Competitive Nucleation Mechanism for CsPbBr3 Perovskite Nanoplatelet Growth

Antisolvent controls the shape and size of anisotropic lead halide perovskite nanocrystals

Colloidal lead halide perovskite nanocrystals have potential for lighting applications due to their optical properties. Precise control of the nanocrystal dimensions and composition is a prerequisite for establishing practical applications. However, the rapid nature of their synthesis precludes a detailed understanding of the synthetic pathways, thereby limiting the optimisation. Here, we deduce the formation mechanisms of anisotropic lead halide perovskite nanocrystals, 1D nanorods and 2D nanoplatelets, by combining in situ X-ray scattering and photoluminescence spectroscopy. In both cases, emissive prolate nanoclusters form when the two precursor solutions are mixed. The ensuing antisolvent addition induces the divergent anisotropy: The intermediate nanoclusters are driven into a dense hexagonal mesophase, fusing to form nanorods. Contrastingly, nanoplatelets grow freely dispersed from dissolving nanoclusters, stacking subsequently in lamellar superstructures. Shape and size control of the nanocrystals are determined primarily by the antisolvent's dipole moment and Hansen hydrogen bonding parameter. Exploiting the interplay of antisolvent and organic ligands could enable more complex nanocrystal geometries in the future.

In situcell-scale probing nucleation, growth and evolution of CsPbBr3nanowires by optical absorption spectroscopy

The growth kinetics of colloidal lead halide perovskite nanomaterials are an integral part of their applications, remains poorly understood due to complex nucleation processes and lack ofin situsize monitoring method. Here we demonstrated that absorption spectra can be used to observein situgrowth processes of ultrathin CsPbBr3nanowires in solution with reference to the effective mass infinite deep square potential well model. By means of this method, we have found that the ultrathin nanowires, fabricated by hot injection method, were firstly formed within one minute. Subsequently, they merge with each other into a thicker structure with increasing reaction time. We revealed that the nucleation, growth, and merging of the CsPbBr3nanowires are determined by the acid concentration and ligand chain length. At lower acidity, the critical nucleation size of the nanowire is smaller, while the shorter the ligand chain length, the faster the merging among the nanowires. Moreover, the merging mode between nanowires changed with their nucleation size. This growth kinetics of CsPbBr3nanowires provides a reference for optimizing the synthesis conditions to obtain the one-dimensional CsPbBr3with desired size, thus enabling accurate control of the nanowire shape.

Kinetically Controlled Synthesis of Quasi-Square CsPbI3 Nanoplatelets with Excellent Stability

Nanoplatelets (NPLs) share excellent luminescent properties with their symmetric quantum dots counterparts and entail special characters benefiting from the shape, like the thickness-dependent bandgap and anisotropic luminescence. However, perovskite NPLs, especially those based on iodide, suffer from poor spectral and phase stability. Here, stable CsPbI3 NPLs obtained by accelerating the crystallization process in ambient-condition synthesis are reported. By this kinetic control, the rectangular NPLs into quasi-square NPLs are tuned, where enlarged width endows the NPLs with a lower surface-area-to-volume ratio (S/V ratio), leading to lower surficial energy and thus improved endurance against NPL fusion (cause for spectral shift or phase transformation). The accelerated crystallization, denoting the fast nucleation and short period of growth in this report, is enabled by preparing a precursor with complete transformation of PbI2 into intermediates (PbI3 -), through an additional iodide supplier (e.g., zinc iodide). The excellent color stability of the materials remains in the light-emitting diodes under various bias stresses.

Synthesis of Organic-Inorganic Hybrid Perovskite/MOF Composites from Pb-MOF Using a Mechanochemical Method

The specific structure and diverse properties of hybrid organic-inorganic perovskite materials make them suitable for use in photovoltaic and sensing fields. In this study, environmentally stable organic-inorganic hybrid perovskite luminescent materials using Pb-MOF as a particular lead source were prepared using a mechanochemical method. Based on the fluorescence intensity of the MAPbBr₃/MOF composite, the mechanized chemical preparation conditions of Pb-MOF were optimized using response surface methodology. Then, the morphological characteristics of the MAPbBr₃/MOF composite at different stages were analyzed using electron microscopy to explore its transformation and growth process. Furthermore, the composite form of MAPbBr₃ with Pb-MOF was studied using XRD and XPS, and the approximate content of MAPbBr₃ in the composite material was calculated. Benefiting from the increase in reaction sites generated from the crush of Pb-MOF during mechanical grinding, more MAPbBr₃ was generated with a particle size of approximately 5.2 nm, although the morphology of the composite was significantly different from the initial Pb-MOF. Optimal performance of MAPbBr₃/MOF was obtained from Pb-MOF prepared under solvent-free conditions, with a milling time of 30 min, milling frequency of 30 Hz and ball-material of 35:1. It was also confirmed that the mechanochemical method had a good universality in preparing organic-inorganic hybrid perovskite/MOF composites.

Synergistic luminescence effect and high-pressure optical properties of CsPbBr2Cl@EuMOFs nanocomposites

Metal-organic frameworks (MOFs) are a class of highly porous materials that have garnered significant attention in the field of optoelectronics due to their exceptional properties. In this study, CsPbBr₂Cl@EuMOFs nanocomposites were synthesized using a two-step method. The fluorescence evolution of the CsPbBr₂Cl@EuMOFs was investigated under high pressure, revealing a synergistic luminescence effect between CsPbBr₂Cl and Eu³⁺. The study found that the synergistic luminescence of CsPbBr₂Cl@EuMOFs remains stable even under high pressure, and there is no energy transfer among different luminous centers. These findings provide a meaningful case for future research on nanocomposites with multiple luminescent centers. Additionally, CsPbBr₂Cl@EuMOFs exhibit a sensitive color-changing mechanism under high pressure, making them a promising candidate for pressure calibration via the color change of the MOF materials.

Diverse CsPbI3 assembly structures: the role of surface acids

Surface ligand engineering, seed introduction and external driving forces play major roles in controlling the anisotropic growth of halide perovskites, which have been widely established in CsPbBr₃ nanomaterials. However, colloidal CsPbI₃ nanocrystals (NCs) have been less studied due to their low formation energy and low electronegativity. Here, by introducing different molar ratios of surface acids and amines to limit the monomer concentration of lead-iodine octahedra during nucleation, we report dumbbell-shaped CsPbI₃ NCs obtained by the in situ self-assembly of nanospheres and nanorods with average sizes of 89 nm and 325 nm, respectively, which showed a high photoluminescence quantum yield of 89%. Structural and surface state analyses revealed that the strong binding of benzenesulfonic acid promoted the formation of a Pb(SO₃^-)_x-rich surface of CsPbI₃ assembly structures. Furthermore, the addition of benzenesulfonic acid increases the supersaturation threshold and the solubility of PbI₂ in a high-temperature reaction system, and controls effectively the lead-iodine octahedron monomer concentration in the second nucleation stage. As a result, the as-synthesized CsPbI₃-S_n NCs exhibited different assembly morphologies and high PLQYs, among which the role of sulfonate groups can be further verified by UV absorption and surface characteristics. The strategy provides a new frontier to rationally control the surface ligand-induced self-assembly structures of perovskites.

Tailoring CsPbBr3 Growth via Non-Polar Solvent Choice and Heating Methods

This study describes an investigation of the role of non-polar solvents on the growth of cesium lead halide (CsPbX₃ X = Br and I) nanoplatelets. We employed two solvents, benzyl ether (BE) and 1-octadecene (ODE), as well as two nucleation and growth mechanisms, one-pot, facilitated by microwave irradiation (MWI)-based heating, and hot-injection, using convection. Using BE and MWI, large mesoscale CsPbBr₃ nanoplatelets were produced, whereas use of ODE produced small crystallites. Differences between the products were observed by optical spectroscopies, which showed first band edge absorptions consistent with thicknesses of ∼9 nm [∼15 monolayer (ML)] for the BE-CsPbBr₃ and ∼5 nm (∼9 ML) for ODE-CsPbBr₃. Both products had orthorhombic crystal structures, with the BE-CsPbBr₃ revealing significant preferred orientation diffraction signals consistent with the asymmetric and two-dimensional platelet morphology. The differences in the final morphology were also observed for products formed via hot injection, with BE-CsPbBr₃ showing thinner square platelets with thicknesses of ∼2 ML and ODE-CsPbBr₃ showing similar morphologies and small crystallite sizes. To understand the role solvent plays in crystal growth, we studied lead plumbate precursor (PbBr_n^2-n) formation in both solvents, as well as solvent plus ligand solutions. The findings suggest that BE dissolves PbBr₂ salts to a higher degree than ODE, and that this BE to precursor affinity persists during growth.

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.

State of the Art and Prospects for Halide Perovskite Nanocrystals

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

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

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

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.

Ligand-engineered bandgap stability in mixed-halide perovskite LEDs

Lead halide perovskites are promising semiconductors for light-emitting applications because they exhibit bright, bandgap-tunable luminescence with high colour purity^1,2. Photoluminescence quantum yields close to unity have been achieved for perovskite nanocrystals across a broad range of emission colours, and light-emitting diodes with external quantum efficiencies exceeding 20 per cent-approaching those of commercial organic light-emitting diodes-have been demonstrated in both the infrared and the green emission channels^1,3,4. However, owing to the formation of lower-bandgap iodide-rich domains, efficient and colour-stable red electroluminescence from mixed-halide perovskites has not yet been realized^5,6. Here we report the treatment of mixed-halide perovskite nanocrystals with multidentate ligands to suppress halide segregation under electroluminescent operation. We demonstrate colour-stable, red emission centred at 620 nanometres, with an electroluminescence external quantum efficiency of 20.3 per cent. We show that a key function of the ligand treatment is to 'clean' the nanocrystal surface through the removal of lead atoms. Density functional theory calculations reveal that the binding between the ligands and the nanocrystal surface suppresses the formation of iodine Frenkel defects, which in turn inhibits halide segregation. Our work exemplifies how the functionality of metal halide perovskites is extremely sensitive to the nature of the (nano)crystalline surface and presents a route through which to control the formation and migration of surface defects. This is critical to achieve bandgap stability for light emission and could also have a broader impact on other optoelectronic applications-such as photovoltaics-for which bandgap stability is required.

Reverse Coarsening and the Control of Particle Size Distribution through Surfactant

The minimization of surface area, as a result of the minimization of (positive) surface energy, is a well-known driving force behind the spontaneous broadening of (nano) particle size distribution. We show that surfactant molecules binding to particle surfaces effectively decrease the surface energy and may change its sign. In this case, contrary to the expected broadening behavior, a minimum of free energy is achieved at the maximum surface area for all particles, i.e., when the particles are identical. Numerical simulations based on the classical Lifshitz–Slyozov–Wagner theory with surfactant-induced surface energy renormalization confirm the collapse of the particle size distribution. As the particle size evolution is much slower than particle nucleation and growth, the manipulation of surface energy with in-situ replacement of surfactant molecules provides a method for controlling particle size distribution with great potential for creating mono-disperse nanoparticles, a key goal of nanotechnology.