Reversible light-mediated compositional and structural transitions between CsPbBr3 and CsPb2Br5 nanosheets

Nanoplatelet Superlattices by Tin-Induced Transformation of FAPbI3 Nanocrystals

The transition from 3D to 2D lead halide perovskites is traditionally led by the lattice incorporation of bulky organic cations. However, the transformation into a coveted 2D superlattice-like structure by cationic substitution at the Pb2+ site of 3D perovskite is unfamiliar. It is demonstrated that the gradual increment of [Sn2+ ] alters the FASnx Pb1- x I3 nanocrystals into the Ruddlesden-Popper-like nanoplatelets (NPLs), with surface-absorbed oleic acid (OA) and oleylamine (OAm) spacer ligand at 80 °C (FA+ : formamidinium cation). These NPLs are stacked either by a perfect alignment to form the superlattice or by offsetting the NPL edges because of their lateral displacements. The phase transition occurs from the Sn/Pb ratio ≥0.011, with 0.64 wt% of Sn2+ species. At and above Sn/Pb = 0.022, the NPL superlattice stacks start to grow along [00l] with a repeating length of 4.37(3) nm, comprising the organic bilayer and the inorganic block having two octahedral layers (n = 2). Besides, a photoluminescence quantum yield of 98.4% is obtained with Sn/Pb = 0.011 (n ≥ 4), after surface passivation by trioctylphosphine (TOP).

Healthy and stable lighting via single-component white perovskite nanoplates

Single-component healthy white light was achieved via Mn²⁺ post-doping into blue perovskite nanoplates (NPLs). The white light consists of two complementary colors, sky-blue (482 nm) and orange-red (610 nm), without harmful deep blue light (400-450 nm), which realizes the Commission Internationale de I'Eclairage (CIE) coordinates of (0.33, 0.33) (standard pure white light) and a color temperature of 6000 K. Benefitting from the lattice shrinking via Mn²⁺ doping, the stability of white NPLs toward long-term storage, UV light, heat, and polar solvents was greatly improved. Finally, a healthy and stable white light-emitting diode (WLED) was fabricated via down-conversion of a UV light LED with our white perovskite NPLs, and the WLED worked continuously for 240 minutes with a color drift of only (±0.006, ±0.004) and with a half lifetime (T₅₀) of 212 minutes.

Highly stable halide perovskites for photocatalysis via multi-dimensional structure design and in situ phase transition

Lead halide perovskite CsPbBr₃ quantum dots (QDs) possess several desirable features which enable them to be promising candidates for photocatalysis. However, the instability caused by their inherent liquid-like ionic properties hampers their further development. Herein, this work employs the surficial molecular modification strategy and a multi-dimensional structure design to ease the instability issue. The additive 2-phenylethanamine bromide (PEABr) can serve as a ligand to compensate for stripping the amine ligands and passivate the surficial bromide vacancy defects of CsPbBr₃ QDs in photocatalysis. In addition, PEABr acts as a reactant to form 2D and quasi-2D perovskite nanosheets. The addition of a small amount of these nanosheets into QDs can enhance their general stability due to their unique layered structures. Moreover, PEABr can trigger the phase transition of cubic CsPbBr₃ into tetragonal CsPb₂Br₅. The newly formed Z-scheme homologous heterojunctions further improve the catalytic performance. Simulated photocatalytic dynamics reveals that our multi-dimensional structure favors decreasing the reaction barrier energy and then facilitating the photocatalytic reaction. Therefore, the electron consumption rate of our multi-dimensional perovskites doubles that of pristine CsPbBr₃ QDs and also has superior long-term stability.

Coupling Localized Laser Writing and Nonlocal Recrystallization in Perovskite Crystals for Reversible Multidimensional Optical Encryption

The ability to generate and manipulate photoluminescence (PL) with high spatial resolution has been of primary importance for applications in micro-optoelectronics, while the emerging metal halide perovskites offer novel material platforms where diverse photonic functionalities and fine structuring are constantly explored. Herein, micro-PL patterns consisting of highly luminescent CsPbBr₃ nanocrystals (NCs) in nonluminescent perovskite crystals are directly fabricated by focused femtosecond laser irradiation. Further modulation with a moisture field leads to the selective dissolution of the laser-destabilized perovskite structures as revealed by density functional theory simulations, thus allowing for facile control of the reversible PL from the recrystallization of moisture-induced CsPbBr₃ NCs. By leveraging the coupled laser writing and moisture modulation, multimodal information encryption is realized by reversible encryption-reading and repeatable erasing-refreshing. This optical storage mechanism is also extended to 3D and 4D by realizing spatially and temporally resolved optical encryption. The coupled multifield modulation on perovskite crystals can enable potential applications in optical storage and encryption, and offer a novel solution for the creation and manipulation of localized PL structures with high temporal and spatial resolutions.

Environmentally Friendly Syntheses of Self-Healed and Printable CsPbBr3 Nanocrystals

Generally, solvents used to synthesize perovskite NCs are toxic, which leads to waste liquid pollution and environmental degradation. Herein, we developed a novel environmentally friendly polar solvent method to synthesize CsPbBr₃ nanocrystals (NCs). Over 65% photoluminescence quantum yield (PLQYs) for NCs could be maintained over 45-850 h of storage time, and a maximum was 78% at 750 h. Such amazing stability in polar solvents is dominated by a ripening process, which heals surface defects. Additionally, their solid films also exhibited good moisture stability. Furthermore, CsPbBr₃ NCs were applied to inkjet-printing to prepare high-quality patterned films.

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.

Temperature-dependent dielectric properties of CsPb2Br5: a 2D inorganic halide perovskite

Two dimensional (2D) CsPb₂Br₅have been successfully synthesized via the chemical precipitation method. Detailed structural, morphological, optical, and dielectric studies of these materials have been performed. These 2D CsPb₂Br₅plates (of thickness around 200-300 nm) are ascribed to a tetragonal lattice system withI4/mcmspace group. The dielectric attributes such as dielectric constant, electrical modulus, loss factor, and the DC, and AC conductivities, are observed to be varying appreciably with temperature over an extensive frequency window of 10 Hz-50 MHz. The Nyquist plots are investigated using the Maxwell-Wagner equivalent circuit model, which shows the impact of grains and grain boundaries on the overall impedance. Both the free charge conductivity and space charge increase with an increment in temperature, as revealed from the modified Cole-Cole plot. The relaxation time and relaxation mechanism of 2D CsPb₂Br₅are estimated using the Kohlrausch-Williams-Watts equation. Variation in DC conductivity and relaxation time, as a function of temperature, closely resembles Arrhenius' behavior. Value of activation energy calculated from the DC conductivity corroborates with the same derived from relaxation time. The observation of high dielectric constant and nominal dielectric loss for CsPb₂Br₅perovskite offers enormous potential in energy harvesting and storage devices.

Photoelectrochemical performance of ligand-free CsPb2Br5 perovskites

A pure aqueous synthesis strategy for ligand-free CsPb2Br5 nanoplatelets and pure-phase single crystal is reported. The CsPb2Br5 perovskite displays excellent photoelectrochemical activity in the absence of other electron acceptors.

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.

A donor-acceptor ligand boosting the performance of FA0.8Cs0.2PbBr3 nanocrystal light-emitting diodes

A donor-acceptor ligand, 3-amino-2-bromo-6-methoxypyridine (ABMeoPy), was introduced to passivate FA0.8Cs0.2PbBr3 nanocrystals (NCs) by a post-processing method. The donor-acceptor interaction can greatly enhance the coordination bond of pyridine-Pb2+, and improve FA0.8Cs0.2PbBr3 NC performance with 95.99% photoluminescence quantum yield (PLQY), 6-month stability in solution, and 26% trap density decrease. In the light of ABMeoPy passivation of FA0.8Cs0.2PbBr3 NCs, the maximum luminance, the maximum current efficiency, and EQE of light-emitting diodes (LEDs) increased 69%, 110%, and 111%, respectively. The strategy of using D-A molecules to passivate perovskite NCs is quite simple and effective, which can be widely promoted in perovskite-based LEDs or solar cells.

Emission Quenching and Recovery of Illuminated Perovskite Quantum Dots Due to Iodide Ion Migration

All-inorganic lead halide perovskite quantum dots (PQDs) are well-known to easily lose their luminescence emission under light illumination, which is often attributed to a photoinduced degradation mechanism. Here, we demonstrate that such quenched emission of PQDs can completely recover in the dark at room temperature, which occurs through a spontaneous "self-healing" mechanism. Our findings indicate that the loss of emission under light illumination is not due to the generally accepted photoinduced degradation, as otherwise the complete recovery would not be possible. Instead, we attribute the emission loss and its subsequent recovery to the migration of iodide ions on the PQD surface. Under illumination, the iodide ions migrate out and associate to adjacent lead ions on the PQD surface, leading to halide vacancies, lattice distortions, and fluorescence quenching. During the recovery process, the migrated iodide ions spontaneously migrate back and fill halide vacancies, resulting in fluorescence recovery. The results of this study offer a useful solution on how to improve the emission and photostability of PQDs.

In Situ Growth of 3D/2D (CsPbBr3/CsPb2Br5) Perovskite Heterojunctions toward Optoelectronic Devices

Two-dimensional (2D) CsPb₂Br₅ exhibits intriguing functions in enhancing the performance of optoelectronic devices in terms of environmental stability and luminescence properties when composited with other perovskites in different dimensionalities. We built a type I three-dimensional (3D) CsPbBr₃/2D CsPb₂Br₅ heterojunction through phase transition where CsPbBr₃ quantum dots in situ grew into 2D CsPb₂Br₅. A thorough growth mechanism study in combination with excited state dynamic investigations via femtosecond spectroscopy and first-principles calculations revealed that the type I hierarchy enhanced the stability of the heterojunction and spurred its luminous quantum yield by prolonging the lifetime of photogenerated carriers. Mixing the heterojunction with other phosphors yielded white-light-emitting diodes with a color rendering index of 94%. The work thus not only offered one new avenue for building heterojunctions by using the "soft crystal" nature of perovskites but also disentangled the enhanced luminescence mechanism of the heterojunction that can be harnessed for promising applications in the luminescence and display fields.

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.

Advances in the Stability of Halide Perovskite Nanocrystals

Colloidal halide perovskite nanocrystals are promising candidates for next-generation optoelectronics because of their facile synthesis and their outstanding and size-tunable properties. However, these materials suffer from rapid degradation, similarly to their bulk perovskite counterparts. Here, we survey the most recent strategies to boost perovskite nanocrystals stability, with a special focus on the intrinsic chemical- and compositional-factors at synthetic and post-synthetic stage. Finally, we review the most promising approaches to address the environmental extrinsic stability of perovskite nanocrystals (PNCs). Our final goal is to outline the most promising research directions to enhance PNCs' lifetime, bringing them a step closer to their commercialization.

Stable, Strongly Emitting Cesium Lead Bromide Perovskite Nanorods with High Optical Gain Enabled by an Intermediate Monomer Reservoir Synthetic Strategy

One-dimensional (1D) semiconductor nanorods are important for numerous applications ranging from optics and electronics to biology, yet the direct synthesis of high-quality metal halide perovskite nanorods remains a challenge. Here, we develop an intermediate monomer reservoir synthetic strategy to realize the controllable growth of uniform and low-defect CsPbBr₃ perovskite nanorods. Intermediates composed of CsPb₂Br₅ and Cs₃In₂Br₉ are obtained through the substitution of Pb²⁺ with In³⁺ cations in the template of CsPbBr₃ nanocubes and act as a precursor reservoir to gradually release monomers, ensuring both the slow growth rate and low defects of nanorods. We have used branched tris(diethylamino)phosphine as a ligand, which not only has unequal binding energies with different crystal faces to promote the orientation growth but also provides strong steric hindrance to shield the nanorods in solution. Because of minor amount of defects and an effective ligand passivation, in addition to significantly enhanced stability, the perovskite nanorods show a high photoluminescence quantum yield of up to 90% and exhibit a net mode gain of 980 cm^-1, the latter being a record value among all the perovskite materials. An extremely low amplified spontaneous emission threshold of 7.5 μJ cm^-2 is obtained under excitation by a nanosecond laser, which is comparable to that obtained using femtosecond lasers in other recent studies.

Surfactant-Induced Anion Exchange and Morphological Evolution for Composition-Controlled Caesium Lead Halide Perovskites with Tunable Optical Properties

Environmentally stable lead halide perovskite nanostructures with engineered composition and morphology are attractive because of their exotic optical properties. Here, we report the synthesis of monodispersed (∼20 nm) CsPbI₃ cubic perovskite nanocrystals (NCs) using edible olive oil as a solvent as well as a chelating reagent. Thereafter, bromide anion exchange reaction using the cetyl trimethyl ammonium bromide surfactant in hexane is carried out at relatively lower temperatures to synthesize caesium lead halide perovskites with variable halide compositions and tunable band gaps. Interestingly, because of the formation of micelles, continuous morphology evolution varying from NCs of different sizes to nanowires (NWs) and nanosheets is observed. The anion exchange temperature has a distinct effect on the morphology of the CsPbBr₃ nanostructure and the anion exchange reaction rate. Finally, an easy solution-processed photoconductive device is demonstrated using as-grown CsPbBr₃ NWs, indicating its potential for optoelectronic applications.

Origin of Luminescent Centers and Edge States in Low-Dimensional Lead Halide Perovskites: Controversies, Challenges and Instructive Approaches

With only a few deep-level defect states having a high formation energy and dominance of shallow carrier non-trapping defects, the defect-tolerant electronic and optical properties of lead halide perovskites have made them appealing materials for high-efficiency, low-cost, solar cells and light-emitting devices. As such, recent observations of apparently deep-level and highly luminescent states in low-dimensional perovskites have attracted enormous attention as well as intensive debates. The observed green emission in 2D CsPb₂Br₅ and 0D Cs₄PbBr₆ poses an enigma over whether it is originated from intrinsic point defects or simply from highly luminescent CsPbBr₃ nanocrystals embedded in the otherwise transparent wide band gap semiconductors. The nature of deep-level edge emission in 2D Ruddlesden-Popper perovskites is also not well understood. In this mini review, the experimental evidences that support the opposing interpretations are analyzed, and challenges and root causes for the controversy are discussed. Shortcomings in the current density functional theory approaches to modeling of properties and intrinsic point defects in lead halide perovskites are also noted. Selected experimental approaches are suggested to better correlate property with structure of a material and help resolve the controversies. Understanding and identification of the origin of luminescent centers will help design and engineer perovskites for wide device applications.

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.

Single pot synthesis of indirect band gap 2D CsPb2Br5 nanosheets from direct band gap 3D CsPbBr3 nanocrystals and the origin of their luminescence properties

Two-dimensional (2D) perovskites recently attracted significant interest due to their unique and novel optoelectronic properties. CsPb2Br5, a 2D inorganic perovskite halide, is an indirect band gap semiconductor, and hence it is not supposed to be luminescent. However, a fundamental understanding of the origin of its luminescence properties is still lacking as there are contradictory literature reports present concerning its luminescence properties. Here, we demonstrate a single pot solution based transformation of 2D CsPb2Br5 nanosheets from the nanocrystals of 3D CsPbBr3 and investigate the origin of its luminescence properties by detailed experiments and density functional theory (DFT) calculations. The photoluminescence of CsPb2Br5 originates from the different amorphous lead bromide ammonium complexes which are present at the surface of the nanosheets. We have also highlighted the formation mechanism of 2D nanosheets from 3D CsPbBr3 nanocrystals. These combined theoretical and experimental studies offer significant insights into the optical properties and formation mechanism of 2D CsPb2Br5 perovskites.

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.

A fractional crystallization technique towards pure mega-size CsPb2Br5 single crystal films

A simple method called fractional crystallization was introduced to prepare pure large-area CsPb₂Br₅ single crystal films with high stability.

Reversible transformation between CsPbBr3 nanowires and nanoparticles

We show that CsPbBr₃ nanowires (NWs) are formed by the hierarchical arrangement of individual nanoparticles (NPs), and reversible transformation from NWs to NPs is also achieved by anion exchange.

Ligand-mediated synthesis of compositionally related cesium lead halide CsPb2X5 nanowires with improved stability

Compositionally related cesium lead halide materials, such as CsPb2X5, have attracted great interest due to their considerable optoelectronic/optical properties as well as improved stability. Currently, CsPb2Br5 nanocrystals can be well-designed by tuning the ligands or precursor ratio, whereas, CsPb2X5 (with Cl- or I-) nanocrystals can only be obtained by the anion exchange method. Herein, we report a method to directly synthesize CsPb2X5 facilitated by thiol ligands. The morphology of CsPb2X5 can be designed as a nanowire. Importantly, the stability of directly synthesized CsPb2X5 nanowires is much improved when compared with the stabilities of the materials obtained by the anion-exchange method. We believe that this method will promote the application of 1D tetragonal CsPb2X5 in optoelectronics, optics and other fields.

Colloidal synthesis of monolayer-thick formamidinium lead bromide perovskite nanosheets with a lateral size of micrometers

We synthesized FAPbBr₃ nanosheets of monolayer thickness and a lateral size of up to micrometers with extremely blue-shifted absorption and emission.

CsPbX3/Cs4PbX6 core/shell perovskite nanocrystals

The synthesis of 3D/0D core/shell lead halide perovskite nanocrystals has been realized using the seeded growth approach for the first time.