High-angle Dark-field Scanning Research Articles

Characterization of L12-Al3Ce phase and its purification mechanism in the Al-Ce-TiCN alloy

Rare earth element Ce has a positive purifying effect on Fe and Si impurities in pure aluminum. In this study, we unveiled the mechanism of Ce purification of Fe and Si impurities in commercial pure aluminum at the atomic scale via utilizing the properties of TiCN nanoparticles, which exhibit distribution along grain boundaries and impede solute atom diffusion at specific cooling rates. The transition phase L12-Al3Ce was characterized in aluminum, which is considered to be an important transition phase to purified products. Furthermore, High Angle Dark Field Scanning Transmission Electron Microscopy (HADDF-STEM) and 3D Atom Probe Tomography (3D-APT) results showed that Ce could respectively enrich the Fe and Si impurities, resulting in the formation of Al-Ce-Fe and Al-Ce-Si clusters. Density functional theory (DFT) results indicated that Fe and Si atoms can incorporate into L12-Al3Ce crystal, forming more stable structures and therefore giving rise to the formation of Al-Ce-Si and Al-Ce-Fe nanoclusters. This study provides atomic-scale insights into the mechanism of Ce purifying Fe and Si impurities in aluminum.

Структурные домены аустенита нестехиометрических сплавов Гейслера на основе Ni-Mn-In

The relationship between antiphase boundaries (APBs), antiphase domains (APDs) and the functional properties of alloys of the Ni-Mn-In system has been studied. The density of APBs was controlled using various thermal treatments. The fine domain structure was observed by transmission electron microscopy (TEM), high-angle dark-field scanning transmission electron microscopy (HAADF-STEM), and electron backscatter diffraction (EBSD). The results obtained are discussed in the context of the possibility of modeling and designing materials with specified characteristics and properties.

Mg29-xPt4+y: Chemical Bonding Inhomogeneity and Structural Complexity.

Mg29–xPt4+y represents the family of complex intermetallic compounds (complex metallic alloys, CMAs). It crystallizes in the cubic non-centrosymmetric space group F4̅3m with a = 20.1068(2) Å and around 400 atoms in a predominantly ordered arrangement. The local disorder around the unit cell origin is experimentally resolved by single-crystal X-ray diffraction in combination with atomic-resolution transmission electron microscopy (TEM, high-angle dark-field scanning TEM) studies. The quantum theory of atoms in molecules-based analysis of atomic charges shows that the unusual mixed Mg/Pt site occupation around the origin results from local charge equilibration in this region of the crystal structure. Chemical bonding analysis reveals for Mg29–xPt4+y—rather unexpected for a crystal structure of this size—space-separated regions of hetero- and homoatomic bonds involving three to six partners (bonding inhomogeneity). Pt-containing 11- and 13-atomic units formed by heteroatomic 3a-, 4a-, and 5a-bonds are condensed via edges and faces to large super-tetrahedrons, which are interlinked by Mg-only 6a-bonds. Spatial separation of the regions with different bonding features is the key difference between the title compound and other CMAs, which are characterized by a predominantly homogeneous distribution of heteroatomic bonds.

Self-organization mechanisms in a Fe-Au film: from isolated core-shell to multicore nanoparticles

Many nanotechnological applications necessitate a high density of nanoparticles (NPs), making NP morphology control highly challenging. In this work, the morphology of bimetallic NPs formed by magnetron sputtering deposition of a Fe(3 nm)-Au(2 nm) bilayer film on an amorphous silica substrate is analyzed using high-angle dark-field scanning transmission electron microscopy (HAADF-STEM). While all the NPs adopt a Fe-Au core-shell chemical order, they can be sorted into three different types. Isolated NPs, displaying either a highly symmetric centered core geometry (CC-type) or an asymmetrical off-centered core geometry (OC-type), are observed despite the rather large metallic volume. The majority of the NPs however displays a multicore geometry, with Fe cores in the 10-12 nm range, larger than the NPs observed in a pure Fe (3nm) film grown under identical conditions. The driving forces leading to the different morphologies are discussed together with the role played by the environnement.

Characterization of nanomaterials with transmission electron microscopy

The field of nanotechnology is about research and development on materials whose at least one dimension is in the range of 1 to 100 nanometers. In recent years, the research activity for developing nano-materials has grown exponentially owing to the fact that they offer better solutions to the challenges faced by various fields such as energy, food, and environment. In this paper, the importance of transmission electron microscopy (TEM) based techniques is demonstrated for investigating the properties of nano-materials. Specifically the nano-materials that are investigated in this report include gold nano-particles (Au-NPs), silver atom-clusters (Ag-ACs), tantalum single-atoms (Ta-SAs), carbon materials functionalized with iron cobalt (Fe-Co) NPs and titania (TiO2) NPs, and platinum loaded Ceria (Pt-CeO2) Nano composite. TEM techniques that are employed to investigate nano-materials include aberration corrected bright-field TEM (BF-TEM), high-angle dark-field scanning TEM (HAADF-STEM), electron energy-loss spectroscopy (EELS), and BF-TEM electron tomography (ET). With the help presented of results in this report, it is proved herein that as many TEM techniques as available in a given instrument are essential for a comprehensive nano-scale analysis of nanomaterials.

The strain models of misfit dislocations at cubic semiconductors hetero-interfaces

The misfit dislocations at cubic semiconductors hetero-interfaces were quantitatively investigated by high angle dark field scanning transmission electron microscopy in combination with geometric phase analysis and dislocation density tensor analysis. A detailed comparison of the extracted information with the available dislocation models (Peierls-Nabarro and Foreman model) in bulk materials shows that these models can be adapted to the misfit dislocations at lattice mismatched interface. A typical illustration is made for GaSb/GaAs, where it is shown that the strain fields of the 60° dislocation follows the Foreman model (a = 1.8), in case of the Lomer and 60° dislocation pair, the Foreman (a = 2.5) and Peierls-Nabarro model apply for εxx and εyy, respectively.

The source of the threading dislocation in GaSb/GaAs hetero-structures and their propagation mechanism

The misfit and threading dislocations during GaSb epitaxy on GaAs substrate were investigated by weak beam dark field and high angle dark field scanning transmission electron microscopy. The geometric phase analysis and dislocation density tensor analysis were next used to analyze the strain around the threading dislocations and quantify the corresponding Burgers vectors. It is shown that there are three types of threading dislocations (mixed, edge, and pair of mixed types) close to the interface, which originate from the 60°, Lomer, and 60° pair misfit dislocations, respectively. During the growth of the epitaxial layer, the edge type as well as the pair of mixed threading dislocations split into two mixed type dislocations for the glide in {111} planes. Eventually, only mixed type dislocations have been observed at the surface of thick GaSb layers.

Tin-alloy heterostructures encapsulated in amorphous carbon nanotubes as hybrid anodes in rechargeable lithium ion batteries

Sn anode in rechargeable lithium ion batteries (LIBs) is currently being intensely investigated due to high reversible capacity and energy density as compared to the commercialized graphite anode. However, large volume change upon cycling causes poor cyclic performance, which prevents the practical application of Sn anode in LIBs. In this study, the nanosized MxSn (M=Ni, Fe, and Cr) alloys were encapsulated in amorphous carbon nanotubes (ACNTs) creating hybrid anode heterostructures. The structure of the hybrid anodes was confirmed by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) and high angle dark field scanning transmission electron microscope (HAADF-STEM), and demonstrated that Ni3Sn4, FeSn2, and Cr2Sn3 alloys exist in the hybrid anodes as both nanowires and nanoparticles. The galvanostatic cycling originating from over 330 charge–discharge cycles indicated that encapsulation of Ni3Sn4, FeSn2, and Cr2Sn3 into ACNTs results in surprisingly excellent cycling performance, high rate capability, and increased initial coulombic efficiency (81.4%). Ex situ HAADF-STEM images of anodes after cycles showed that one-dimensional ACNTs as well as electrochemically inactive phase M (Ni, Fe, and Cr) in MxSn function as good matrices, offering “buffer zone” to effectively accommodate the mechanical stress induced by Sn anode expansion and shrinkage. Importantly, ACNTs enable electrical contact of Sn nanoparticles with the current collectors. Therefore, our design can significantly overcome electrochemical degradation of anodes with large volume change, resulting in increased LIB performance.

Stability of Supported Organometallic Clusters Probed by a Mass-Sensitive TEM Technique

Using a refined quantitative TEM technique, we measure the relative mass of individual organometallic rhenium clusters on a graphite support. We observe that whereas 6-Re atom species preserve integrity on precipitation from solution, fragmentation and sintering occur in 7- and 8-Re atoms cluster species. These organometallics are candidates as catalytic materials. The technique is based on high-angle dark-field scanning transmission electron microscope (STEM) imaging. The emphasis in this analysis is on using very high angles of scattering to suppress coherence effects from these clusters and on reliably measuring their relative experimental elastic scattering cross sections.

Optimum collection angle for detection of small catalyst particles in high-angle dark-field Scanning Transmission Electron Microscopy

Since Crewe et al demonstrated the detection of single heavy atoms on thin carbon support films, there has been considerable interest in the use of high angle annular dark field detectors(HADD) to study particles in the subnanometer size range. A recent contribution to this subject is the work of Treacy and Rice.Increased interest in this technique can be attributed to the minimization of diffraction and phase contrast effects in the imaging of small atom catalyst particles .In this study we calculate the expected signal using realistic electron scattering cross sections. For the high angle dark field considered, it is reasonable to assume single scattering and to consider dynamical effects averaged out by the averaging over annular aperture.The electron scattering amplitude used is given by the Mott formula;