Insight into the Ligand-Mediated Synthesis of Colloidal CsPbBr3 Perovskite Nanocrystals: The Role of Organic Acid, Base, and Cesium Precursors

Highly Stable CsSnCl3 Quantum Dots Grown in an Ionic Liquid/Gelatin Composite System through an In Situ Method

Lead halide perovskite quantum dots (QDs) are controversial due to their high lead content. Tin, a low-toxic element with an outer electronic structure similar to that of Pb, becomes a strong candidate for preparing lead-free perovskite QDs. However, tin-based perovskite QDs, especially CsSnCl₃ QDs, exhibit poor environmental stability. Herein, we proposed an strategy for highly stable CsSnCl₃ QDs using an ionic liquid as a solvent and antioxidant and gelatin as a multidentate ligand and coating material through an in situ method ([AMIM]Cl/gelatin-QDs). The results showed that the abundant active groups of gelatin served as the nucleation growth center for QDs and further passivated QDs. At the same time, the long molecular chain of gelatin can coat the QDs to isolate the environment and fully protect QDs, and the size of QDs grown in gelatin was 5-10 nm. In addition, the oxidation resistance of ionic liquids and the halogen-rich environment formed also played an important role. Even if [AMIM]Cl/gelatin-QDs were treated with water and ultraviolet light simultaneously, its remaining fluorescence intensity was still above 60% within 72 h. Meaningfully, QDs endowed the composite system mildew resistance, which can resist the erosion of gelatin by molds, thereby realizing the system's long-term protection toward CsSnCl₃ QDs.

Colloidal Metal-Halide Perovskite Nanoplatelets: Thickness-Controlled Synthesis, Properties, and Application in Light-Emitting Diodes

Colloidal metal-halide perovskite nanocrystals (MHP NCs) are gaining significant attention for a wide range of optoelectronics applications owing to their exciting properties, such as defect tolerance, near-unity photoluminescence quantum yield, and tunable emission across the entire visible wavelength range. Although the optical properties of MHP NCs are easily tunable through their halide composition, they suffer from light-induced halide phase segregation that limits their use in devices. However, MHPs can be synthesized in the form of colloidal nanoplatelets (NPls) with monolayer (ML)-level thickness control, exhibiting strong quantum confinement effects, and thus enabling tunable emission across the entire visible wavelength range by controlling the thickness of bromide or iodide-based lead-halide perovskite NPls. In addition, the NPls exhibit narrow emission peaks, have high exciton binding energies, and a higher fraction of radiative recombination compared to their bulk counterparts, making them ideal candidates for applications in light-emitting diodes (LEDs). This review discusses the state-of-the-art in colloidal MHP NPls: synthetic routes, thickness-controlled synthesis of both organic-inorganic hybrid and all-inorganic MHP NPls, their linear and nonlinear optical properties (including charge-carrier dynamics), and their performance in LEDs. Furthermore, the challenges associated with their thickness-controlled synthesis, environmental and thermal stability, and their application in making efficient LEDs are discussed.

Synthesis of Stable Lead-Free Cs3 Sb2 (Brx I1- x )9 (0 ≤ x ≤ 1) Perovskite Nanoplatelets and Their Application in CO2 Photocatalytic Reduction

Exploring photocatalysts to foster CO₂ photoreduction into high value-added chemicals is of great significance. Lead halide perovskites (LHPs) have recently been extensively investigated as photocatalysts, owing to their facile fabrication and prominent optoelectronic properties. However, the toxicity of lead and instability will hinder their future large-scale applications. To address these challenges, a series of lead-free Sb-based all-inorganic mixed halide perovskite Cs₃ Sb₂ (Br_x I_{1- x} )₉ (0 ≤ x ≤ 1) nanoplatelets (NPLs) is synthesized. The perovskite NPLs are prepared using a ligand-assisted re-precipitation approach at 50 °C. The authors observe the tunability of their optical band gaps from 2.1 to 2.5 eV, and they can maintain the excellent stability over 120 h under heating at 100 °C or UV light irradiation. The resultant materials are employed as efficient photocatalysts for visible-light driven CO₂ reduction at the gas-solid interface. The Cs₃ Sb₂ (Br_0.7 I_0.3 )₉ perovskite NPLs afford an impressive overall yield of 27.7 µmol g^-1 for the selective photocatalytic conversion of CO₂ into CO. This study represents a significant demonstration for practical artificial photosynthesis by using LHP materials as photocatalysts.

Transformation of Metal Halides to Facet-Modulated Lead Halide Perovskite Platelet Nanostructures on A-Site Cs-Sublattice Platform

The conversion of metal halides to lead halide perovskites with B-site metal ion diffusion has remained a convenient approach for obtaining shape-modulated perovskite nanocrystals. These transformations are typically observed for materials having a common A-site Cs-sublattice platform. However, due to the fast reactions, trapping the interconversion process has been difficult. In an exploration of the tetragonal phase of Cs₇Cd₃Br₁₃ platelets as the parent material, herein, a slower diffusion of Pb(II) leading to facet-modulated CsPbBr₃ platelets is reported. This was expected due to the presence of Cd(II) halide octahedra along with Cd(II) halide tetrahedra in the parent material. This helped in microscopically monitoring their phase transformation via an epitaxially related core/shell intermediate heterostructure. The transformation was also derived and predicted by density functional theory calculations. Further, when the reaction chemistry was tuned, core/shell platelets were transformed to different facet-modulated and hollow CsPbBr₃ platelet nanostructures. These platelets having different facets were also explored for catalytic CO₂ reduction, and their catalytic rates were compared.

Efficient and Stable CF3PEAI-Passivated CsPbI3 QDs toward Red LEDs

Oleylamine and oleic acid are common organic capping ligands used in the hot injection preparation of perovskite quantum dots (QDs). Their labile nature is responsible for the poor colloidal stability and conductivity that affect the performance of perovskite QD light-emitting diodes (LEDs). We introduced 4-trifluoro phenethylammonium iodide (CF₃PEAI) directly in the synthesis and found that CF₃PEAI efficiently modified the I^- vacancy defects on the QD surface and partially substituted the surface capping ligand oleylamine. The strong electron pulling ability of F in CF₃PEAI results in a more positive -NH₃⁺ terminal compared to that of PEAI, which promotes tight bonding of CF₃PEAI on the surface of CsPbI₃ QDs. As a result, we achieved bright QDs with a photoluminescence quantum yield of 92% and efficient red LEDs. The maximal luminance was improved to 4550 cd m^-2 for 685 nm red light, which was nearly 4.6-fold of the LEDs with plain CsPbI₃ QDs. Additionally, the peak external quantum efficiency reached 12.5%.

Highly luminescent MAPbI3 perovskite quantum dots with a simple purification process via ultrasound-assisted bead milling

Organic-inorganic hybrid lead halide perovskite quantum dots (QDs) have various excellent optical properties, and they have drastically enhanced the field of light-emitting diode (LED) research. However, red-emissive CH3NH3 (MA) PbI3 QDs have worse optical properties compared with those of green-emissive MAPbBr3 QDs due to their instability under high-moisture and high-temperature conditions. Therefore, it is quite difficult to prepare MAPbI3 QDs with good optical properties via bottom-up methods using conditions involving high temperature and high-solubility solvents. On the other hand, top-down methods for preparing MAPbI3 QDs under an air atmosphere have attracted attention; however, there are issues, such as PL emission with a wide FWHM being obtained due to the wide particle-size distribution. In this research, red-emissive MAPbI3 QDs were prepared via an ultrasound-assisted bead milling (UBM) method, and the MAPbI3 QDs were purified using various carboxylate esters. As a result, we solved the issue of the wide particle-size distribution unique to top-down methods via purifying the MAPbI3 QDs, and they achieved the following excellent optical properties: a FWHM of 44 to 48 nm and a PLQY of over 60%. Notably, a fabricated LED device with MAPbI3 QDs purified using methyl acetate showed a PL peak at 738 nm and a FWHM of 49 nm, resulting in an excellent EQE value of 3.2%.

Scratching the Surface: Passivating Perovskite Nanocrystals for Future Device Integration

Metal halide perovskite materials have recently upended the field of photovoltaics and are aiming to make waves across a multitude of other fields and applications. Recently, perovskite nanocrystals have been synthesized and are rapidly outpacing traditional semiconductor nanocrystals in application driven fields due to their inherent defect tolerance and facile tunability, resulting in high photoluminescent quantum yields and efficient devices. Future improvements to perovskite nanocrystals toward device driven applications must come at the perovskite surface. The last half decade has resulted in considerable progress in tailoring the perovskite nanocrystal/ligand surface toward maximizing the optoelectronic performance. Here, we review the current progress and discuss how further improvements could be made to further improve this bright class of materials.

Self-Assembly and Regrowth of Metal Halide Perovskite Nanocrystals for Optoelectronic Applications

Over the past decade, the impressive development of metal halide perovskites (MHPs) has made them leading candidates for applications in photovoltaics (PVs), X-ray scintillators, and light-emitting diodes (LEDs). Constructing MHP nanocrystals (NCs) with promising optoelectronic properties using a low-cost approach is critical to realizing their commercial potential. Self-assembly and regrowth techniques provide a simple and powerful “bottom-up” platform for controlling the structure, shape, and dimensionality of MHP NCs. The soft ionic nature of MHP NCs, in conjunction with their low formation energy, rapid anion exchange, and ease of ion migration, enables the rearrangement of their overall appearance via self-assembly or regrowth. Because of their low formation energy and highly dynamic surface ligands, MHP NCs have a higher propensity to regrow than conventional hard-lattice NCs. Moreover, their self-assembly and regrowth can be achieved simultaneously. The self-assembly of NCs into close-packed, long-range-ordered mesostructures provides a platform for modulating their electronic properties (e.g., conductivity and carrier mobility). Moreover, assembled MHP NCs exhibit collective properties (e.g., superfluorescence, renormalized emission, longer phase coherence times, and long exciton diffusion lengths) that can translate into dramatic improvements in device performance. Further regrowth into fused MHP nanostructures with the removal of ligand barriers between NCs could facilitate charge carrier transport, eliminate surface point defects, and enhance stability against moisture, light, and electron-beam irradiation. However, the synthesis strategies, diversity and complexity of structures, and optoelectronic applications that emanate from the self-assembly and regrowth of MHPs have not yet received much attention. Consequently, a comprehensive understanding of the design principles of self-assembled and fused MHP nanostructures will fuel further advances in their optoelectronic applications.

In this Account, we review the latest developments in the self-assembly and regrowth of MHP NCs. We begin with a survey of the mechanisms, driving forces, and techniques for controlling MHP NC self-assembly. We then explore the phase transition of fused MHP nanostructures at the atomic level, delving into the mechanisms of facet-directed connections and the kinetics of their shape-modulation behavior, which have been elucidated with the aid of high-resolution transmission electron microscopy (HRTEM) and first-principles density functional theory calculations of surface energies. We further outline the applications of assembled and fused nanostructures. Finally, we conclude with a perspective on current challenges and future directions in the field of MHP NCs.

A dual-readout sandwich immunoassay based on biocatalytic perovskite nanocrystals for detection of prostate specific antigen

Nanozymes have been regarded as an excellent alternative for natural enzymes because of their high stability, low cost, and high activity. However, their use in disease diagnosis is still challenging, since the complex biological samples foul the nanozymes' surface and generate interference signals, thereby compromising the performance of nanozyme-based assays. Here, we report a dual-readout, CsPbBr₃ NCs-based sandwich immunoassay for the detection of prostate specific antigen (PSA). Thanks to their excellent fluorescence and intrinsic peroxidase-like catalytic activity, the designed phospholipid-coated CsPbBr₃ NCs (PL-CsPbBr₃ NCs) served as an attractive dual signal generator (fluorescent and colorimetric), which is hardly achieved by other nanozymes. The Michaelis-Menten constant (K_M) values of PL-CsPbBr₃ NCs for H₂O₂ and tetramethylbenzidine are 2.85 mM and 1.42 mM, respectively. Meanwhile, the lipid shell around CsPbBr₃ NCs not only greatly improves their aqueous stability, but also helps them resist the unspecific adsorption of biological impurities. Thus, the proposed dual-readout immunoassay enables precise, cost-effective, and anti-jamming detection of PSA in real serum samples with a low detection limit of 0.29 ng mL^-1 (colorimetric) and 0.081 ng mL^-1 (fluorescence). This enhanced immunoassay opens new insights for the application of perovskites in bioanalysis, especially for protein assay, holding great potential for disease diagnosis.

A-Site Substitute for Fabricating All-Inorganic Perovskite CsPbCl3 with Application in Self-Powered Ultraviolet Photodetectors

Because of its stable chemical properties and wide band gap, CsPbCl₃ perovskite has shown great application prospects in ultraviolet photodetectors (UPDs). However, the poor solubility of CsCl in organic solvents impedes the fabrication of high-quality CsPbCl₃ films. Herein, we introduced an A-site substitute route for fabricating a high-quality CsPbCl₃ microcrystalline (MC) film by spin-coating cesium acetate on a MAPbCl₃ MC film followed by a high-temperature annealing process. To enhance the device performance of the FTO/SnO₂/CsPbCl₃ MCs/carbon structure UPD, a pressure-assisted annealing strategy was carried out, which reduced the void density and surface roughness of the microcrystal film. Finally, our optimized PDs showed high device performances with an on/off ratio of 6 × 10⁴, a responsivity of 0.13 A W^-1, a detectivity of as high as 1.07 × 10¹² Jones, and a rise/fall time of 10/24 μs. Moreover, our unpacked PDs showed good storage and light stability. Our results lay a foundation for the application of all inorganic perovskite in the ultraviolet region.

Organic building blocks at inorganic nanomaterial interfaces

This tutorial review presents our perspective on designing organic molecules for the functionalization of inorganic nanomaterial surfaces, through the model of an "anchor-functionality" paradigm. This "anchor-functionality" paradigm is a streamlined design strategy developed from a comprehensive range of materials (e.g., lead halide perovskites, II-VI semiconductors, III-V semiconductors, metal oxides, diamonds, carbon dots, silicon, etc.) and applications (e.g., light-emitting diodes, photovoltaics, lasers, photonic cavities, photocatalysis, fluorescence imaging, photo dynamic therapy, drug delivery, etc.). The structure of this organic interface modifier comprises two key components: anchor groups binding to inorganic surfaces and functional groups that optimize their performance in specific applications. To help readers better understand and utilize this approach, the roles of different anchor groups and different functional groups are discussed and explained through their interactions with inorganic materials and external environments.

Large-scale continuous preparation of highly stable α-CsPbI3/m-SiO2 nanocomposites by a microfluidics reactor for solid state lighting application

CsPbI3-Mesoporous SiO2 nanocomposites with ultrahigh chemical stability were fabricated by the microfluidic technology for large-scale continuous production.

Enhance luminescence or change morphology: effect of the doping method on Cu2+-doped CsPbBr3 perovskite nanocrystals

The effect of Cu2+ on CsPbBr3 nanocrystals is compared between the hot-injection method and postsynthetic cation-exchange reaction.

Ligand-modified synthesis of shape-controllable and highly luminescent CsPbBr3 perovskite nanocrystals under ambient conditions

The intrinsic insights of ligand-modified shape-transformation of CsPbBr3 nanocrystals between nanocubes and nanorods are revealed systematically, which can accelerate their practical applications in the optoelectronic field.

2D/2D Schottky heterojunction of in-situ growth FAPbBr3/Ti3C2 composites for enhancing photocatalytic CO2 reduction

Mimicking the natural photosynthesis process to convert carbon dioxide into value-added chemicals is vital to solving both the climate crisis worldwide and the depletion of fossil fuels. Herein, we explore the synthesis of 2D FAPbBr₃ nanoplate combined with 2D Ti₃C₂ nanosheet to form a 2D/2D FAPbBr₃/Ti₃C₂ Schottky heterojunction using facile hot-injection and in-situ growth approaches. The Schottky heterojunction of FAPbBr₃/Ti₃C₂ over large interfacial contact provides abundant channels for transferring photogenerated carriers from FAPbBr₃ nanoplate to Ti₃C₂ nanosheet. The experimental results showed a CO yield of 93.82 μmol·g^-1·h^-1 with ethyl acetate/deionization water as a sacrificial reagent for FAPbBr₃/Ti₃C₂ composite, which was 1.25-fold enhancement that on pristine FAPbBr₃ nanoplates. The large 2D heterointerface can efficiently accelerate the spatial separation and transfer of photogenerated carriers and result in the superior photocatalytic activity and favorable stability of FAPbBr₃/Ti₃C₂ photocatalysts, which are proved by in-situ X-ray photoelectron spectroscopy, photoluminescence, transient absorption spectra, and Mott-Schottky measurement. Thus, this work unveils that 2D/2D Schottky heterostructures would significantly improve the reaction activities of halide perovskite-based photocatalysts.

High Efficiency and Narrow Emission Band Pure-Red Perovskite Colloidal Quantum Wells

Colloidal quantum wells (CQWs) have excellent optical performance, such as ultranarrow emission, due to strong quantum confinement in the vertical direction. However, there are few reports on metal halide perovskite CQWs with high-purity red emission (∼630 nm) for the display application, owing to the broadened photoluminescence spectrum and beyond the red emission range. Herein, we successfully synthesize high-quality CsPbI₃ CQWs in a strong electronegative solvent (mesitylene), which cut the chemical reaction rate of the precursor system to slow down the crystal nuclei growth. The number of PbI₆^4- octahedral layers (n) of CQWs can be regulated to achieve the emission of the CQW shift from orange (596 nm, n = 3) to red (626 nm, n = 4). The pure-red-emitting CQWs have a high photoluminescence quantum yield of 99% and a narrow emission bandwidth (28 nm). The Commission Internationale de l'Eclairage coordinates (0.69, 0.31) meet the requirement of the Rec. 2020 standard.

Biocatalytic CsPbX3 Perovskite Nanocrystals: A Self-Reporting Nanoprobe for Metabolism Analysis

CsPbX₃ perovskite nanocrystals (NCs), with excellent optical properties, have drawn considerable attention in recent years. However, they also suffer from inherent vulnerability and hydrolysis, causing the new understanding or new applications to be difficultly explored. Herein, for the first time, it is discovered that the phospholipid membrane (PM)-coated CsPbX₃ NCs have intrinsic biocatalytic activity. Different from other peroxidase-like nanozymes relying on extra chromogenic reagents, the PM-CsPbX₃ NCs can be used as a self-reporting nanoprobe, allowing an "add-to-answer" detection model. Notably, the fluorescence of PM-CsPbX₃ NCs can be rapidly quenched by adding H₂ O₂ and then be restored by removing excess H₂ O₂ . Initiated from this unexpected observation, the PM-CsPbX₃ NCs can be explored to prepare multi-color bioinks and metabolite-responsive paper analytical devices, demonstrating the great potential of CsPbX₃ NCs in bioanalysis. This is the first report on the discovery of nanozyme-like property of all-inorganic CsPbX₃ perovskite NCs, which adds another piece to the nanozyme puzzle and opens new avenues for in vitro disease diagnostics.

Surface ligand engineering of CsPbBr3 perovskite nanowires for high-performance photodetectors

Surface ligand engineering is of great importance for the preparation of one-dimensional (1D) CsPbBr₃ nanowires for high-performance photodetectors. The traditional long-chain terminated ligands such as oleylamine/oleic acid (C18) used in the preparation of CsPbBr₃ nanowires will form an electrically insulating layer on the surface of the nanowires, which hinders the effective transport of charge carriers in optoelectronic devices. In this paper, short-chain ligands, including dodecylamine/dodecanoic acid (C12), octylamine/octanoic acid (C8) and hexylamine/hexanoic acid (C6), are introduced to partially replace long-chain ligands (C18) to successfully prepare various CsPbBr₃ nanowires via a solvothermal method. Microstructure characterization indicates that the four kinds of nanowires before/after surface ligand engineering, which are named as C18-CsPbBr₃, C12/18-CsPbBr₃, C8/18-CsPbBr₃ and C6/18-CsPbBr₃, all have high aspect ratio and purity. As compared with CsPbBr₃ with long-chain terminated ligands, the C8/18-CsPbBr₃ and C6/18-CsPbBr₃ nanowires with shorter chain ligands exhibit superior photoluminescence (PL) performance and stability under adverse conditions such as ultraviolet irradiation and high temperature. The constructed photodetectors based on C8/18-CsPbBr₃ and C6/18-CsPbBr₃ nanowires have shown improved performances. This work provides a new idea for the preparation of CsPbBr₃ nanowires with high optical properties, stability and charge transport, and the prepared CsPbBr₃ nanowires have potential application prospects in optoelectronic devices.

Cs-Lattice Extension and Expansion for Inducing Secondary Growth of CsPbBr3 Perovskite Nanocrystals

The increase of the stability of perovskite nanocrystals with respect to exposure to polar media, layers growth, or shelling with different materials is in demand. While these are widely studied for metal chalcogenide nanocrystals, it has yet to be explored for perovskite nanocrystals. Even growth of a single monolayer on any facet or on the entire surface of these nanocrystals could not be established yet. To address this, herein, a secondary growth approach leading to creation of a secondary lattice with subsequent expansion on preformed CsPbBr₃ perovskite nanocrystals is reported. As direct layer growth by adding precursors was not successful, Cs-lattice extension to preformed CsPbBr₃ nanocrystals was performed by coupling CsBr to these nanocrystals. Opening both {110}/{002} and {200} facets of parent CsPbBr₃ nanocrystals, CsBr was observed to be connected with lattice matching to the {200} facets. Further with Pb(II) incorporation, the Cs-sublattices of CsBr were expanded to CsPbBr₃ and led to cube-couple nanocrystals. However, as cubes in these nanostructures were differently oriented, these showed lattice mismatch at their junctions. This lattice mismatch though restricted complete shelling but successfully favored the secondary growth on specific facets of parent CsPbBr₃ nanocrystals. Details of this secondary growth via lattice extension and expansion are microscopically analyzed and reported. These results further suggest that lead halide perovskite nanocrystals can be epitaxially grown under proper reaction design and more complex as well as heterostructures of these materials can be fabricated to meet the current demands.

Defect Passivation for Perovskite Solar Cells: from Molecule Design to Device Performance

Perovskite solar cells (PSCs) are a promising third-generation photovoltaic (PV) technology developed rapidly in recent years. Further improvement of their power conversion efficiency is focusing on reducing the non-radiative charge recombination induced by the defects in metal halide perovskites. So far, defect passivation by the organic small molecule has been considered as a promising approach for boosting the PSC performance owing to their large structure flexibility adapting to passivating variable kinds of defect states and perovskite compositions. Here, the recent progress of defect passivation toward efficient and stable PSCs was reviewed from the viewpoint of molecular structure design and device performance. To comprehensively reveal the structure-performance correlation of passivation molecules, it was separately discussed how the functional groups, organic frameworks, and side chains affect the corresponding PV parameters of PSCs. Finally, a guideline was provided for researchers to select more suitable passivation agents, and a perspective was given on future trends in development of passivation strategies.

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.

Copper-doping defect-lowered perovskite nanosheets for deep-blue light-emitting diodes

Mixed-halide blue perovskites CsPb(Br/Cl)₃ are considered promising candidates for developing efficient deep-blue perovskite light-emitting diodes (PeLEDs), but their low photoluminescence quantum yield (PLQY), environmental instability, and poor device performance gravely inhibit their future development. Here, we employ a heteroatomic Cu²⁺ doping strategy combined with post-treatment Br^- anion exchange to prepare high-performance deep-blue perovskites CsPb(Br/Cl)₃. The Cu²⁺ doping strategy significantly decreases the intrinsic chlorine defects, ensuring that the inferior CsPbCl₃ quantum dots are transformed into two-dimensional nanosheets with enhanced violet photoluminescence and increased exciton binding energy. Further, with the post-treatment Br^- anion exchange, the as-prepared CsPb(Br/Cl)₃ nanosheets with more radiation recombination and less ion migration present an enhanced PLQY of 94% and better humidity stability of 30 days. Based on the optimized CsPb(Br/Cl)₃, we fabricated deep-blue PeLEDs with luminescence emission at 462 nm, a maximum luminance of 761 cd m^-2, and a current density of 205 mA cm^-2. This work puts forward a feasible synthesis strategy to prepare efficient and stable mixed-halide blue perovskite CsPb(Br/Cl)₃ and related blue PeLEDs, which may promote the further application of mixed-halide perovskites in the blue light range.

High-Resolution Colloidal Quantum Dot Film Photolithography via Atomic Layer Deposition of ZnO

High-resolution patterning of quantum dot (QD) films is one of the preconditions for the practical use of QD-based emissive display platforms. Recently, inkjet printing and transfer printing have been actively developed; however, high-resolution patterning is still limited owing to nozzle-clogging issues and coffee ring effects during the inkjet printing and kinetic parameters such as pickup and peeling speed during the transfer process. Consequently, employing direct optical lithography would be highly beneficial owing to its well-established process in the semiconductor industry; however, exposing the photoresist (PR) on top of the QD film deteriorates the QD film underneath. This is because a majority of the solvents for PR easily dissolve the pre-existing QD films. In this study, we present a conventional optical lithography process to obtain solvent resistance by reacting the QD film surface with diethylzinc (DEZ) precursors using atomic layer deposition. It was confirmed that, by reacting the QD surface with DEZ and coating PR directly on top of the QD film, a typical photolithography process can be performed to generate a red/green/blue pixel of 3000 ppi or more. QD electroluminescence devices were fabricated with all primary colors of QDs; moreover, compared to reference QD-LED devices, the patterned QD-LED devices exhibited enhanced brightness and efficiency.

Precise Control of Green to Blue Emission of Halide Perovskite Nanocrystals Using Terbium Chloride as Chlorine Source

CsPbCl_xBr_3-x nanocrystals were prepared by ligand-assisted deposition at room temperature, and their wavelength was accurately adjusted by doping TbCl₃. The synthesized nanocrystals were monoclinic and the morphology was almost unchanged after doping. The fluorescence emission of CsPbCl_xBr_3-x nanocrystals was easily controlled from green to blue by adjusting the amount of TbCl₃, which realizes the continuous and accurate spectral regulation in the range of green to blue. This method provides a new scheme for fast anion exchange of all-inorganic perovskite nanocrystals in an open environment at room temperature.

Enhancement of photoluminescence and the stability of CsPbX3 (X = Cl, Br, and I) perovskite nanocrystals with phthalimide passivation

Cesium lead halide perovskite nanocrystals (CsPbX₃ NCs) have been the flourishing area of research in the field of photovoltaic and optoelectronic applications because of their excellent optical and electronic properties. However, they suffer from low stability and deterioration of photoluminescence (PL) properties post-synthesis. In this work, we demonstrate that incorporating an additional ligand can further enhance the optical properties and stability of NCs. Here, we introduced phthalimide as a new surface passivation ligand into the oleic acid/oleylamine system in situ to get near-unity photoluminescence quantum yield (PLQY) of CsPbBr₃ and CsPbI₃ perovskite NCs. Phthalimide passivation dramatically improves the stability of CsPbCl₃, CsPbBr₃, and CsPbI₃ NCs under ambient light and UV light. The PL intensity was recorded for one year, which showed a dramatic improvement for CsPbBr₃ NCs. Nearly 11% of PL can be retained even after one year with phthalimide passivation. CsPbCl₃ NCs exhibit 3 times higher PL with phthalimide and retain 12% PL intensity even after two months, while PL of as-synthesized NCs completely diminishes. Under continuous UV light illumination, the PL intensity of phthalimide passivated NCs is well preserved, while the as-synthesized NCs exhibit negligible PL emission in 2 days. About 40% and 25% of initial PL is preserved for CsPbBr₃ and CsPbCl₃ NCs in the presence of phthalimide. CsPbI₃ NCs with phthalimide exhibit PL even after 2 days, while PL for as-synthesized NCs rapidly declined in the first 10 h. The presence of phthalimide in CsPbI₃ NCs could maintain stability even after a week, while the as-synthesized NCs underwent a transition to the non-luminescent phase within 4 days. Furthermore, blue, green, yellow, and red-emitting diodes using CsPbCl_1.5Br_1.5, CsPbBr₃, CsPbBr_1.5I_1.5, CsPbI₃ NCs, respectively, are fabricated by drop-casting NCs onto blue LED lights, which show great potential in the field of display and lighting technologies.

Two-dimensional halide perovskites: synthesis, optoelectronic properties, stability, and applications

Halide perovskites are promising materials for light-emitting and light-harvesting applications. In this context, two-dimensional perovskites such as nanoplatelets or Ruddlesden-Popper and Dion-Jacobson layered structures are important because of their structural flexibility, electronic confinement, and better stability. This review article brings forth an extensive overview of the recent developments of two-dimensional halide perovskites both in the colloidal and non-colloidal forms. We outline the strategy to synthesize and control the shape and discuss different crystalline phases and optoelectronic properties. We review the applications of two-dimensional perovskites in solar cells, light-emitting diodes, lasers, photodetectors, and photocatalysis. Besides, we also emphasize the moisture, thermal, and photostability of these materials in comparison to their three-dimensional analogs.

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.

Oriented Halide Perovskite Nanostructures and Thin Films for Optoelectronics

Oriented semiconductor nanostructures and thin films exhibit many advantageous properties, such as directional exciton transport, efficient charge transfer and separation, and optical anisotropy, and hence these nanostructures are highly promising for use in optoelectronics and photonics. The controlled growth of these structures can facilitate device integration to improve optoelectronic performance and benefit in-depth fundamental studies of the physical properties of these materials. Halide perovskites have emerged as a new family of promising and cost-effective semiconductor materials for next-generation high-power conversion efficiency photovoltaics and for versatile high-performance optoelectronics, such as light-emitting diodes, lasers, photodetectors, and high-energy radiation imaging and detectors. In this Review, we summarize the advances in the fabrication of halide perovskite nanostructures and thin films with controlled dimensionality and crystallographic orientation, along with their applications and performance characteristics in optoelectronics. We examine the growth methods, mechanisms, and fabrication strategies for several technologically relevant structures, including nanowires, nanoplates, nanostructure arrays, single-crystal thin films, and highly oriented thin films. We highlight and discuss the advantageous photophysical properties and remarkable performance characteristics of oriented nanostructures and thin films for optoelectronics. Finally, we survey the remaining challenges and provide a perspective regarding the opportunities for further progress in this field.

Color-Stable Blue Light-Emitting Diodes Enabled by Effective Passivation of Mixed Halide Perovskites

Bandgap tuning through mixing halide anions is one of the most attractive features for metal halide perovskites. However, mixed halide perovskites usually suffer from phase segregation under electrical biases. Herein, we obtain high-performance and color-stable blue perovskite LEDs (PeLEDs) based on mixed bromide/chloride three-dimensional (3D) structures. We demonstrate that the color instability of CsPb(Br_1-xCl_x)₃ PeLEDs results from surface defects at perovskite grain boundaries. By effective defect passivation, we achieve color-stable blue electroluminescence from CsPb(Br_1-xCl_x)₃ PeLEDs, with maximum external quantum efficiencies of up to 4.5% and high luminance of up to 5351 cd m^-2 in the sky-blue region (489 nm). Our work provides new insights into the color instability issue of mixed halide perovskites and can spur new development of high-performance and color-stable blue PeLEDs.

Multidentate ligand approach for conjugation of perovskite quantum dots to biomolecules

Building compatible surface on perovskite quantum dots (PQDs) for applications like sensing analytes in aqueous medium is highly challenging and if achieved by simple means can revolutionize disease diagnostics. The present work reports the surface engineering of CsPbBr₃ QDs via "simple ligand exchange process" to achieve water-compatible QDs towards detection of biomolecules. The monodentate oleic acid ligand in CsPbBr₃ QDs is exchanged with dicarboxylic acid containing (bidentate) ligands such as folic acid (FA), ethylenediamine tetra-acetic acid (EDTA), succinic acid (SA) and glutamic acid (GA) to develop an efficient water-compatible PQD-ligand system. optical and theoretical studies showed the existence of a stronger binding between the perovskite and succinic acid ligand as compared to oleic acid (OA) and all other ligands. Replacement of OA with SA and retention of crystal structure is validated using spectroscopic and microscopic tools. It is observed that SA ligands facilitate better electronic coupling with PQDs and show significant improvement in fluorescence and stability. Further N-Hydroxy succinimide (NHS), which is a well-known compound to activate carboxyl groups, is used to bind onto SA PQDs as multidentate ligand, to form water stable PQDs. SA PQDs react with NHS (in water) to form multidentate ligand passivated PQDs that show very high photoluminescence (PL) as compared to OA PQDs in toluene. This also results in the formation of an NHS ester that allows bioconjugation with PQDs. This simple probe in water is further utilized for sensing a highly hydrophilic bovine serum albumin (BSA) protein as a model target to demonstrate the potential and effectiveness of this process to create compatible QDs for the successful conjugation of biomolecules. Although the focus of this work is to demonstrate bioconjugation and not achieving higher sensitivity levels, the intrinsic sensing level of these compatible QDs towards BSA shows a detection limit of 51.47 nM, which is above par with other reports in literature.

Poly(catecholamine) coated CsPbBr3 perovskite microlasers: lasing in water and biofunctionalization

Lead halide perovskite (LHP) is a promising material for various optoelectronic applications. Surface coating on particles is a common strategy to improve their functionality and environmental stability, but LHP is not amenable to most coating chemistries because of its intrinsic weakness against polar solvents. Here, we describe a novel method of synthesizing LHP microlasers in a super-saturated polar solvent using sonochemistry and applying various functional coatings on individual microlasers in situ. We synthesize cesium lead bromine perovskite (CsPbBr3) microcrystals capped with organic poly-norepinephrine (pNE) layers. The catechol group of pNE coordinates to bromine-deficient lead atoms, forming a defect-passivating and diffusion-blocking shell. The pNE layer enhances the material lifetime of CsPbBr3 in water by 2,000-folds, enabling bright luminescence and lasing from single microcrystals in water. Furthermore, the pNE shell permits biofunctionalization with proteins, small molecules, and lipid bilayers. Luminescence from CsPbBr3 microcrystals is sustained in water over 1 hour and observed in live cells. The functionalization method may enable new applications of LHP laser particles in water-rich environments.

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.

Recent Advances in Ligand Design and Engineering in Lead Halide Perovskite Nanocrystals

Lead halide perovskite (LHP) nanocrystals (NCs) have recently garnered enhanced development efforts from research disciplines owing to their superior optical and optoelectronic properties. These materials, however, are unlike conventional quantum dots, because they possess strong ionic character, labile ligand coverage, and overall stability issues. As a result, the system as a whole is highly dynamic and can be affected by slight changes of particle surface environment. Specifically, the surface ligand shell of LHP NCs has proven to play imperative roles throughout the lifetime of a LHP NC. Recent advances in engineering and understanding the roles of surface ligand shells from initial synthesis, through postsynthetic processing and device integration, finally to application performances of colloidal LHP NCs are covered here.

Tailoring the Structure of Low-Dimensional Halide Perovskite through a Room Temperature Solution Process: Role of Ligands

In this study, halide perovskite nanocrystals are synthesized by controlling the ligand length and amount, and investigated the effects on the change in the ligand length and amount on the shape, size, crystal structure, and optical properties of the perovskite nanocrystals. The results reveal the tendency and respective effects of amine and acid ligands on perovskite nanocrystals. The amine ligands bind directly to the perovskite nanocrystals. Consequently, the amine ligands with longer chains interfere with the aggregation of the initially formed nanocrystals, thus limiting the size of the halide perovskite nanocrystals. Similar to the amine ligands, the acid ligands directly bond with the perovskite nanocrystals; however, they are also indirectly distributed around the nanocrystals, thus affecting their structure and dispersion. Consequently, the acid ligands affect the assembly of the initially formed nanocrystals, which determine the shape and crystal structure of the nanocrystals. It is believed that the report will provide useful insight on the synthesis of halide perovskites for application in optoelectronic devices.

Preparation of perovskite CsPb(Br x I1-x )3 quantum dots at room temperature

Here, we propose a method for preparing red perovskite CsPb(Br_xI_1−x)₃ quantum dots (QDs) at room temperature. The PL emission peak of the QDs is close to 650 nm, and the full width at half maximum reaches 61 nm. The red CsPb(Br_xI_1−x)₃ QDs with higher water stability are obtained successfully at room temperature by adjusting the proportion of halogen elements and incorporating polystyrene. At the same time, the red QDs are combined with a blue light-emitting diode to prepare a white light-emitting device (WLED). Compared with the WLED based on the Ce-doped Yttrium Aluminum Garnet (YAG:Ce) phosphor alone, the new WLED based on QDs exhibits a reduced correlated color temperature of 2976 K, while achieving a lumen efficiency of 55.3 lm W⁻¹, which provides a feasible solution for future research on light-emitting diodes (LEDs).

Here, we propose a method for preparing red perovskite CsPb(Br_xI_1−x)3 quantum dots (QDs) at room temperature and successfully applied to WLEDs.

Tunable Dual-Color Emission Perovskites via Post-Synthetic Modification Strategy for Near-Unity Photoluminescence Quantum Yield

Lead halide perovskites (LHPs) with excellent performance have become promising materials for optoelectrical devices. However, as for the dual-color emission LHPs (DELHPs), the low photoluminescence quantum yield (PLQY) hinders their applications. Herein, a simple low-cost room-temperature post-synthetic modification strategy is used to achieve a near-unity PLQY of DELHPs. It is proven that ZnBr₂ plays an important role as an inorganic ligand in reducing surface defects to induce a 95.4% increase in the radiative decay rate and a 99.5% decrease in the nonradiative decay rate in the treated DELHPs compared with the pristine DELHPs. The performance of the blue emission from the surface lattice is greatly improved via the modification of ZnBr₂. DELHPs with different ratios of blue and green emissions are obtained by changing the specific surface area and ZnBr₂ concentration. The distribution and mechanism of Zn²⁺ are discussed using the research model based on these DELHPs. The first example of the single-layer dual-color perovskite electroluminescence device is realized from DELHPs. This work provides a new perspective for improving the performance of DELHPs, which will greatly accelerate the development of emission materials of LHPs.

Efficient and Stable Red Perovskite Light-Emitting Diodes with Operational Stability >300 h

The long-term operational stability of perovskite light-emitting diodes (PeLEDs), especially red PeLEDs with only several hours typically, has always faced great challenges. Stable β-CsPbI₃ nanocrystals (NCs) are demonstrated for highly efficient and stable red-emitting PeLEDs through incorporation of poly(maleic anhydride-alt-1-octadecene) (PMA) in synthesizing the NCs. The PMA can chemically interact with PbI₂ in the precursors via the coupling effect between O groups in PMA and Pb²⁺ to favor crystallization of stable β-CsPbI₃ NCs. Meanwhile, the cross-linked PMA significantly reduces the Pb_Cs anti-site defect on the surface of the β-CsPbI₃ NCs. Benefiting from the improved crystal phase quality, the photoluminescence quantum yield for β-CsPbI₃ NCs films remarkably increases from 34% to 89%. The corresponding red-emitting PeLEDs achieves a high external quantum efficiency of 17.8% and superior operational stability with the lifetime, the time to half the initial electroluminescence intensity (T₅₀ ) reaching 317 h at a constant current density of 30 mA cm^-2 .

Revealing the Aging Effect of Metal-Oleate Precursors on the Preparation of Highly Luminescent CsPbBr3 Nanoplatelets

Due to the ultrafast crystallization process in the triple-source ligand-assisted reprecipitation (TSLARP) technique the [L_yPbBr_x] octahedra is easily distorted, resulting in anisotropic two-dimensional nanoplatelets (NPLs) with low photoluminescence quantum yield (PLQY) and poor stability. Unexpectedly, we obtain CsPbBr₃ NPLs with PLQY approaching unity and high stability using the TSLARP technique through aging the metal-oleate precursors. We find that the significant enhancement of the PLQY is related to the change of solution chemistry of the Pb-oleate precursor in the aging process. While hybrid CsPbBr₃@Cs₄PbBr₆ NPLs with low PLQY (28%) are formed with fresh Pb-oleate precursor, phase-pure CsPbBr₃ NPLs with PLQY of 97.4% are obtained with the aged Pb-oleate precursor. A model that takes into account the transformation of the Pb-oleate in toluene from isolated molecules into clusters after aging is proposed to explain the phenomenon. Our finding highlights the importance of understanding the solution chemistry for the synthesis of the highly luminescent NPLs and provides a new way to break the "blue-wall" in perovskite light-emitting 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.

Highly Photoluminescent CsPbBr3/CsPb2Br5 NCs@TEOS Nanocomposite in Light-Emitting Diodes

All-inorganic halide perovskite (CsPb₂Br₅) nanocrystals (NCs) have received widespread attention owing to their unique photoelectric properties. This work reports a novel strategy to control the phase transition from CsPbBr₃ to CsPb₂Br₅ and investigates the effects of different treatment times and treatment temperatures on perovskite NCs formation. By controlling the volume of tetraethoxysilane (TEOS) added, the formation of different phases of perovskite powder can be well controlled. In addition, a white light-emitting diode (WLED) device is designed by coupling the CsPbBr₃/CsPbBr₃-CsPb₂Br₅ NCs@TEOS nanocomposite and CaAlSiN₃:Eu²⁺ commercial phosphor with a 460 nm InGaN blue chip, exhibiting a high luminous efficiency of 57.65 lm/W, color rendering index (CRI) of 91, and a low CCT of 5334 K. The CIE chromaticity coordinates are (0.3363, 0.3419). This work provides a new strategy for the synthesis of CsPbBr₃/CsPbBr₃-CsPb₂Br₅ NCs@TEOS nanocomposite, which can be applied to the field of WLEDs and display devices.

Synthesis of Lead-Free Cs2AgBiX6 (X = Cl, Br, I) Double Perovskite Nanoplatelets and Their Application in CO2 Photocatalytic Reduction

Morphology control represents an important strategy for the development of functional nanomaterials and has yet to be achieved in the case of promising lead-free double perovskite materials so far. In this work, high-quality Cs₂AgBiX₆ (X = Cl, Br, I) two-dimensional nanoplatelets were synthesized through a newly developed synthetic procedure. By analyzing the optical, morphological, and structural evolutions of the samples during synthesis, we elucidated that the growth mechanism of lead-free double perovskite nanoplatelets followed a lateral growth process from mono-octahedral-layer (half-unit-cell in thickness) cluster-based nanosheets to multilayer (three to four unit cells in thickness) nanoplatelets. Furthermore, we demonstrated that Cs₂AgBiBr₆ nanoplatelets possess a better performance in photocatalytic CO₂ reduction compared with their nanocube counterpart. Our work demonstrates the first example with two-dimensional morphology of this important class of lead-free perovskite materials, shedding light on the synthetic manipulation and the application integration of such promising materials.

Highly-stable tin-based perovskite nanocrystals produced by passivation and coating of gelatin

Lead-halide perovskite nanocrystals (NCs) are limited in commercial applications due to their high lead content. Developing lead-free perovskite NCs becomes a new choice. Among them, the tin-halide perovskite NCs exhibit the excellent photoelectric conversion efficiency, but has worse stability. Herein we describe an effective approach to the preparation of highly-stable all-inorganic tin-based perovskite NCs by using gelatin via interfacial passivation and coating, which leads to the retention of 77.46% of photoluminescence intensity even after the dispersion of the NCs in water for 3 d. The results show that gelatin form a "rich ligand" state on NC surface, such as amino-Sn, carboxylate-Sn and halogen-ammonium hydrogen-bonding interactions. The amino-Sn coordination would be replaced by carboxylate-Sn coordination when NCs are dispersed in polar-media. Meanwhile, gelatin is imparted excellent anti-mildew properties by NCs, which ensures long-lasting effect to NCs. This will promote the stability and sustainable development of the perovskite device.

The Surface Chemistry and Structure of Colloidal Lead Halide Perovskite Nanocrystals

ConspectusSince the initial discovery of colloidal lead halide perovskite nanocrystals, there has been significant interest placed on these semiconductors because of their remarkable optoelectronic properties, including very high photoluminescence quantum yields, narrow size- and composition-tunable emission over a wide color gamut, defect tolerance, and suppressed blinking. These material attributes have made them attractive components for next-generation solar cells, light emitting diodes, low-threshold lasers, single photon emitters, and X-ray scintillators. While a great deal of research has gone into the various applications of colloidal lead halide perovskite nanocrystals, comparatively little work has focused on the fundamental surface chemistry of these materials. While the surface chemistry of colloidal semiconductor nanocrystals is generally affected by their particle morphology, surface stoichiometry, and organic ligands that contribute to the first coordination sphere of their surface atoms, these attributes are markedly different in lead halide perovskite nanocrystals because of their ionicity.In this Account, emerging work on the surface chemistry of lead halide perovskite nanocrystals is highlighted, with a particular focus placed on the most-studied composition of CsPbBr₃. We begin with an in-depth exploration of the native surface chemistry of as-prepared, 0-D cuboidal CsPbBr₃ nanocrystals, including an atomistic description of their surface termini, vacancies, and ionic bonding with ligands. We then proceed to discuss various post-synthetic surface treatments that have been developed to increase the photoluminescence quantum yields and stability of CsPbBr₃ nanocrystals, including the use of tetraalkylammonium bromides, metal bromides, zwitterions, and phosphonic acids, and how these various ligands are known to bind to the nanocrystal surface. To underscore the effect of post-synthetic surface treatments on the application of these materials, we focus on lead halide perovskite nanocrystal-based light emitting diodes, and the positive effect of various surface treatments on external quantum efficiencies. We also discuss the current state-of-the-art in the surface chemistry of 1-D nanowires and 2-D nanoplatelets of CsPbBr₃, which are more quantum confined than the corresponding cuboidal nanocrystals but also generally possess a higher defect density because of their increased surface area-to-volume ratios.

Halide Ion Migration in Perovskite Nanocrystals and Nanostructures

ConspectusThe optical and electronic properties of metal halide perovskites provide insight into the operation of solar cells as well as their long-term operational stability. Halide mobility in perovskite films is an important factor influencing solar cell performance. One can visualize halide ion migration through halide exchange between two nanocrystal suspensions or between physically paired films of two different metal halide perovskites. The ability to tune band gap by varying halide ratios (Cl:Br or Br:I) allows the synthesis of mixed halide perovskites with tailored absorption and emission across the entire visible spectrum. Interestingly, mixed halide (e.g., MAPb(Br_0.5I_0.5)₃) films undergo phase segregation to form Br-rich and I-rich sites under steady state illumination. Upon halting illumination, segregated phases mix to restore original mixed halide compositions. Introducing multiple cations (Cs, formamidinium) at the A site or alloying with Cl greatly suppresses halide mobilities. Long-term irradiation of MAPb(Br_0.5I_0.5)₃ films also cause expulsion of iodide leaving behind Br-rich phases. Hole trapping at I-rich sites in MAPb(Br_0.5I_0.5)₃ is considered to be an important step in inducing halide mobility in photoirradiated films. This Account focuses on halide ion migration in nanocrystals and nanostructured films driven by entropy of mixing in dark and phase segregation under light irradiation.