Reversible Emission Tunability from 2D‐Layered Perovskites with Conjugated Organic Cations

The structural flexibility of 2D‐layered halide perovskites provides unprecedented opportunities for tuning their optical properties. For example, lattice distortions facilitate white emission that stems from self‐trapped excitons or defects, and organic cations and halides determine structural stability and emission range. Herein, the optical properties of a set of single‐layer thiophene‐based 2D lead bromide platelets are investigated. Blue‐ and white‐emitting materials based on the choice of thiophene cation and HBr concentration in the synthesis and reversible white to blue color switching by sequential washing and precursor exposure of the fabricated samples are obtained. The photophysical and structural studies indicate that the key to color switching is the formation and suppression of self‐trapped excitons by the supply and removal of cations and halides in acetone. The range of emission color from these materials is extended to red by efficient Mn doping that leads to an additional strong emission peak centered at 620 nm. The findings stimulate the development of color‐tunable and switchable light emitters based on a single material

Про конституційно-правові засади національного суверенітету в Україні

Розглядаються характеристики народу та нації як суб’єктів конституційно-правових відносин, зв’язок етнічного і політичного чинників у змісті національного суверенітету.Рассматриваются характеристики народа и нации как субъектов конституцион­но-правовых отношений, связь этнического и политического в содержании националь­ного суверенитета.Describe the nations and the peoples as subjects of constitutionals law relatios, the con­nection between ethnos and political in content of national sovereignty

Understanding and tailoring ligand interactions in the self-assembly of branched colloidal nanocrystals into planar superlattices

Colloidal nanocrystals can self-assemble into highly ordered superlattices. Recent studies have focused on changing their morphology by tuning the nanocrystal interactions via ligand-based surface modification for simple particle shapes. Here we demonstrate that this principle is transferable to and even enriched in the case of a class of branched nanocrystals made of a CdSe core and eight CdS pods, so-called octapods. Through careful experimental analysis, we show that the octapods have a heterogeneous ligand distribution, resembling a cone wrapping the individual pods. This induces location-specific interactions that, combined with variation of the pod aspect ratio and ligands, lead to a wide range of planar superlattices assembled at an air–liquid interface. We capture these findings using a simple simulation model, which reveals the necessity of including ligand-based interactions to achieve these superlattices. Our work evidences the sensitivity that ligands offer for the self-assembly of branched nanocrystals, thus opening new routes for metamaterial creation

Temperature Driven Transformation of CsPbBr3_3 Nanoplatelets into Mosaic Nanotiles in Solution through Self-Assembly

Two-dimensional colloidal halide perovskite nanocrystals are promising materials for light emitting applications. In addition, they can be used as components to create a variety of materials through physical and chemical transformations. Recent studies focused on nanoplatelets that are able to self-assemble and transform on solid substrates. Yet, the mechanism behind the process and the atomic arrangement of their assemblies remain unclear. Here, we present the transformation of self-assembled stacks of CsPbBr$_3$ nanoplatelets in solution, capturing the different stages of the process by keeping the solutions at room temperature and monitoring the nanocrystal morphology over a period of a few months. 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 particles surfaces, causing the merging of nanoplatelet stacks, which first form nanobelts; ii) merging of neighboring nanobelts that form more extended nanoplates; and iii) attachment of nanobelts and nanoplates, which create objects with an atomic structure that resemble a mosaic made of broken nanotiles. We reveal that the starting nanoplatelets merge seamlessly and defect-free on an atomic scale in small and thin nanobelts. However, aged nanobelts and nanoplates, which are mainly stabilized by amine/ammonium ions, link through a bilayer of CsBr. In this case, the atomic columns of neighboring perovskite lattices shift by a half-unit-cell, forming Ruddlesden-Popper planar faults.Comment: 28 pages, 5 Figure

Core/Shell CdSe/CdS bone‐shaped nanocrystals with a thick and anisotropic shell as optical emitters

Colloidal core/shell nanocrystals are key materials for optoelectronics, enabling control over essential properties via precise engineering of the shape, thickness, and crystal structure of their shell. Here, the growth protocol for CdS branched nanocrystals is applied on CdSe nanoplatelet seeds and bone-shaped heterostructures are obtained with a highly anisotropic shell. Surprisingly, the nanoplatelets withstand the high growth temperature of 350 degrees C and structures with a CdSe nanoplatelet core that is overcoated by a shell of cubic CdS are obtained, on top of which tetrahedral CdS structures with hexagonal lattice are formed. These complex core/shell nanocrystals show a band-edge emission around 657 nm with a photoluminescence quantum yield of approximate to 42% in solution, which is also retained in thin films. Interestingly, the nanocrystals manifest simultaneous red and green emission and the relatively long wavelength of the green emission indicates charge recombination at the cubic/hexagonal interface of the CdS shell. The nanocrystal films show amplified spontaneous emission, random lasing, and distributed feedback lasing when the material is deposited on suitable gratings. This work stimulates the design and fabrication of more exotic core/shell heterostructures where charge carrier delocalization, dipole moment, and other optical and electrical properties can be engineered

Real-Time In Situ Observation of CsPbBr3 Perovskite Nanoplatelets Transforming into Nanosheets

The manipulation of nano-objects through heating is an effective strategy for inducing structural modifications and therefore changing the optoelectronic properties of semiconducting materials. Despite its potential, the underlying mechanism of the structural transformations remains elusive, largely due to the challenges associated with their in situ observations. To address these issues, we synthesize temperature-sensitive CsPbBr3 perovskite nanoplatelets and investigate their structural evolution at the nanoscale using in situ heating transmission electron microscopy. We observe the morphological changes that start from the self-assembly of the nanoplatelets into ribbons on a substrate. We identify several paths of merging nanoplates within ribbons that ultimately lead to the formation of nanosheets dispersed randomly on the substrate. These observations are supported by molecular dynamics simulations. We correlate the various paths for merging to the random orientation of the initial ribbons along with the ligand mobility (especially from the edges of the nanoplatelets). This leads to the preferential growth of individual nanosheets and the merging of neighboring ones. These processes enable the creation of structures with tunable emission, ranging from blue to green, all from a single material. Our real-time observations of the transformation of perovskite 2D nanocrystals reveal a route to achieve large-area nanosheets by controlling the initial orientation of the self-assembled objects with potential for large-scale applications

Understanding and tailoring ligand interactions in the self-assembly of branched colloidal nanocrystals into planar superlattices

Colloidal nanocrystals can self-assemble into highly ordered superlattices. Recent studies have focused on changing their morphology by tuning the nanocrystal interactions via ligand-based surface modification for simple particle shapes. Here we demonstrate that this principle is transferable to and even enriched in the case of a class of branched nanocrystals made of a CdSe core and eight CdS pods, so-called octapods. Through careful experimental analysis, we show that the octapods have a heterogeneous ligand distribution, resembling a cone wrapping the individual pods. This induces location-specific interactions that, combined with variation of the pod aspect ratio and ligands, lead to a wide range of planar superlattices assembled at an air-liquid interface. We capture these findings using a simple simulation model, which reveals the necessity of including ligand-based interactions to achieve these superlattices. Our work evidences the sensitivity that ligands offer for the self-assembly of branched nanocrystals, thus opening new routes for metamaterial creation