Abstract: Copper chalcogenides find applications in different domains including photonics, photothermal therapy and photovoltaics. CuTe nanocrystals have been proposed as an alternative to noble metal particles for plasmonics. Although it is known that deviations from stoichiometry are a prerequisite for plasmonic activity in the near-infrared, an accurate description of the material and its (optical) properties is hindered by an insufficient understanding of the atomic structure and the influence of defects, especially for materials in their nanocrystalline form. We demonstrate that the structure of Cu1.5±xTe nanocrystals canbe determined using electron diffraction tomography. Real-space high-resolution electron tomography directly reveals the three-dimensional distribution of vacancies in the structure. Through first-principles density functional theory, we furthermore demonstrate that the influence of these vacancies on the optical properties of the nanocrystals is determined. Since our methodology is applicable to a variety of crystalline nanostructured materials, it is expected to provide unique insights concerning structure–property correlations.

Abstract: In materials science, high angle annular dark field scanning transmission electron microscopy is often used for tomography at the nanometer scale. In this work, it is shown that a thickness dependent, non-linear damping of the recorded intensities occurs. This results in an underestimated intensity in the interior of reconstructions of homogeneous particles, which is known as the cupping artifact. In this paper, this non-linear effect is demonstrated in experimental images taken under common conditions and is reproduced with a numerical simulation. Furthermore, an analytical derivation shows that these non-linearities can be inverted if the imaging is done quantitatively, thus preventing cupping in the reconstruction.

Abstract: Nanometer sized ZnO clusters are produced in the gas phase and subsequently deposited on clean Au(111) surfaces under ultra-high vacuum conditions. The zinc blende atomic structure of the approximately spherical ZnO clusters is resolved by high resolution scanning transmission electron microscopy. The large band gap and weak n-type conductivity of individual clusters are determined by scanning tunnelling microscopy and spectroscopy at cryogenic temperatures. The conduction band is found to exhibit clear quantization into discrete energy levels, which can be related to finite-size effects reflecting the zero-dimensional confinement. Our findings illustrate that gas phase cluster production may provide unique possibilities for the controlled fabrication of high purity quantum dots and heterostructures that can be size selected prior to deposition on the desired substrate under controlled ultra-high vacuum conditions.

Abstract: Very uniform and well shaped Mn3O4 nano-octahedra are synthesized using a simple hydrothermal method under the help of polyethylene glycol (PEG200) as a reductant and shape-directing agent. The nano-octahedra formation mechanism is monitored. The shape and crystal orientation of the nanoparticles is reconstructed by scanning electron microscopy and electron tomography, which reveals that the nano-octahedra only selectively expose {101} facets at the external surfaces. The magnetic testing demonstrates that the Mn3O4 nano-octahedra exhibit anomalous magnetic properties: the Mn3O4 nano-octahedra around 150 nm show a similar Curie temperature and blocking temperature to Mn3O4 nanoparticles with 10 nm size because of the vertical axis of [001] plane and the exposed {101} facets. With these Mn3O4 nano-octahedra as a catalyst, the photodecomposition of rhodamine B is evaluated and it is found that the photodecomposition activity of Mn3O4 nano-octahedra is much superior to that of commercial Mn3O4 powders. The anomalous magnetic properties and high superior photodecomposition activity of well shaped Mn3O4 nano-octahedra should be related to the special shape of the nanoparticles and the abundantly exposed {101} facets at the external surfaces. Therefore, the shape preference can largely broaden the application of the Mn3O4 nano-octahedra.

Abstract: Pb cations in PbS quantum rods made from CdS quantum rods by successive complete cationic exchange reactions are partially re-exchanged for Cd cations. Using STEM-HAADF, we show that this leads to the formation of unique multiple dot-in-rod PbS/CdS heteronanostructures, with a photoluminescence quantum yield of 4555%. We argue that the formation of multiple dot-in-rods is related to the initial polycrystallinity of the PbS quantum rods, where each PbS crystallite transforms in a separate PbS/CdS dot-in-dot. Effective mass modeling indicates that electronic coupling between the different PbS conduction band states is feasible for the multiple dot-in-rod geometries obtained, while the hole states remain largely uncoupled.

Abstract: Branched nanostructures are of great interest because of their promising optical and electronic properties. For successful and reliable integration in applications such as photovoltaic devices, the thermal stability of the nanostructures is of major importance. Here the different domains (CdSe cores, CdS pods) of the heterogeneous octapods are shown to have different thermal stabilities, and heating is shown to induce specific shape changes. The octapods are heated from room temperature to 700 °C, and investigated using (analytical and tomographic) transmission electron microscopy (TEM). At low annealing temperatures, pure Cd segregates in droplets at the outside of the octapods, indicating non-stochiometric composition of the octapods. Furthermore, the tips of the pods lose their faceting and become rounded. Further heating to temperatures just below the sublimation temperature induces growth of the zinc blende core at the expense of the wurtzite pods. At higher temperatures, (500700 °C), sublimation of the octapods is observed in real time in the TEM. Three-dimensional tomographic reconstructions reveal that the four pods pointing into the vacuum have a lower thermal stability than the four pods that are in contact with the support.

Abstract: Using electron tomography combined with electron energy loss spectroscopy (EELS), we are able to map the valency of the Ce ions in CeO2-x nanocrystals in three dimensions. Our results show a clear facet-dependent reduction shell at the surface of ceria nanoparticles; {111} surface facets show a low surface reduction, whereas at {001} surface facets, the cerium ions are more likely to be reduced over a larger surface shell. Our generic tomographic technique allows a full 3D data cube to be reconstructed, containing an EELS spectrum in each voxel. This possibility enables a three-dimensional investigation of a plethora of material-specific physical properties such as valency, chemical composition, oxygen coordination, or bond lengths, triggering the synthesis of nanomaterials with improved properties.

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: Surface plasmon modes in metallic nanostructures largely determine their optoelectronic properties. Such plasmon modes can be manipulated by changing the morphology of the nanoparticles or by bringing plasmonic nanoparticle building blocks close to each other within organized assemblies. We report the EELS mapping of such plasmon modes in pure Ag nanocubes, Au@Ag coreshell nanocubes, and arrays of Au@Ag nanocubes. We show that these arrays enable the creation of interesting plasmonic structures starting from elementary building blocks. Special attention will be dedicated to the plasmon modes in a triangular array formed by three nanocubes. Because of hybridization, a combination of such nanotriangles is shown to provide an antenna effect, resulting in strong electrical field enhancement at the narrow gap between the nanotriangles.

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: Oriented attachment of PbSe nanocubes can result in the formation of two-dimensional (2D) superstructures with long-range nanoscale and atomic order. This questions the applicability of classic models in which the superlattice grows by first forming a nucleus, followed by sequential irreversible attachment of nanocrystals, as one misaligned attachment would disrupt the 2D order beyond repair. Here, we demonstrate the formation mechanism of 2D PbSe superstructures with square geometry by using in situ grazing-incidence X-ray scattering (small angle and wide angle), ex situ electron microscopy, and Monte Carlo simulations. We observed nanocrystal adsorption at the liquid/gas interface, followed by the formation of a hexagonal nanocrystal monolayer. The hexagonal geometry transforms gradually through a pseudo-hexagonal phase into a phase with square order, driven by attractive interactions between the {100} planes perpendicular to the liquid substrate, which maximize facet-to-facet overlap. The nanocrystals then attach atomically via a necking process, resulting in 2D square superlattices.

Abstract: Magnetic hyperthermia has the potential to play an important role in cancer therapy and its efficacy relies on the nanomaterials selected. Superparamagnetic iron oxide nanoparticles (SPIONs) are excellent candidates due to the ability of producing enough heat to kill tumor cells by thermal ablation. However, their heating properties depend strongly on crystalline structure and size, which may not be controlled and tuned during the synthetic process; therefore, a postprocessing is needed. We show how thermal annealing can be simultaneously coupled with ligand exchange to stabilize the SPIONs in polar solvents and to modify their crystal structure, which improves hyperthermia behavior. Using high-resolution transmission electron microscopy, X-ray diffraction, Mossbauer spectroscopy, vibrating sample magnetometry, and lock-in thermography, we systematically investigate the impact of size and ligand exchange procedure on crystallinity, their magnetism, and heating ability. We describe a valid and simple approach to optimize SPIONs for hyperthermia by carefully controlling the size, colloidal stability, and crystallinity.