective way to significantly improve the physicochemical properties of cesium lead halide PeNCs, which would have profound implications for a range of applications. In this review, we provide a brief overview of the most recent advances in cation-doped all-inorganic caesium lead halide PeNCs, aimed at developing high-performance and long-term stable optoelectronic and photovoltaic devices, which covers areas from their fundamental considerations of cation doping, controlled synthesis methodology and novel physicochemical properties to the optoelectronic applications with an emphasis on perovskite-based LEDs and solar cells. And finally, some possible directions of future efforts toward this active research field are also proposed.

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.

Transforming colloidal Cs4PbBr6 nanocrystals with poly(maleic anhydride-alt-1-octadecene) into stable CsPbBr3 perovskite emitters through intermediate heterostructures

The preparation of strongly emissive CsPbBr3 perovskite nanocrystals with robust surface passivation is a challenge in the field of lead halide perovskite nanomaterials. We report an approach to prepare polymer-capped CsPbBr3 perovskite nanocrystals by reacting oleylammonium/oleate-capped Cs4PbBr6 nanocrystals with poly(maleic anhydride-alt-1-octadecene) (PMAO). PMAO contains succinic anhydride units that are reactive towards the oleylamine species present on the Cs4PbBr6 nanocrystals' surface and produces polysuccinamic acid, which, in turn, triggers the Cs4PbBr6 to CsPbBr3 conversion. The transformation occurs through the formation of Cs4PbBr6-CsPbBr3 heterostructures as intermediates, which are captured because of the mild reactivity of PMAO and are investigated by high-resolution electron microscopy. The Cs4PbBr6-CsPbBr3 heterostructures demonstrate a dual emission at cryogenic temperature with an indication of the energy transfer from Cs4PbBr6 to CsPbBr3. The fully-transformed CsPbBr3 NCs have high photoluminescence quantum yield and enhanced colloidal stability, which we attribute to the adhesion of polysuccinamic acid to the NC surface through its multiple functional groups in place of oleate and alkylammonium ligands. The PMAO-induced transformation of Cs4PbBr6 NCs opens up a strategy for the chemical modification of metal halide NCs initially passivated with nucleophilic amines.

All-Inorganic CsPbBr3 Perovskite Films Prepared by Single Source Thermal Ablation

Hybrid organo-lead halide perovskites are becoming the benchmark material for next generation photovoltaics and a very important player for other applications such as photodetectors and light emitting diodes. Nevertheless, the most important issue hindering the large-scale application of these materials remains their intrinsic instability due to the organic cation. Although the substitution with inorganic cesium (Cs) enhances stability, in most cases solution deposition methods of fully inorganic perovskites result in high surface roughness and poor surface coverage. This work reports on the evaporation of the CsPbBr₃ precursor by Single Source Thermal Ablation, showing that just after deposition films consist of a mixture of CsPbBr₃, CsPb₂Br₅, and Cs₄PbBr₆ due to a vertical composition gradient. We point out that mild post deposition treatments lead to the conversion of CsPb₂Br₅ and Cs₄PbBr₆ into CsPbBr₃ due to its higher thermodynamic stability. Conversion results into smooth and pinhole-free CsPbBr₃ films with good light absorption and emission properties. We demonstrate the suitability of obtained films for planar devices by preparing perovskite-based pure-green light emitting diodes, thus promoting Single Source Thermal Ablation as a promising alternative deposition technique for all-inorganic perovskite-based devices.

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.

Low-temperature direct synthesis of perovskite nanocrystals in water and their application in light-emitting diodes

Cesium lead halide perovskite nanocrystals (PNCs) have aroused tremendous research attention because of their excellent optoelectronic properties. Herein, we developed a facile and green low-temperature strategy free of organic solvents, in which only pure water was adopted as the solvent, to synthesize CsPbBr3 NCs. Intriguingly, although formed with the assistance of water, the obtained CsPbBr3 NCs present a cubic crystal structure, photoluminescence quantum yield (PLQY) of 75%, and narrow emission line width for bright green emission. Furthermore, both electroluminescence (EL) and photoluminescence (PL)-based light-emitting diodes (LEDs) present intrinsic green emission originating from the as-prepared CsPbBr3 NCs. Hence, this work offered a novel eco-friendly avenue for the preparation of perovskite NCs for their practical applications in LEDs.