Koh Saitoh

Quasi-Unit-Cell Model for an Al-Ni-Co Ideal Quasicrystal based on Clusters with Broken Tenfold Symmetry

We present new evidence supporting the quasi-unit-cell description of the Al72Ni20Co8 decagonal quasicrystal which shows that the solid is composed of repeating, overlapping decagonal cluster columns with broken tenfold symmetry. We propose an atomic model which gives a significantly improved fit to electron microscopy experiments compared to a previous proposal by us and to alternative proposals with tenfold symmetric clusters.

Structural Study of an Al 72Ni 20Co 8 Decagonal Quasicrystal Using the High-Angle Annular Dark-Field Method

High-angle annular dark-field (HAADF) images of an Al72Ni20Co8 decagonal quasicrystal have been taken for the first time. It has been found that the transition metals form pentagonal and star-shaped cluster columns with radii of about 0.4 nm. The atom-cluster columns with a 2 nm diameter of the alloy, which have been considered as the basic structural unit with decagonal symmetry so far, have been revealed to have no decagonal symmetry. These atom-cluster columns have been found to be decomposed into the pentagonal and star-shaped cluster columns, corresponding clusters existing in the Al13Fe4 monoclinic approximant. The tiling of the pentagonal and star-shaped clusters has been found to agree with the rhombic Penrose lattice for an area with a diameter greater than 50 nm.

Electron microscope study of the symmetry of the basic atom cluster and a structural change of decagonal quasicrystals of Al-Cu-Co alloys

Abstract The symmetry of the basic atom cluster and the space group of the decagonal quasicrystal of an annealed Al65Cu15Co20, alloy have been investigated by high-resolution electron microscopy and convergent-beam electron diffraction. It has been revealed that the symmetry of the clusters projected along the c-axis (decagonal axis) is pentagonal and that the space group is P105/mmc. Decagonal quasicrystals of melt-quenched Al-Cu-Co alloys, which belong to the space group of P 10 m2, were previously found to have pentagonal clusters (Saitoh et al. 1994, Mater. Sci. Engng A, 181–182, 805). Therefore, the symmetry of the atom clusters projected along the c-axis is pentagonal in the Al-Cu-Co quasicrystals irrespective of specimen preparation methods. The space group of melt-quenched Al-Cu-Co alloys changes from P 10 m2 to P105/mmc at about 600°C. In the P 10 m2 phase, the inversion domains are several tens of nanometres in size and all the clusters in one domain have the same sense of polarity. Thus, either...

30-kV spin-polarized transmission electron microscope with GaAs–GaAsP strained superlattice photocathode

A spin-polarized electron beam has been used as the probe beam in a transmission electron microscope by using a photocathode electron gun with a photocathode made of a GaAs–GaAsP strained superlattice semiconductor with a negative electron affinity (NEA) surface. This system had a spatial resolution of the order of 1 nm for at 30 keV and it can generate an electron beam with an energy width of 0.24 eV without employing monochromators. This narrow width suggests that a NEA photocathode can realize a high energy resolution in electron energy-loss spectroscopy and a longitudinal coherence of 3 × 10−7 m.

New structural model of an Al72Ni20Co8 decagonal quasicrystal

A new structural model of an Al 72 Ni 20 Co 8 decagonal quasicrystal is constructed on the basis of high-angle annular dark-field images and high-resolution electron microscope images. The model is composed of two types of atom clusters with about a 0.4 nm radius as basic structural units, which are similar to the unit existing in the Al 13 Fe 4 monoclinic approximant. The two clusters are located at the lattice points of the rhombic Penrose lattice. The model satisfies the symmetry of space group P 10 5 / m m c . High-resolution electron microscope images calculated from the model agree well with the experimental ones.

Structure of Al-Ni-Co decagonal quasicrystals

Al-Ni-Co stable decagonal quasicrystals had been believed to belong to P10 5 /mmc and to have decagonal symmetry of the basic atom-cluster column when projected along the decagonal axis. The space group and the projection symmetry of the cluster columns of Al 70 Ni 30x Cox (x = 15 or 20) quasicrystals were re-examined by convergent-beam electron diffraction and high-resolution electron microscopy (HREM). It has been revealed that the space group is P10 5 /mmc but the projection symmetry is pentagonal. The quasicrystals have been found to be isomorphic with Al-Cu-Co decagonal quasicrystals by Saitoh et al. in 1996 (Phil. Mag. A, 73, 381). The symmetries of the HREM image of the cluster column have been revealed to be pentagonal and decagonal at accelerating voltages of 200 and 300 kV respectively. The strong contribution of the 13420-type reflections at 300kV smears the polar (pentagonal) image of the cluster column because electron waves scattered from all the atoms are superposed nearly in phase for the reflections with a lattice spacing of about 0.2nm. It is demonstrated by computer simulations that a pentagonal atom-cluster-column model can produce pentagonal and decagonal HREM images at accelerating voltages of 200 and 300 kV respectively.

Direct observation of oxygen atoms in rutile titanium dioxide by spherical aberration corrected high-resolution transmission electron microscopy

We have developed a spherical aberration corrected transmission electron microscopy (Cs-corrected TEM) technique that allows us to obtain clearer images in real space than ever before. We applied this technique to titanium oxide, in which light elements such as oxygen are difficult to observe using TEM because of its small cross section and electronic damage. In the present study, we successfully observed oxygen atoms in rutile TiO2. In addition, this direct observation of oxygen atoms enabled us to study the Magn?li structure (TinO2n?1), which is caused by oxygen vacancies. These vacancies caused an atomic relaxation of the titanium and oxygen atoms. The relaxed atoms formed a characteristic shear structure of rutile titanium dioxide phase. This shear structure of the Magn?li structure (TinO2n?1) was visualized with a spatial resolution of 0.119?nm. At the same time, the selected area diffraction (SAD) pattern of the defect structure was obtained. Additional spots were shown inside the rutile [110] spot. We made structural models of the shear structure and simulated the diffraction pattern and images using a multi-slice simulation. Additional spots in the simulated diffraction patterns accurately reconstructed the experimental data. We also considered the possibility of the real-space analysis of local structures using spherical aberration corrected transmission electron microscopy.

Dynamic environmental transmission electron microscopy observation of platinum electrode catalyst deactivation in a proton-exchange-membrane fuel cell

Spherical-aberration-corrected environmental transmission electron microscopy (AC-ETEM) was applied to study the catalytic activity of platinum/amorphous carbon electrode catalysts in proton-exchange-membrane fuel cells (PEMFCs). These electrode catalysts were characterized in different atmospheres, such as hydrogen and air, and a conventional high vacuum of 10(-5) Pa. A high-speed charge coupled device camera was used to capture real-time movies to dynamically study the diffusion and reconstruction of nanoparticles with an information transfer down to 0.1 nm, a time resolution below 0.2 s and an acceleration voltage of 300 kV. With such high spatial and time resolution, AC-ETEM permits the visualization of surface-atom behaviour that dominates the coalescence and surface-reconstruction processes of the nanoparticles. To contribute to the development of robust PEMFC platinum/amorphous carbon electrode catalysts, the change in the specific surface area of platinum particles was evaluated in hydrogen and air atmospheres. The deactivation of such catalysts during cycle operation is a serious problem that must be resolved for the practical use of PEMFCs in real vehicles. In this paper, the mechanism for the deactivation of platinum/amorphous carbon electrode catalysts is discussed using the decay rate of the specific surface area of platinum particles, measured first in a vacuum and then in hydrogen and air atmospheres for comparison.

Measuring the orbital angular momentum of electron vortex beams using a forked grating.

The present study experimentally examines how an electron vortex beam with orbital angular momentum (OAM) undergoes diffraction through a forked grating. The nth-order diffracted electron vortex beam after passing through a forked grating with a Burgers vector of 1 shows an OAM transfer of nℏ. Hence, the diffraction patterns become mirror asymmetric owing to the size difference between the electron beams. Such a forked grating, when used in combination with a pinhole located at the diffraction plane, could act as an analyzer to measure the OAM of input electrons.

Production of electron vortex beams carrying large orbital angular momentum using spiral zone plates

We report the production of electron vortex beams carrying large orbital angular momentum (OAM) using micro-fabricated spiral zone plates. A series of the spherical waves, focussing onto different positions along the propagating direction of the electron beam, were observed. The nth order vortex beam has an OAM n times larger than that of the first-order vortex beam. We observed an electron vortex with an OAM up to in a high-order diffracted wave. A linear dependence of the diameter of the vortex beam on the OAM was observed, being consistent to numerical simulations.