Annular Electron Research Articles

Quantitative microstructural and spectroscopic investigation of inversion domain boundaries in sintered zinc oxide ceramics doped with iron oxide

Abstract It is known that sintering of powders of zinc oxide (ZnO) with small additions of iron oxide results in a ceramic with grains exhibiting a characteristic inversion domain micro-structure with planar inversion domain boundaries (IDBs) on two different habit planes. This study concentrates on a quantitative analysis, by a combination of different transmission electron microscopy methods, of those IDBs that are parallel to {0001} basal planes of the wurtzite structure of ZnO. Electron diffraction and dark-field imaging prove the nature of the inversion. High-resolution annular dark field scanning transmission electron microscopy allows measurement of the rigid body displacements across these IDBs and of the local lattice contraction related to the octahedral interstices that form the boundaries. Energy-selected imaging, electron energy-loss spectroscopy and energy-dispersive X-ray spectroscopy have been combined to determine the chemical composition of the IDBs quantitatively. It is thus shown unambiguously that every such fault consists of precisely one basal plane of octahedral interstices that are completely occupied by Fe3+ ions and that these FeO6 octahedra are themselves contracted along the <0001> direction. A local charge balance model explains the observations.

Determination of Dy substitution site in Nd2\u2212xDyxFe14B by HAADF-STEM and illustration of magnetic anisotropy of \u201cg\u201d and \u201cf\u201d sites, before and after substitution

Nd2Fe14B and Nd2−xDyxFe14B (x = 0.25, 0.50) particles were prepared by the modified co-precipitation followed by reduction–diffusion process. Bright field scanning transmission electron microscope (BF-STEM) image revealed the formation of Nd–Fe–B trigonal prisms in [− 101] viewing zone axis, confirming the formation of Nd2Fe14B/Nd2−xDyxFe14B. Accurate site for the Dy substitution in Nd2Fe14B crystal structure was determined as “f” site by using high-angle annular dark field scanning transmission electron microscope (HAADF-STEM). It was found that all the “g” sites are occupied by the Nd, meanwhile Dy occupied only the “f” site. Anti-ferromagnetic coupling at “f” site decreased the magnetic moment values for Nd1.75Dy0.25Fe14B (23.48 μB) and Nd1.5Dy0.5Fe14B (21.03 μB) as compared to Nd2Fe14B (25.50 μB). Reduction of magnetic moment increased the squareness ratio, coercivity and energy product. Analysis of magnetic anisotropy at constant magnetic field confirmed that “f” site substitution did not change the patterns of the anisotropy. Furthermore, magnetic moment of Nd2Fe14B, Nd2−xDyxFe14B, Nd (“f” site), Nd (“g” site) and Dy (“f” site) was recorded for all angles between 0° and 180°.

Novel kind of decagonal ordering in Al74Cr15Fe11

A high-angle annular dark field scanning transmission electron microscopy study of the intermetallic compound Al74Cr15Fe11 reveals a quasiperiodic structure significantly differing from the ones known so far. In contrast to the common quasi-unit-cells based on Gummelt decagons, the present structure is related to a covering formed by Lück decagons, which can also be described by a Hexagon-Bow-Tie tiling.

Sequential conversion from line defects to atomic clusters in monolayer WS2

Transition metal dichalcogenides (TMD), which is composed of a transition metal atom and chalcogen ion atoms, usually form vacancies based on the knock-on threshold of each atom. In particular, when electron beam is irradiated on a monolayer TMD such as MoS2 and WS2, S vacancies are formed preferentially, and they are aligned linearly to constitute line defects. And then, a hole is formed at the point where the successively formed line defects collide, and metal clusters are also formed at the edge of the hole. This study reports a process in which the line defects formed in a monolayer WS2 sheet expends into holes. Here, the process in which the W cluster, which always occurs at the edge of the formed hole, goes through a uniform intermediate phase is explained based on the line defects and the formation behavior of the hole. Further investigation confirms the atomic structure of the intermediate phase using annular dark field scanning transition electron microscopy (ADF-STEM) and image simulation.

Epitaxy from a Periodic Y–O Monolayer: Growth of Single-Crystal Hexagonal YAlO3 Perovskite

The role of an atomic-layer thick periodic Y–O array in inducing the epitaxial growth of single-crystal hexagonal YAlO3 perovskite (H-YAP) films was studied using high-angle annular dark-field and annular bright-field scanning transmission electron microscopy in conjunction with a spherical aberration-corrected probe and in situ reflection high-energy electron diffraction. We observed the Y–O array at the interface of amorphous atomic layer deposition (ALD) sub-nano-laminated (snl) Al2O3/Y2O3 multilayers and GaAs(111)A, with the first film deposition being three cycles of ALD-Y2O3. This thin array was a seed layer for growing the H-YAP from the ALD snl multilayers with 900 °C rapid thermal annealing (RTA). The annealed film only contained H-YAP with an excellent crystallinity and an atomically sharp interface with the substrate. The initial Y–O array became the bottom layer of H-YAP, bonding with Ga, the top layer of GaAs. Using a similar ALD snl multilayer, but with the first film deposition of three ALD-Al2O3 cycles, there was no observation of a periodic atomic array at the interface. RTA of the sample to 900 °C resulted in a non-uniform film, mixing amorphous regions and island-like H-YAP domains. The results indicate that the epitaxial H-YAP was induced from the atomic-layer thick periodic Y–O array, rather than from GaAs(111)A.

In situ atomic level studies of thermally controlled interlayer stacking shifts in 2D transition metal dichalcogenide bilayers

Abstract

Rendering hydrophobic nanoclusters water-soluble and biocompatible.

Hydrophobic and hydrophilic nanoclusters embody complementary superiorities. The means to amalgamate these superiorities, i.e., the atomic precision of hydrophobic clusters and the water dissolvability of hydrophilic clusters, remains challenging. This work presents a versatile strategy to render hydrophobic nanoclusters water-soluble—the micellization of nanoclusters in the presence of solvent-conjoined Na+ cations—which overcomes the above major challenge. Specifically, although [Ag29(SSR)12(PPh3)4]3− nanoclusters are absolutely hydrophobic, they show good dissolvability in aqueous solution in the presence of solvent-conjoined Na+ cations (Na1(NMP)5 or Na3(DMF)12). Such cations act as both counterions of these nanoclusters and surface cosolvent of cluster-based micelles in the aqueous phase. A combination of DLS (dynamic light scattering) and aberration-corrected HAADF-STEM (high angle annular dark field detector scanning transmission electron microscopy) measurements unambiguously shows that the phase-transfer of hydrophobic Ag29 into water is triggered by the micellization of nanoclusters. Owing to the excellent water solubility and stability of [Ag29(SSR)12(PPh3)4]3−[Na1(NMP)5]3+ in H2O, its performance in cell staining has been evaluated. Furthermore, the general applicability of the micellization strategy has been verified. Overall, this work presents a convenient and efficient approach for the preparation of cluster-based, biocompatible nanomaterials.

Effect of the Azimuthal Inhomogeneity of Beam Density on the Dynamics of Electron Bunches in Multiwave Cherenkov Oscillators

Data obtained in a series of computational experiments are used to study features of the formation of charged-particle bunches with strong azimuthal density inhomogeneity of the annular relativistic electron beam in multiwave Cherenkov oscillators. It is shown that the azimuthal motion of particles is negligible under the chosen parameter values. The oscillator radiation remains coherent.

Morphology visualization of irregular shape bacteria by electron holography and tomography

In the current work, irregular morphology of Staphylococcus aureus bacteria has been visualized by phase retrieval employing off-axis electron holography (EH) and 3D reconstruction electron tomography using high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Bacteria interacting with gold nanoparticles (AuNP) acquired a shrunken or irregular shape due to air dehydration processing. STEM imaging shows the attachment of AuNP on the surface of cells and suggests an irregular 3D morphology of the specimen. The phase reconstruction demonstrates that off-axis electron holography can reveal with a single hologram the morphology of the specimen and the distribution of the functionalized AuNPs. In addition, EH reduces significantly the acquisition time and the cumulative radiation damage (in three orders of magnitude) over biological samples in comparison with multiple tilted electron expositions intrinsic to electron tomography, as well as the processing time and the reconstruction artifacts that may arise during tomogram reconstruction.

Simulation of the development and interaction of instabilities in a relativistic electron beam under variation of the beam wall thickness

The development and interaction of Bursian and diocotron instabilities in an annular relativistic electron beam propagating in a cylindrical drift chamber are investigated analytically and numerically as functions of the beam wall thickness and the magnitude of the external uniform magnetic field. It is found that the interaction of instabilities results in the formation of a virtual cathode with a complicated rotating helical structure and several reflection regions (electron bunches) in the azimuthal direction. It is shown that the number of electron bunches in the azimuthal direction increases with decreasing beam wall thickness and depends in a complicated manner on the magnitude of the external magnetic field.

Comparison of the effect of annular and solid electron beams on linear and nonlinear traveling wave tube

Abstract The present paper, compares the effect of the annular and solid electron beam on the efficiency of linear and nonlinear TWTs. To do this, first we introduce four different geometric structure of the beam-helix. Then, we calculate the output power of each structure, in linear and nonlinear modes, at different frequencies using the numerical solution of the mathematical equations of the multi-frequency Eulerian model. Now, plot the output power in terms of distance for each structure at different frequencies and compare them. In a linear tube, the effect of annular beams on the output power is better than the solid beam, while this affects the frequency in nonlinear tubes. It is shown that in linear regime the power increase linearly with frequency but for nonlinear regimes is nonlinear.

Atomic-scale study of {1 1 2} twin boundary structure in a β-Ti alloy

The atomic structure of {1 1 2} twin boundary in a specialty β-Ti alloy, gum metal, has been studied by means of conventional transmission electron microscopy, bright-field and high-angle annular dark field scanning transmission electron microscopy imaging. The observations provide direct evidence that the {1 1 2} twin boundary has a complex, multiple-layer structure, which is composed of various structural units. These structural units are formed as a result of different degrees of inner atomic-column shifting. The complex structure of the {1 1 2} twin boundary is different to the atomic structure of the previously reported ω phase.

Synthesis of S-rich flower-like Fe2O3-MoS2 for Cr(VI) removal

ABSTRACTA novel Fe2O3-MoS2 nanocomposite was synthesized directly via the solvothermal method. Scanning electron microscopy (SEM) results showed the as-prepared Fe2O3-MoS2 had a uniform 3D blooming flower-like nanostructure with a MoS2 substrate. The high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) confirmed the Fe2O3 nanostructures were well-dispersed on the surface of the layered MoS2. The elemental mapping results revealed Fe, O, Mo and S elements coexisted in the Fe2O3-MoS2 nanocomposite. X-ray photoelectron spectroscopy (XPS) results displayed an S-rich MoS2 structure had been formed in the Fe2O3-MoS2 nanocomposite. As expected, the S-rich Fe2O3-MoS2 nanocomposite had better photocatalytic performance on Cr(VI) reduction than that of bare Fe2O3, MoS2 and TiO2 P25.

Synthesis and characterization of a novel binuclear iron phthalocyanine/reduced graphene oxide nanocomposite for non-precious electrocatalyst for oxygen reduction

Binuclear iron phthalocyanine/reduced graphene oxide (bi-FePc/RGO) nanocomposite with good electrocatalytic activity for ORR in alkaline medium was prepared in one step. High angle annular dark field image scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy element mapping results show bi-FePc was uniformly distributed on RGO. An obvious cathodic peak located at about −0.23 V (vs. SCE) in CV and an onset potential of −0.004 V (vs. SCE) in LSV indicate the as-prepared bi-FePc/RGO nanocomposite possesses high activity which is closed to Pt/C for ORR. The ORR on bi-FePc/RGO nanocomposite follows four-electron transfer pathway in alkaline medium. Compared with Pt/C, there is only a slight decrease (about 0.02 V vs. SCE) for bi-FePc/RGO nanocomposite when the methanol exists. The excellent activity and methanol tolerance in alkaline solutions proves that bi-FePc/RGO nanocomposite could be considered as a promising cathode catalyst for alkaline fuel cells.

Annular dark field transmission electron microscopy for protein structure determination

Recently annular dark field (ADF) transmission electron microscopy (TEM) has been advocated as a means of recording images of biological specimens with better signal to noise ratio (SNR) than regular bright field images. I investigate whether and how such images could be used to determine the three-dimensional structure of proteins given that an ADF aperture with a suitable pass-band can be manufactured and used in practice. I develop an approximate theory of ADF-TEM image formation for weak amplitude and phase objects and test this theory using computer simulations. I also test whether these simulated images can be used to calculate a three-dimensional model of the protein using standard software and discuss problems and possible ways to overcome these.

Effect of Topological Bumpy Surface Underlayer on Compositionally Modulated Atomic Layer Stacking in High <inline-formula> <tex-math notation="LaTeX">${K}_{u}$ </tex-math></inline-formula> hcp Co<sub>80</sub>Pt<sub>20</sub> Film With (00.2) Crystallographic Texture Orientation

The effect of topological bumpy surface underlayer on compositionally modulated atomic layer stacking in high uniaxial magnetocrystalline anisotropy ( $K_u$ ) hcp Co80Pt20 film with (00.2) crystallographic texture orientation was investigated. In this paper, the following has been discussed. Concerning preparation of underlayer with various morphologies, an annealing process prior to the deposition of Co80Pt20 film, and introduction of a metal-oxide buffer layer are effective to obtain underlayers with surface roughness ranging from $\sim 0.4$ to 2 nm and grain size ranging from $\sim 6.7$ to 12.5 nm. To realize a compositionally modulated atomic layer stacking structure, which is confirmed by the high angle annular dark field detector scanning transmission electron microscopy observation, an underlayer with surface roughness less than $\sim 1$ nm and/or grain size larger than $\sim 8$ nm is required. The compositional modulation shows the same normal direction as substrate normal and the c-axis direction of the hcp grain. To reach Ku of $\sim 1.7 \,\times \,10^{7}$ erg/cm $^3$ , surface roughness reduction to $ \sim 0.5$ nm and grain size increase to $ \sim 12$ nm are needed.

Observation of Au nanoparticles on the surface of silicon nanowire grown by rapid thermal chemical vapour deposition

The size evolution of gold (Au) nanoparticles (NPs) on the sidewall surface of silicon (Si) nanowires (NWs) has been investigated by thermal treatments, using high-angle annular dark field scanning transmission electron microscopy. The Si NWs grown at 550°C by rapid thermal chemical vapour deposition have been observed to be surrounded by Au NPs with less than 5 nm diameter and ∼1012 cm−2 density on the whole Si NW surface. To explore the size change of Au NPs, the Au NPs on the Si NW were annealed ex situ at the temperature range of 700–900°C for 20 min. The sizes of NPs for samples annealed at 700, 800 and 900°C represent Gaussian distribution with the average size of 4, 6 and 7 nm, respectively, while at high temperatures above 900°C, they change to a bimodal distribution. It is suggested that the surface diffusion rate of Au NPs on Si NW is much lower than that on the Si substrate because of the substitutional diffusion mechanism.

Combining HAADF STEM tomography and electron diffraction for studies of α-Al(Fe,Mn)Si dispersoids in 3xxx aluminium alloys

A new methodology to study the precipitation crystallography of dispersoids in an Al matrix is proposed. By combining high angle annular dark field tomography and electron diffraction studies, the three-dimensional morphology, orientation relationship (OR) with Al matrix and habit planes of the dispersoids can be achieved simultaneously. This approach has been applied to investigate the -Al(Mn,Fe)Si dispersoids precipitated in an AA3xxx alloy. Most dispersoids have a plate-shaped morphology after low-temperature homogenization at 450C. The largest proportion of the dispersoids follows the previously described OR with the Al matrix . Two plate-shaped dispersoids have been studied in detail. The dispersoid following the commonly observed orientation had habit planes . The dispersoid not following the commonly observed OR had habit planes , with (OR) , .

Ferroelectric Cathode Electron Emission Dependence on Magnetic Field

Many experiments on microwave and millimeter wave tubes, which are based on ferroelectric (FE) cathodes include a magnetic field in the emission region. For such electron devices, an optimal magnetic field leads to overall performance enhancement. In this paper, the dependence of the electron emission from an FE cathode on the magnetic field is experimentally examined and presented. The electron current emitted from an annular FE electron gun, suitable for gyrotron tubes, was measured with different magnetic fields. It was found that adding magnetic field extends the emitted current until a maximal working point is reached. Magnetic field above this point reduces the emission. A ~20 % enhancement in the emitted current relative to the emission in the absence of a magnetic field is obtained. Another observation relates to the gap closure and breakdown effects. The influence of the magnetic field on these effects is also reported. A suitable working point for an electron tube is chosen according to these observations.

Strain Measurement with Nanometre Resolution by Transmission Electron Microscopy

Strain is routinely used in state-of-the-art semiconductor devices in order to improve their electrical performance. Here we present experimental strain measurements obtained by different transmission electron microscopy (TEM) based techniques. Dark field electron holography, nanobeam electron diffraction (NBED) and high angle annular dark field scanning electron microscopy (HAADF STEM) are demonstrated. In this paper we demonstrate the spatial resolution and sensitivity of these different techniques on a simple calibration specimen where the accuracy of the measurement can easily be assessed.