A variety of colloidal chemical approaches has been developed in the last few decades for the controlled synthesis of nanostructured materials in either water or organic solvents. Besides the precursors, the solvents, reducing agents, and the choice of surfactants are crucial for tuning the composition, morphology and other properties of the resulting nanoparticles. The ligands employed include thiols, amines, carboxylic acids, phosphines and phosphine oxides. Generally, adding a single ligand to the reaction mixture is not always adequate to yield the desired features. In this review, we discuss in detail the role of the oleic acid/oleylamine ligand pair in the chemical synthesis of nanoparticles. The combined use of these ligands belonging to two different categories of molecules aims to control the size and shape of nanoparticles and prevent their aggregation, not only during their synthesis but also after their dispersion in a carrier solvent. We show how the different binding strengths of these two molecules and their distinct binding modes on specific facets affect the reaction kinetics toward the production of nanostructures with tailored characteristics. Additional functions, such as the reducing function, are also noted, especially for oleylamine. Sometimes, the carboxylic acid will react with the alkylamine to form an acid-base complex, which may serve as a binary capping agent and reductant; however, its reducing capacity may range from lower to much lower than that of oleylamine. The types of nanoparticles synthesized in the simultaneous presence of oleic acid and oleylamine and discussed herein include metal oxides, metal chalcogenides, metals, bimetallic structures, perovskites, upconversion particles and rare earth-based materials. Diverse morphologies, ranging from spherical nanoparticles to anisotropic, core-shell and hetero-structured configurations are presented. Finally, the relation between tuning the resulting surface and volume nanoparticle properties and the relevant applications is highlighted.

Oleic acid/oleylamine ligand pair: a versatile combination in the synthesis of colloidal nanoparticles.

Over the past few years, metal halide perovskite nanocrystals have been at the forefront of colloidal semiconductor nanomaterial research because of their fascinating properties and potential applications. However, their intrinsic phase instability and chemical degradation under external exposures (high temperature, water, oxygen, and light) are currently limiting the real-world applications of perovskite optoelectronics. To overcome these stability issues, researchers have reported various strategies such as doping and encapsulation. The doping improves the optical and photoactive phase stability, whereas the encapsulation protects the perovskite NCs from external exposures. This perspective discusses the rationale of various strategies to enhance the stability of perovskite NCs and suggests possible future directions for the fabrication of optoelectronic devices with long-term stability while maintaining high efficiency.

/pdf/enhancing-the-intrinsic-and-extrinsic-stability-of-halide-31c6rrvt.pdf

Enhancing the Intrinsic and Extrinsic Stability of Halide Perovskite Nanocrystals for Efficient and Durable Optoelectronics

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 CsPbBrxI3-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.

/pdf/elucidating-the-role-of-antisolvents-on-the-surface-1vrs4v6a.pdf

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

Phosphor-converted white-light-emitting diodes (pc-WLEDs) are a commercially successful product for energy-efficient illumination. A large number of reported phosphors exhibit high photoluminescence efficiency but struggle in pc-WLED applications. So what kind of phosphors are required to upgrade the existing pc-WLED? An answer to this question is provided here. The industry is moving toward high-power pc-WLED. For that, issues of thermal instability (at ∼200 °C) and luminescence saturation of phosphors, along with the efficiency drop of the InGaN chip, need to be addressed. Another research direction is to reduce excessive blue light from the present commercial pc-WLED, by replacing the blue-emitting InGaN chip with an ultraviolet (UV)-violet chip. Designing phosphors for high-power pc-WLED with UV-violet chip still remains a challenge. This Perspective highlights the present challenges and provides insights to address them in the future. 

Challenges and Strategies to Design Phosphors for Future White Light Emitting Diodes

: Two-dimensional (2D) colloidal perovskite CsPbBr 3 nanoplatelets (NPLs) have recently attracted attention as promising blue-emitting materials because of their large exciton binding energy and precisely controlled thickness. However, synthesizing NPLs that are homogeneous and highly ﬂ uorescent while exhibiting long-term stability has been challenging, which has limited the successful integration of such materials in optoelectronic devices (e.g., light-emitting devices). Herein, we introduce a strategy relying on surface ligand-engineering, using PbBr 2 -complexed polymers as coating ligands that impart morphological and colloidal stability while enhancing the photoluminescence quantum yield of perovskite NPLs. The polymers are salt-rich compounds that present several ammonium (or imidazolium) bromide anchoring groups and alkyl chains with di ﬀ erent lengths as solubilizing blocks. The presence of quaternary ammonium salts facilitates the dissolution of PbBr 2 in organic solutions, forming polysalt-[Pb x Br y ] 2 x − y complexes. These complexes strongly interact with the NPLs and eliminate surface defects, compared to the as-grown materials, and improve their structural and morphological stability. This strategy has yielded high-quality blue-emitting NPLs that maintain color purity (with narrow pro ﬁ les at ∼ 460 nm) with photoluminescence quantum yield (PLQY) up to 80%. Additionally, signi ﬁ cantly enhanced long-term stability of the nanoplatelets under several challenging conditions, including UV irradiation and highly diluted conditions, has been achieved. The inset shows plots of the optical densities at 450 and 320 nm as a function of the added ligand mass. Reduction in the absorption at the ﬁ rst exciton peak (at 450 nm), due to etching of the NPLs, is coupled to the formation of [Pb x Br y ] 2 x − y complexes at 320 nm.

Engineering Highly Fluorescent and Colloidally Stable Blue-Emitting CsPbBr3 Nanoplatelets Using Polysalt/PbBr2 Ligands

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. This article is protected by copyright. All rights reserved.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adma.202107105

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. 

https://www.repository.cam.ac.uk/bitstream/1810/334837/2/adma.202107105.pdf

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

We report here fast A-site cation cross-exchange between APbX 3 perovskite nanocrystals (NCs) made of different A-cations (Cs (Cesium), FA (formamidinium), and MA (methylammonium)) at room temperature. Surprisingly, the A-cation cross-exchange proceeds as fast as the halide (X=Cl, Br, or I) exchange with the help of free A-oleate complexes present in the freshly prepared colloidal perovskite NC solutions. It enabled the preparation of double (MACs, MAFA, CsFA)- and triple (MACsFA)-cation perovskite NCs with an optical bandgap that is finely tunable by their A-site composition. The optical spectroscopy together with structural analysis using XRD and atomically resolved high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and integrated differential phase contrast (iDPC) STEM indicates the homogeneous distribution of different cations in the mixed perovskite NC lattice. Unlike halide ions, the A-cations do not phase segregate under light illumination. 

/pdf/fast-a-site-cation-cross-exchange-at-room-temperature-single-2u489lg5.pdf

Bernardo Camou

Papers

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

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

Fast A‐Site Cation Cross‐Exchange at Room Temperature: Single‐to Double‐ and Triple‐Cation Halide Perovskite Nanocrystals

Composition-Tuning in Colloidal Lead Halide Perovskite Nanocrystals by cation-Exchange