Exploring the surface chemistry of cesium lead halide perovskite nanocrystals

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

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

Ionic liquid assisted preparation and modulation of the photoluminescence kinetics for highly efficient CsPbX3 nanocrystals with improved stability

CsPbX₃ (X = Cl, Br, I) nanocrystals (NCs) are competitive fluorescent materials for lighting and displays owing to their excellent photophysical properties. However, the stability and optoelectronic performance of the perovskite NCs are severely limited by the highly dynamic binding feature of the present ligand strategy. Herein, a facile approach was employed to synthesize CsPbBr₃ NCs with the assistance of the ionic liquid (IL) 1-butyl-3-methylimidazolium bromide ([Bmim]Br). By strictly controlling the addition dose of [Bmim]Br (n_IL/n_Pb = 0.125) into the reaction precursor, it is possible to obtain the desired cube-shaped and monodisperse CsPbBr₃ NCs with simultaneous enhancement of the storage and irradiation stability as well as photoluminescence quantum yields (PLQYs, ∼91%). Stability tests show that the emission intensity of the parent CsPbBr₃ NCs drops to 50% of its initial emission intensity after storage under an open atmosphere for 91 days, while the sample prepared with the assistance of [Bmim]Br can maintain 82% of the PL intensity. Meanwhile, the modified CsPbBr₃ NCs also present superior photo-stability, and still maintain 81% of the original PL intensity after continuous illumination under an ultraviolet lamp for 24 h, but the intensity of the parent CsPbBr₃ NCs reduces to 35% of the original intensity. Through the morphology, composition, and luminescence kinetics evolution of CsPbBr₃ NCs, these benefits were attributed to the modulation by [Bmim]Br, which could effectively provide Br ions for the formation and growth of NCs, resulting in the reduction of surface traps. Moreover, [Bmim]Br exhibited strong interactions with NCs, and the deprotonation of oleic acid (OA) was inhibited, resulting in the effective passivation of surface defects. Finally, CsPbX₃ NCs with different compositions were obtained via a facile anion exchange reaction, leading to the tunable emission in the range of 462-665 nm and a wide colour gamut (129.65% NTSC standard). This work opens a new avenue for modulating the surface properties of CsPbX₃ NCs, which will create opportunities for their application in the photoelectric field.

Temperature-Driven Transformation of CsPbBr3 Nanoplatelets into Mosaic Nanotiles in Solution through Self-Assembly

Two-dimensional colloidal halide perovskite nanocrystals are promising materials for light-emitting applications. Recent studies have focused on nanoplatelets that are able to self-assemble and transform on solid substrates. However, the mechanism behind the process and the atomic arrangement of their assemblies remain unclear. Here, we present a detailed analysis of the transformation of self-assembled stacks of CsPbBr₃ nanoplatelets in solution over a period of a few months by using ex situ transmission electron microscopy and surface analysis. We demonstrate that the transformation mechanism can be understood as oriented attachment, proceeding through the following steps: (i) desorption of the ligands from the surfaces of the particles, causing the seamless atomic merging of nanoplatelet stacks into nanobelts; (ii) merging of neighboring nanobelts that form more extended nanoplates; and (iii) attachment of nanobelts and nanoplates, forming objects with an atomic structure that resembles a mosaic made of broken nanotiles. We reveal that aged nanobelts and nanoplates, which are mainly stabilized by amine/ammonium ions, link through a bilayer of CsBr, with the atomic columns of neighboring perovskite lattices shifted by a half-unit-cell, forming Ruddlesden-Popper planar faults. We also show, via in situ monitoring of the nanocrystal photoluminescence combined with transmission electron microscopy analysis, that the transformation is temperature driven and that it can take place within tens of minutes in solution and in spin-coated films. Understanding this process gives crucial information for the design and fabrication of perovskite materials, where control over the type and density of defects is desired, stimulating the development of perovskite nanocrystal structures with tailored electronic properties.

A relationship between the surface composition and spectroscopic properties of cesium lead bromide (CsPbBr3) perovskite nanocrystals: focusing on photoluminescence efficiency

We have previously developed CsPbBr3 NCs exhibiting a tremendously high photoluminescence (PL) and structural stability by adding ZnBr2. However, understanding of these outstanding properties is lacking due to the absence of spectroscopic analyses, such as spectral or dynamical characteristics. In this work, we conducted a comparative analysis of photophysical properties for conventional-CsPbBr3 NCs and ZnBr2-CsPbBr3 NCs. First, we analyzed the blinking traces by comparing the single crystal PL intermittency. It has been found that the PL quantum yield of CsPbBr3 NCs is gradually decreasing at the ensemble level, resulting from a significant activation of the Auger-induced blinking. Furthermore, the time-resolved TA dynamics supports the fact that Auger-type energy transfer accelerates the hot carrier cooling time, and thereby the Auger-induced blinking behavior in the band-edge state becomes dominant over time. Here, ZnBr2-CsPbBr3 NCs showed a low multiexciton Auger amplitude and therefore had a stable PL emission compared with conventional-CsPbBr3 NCs. Finally, we suggest that both NCs differ in intraband spacing possibly due to capping ligands, finally leading to a suppressed Auger process and higher stability for ZnBr2-CsPbBr3 NCs.

Insights into the role of the lead/surfactant ratio in the formation and passivation of cesium lead bromide perovskite nanocrystals

This study aims at rationalizing the effects of the lead/surfactant ratio on the structural evolution of cesium lead-bromide perovskite nanocrystals (NCs), ascertaining how their shape and surface composition can be modulated by suitably adjusting the ligand amount (an equivolumetric mixture of oleic acid and oleyl amine) relatively to lead bromide. The tailoring of the reaction conditions allows the obtainment of blue-emitting CsPbBr3 nanoplatelets in the presence of ligand excess, while green-emitting nanocubes are achieved under low-surfactant conditions. An insight into the NC's shape evolution dictated by the different reaction conditions suggests that the generation of CsPbBr3 nanoplatelets is controlled by the dimensions of [(RNH3)2(PbBr4)]n layers formed before the injection of cesium oleate. The growth step promoted by preformed layers is concomitant to (but independent from) the nucleation process of lead-based species, leading to centrosymmetric nanocubes (prevalent in low-surfactant regimes) or Cs4PbBr6 NCs (prevalent in high-surfactant regimes). The proposed NC growth is supported by the analysis of the optical properties of non-purified samples, which reveal the selective presence of structures endowed with four cell unit average thickness accompanying larger emissive nanocubes. By combining nuclear magnetic resonance (NMR) and UV-Vis spectroscopy techniques, it is ascertained that the lead/surfactant ratio also controls the relative proportion between lead-based species (PBr2, PbBr3-, PbBr42- and plausibly PbBr53- or PbBr64-) formed before cesium injection, which regulate the size of [(RNH3)2(PbBr4)]n layers as well as the formation of Cs4PbBr6 NCs during the nucleation stage. The surface chemistry of the differently structured perovskite NCs is investigated by correlating the elemental composition of the nanoparticles with specific NMR signals ascribable to the surface ligands. This level of investigation also sheds light on the stability of the time-dependent fluorescence exhibited by differently composed perovskite NCs before the loss of their colloidal integrity. Our findings can bring about a fine tuning of the synthetic methods currently employed for controlling the shape and surface chemistry of perovskite NCs.

Understanding the Ligand Effects on Photophysical, Optical, and Electroluminescent Characteristics of Hybrid Lead Halide Perovskite Nanocrystal Solids

There has been a tremendous amount of interest in developing high-efficiency light-emitting diodes (LEDs) based on colloidal nanocrystals (NCs) of hybrid lead halide perovskites. Here, we systematically investigate the ligand effects on EL characteristics by tuning the hydrophobicity of primary alkylamine ligands used in NC synthesis. By increasing the ligand hydrophobicity, we find (i) a reduced NC size that induces a higher degree of quantum confinement, (ii) a shortened exciton lifetime that increases the photoluminescence quantum yield, (iii) a lowering of refractive index that increases the light outcoupling efficiency, and (iv) an increased thin-film resistivity. Accordingly, ligand engineering allows us to demonstrate high-performance green LEDs exhibiting a maximum external quantum efficiency up to 16.2%. The device operational lifetime, defined by the time lasted when the device luminance reduces to 85% of its initial value, LT85, reaches 243 min at an initial luminance of 516 cd m^-2.

Stable Ultraconcentrated and Ultradilute Colloids of CsPbX3 (X = Cl, Br) Nanocrystals Using Natural Lecithin as a Capping Ligand

Attaining thermodynamic stability of colloids in a broad range of concentrations has long been a major thrust in the field of colloidal ligand-capped semiconductor nanocrystals (NCs). This challenge is particularly pressing for the novel NCs of cesium lead halide perovskites (CsPbX₃; X = Cl, Br) owing to their highly dynamic and labile surfaces. Herein, we demonstrate that soy lecithin, a mass-produced natural phospholipid, serves as a tightly binding surface-capping ligand suited for a high-reaction yield synthesis of CsPbX₃ NCs (6–10 nm) and allowing for long-term retention of the colloidal and structural integrity of CsPbX₃ NCs in a broad range of concentrations—from a few ng/mL to >400 mg/mL (inorganic core mass). The high colloidal stability achieved with this long-chain zwitterionic ligand can be rationalized with the Alexander–De Gennes model that considers the increased particle–particle repulsion due to branched chains and ligand polydispersity. The versatility and immense practical utility of such colloids is showcased by the single NC spectroscopy on ultradilute samples and, conversely, by obtaining micrometer-thick, optically homogeneous dense NC films in a single spin-coating step from ultraconcentrated colloids.

Delayed Photoluminescence and Modified Blinking Statistics in Alumina-Encapsulated Zero-Dimensional Inorganic Perovskite Nanocrystals

We demonstrate enhancement of the photoluminescence (PL) properties of individual zero-dimensional (0D) Cs₄PbBr₆ perovskite nanocrystals (PNCs) upon encapsulation by alumina using an appropriately modified atomic layer deposition method. In addition to the increased PL intensity and improved long-term stability of encapsulated PNCs, our single-particle studies reveal substantial changes in the PL blinking statistics and the persistent appearance of the long-lived, "delayed" PL components. The blinking patterns exhibit a modification from the fast switching between fluorescent ON and OFF states found in bare PNCs to a behavior with longer ON states and more isolated OFF states in alumina-encapsulated PNCs. Controlled exposure of 0D nanocrystals to moisture suggests that the observed PL lifetime changes may be related to water-induced "reservoir" states that allow for longer-lived charge storage with subsequent back-feeding into the emissive states. Viable encapsulation of PNCs with metal oxides that can preserve and even enhance their PL properties can be utilized in the fabrication of extended structures on their basis for optoelectronic and photonic applications.

Highly Luminescent and Water-Resistant CsPbBr3-CsPb2Br5 Perovskite Nanocrystals Coordinated with Partially Hydrolyzed Poly(methyl methacrylate) and Polyethylenimine

All inorganic lead halide perovskite nanocrystals (PNCs) typically suffer from poor stability against moisture and UV radiation as well as degradation during thermal treatment. The stability of PNCs can be significantly enhanced through polymer encapsulation, often accompanied by a decrease of photoluminescence quantum yield (PLQY) due to the loss of highly dynamic oleylamine/oleic acid (OLA/OA) ligands. Herein, we propose a solution for this problem by utilizing partially hydrolyzed poly(methyl methacrylate) (h-PMMA) and highly branched poly(ethylenimine) (b-PEI) as double ligands stabilizing the PNCs already during the mechanochemical synthesis (grinding). The hydrophobic polymer of h-PMMA imparts excellent film-forming properties and water stability to the resulting NC-polymer composite. In its own turn, the b-PEI forms an amino-rich, strongly binding ligand layer on the surface of the PNCs being responsible for the significant improvement of the PLQY and the stability of the resulting material. Moreover, the introduction of b-PEI promotes a partial phase conversion from CsPbBr₃ to CsPb₂Br₅ to obtain CsPbBr₃/CsPb₂Br₅ nanocrystals with a core-shell-like structure. As-prepared PNCs solutions are directly processable as inks, while their PLQY drops only slightly from 75% in colloidal solution to 65% in films. Moreover, the final PNC-polymer film exhibits excellent stability against water, heat, and ultraviolet light irradiation. These superior properties allowed us to fabricate a proof of concept thin film OLED with h-PMMA/b-PEI-stabilized PNCs as an easily processable, narrowly emitting color conversion composite material.

The dynamic surface chemistry of colloidal metal chalcogenide quantum dots

The chemical species (ligands) at the surface of colloidal inorganic semiconductor nanocrystals (QDs) mediate their interactions with the surroundings. The solvation of the QDs reflects a subtle interplay between ligand-solvent and ligand-ligand interactions, which eventually compete with the coordination of the ligands at the QD surface. The QD surface coordination and solvation are indeed fundamental to preserve their optoelectronic properties and to foster the effective application of QD-based inks and nanocomposites. Here we investigate such ligand interactions by exploiting diffusion ordered NMR spectroscopy (DOSY), which is suggested as an essential complement to spectral line width analysis. To this end, we use colloidal metal chalcogenide (CdS, CdSe, and PbS) QDs with (metal-)oleate ligands at their surface in several solvents exhibiting different viscosities and polarities. We demonstrate that the ligand shell is dynamically bound to the metal chalcogenide QDs, and is thus in equilibrium between the QD surface and the surrounding solvent. Such dynamic equilibria depend on ligand-solvent interactions, which are more prominent in aliphatic, rather polar solvents that favor the solvation of the ligands and, as a consequence, their displacement from the QD surface. In addition, the ligand-ligand interactions, which are more relevant for larger QDs, contribute to the stabilization of the ligand bonding at the QD surface.

Highly stable hybrid perovskite light-emitting diodes based on Dion-Jacobson structure

Organic-inorganic hybrid halide perovskites are emerging as promising materials for next-generation light-emitting diodes (LEDs). However, the poor stability of these materials has been the main obstacle challenging their application. Here, we performed first-principles calculations, revealing that the molecule dissociation energy of Dion-Jacobson (DJ) structure using 1,4-bis(aminomethyl)benzene molecules as bridging ligands is two times higher than the typical Ruddlesden-Popper (RP) structure based on phenylethylammonium ligands. Accordingly, LEDs based on the DJ structure show a half-lifetime over 100 hours, which is almost two orders of magnitude longer compared with those based on RP structural quasi-two-dimensional perovskite. To the best of our knowledge, this is the longest lifetime reported for all organic-inorganic hybrid perovskites operating at the current density, giving the highest external quantum efficiency (EQE) value. In situ tracking of the film composition in operation indicates that the DJ structure was maintained well after continuous operation under an electric field.