Surface passivation of mixed-halide perovskite CsPb(BrxI1-x)3 nanocrystals by selective etching for improved stability

Technological Breakthroughs in Chip Fabrication, Transfer, and Color Conversion for High-Performance Micro-LED Displays

The implementation of high-efficiency and high-resolution displays has been the focus of considerable research interest. Recently, micro light-emitting diodes (micro-LEDs), which are inorganic light-emitting diodes of size <100 µm2 , have emerged as a promising display technology owing to their superior features and advantages over other displays like liquid crystal displays and organic light-emitting diodes. Although many companies have introduced micro-LED displays since 2012, obstacles to mass production still exist. Three major challenges, i.e., low quantum efficiency, time-consuming transfer, and complex color conversion, have been overcome with technological breakthroughs to realize cost-effective micro-LED displays. In the review, methods for improving the degraded quantum efficiency of GaN-based micro-LEDs induced by the size effect are examined, including wet chemical treatment, passivation layer adoption, LED structure design, and growing LEDs in self-passivated structures. Novel transfer technologies, including pick-up transfer and self-assembly methods, for developing large-area micro-LED displays with high yield and reliability are discussed in depth. Quantum dots as color conversion materials for high color purity, and deposition methods such as electrohydrodynamic jet printing or contact printing on micro-LEDs are also addressed. This review presents current status and critical challenges of micro-LED technology and promising technical breakthroughs for commercialization of high-performance displays.

Elucidating the Role of Antisolvents on the Surface Chemistry and Optoelectronic Properties of CsPbBrxI3-x Perovskite Nanocrystals

Colloidal lead-halide perovskite nanocrystals (LHP NCs) have emerged over the past decade as leading candidates for efficient next-generation optoelectronic devices, but their properties and performance critically depend on how they are purified. While antisolvents are widely used for purification, a detailed understanding of how the polarity of the antisolvent influences the surface chemistry and composition of the NCs is missing in the field. Here, we fill this knowledge gap by studying the surface chemistry of purified CsPbBr_xI_3-x NCs as the model system, which in itself is considered a promising candidate for pure-red light-emitting diodes and top-cells for tandem photovoltaics. Interestingly, we find that as the polarity of the antisolvent increases (from methyl acetate to acetone to butanol), there is a blueshift in the photoluminescence (PL) peak of the NCs along with a decrease in PL quantum yield (PLQY). Through transmission electron microscopy and X-ray photoemission spectroscopy measurements, we find that these changes in PL properties arise from antisolvent-induced iodide removal, which leads to a change in halide composition and, thus, the bandgap. Using detailed nuclear magnetic resonance (NMR) and Fourier-transform infrared spectroscopy (FTIR) measurements along with density functional theory calculations, we propose that more polar antisolvents favor the detachment of the oleic acid and oleylamine ligands, which undergo amide condensation reactions, leading to the removal of iodide anions from the NC surface bound to these ligands. This work shows that careful selection of low-polarity antisolvents is a critical part of designing the synthesis of NCs to achieve high PLQYs with minimal defect-mediated phase segregation.

Luminescent Thin Films Enabled by CsPbX3 (X=Cl, Br, I) Precursor Solution

Inorganic cesium lead halide perovskite nanocrystals are candidates for lighting and display materials due to their outstanding optoelectronic properties. However, the dissolution issue of perovskite nanocrystals in polar solvents remains a challenge for practical applications. Herein, we present a newly designed one-step spin-coating strategy to prepare a novel multicolor-tunable CsPbX₃ (X=Cl, Br, I) nanocrystal film, where the CsPbX₃ precursor solution was formed by dissolving PbO, Cs₂ CO₃ , and CH₃ NH₃ X into the ionic liquid n-butylammonium butyrate. The as-designed CsPbX₃ nanocrystal films show high color purity with a narrow emission width. Also, the blue CsPb(Cl/Br)₃ film demonstrates an absolute photoluminescence quantum yields (PLQY) of 15.6 %, which is higher than 11.7 % of green CsPbBr₃ and 8.3 % of red CsPb(Br/I)₃ film. This study develops an effective approach to preparing CsPbX₃ nanocrystal thin films, opening a new avenue to design perovskite nanocrystals-based devices for lighting and display applications.

Additive-Assisted Optimization in Morphology and Optoelectronic Properties of Inorganic Mixed Sn-Pb Halide Perovskites

Halide perovskite solar cells (HPSCs) are promising photovoltaic materials due to their excellent optoelectronic properties, low cost, and high efficiency. Here, we demonstrate atmospheric solution processing and stability of cesium tin-lead triiodide (CsSnPbI₃) thin films for solar cell applications. The effect of additives, such as pyrazine and guanidinium thiocyanate (GuaSCN), on bandgap, film morphology, structure, and stability is investigated. Our results indicate the formation of a wide bandgap (>2 eV) structure with a mixed phase of tin oxide (SnO₂) and Cs(Sn, Pb)I₃. The addition of pyrazine decreases the intensity of SnO₂ peaks, but the bandgap does not change much. With the addition of GuaSCN, the bandgap of the films reduces to 1.5 eV, and a dendritic structure of Cs(Sn, Pb)I₃ is observed. GuaSCN addition also reduces the oxygen content in the films. To enable uniform film crystallization, cesium chloride (CsCl) and dimethyl sulfoxide (DMSO) additives are used in the precursor. Both CsCl and DMSO suppress dendrite formation with the latter resulting in uniform polycrystalline films with a bandgap of 1.5 eV. Heat and light soaking (HLS) stability tests at 65 °C and 1 sun for 100 h show all film types are stable with temperature but result in phase segregation with light exposure.

Atomic Level Insights into Metal Halide Perovskite Materials by Scanning Tunneling Microscopy and Spectroscopy

Metal halide perovskite materials (MHPMs) have attracted significant attention because of their superior optoelectronic properties and versatile applications. The power conversion efficiency of MHPM solar cells (PSCs) has skyrocketed to 25.5 %. Although the performance of PSCs is already competitive, several important challenges still need to be solved to realize commercial applications. A thorough understanding of surface atomic structures and structure-property relationships is at the heart of these remaining issues. Scanning tunneling microscopy (STM) and spectroscopy (STS) can be used to characterize the surface properties of MHPMs, which can offer crucial insights into MHPMs at the atomic scale. This Review summarizes recent progress in STM and STS studies on MHPMs, with a focus on the surface properties. We provide understanding from the comparative perspective of several different MHPMs. We also highlight a series of novel phenomena observed by STM and STS. Finally, we outline a few research topics of primary importance for future studies.

A universal synthesis strategy for stable CsPbX3@oxide core-shell nanoparticles through bridging ligands

CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) have shown great potential in various optoelectronic devices due to their excellent photophysical properties. However, the poor stability has severely impeded their practical applications. Much effort has been devoted to the preparation of monodisperse core-shell NCs to improve the stability of CsPbX3 NCs. However, it is still challenging to develop a general method to coat CsPbX3 NCs with oxides at the single-particle level. In this work, we report a simple way to prepare monodisperse CsPbX3@SiO2/Ta2O5/ZrO2 core-shell structure NCs using 3-aminopropyl triethoxysilane (APTES) as a bridging ligand. It has been found that careful control of the hydrolysis and condensation process of oxide precursors is critical for the successful preparation of CsPbX3@oxide core-shell NCs. The stability of CsPbI3 NCs upon attack of water, UV-light irradiation, and heating before and after the oxide shell growth has been investigated, demonstrating the efficient protective effect of oxide shells. This work not only provides a novel and universal approach for coating the individual CsPbX3 nanocrystal with various oxide shells but also paves the way for potential practical applications of CsPbX3 NCs because of the enhanced 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.

Suppressing ion migration of CsPbBrxI3-xnanocrystals by Nickel doping and the application in high-efficiency WLEDs

All inorganic perovskite nanocrystals CsPbX₃(X = Cl, Br, I) are the great potential candidates for the application of high-performance light emitting diodes (LED) due to their high Photoluminescence Quantum Yield (PLQY), high defect tolerance, narrow full-width half-maximum and tunable wavelength of 410-700 nm. However, the application of red-emitting (630-650 nm) CsPbBr_xI_3-xnanocrystals are perplexed by phase segregation due to the composition of mixed halides and the difference in halide ion mobility. Herein, we provide an effective strategy to suppressing the migration of Br/I ions through Ni²⁺doping via a facile Hot-Injection method and the PLQY was improved as well. DFT calculations show that the introduction of Ni²⁺causes a slight contraction of the host crystal structure, which improves the bond energy between Pb and halides and reduces the level of surface defects. Therefore, the phase stability is improved by Ni²⁺doping because the phase segregation caused by ion migration in the mixed phase is effectively inhibited. Meanwhile, the non-radiative recombination in the exciton transition process is reduced and the PLQY is improved. What's more, benefiting from the suppressed ion migration and enhanced PLQY, we combine the Ni²⁺-doped CsPbBr_xI_3-xnanocrystals with different Br/I ratios and YAG: Ce³⁺phosphors as color conversion layers to fabricate high efficiency WLED. When the ratio of Br/I is 9:11, WLED has a color coordinate of (0.3621, 0.3458), the color temperature of 4336 K and presents a high luminous efficiency of 113.20 lm W^-1, color rendering index of 94.9 under the driving current of 20 mA and exhibits excellent stability, which shows great potential in the application of LED.

Nano Ball-Milling Using Titania Nanoparticles to Anchor Cesium Lead Bromine Nanocrystals and Energy Transfer Characteristics in TiO2 @CsPbBr3 Architecture

Recently, all-inorganic halide perovskite (CsPbX₃ , (X = Cl, Br, and I)) nanocrystals (NCs) based hybrid architectures have attracted extensive attention owing to their distinct luminescence characteristics. However, due to stress and lattice mismatch, it is still a challenge to construct heterojunctions between perovskite NCs and the nanostructures with different lattice parameters and non-cubic contour. In this work, a room temperature mechanochemical method is presented to construct TiO₂ @CsPbBr₃ hybrid architectures, in which TiO₂ nanoparticles (NPs) with a hard lattice as nano "balls" mill off the angles and anchor to the CsPbBr₃ NCs with a soft lattice. On the contrary, to ball-mill without TiO₂ or with conventional ceramics balls replacing TiO₂ , CsPbBr₃ NCs still maintain cubic contour deriving from their cubic crystal structures. Moreover, the TiO₂ @CsPbBr₃ architectures display distinct improvement of photoluminescence quantum yields and more excellent thermal stability in contrast with pristine CsPbBr₃ owing to the passivation of surface defect, small surface area, and energy transfer from CsPbBr₃ to TiO₂ . Meanwhile, there is distinct luminous decay characteristic under the radiation of UV and visible light due to the "on" and "off" TiO₂ response. The method provides an alternative strategy to acquire other anchoring heterojunctions based on perovskite NCs for further regulating their luminescent characteristics.

High-Performance Perovskite Light-Emitting Diodes with Surface Passivation of CsPbBrxI3-x Nanocrystals via Antisolvent-Triggered Ion-Exchange

Inorganic lead halide perovskite nanocrystals (PeNCs) have intensively drawn attention as efficient light-emitting materials for optoelectronic applications due to their fine optoelectronic properties with a high photoluminescence quantum yield and easily tunable saturated emission color. However, the poor stability of the red-emitting PeNCs has become an obstacle because of the uncontrollable iodine substitution from the PeNCs due to weak Pb-I bonding. In this work, we have demonstrated a ligand-mediated post-treatment (LMPT) method using a halide ion-pair ligand, tridodecylmethyl ammonium iodide (TrDAI), for the air stable and high-quality red-emitting PeNCs. Through the LMPT method, the optoelectronic properties of red-emitting PeNCs are dramatically improved resulting in a PLQY of 88.7% at 637 ± 2 nm emission with an increased carrier lifetime from 20.77 to 31.52 ns. We achieve highly efficient red perovskite light-emitting diodes exhibiting a maximum current efficiency of 7.69 cd A^-1 and an external quantum efficiency of 6.36% at 637 ± 2 nm electroluminescence emission with a sharp full-width at half maximum of 31 nm.

Strong fluorescence blinking of large-size all-inorganic perovskite nano-spheres

We demonstrated strong fluorescence blinking on large all-inorganic perovskite (CsPbBr₃) nano-spheres. By performing (time-resolved) micro-photoluminescence (μ-PL) measurements, the unique blinking characteristics of the as-grown nano-spheres with diameters of hundred nanometers, are clearly observed. Blinking has no obvious on/off states, which is different from the blinking characteristics of quantum dots. It is believed that the blinking of fluorescence is caused by metastable defect-induced trapping of carriers on the surface of the nano-spheres, because dramatically suppressed fluorescence blinking and the decay rates of ultrafast carriers are realized by surface passivation of the nano-spheres. Surface defects are closely related to the ambient atmosphere, which has been further confirmed by PL measurements of the as-grown nano-spheres in vacuum. Additionally, we also found that the fluorescence blinking was significantly suppressed as the sample size increased, which can be attributed to the large-size induced average effect on fluorescence blinking. These results may be important for understanding the mechanism of the fluorescence blinking of perovskite materials and for developing optical devices with good fluorescence stability.

A "Cocktail" Approach to Effective Surface Passivation of Multiple Surface Defects of Metal Halide Perovskites Using a Combination of Ligands

Surface passivation of metal halide perovskites (MHPs) is essential for their stability and various properties as well as functionalities, including optical and electronic. Passivation is important for both stabilizing intrinsic defects and preventing extrinsic damaging species from reaching the perovskite (PVK), such as water and oxygen. Because of the ternary nature of their chemical composition, multiple surface defects exist for both bulk and nanostructured PVKs, with the latter particularly prominent because of their extremely large surface-to-volume ratio. To effectively passivate the different surface defects, a multitude of different ligands are necessary because each type of defect likely requires a different ligand for optimal passivation, as has been successfully demonstrated in a number of systems in essentially a "cocktail" approach. Characteristics of the ligands that affect effectiveness of passivation include size, shape, charge and charge distribution, orientation, conductivity, and interligand interaction. Examples of ligands for MHPs include both cationic and anionic or zwitterionic species with varied valences. The challenge is to identify the most effective ligand for each type of defect, and addressing this will require further experimental and theoretical study.

Moisture-Induced Crystallinity Improvement for Efficient CsPbI3-xBrx Perovskite Solar Cells with Excess Cesium Bromide

All-inorganic perovskite materials emerge to be potential promising candidates for photoelectric applications. However, their instability to moisture has seriously limited future applications. It is imperative to develop new fabrication methods to make the perovskite films moisture-tolerant. In this work, it is demonstrated that both crystallinity and photoelectronic performance of the CsPbI_3-xBr_x (x < 1) perovskite were enhanced with the existence of excess CsBr and employing moisture treatment. An intermediate product of CsI_1-yBr_y/CsPbI_3-xBr_x was obtained by using an extra amount of CsBr, and the structural evolution toward a uniform CsPbI_3-zBr_z layer (as the Br^- incorporation, the mole ratio increases from x to z) was initiated by moisture treatment, forming new (PbX₆)^4- octahedra with the bonding of CsI_1-yBr_y to Pb²⁺. Attributed to the homogeneity of the stoichiometry and crystallinity, the photovoltaic performance of the moisture-treated perovskite films was improved, with the open-circuit voltage increased from 0.97 to 1.10 V and the power conversion efficiency from 10.89 to 13.09%.

Unusual Stability and Temperature-Dependent Properties of Highly Emissive CsPbBr3 Perovskite Nanocrystals Obtained from in Situ Crystallization in Poly(vinylidene difluoride)

All-inorganic cesium lead halide perovskite nanocrystals (CsPbX₃, X = Cl, Br, or I) present broad applications in the field of optoelectronics due to their excellent photoluminescence (PL), narrow spectral bandwidth, and wide spectral tunability. However, their poor stability limits their practical application. In this work, we successfully use an in situ crystallization strategy for growing and cladding CsPbBr₃ perovskite nanocrystals in poly(vinylidene difluoride) (PVDF). The CsPbBr₃ nanocrystals in the as-fabricated CsPbBr₃@PVDF composites have an average diameter of 16-18 nm and a strong PL emission (537 nm), with a photoluminescence quantum yield exceeding 30%. In addition, the fabricated CsPbBr₃@PVDF composites present improved resistance to heat and water preserving with remarkable optical performance, owing to the effective protection of PVDF. Moreover, the CsPbBr₃ nanocrystals generated in PVDF can withstand temperatures up to 170 °C and can be completely immersed in water for 60 days while still retaining high PL intensity, which facilitate the practical application of CsPbBr₃ perovskite nanocrystals. These CsPbBr₃@PVDF composite films with remarkable optical performances and superior anti-interference ability have broad application prospects in optoelectronics as well as good potential as temperature sensors in mechanical engineering.

Postsynthesis Spontaneous Coalescence of Mixed-Halide Perovskite Nanocubes into Phase-Stable Single-Crystalline Uniform Luminescent Nanowires

All inorganic mixed-halide perovskite, CsPb(Br _xI_{1- x})₃ (0 ≤ x ≤ 1), nanocrystals possess tunable photoluminescence with high quantum yield in the visible window. However, the photoluminescence degrades rapidly with postsynthetic aging due to the spontaneous ion separation and phase instability. Here we show that the postsynthetic aging of CsPb(Br _xI_{1- x})₃ nanocubes spontaneously forms highly uniform single-crystalline nanowires with a diameter of 9 ± 0.5 nm and length of up to several micrometers. The nanowires show bright photoluminescence with an absolute photoluminescence quantum yield of 41%. Rietveld refinement identifies the stable orthorhombic phase of the nanowires, implying a phase transition from the cubic crystallographic phase of the nanocubes during the morphology evolution. Transient absorption spectroscopy reveals a faster excited-state decay dynamic with a large exciton delocalization length in 1D nanowires. Our findings elucidate the insights into the postsynthesis morphology evolution of mixed-halide perovskite nanocrystals leading to luminescent nanowires with excellent crystal phase stability for potential optoelectronic applications.

A systematic study of the synthesis of cesium lead halide nanocrystals: does Cs4PbBr6 or CsPbBr3 form?

Cs4PbBr6 nanocrystals (NCs) have been recently studied as they can enhance the light emitting efficiency and stability of CsPbBr3 NCs. However, the synthesis of Cs4PbBr6 NCs is often accompanied by the generation of CsPbBr3 NCs, and less attention has been paid to how to exactly control their formation. Here, we investigated the key factors in deciding the final products in the hot-injection synthesis by a modified amine-free method. We found that the molarity ratio of Cs/Pb dominated the final products, while the amount of bromine had a relatively small effect. In addition, introducing a certain amount of oleylamine into the amine-free reaction leads to the formation of Cs4PbBr6, instead of CsPbBr3 NCs. This clearly shows that the protection ligand oleylamine also plays an important role in the formation of Cs4PbBr6 NCs. This well understood and fine control of the synthesis may inspire a new CsPbBr3@Cs4PbBr6 core-shell structure, with the same chemical elements but a different crystal phase in the core and the shell. This nanostructure would open a new avenue for enhancing the stability of perovskite nanocrystals.

Exploring the surface chemistry of cesium lead halide perovskite nanocrystals

Colloidal nanocrystals (NCs) of cesium lead halide perovskites (CsPbX3, X = Cl, Br or I) are emerging as an exciting class of optoelectronic materials, but the retention of their colloidal and structural integrity during isolation, purification and handling still represents a critical issue. The impelling questions concerning their intrinsic chemical instability are connected to the dynamic nature of the bonding between the inorganic surface and the long-chain capping ligands. However, the key aspects of CsPbX3's surface chemistry that directly impact their stability remain elusive. In this contribution, we provide an in-depth investigation of the surface properties of differently composed CsPbX3 NCs, prepared by traditional hot-injection methods. The study, mainly relying on solution NMR spectroscopy, is backed up by elemental analysis as well as morphological, structural and optical investigations. We ascertained that the nature of the ligand adsorption/desorption processes at the NC surface is dependent on its elemental composition, thus explaining the origin of the instability afflicting CsPbI3 NCs. We also evaluated the effect of NC purification as well as of the degradation pathways involving the organic shell on the surface chemistry of CsPbX3 NCs. This study paves the way for new post-functionalization strategies for this promising class of nanomaterials.

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.

Understanding the Surface Properties of Halide Exchanged Cesium Lead Halide Nanoparticles

This report describes a characterization study of the surfaces of CsPbBr₃ and CsPbBr_{3- x}I _x perovskite nanoparticles (NPs) obtained via a simultaneous purification and halide exchange (HE) postsynthetic processing technique. We studied composition-dependent NP-ligand interactions via diffusion ordered NMR (DOSY) and quantified resulting photoluminescence quantum yield (QY) as a function of halide exchange as well as ligand exchange. Importantly, ligand binding strength and QY were found to decrease when successive purification and/or halide/ligand exchange steps were taken without careful concurrent additions of acid and base ligands. This suggests that ligands added during postsynthetic processing steps are localized at the surface of the NP, passivating open surface sites. Further, we show that CsPbBr_{3- x}I _x with increasing CsPbI₃ character, obtained via the same method, have decreasing ligand density, from 6.4 to 1.4 to 0.2 nm^-2, indicating the composition-dependence of surface ligand binding, which also has consequences on the QY of the resulting mixed-halide NPs. These results shed further light on the importance of ion-ligand moiety additions during purification and halide exchange of highly emissive CsPbBr₃ NPs to maintain their as-synthesized properties, as well as the intrinsic differences in surfaces binding and photostability between near-unity QY CsPbBr₃ and mixed-halide CsPbBr_{3- x}I _x NPs.

Interfacial Synthesis of Highly Stable CsPbX3/Oxide Janus Nanoparticles

The poor stability of CsPbX₃ (X = Cl, Br, I) nanocrystals (NCs) has severely impeded their practical applications. Although there are some successful examples on encapsulating multiple CsPbX₃ NCs into an oxide or polymer matrix, it has remained a serious challenge for the surface modification/encapsulation using oxides or polymers at a single particle level. In this work, monodisperse CsPbX₃/SiO₂ and CsPbBr₃/Ta₂O₅ Janus nanoparticles were successfully prepared by combining a water-triggered transformation process and a sol-gel method. The CsPbBr₃/SiO₂ NCs exhibited a photoluminescence quantum yield of 80% and a lifetime of 19.8 ns. The product showed dramatically improved stability against destruction by air, water, and light irradiation. Upon continuous irradiation by intense UV light for 10 h, a film of the CsPbBr₃/SiO₂ Janus NCs showed only a slight drop (2%) in the PL intensity, while a control sample of unmodified CsPbBr₃ NCs displayed a 35% drop. We further highlighted the advantageous features of the CsPbBr₃/SiO₂ NCs in practical applications by using them as the green light source for the fabrication of a prototype white light emitting diode, and demonstrated a wide color gamut covering up to 138% of the National Television System Committee standard. This work not only provides a novel approach for the surface modification of individual CsPbX₃ NCs but also helps to address the challenging stability issue; therefore, it has an important implication toward their practical applications.

Improving the Stability and Size Tunability of Cesium Lead Halide Perovskite Nanocrystals Using Trioctylphosphine Oxide as the Capping Ligand

Recently, all-inorganic cesium lead halide (CsPbX₃, X = Cl, Br, and I) nanocrystals (NCs) have drawn wide attention because of their excellent optoelectronic properties and potential applications. However, one of the most significant challenges of such NCs is their low stability against protonic solvents. In this work, we demonstrate that by incorporating a highly branched capping ligand, trioctylphosphine oxide (TOPO), into the traditional oleic acid/oleylamine system, monodisperse CsPbX₃ NCs with excellent optoelectronic properties can be achieved at elevated temperatures (up to 260 °C). The size of such NCs can be varied in a relatively wide range. The capping of TOPO on NCs has been verified through Fourier transform infrared spectroscopy measurement. More importantly, the presence of TOPO can dramatically improve the stability of CsPbX₃ NCs against ethanol treatment. After ethanol treatment for 100 min, the emission intensity of the TOPO-capped sample dropped only 5%, whereas that of non-TOPO-capped NCs dropped up to 86%. This work may shed some light on the preparation and application of CsPbX₃ NCs with higher stability.

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

We report a novel CsX-stripping mechanism that enables the efficient chemical transformation of nonluminescent Cs₄PbX₆ (X = Cl, Br, I) nanocrystals (NCs) to highly luminescent CsPbX₃ NCs. During the transformation, Cs₄PbX₆ NCs dispersed in a nonpolar solvent are converted into CsPbX₃ NCs by stripping CsX through an interfacial reaction with water in a different phase. This process takes advantage of the high solubility of CsX in water as well as the ionic nature and high ion diffusion property of Cs₄PbX₆ NCs, and produces monodisperse and air-stable CsPbX₃ NCs with controllable halide composition, tunable emission wavelength covering the full visible range, narrow emission width, and high photoluminescent quantum yield (up to 75%). An additional advantage is that this is a clean synthesis as Cs₄PbX₆ NCs are converted into CsPbX₃ NCs in the nonpolar phase while the byproduct of CsX is formed in water that could be easily separated from the organic phase. The as-prepared CsPbX₃ NCs show enhanced stability against moisture because of the passivated surface. Our finding not only provides a new pathway for the preparation of highly luminescent CsPbX₃ NCs but also adds insights into the chemical transformation behavior and stabilization mechanism of these emerging perovskite nanocrystals.

Lead Halide Perovskite Nanocrystals in the Research Spotlight: Stability and Defect Tolerance

This Perspective outlines basic structural and optical properties of lead halide perovskite colloidal nanocrystals, highlighting differences and similarities between them and conventional II-VI and III-V semiconductor quantum dots. A detailed insight into two important issues inherent to lead halide perovskite nanocrystals then follows, namely, the advantages of defect tolerance and the necessity to improve their stability in environmental conditions. The defect tolerance of lead halide perovskites offers an impetus to search for similar attributes in other related heavy metal-free compounds. We discuss the origins of the significantly blue-shifted emission from CsPbBr₃ nanocrystals and the synthetic strategies toward fabrication of stable perovskite nanocrystal materials with emission in the red and infrared parts of the optical spectrum, which are related to fabrication of mixed cation compounds guided by Goldschmidt tolerance factor considerations. We conclude with the view on perspectives of use of the colloidal perovskite nanocrystals for applications in backlighting of liquid-crystal TV displays.