Abstract: Galvanic replacement reactions on metal nanoparticles are often used for the preparation of hollow nanostructures with tunable porosity and chemical composition, leading to tailored optical and catalytic properties. However, the precise interplay between the three-dimensional (3D) morphology and chemical composition of nanostructures during galvanic replacement is not always well understood as the 3D chemical imaging of nanoscale materials is still challenging. It is especially far from straightforward to obtain detailed information from the inside of hollow nanostructures using electron microscopy techniques such as SEM or TEM. We demonstrate here that a combination of state-of-the-art EDX mapping with electron tomography results in the unambiguous determination of both morphology transformation and elemental composition of nanostructures in 3D, during galvanic replacement of Ag nanocubes. This work provides direct and unambiguous experimental evidence toward understanding the galvanic replacement reaction. In addition, the powerful approach presented here can be applied to a wide range of nanoscale transformation processes, which will undoubtedly guide the development of novel nanostructures.

Abstract: The three-dimensional (3D) atomic structure of nanomaterials, including strain, is crucial to understand their properties. Here, we investigate lattice strain in Au nanodecahedra using electron tomography. Although different electron tomography techniques enabled 3D characterizations of nanostructures at the atomic level, a reliable determination of lattice strain is not straightforward. We therefore propose a novel model-based approach from which atomic coordinates are measured. Our findings demonstrate the importance of investigating lattice strain in 3D.

Abstract: A thorough understanding of the three-dimensional (3D) atomic structure and composition of coreshell nanostructures is indispensable to obtain a deeper insight on their physical behavior. Such 3D information can be reconstructed from two-dimensional (2D) projection images using electron tomography. Recently, different electron tomography techniques have enabled the 3D characterization of a variety of nanostructures down to the atomic level. However, these methods have all focused on the investigation of nanomaterials containing only one type of chemical element. Here, we combine statistical parameter estimation theory with compressive sensing based tomography to determine the positions and atom type of each atom in heteronanostructures. The approach is applied here to investigate the interface in coreshell Au@Ag nanorods but it is of great interest in the investigation of a broad range of nanostructures.

Abstract: It is widely accepted that the physical properties of nanostructures depend on the type of surface facets1, 2. For Au nanorods, the surface facets have a major influence on crucial effects such as reactivity and ligand adsorption and there has been controversy regarding facet indexing3, 4. Aberration-corrected electron microscopy is the ideal technique to study the atomic structure of nanomaterials5, 6. However, these images correspond to two-dimensional (2D) projections of 3D nano-objects, leading to an incomplete characterization. Recently, much progress was achieved in the field of atomic-resolution electron tomography, but it is still far from being a routinely used technique. Here we propose a methodology to measure the 3D atomic structure of free-standing nanoparticles, which we apply to characterize the surface facets of Au nanorods. This methodology is applicable to a broad range of nanocrystals, leading to unique insights concerning the connection between the structure and properties of nanostructures.

Abstract: Polyester-grafted polyhedral oligomeric silsesquioxane (POSS) nanohybrids selectively produced by ring-opening polymerization of ε-caprolactone and L,L-lactide (A.-L. Goffin, E. Duquesne, S. Moins, M. Alexandre, Ph. Dubois, Eur. Polym. Journal, 2007, 43, 4103) were studied as masterbatches by melt-blending within their corresponding commercial polymeric matrices, i.e., poly(ε-caprolactone) (PCL) and poly(L,L-lactide) (PLA). For the sake of comparison, neat POSS nanoparticles were also dispersed in PCL and PLA. The objective was to prepare aliphatic polyester-based nanocomposites with enhanced crystallization behavior, and therefore, enhanced thermo-mechanical properties. Wide-angle X-ray scattering and transmission electron microscopy attested for the dispersion of individualized POSS nanoparticles in the resulting nanocomposite materials only when the polyester-grafted POSS nanohybrid was used as a masterbatch. The large impact of such finely dispersed (grafted) nanoparticles on the crystallization behavior for the corresponding polyester matrices was noticed, as evidenced by differential scanning calorimetry analysis. Indeed, well-dispersed POSS nanoparticles acted as efficient nucleating sites, significantly increasing the crystallinity degree of both PCL and PLA matrices. As a result, a positive impact on thermo-mechanical properties was highlighted by dynamic mechanical thermal analysis.

Abstract: AlGaN/GaN heterostructures were grown on “on-axis” and 2° off (0001) 4H-SiC substrates by metalorganic vapor phase epitaxy (MOVPE). Structural characterization was performed by transmission electron microscopy. The dislocation density, being greater in the on-axis case, is gradually reduced in the GaN layer and is forming
dislocation loops in the lower region. Steps aligned along [11̅00] in the off-axis case give rise to simultaneous defect formation. In the on-axis case, an almost zero density of steps is observed, with the main origin of defects probably being the orientation mismatch at the grain boundaries between the small not fully coalesced AlN grains. V-shaped formations are observed in the AlN nucleation layer, but are more frequent in the off-axis case, probably enhanced by the presence of steps. These V-shaped formations are completely overgrown by the GaN layer, during the subsequent deposition, presenting AlGaN areas in the walls of the defect, indicating an interdiffusion between the layers. Finally, at the AlGaN/GaN heterostructure surface in the on-axis case, V-shapes are observed, with the AlN spacer and AlGaN (21% Al) thickness on relaxed GaN exceeding the critical thickness for relaxation. On the other hand, no relaxation in the form of V-shape creation is observed in the off-axis case, probably due to the smaller AlGaN thickness (less than 21% Al). The AlN spacer layer, grown in between the heterostructure, presents a uniform thickness and clear interfaces.

Abstract: Au particles were prepared by evaporation in ultra high vacuum at high temperature, on the surfaces of TiO2 micro-spheres with the anatase structure. The morphology and the structural deformation in Au deposits were studied by high resolution transmission electron microscopy and image simulations by the multislice technique. The particles were polyhedral, limited by (100) and (111) faces. Patches with a hexagonal lattice were found around the particles, which was interpreted as thin Au islands on the surface. In these islands the Au lattice was deformed and perfectly accommodated to the (110) surface of TiO2. (C) 2001 Elsevier Science B.V. All rights reserved.

Abstract: In-situ fabrication of palladium(0) nanoparticles inside zeolitic imidazolate frameworks (ZIF-8) has been established via one-step facile spray-dry technique. Crystal structures and morphologies of the Pd@ZIF-8 samples are investigated by powder XRD, TEM, SAED, STEM, and EDX techniques. High angle annular dark field scanning transmission electron microscopy (HAAD-STEM) and 3D tomographic analysis confirm the presence of palladium nanoparticles inside the ZIF-8 structure. The porosity, surface area and N-2 physisorption properties are evaluated for Pd@ZIF-8 with various palladium contents. Furthermore, Pd@ZIF-8 samples are effectively applied as heterogeneous catalysts in alkenes hydrogenation. This straightforward method is able to speed up the synthesis of encapsulation of metal nanoparticles in metal organic frameworks. (C) 2017 Elsevier B.V. All rights reserved.

Abstract: Nano-scale ordering may be revealed in transmission electron microscopy (TEM) by at least three techniques that will be presented in this work: selected area electron diffraction, conventional TEM and high-resolution TEM. Digital image processing is used to extract additional information from the high-resolution micrographs. The described methods are illustrated in a microstructural and compositional study of a 90%Pb(Mg1/3Nb2/3)O-3-10%PbTiO2 ceramic sample. High-resolution images reveal the presence of ordered compositional nano-domains, observable in two specific crystallographic orientations. Antiphase boundaries lying in the (111) planes separate them, while (100) and (111) facets separate the ordered domains from the disordered matrix.

Abstract: The combination of AgNO3 with NaH results in Ag nanoparticles that can selectively perform alcohol aminations under mild reaction conditions (110 °C). NaH not only serves as a reducing agent for the Ag salt, but also activates the alcohol for dehydrogenation to the corresponding ketone/aldehyde. The stability of the particles can be improved by immobilizing them onto mixed Al2O3Ga2O3 supports; the combination of Ga and Al provides materials with stronger Lewis acidic sites compared to pure alumina or gallium oxide supports. This leads to catalysts with enhanced activities, without the necessity of adding external Lewis acids. Detailed TEM characterization also reveals a close interaction between the Ag NPs and the gallium oxide phase. The obtained catalysts are recyclable and show activity for the alcohol amination using a variety of aliphatic and aromatic amines under mild conditions.

Abstract: Mixed-valence polycyanides (Prussian Blue analogues) possess a rich palette of properties spanning from room-temperature ferromagnetism to zero thermal expansion, which can be tuned by chemical modifications or the application of external stimuli (temperature, pressure, light irradiation). While molecule-based materials can combine physical and chemical properties associated with molecular-scale building blocks, their successful integration into real devices depends primarily on higher-order properties such as crystal size, shape, morphology, and organization. Herein a study of a new reduced-dimensionality system based on Prussian Blue analogues (PBAs) is presented. The system is built up by means of a modified Langmuir-Blodgett technique, where the PBA is synthesized from precursors in a self-limited reaction on a clay mineral surface. The focus of this work is understanding the magnetic properties of the PBAs in different periodic, low-dimensional arrangements, and the influence of the “on surface” synthesis on the final properties and dimensionality of the system.