Kouichi Hayashi

Long-period ordered structure in a high-strength nanocrystalline Mg-1 at% Zn-2 at% Y alloy studied by atomic-resolution Z-contrast STEM

The microstructure of a nanocrystalline Mg97Zn1Y2 (at%) bulk alloy prepared by warm extrusion of rapidly solidified powders has been investigated by a combination of techniques, such as conventional high-resolution transmission electron microscopy (HRTEM), atomic-resolution high-angle annular dark field scanning-TEM (HAADF-STEM) with Z-contrast and energy-dispersive X-ray spectroscopy (EDS) with a sub-nanometer electron probe. We show that a novel long-period ordered structure is formed in the alloy, whose unit cell is composed of six close-packed planes of the magnesium crystal with a stacking sequence of ABCBCB′ where A and B′ layers are significantly enriched by Zn and Y. The lattice is distorted from an ideal hexagonal lattice of 6H-type (ABCBCB), which is probably due to an asymmetry of the chemical order with respect to the 6H-type stacking order. The present results demonstrate that the additional elements of a few atomic percent to Mg lead to formation of a long-period chemical-ordered as well as stacking-ordered structure, as directly revealed by a unique Z-contrast method.

Emission color tunable light-emitting diodes composed of InGaN multifacet quantum wells

We demonstrate that the apparent emission colors of InGaN-based light-emitting diodes using microstructured multifacet quantum wells as active layers can externally be controlled over a wide spectral range that encompasses green to blue or white at a color temperature of 4000K to blue along the Planckian locus. The controllability relies on facet-dependent polychromatic emissions. The pulsed current operation with the appropriate duties varied their relative intensities and the consequent apparent colors without seriously affecting the total number of emitted photons, particularly for the blue to green variation.

X-ray fluorescence holography

X-ray fluorescence holography (XFH) is a method of atomic resolution holography which utilizes fluorescing atoms as a wave source or a monitor of the interference field within a crystal sample. It provides three-dimensional atomic images around a specified element and has a range of up to a few nm in real space. Because of this feature, XFH is expected to be used for medium-range local structural analysis, which cannot be performed by x-ray diffraction or x-ray absorption fine structure analysis. In this article, we explain the theory of XFH including solutions to the twin-image problem, an advanced measuring system, and data processing for the reconstruction of atomic images. Then, we briefly introduce our recent applications of this technique to the analysis of local lattice distortions in mixed crystals and nanometer-size clusters appearing in the low-temperature phase of a shape-memory alloy.

Refracted X-rays propagating near the surface under grazing incidence condition ☆

Abstract X-ray refractions through a silicon wafer and an organic thin film, n-C 33 H 68 /Si, were measured using white incident X-rays under a grazing incidence condition. The energy variation of a refracted X-ray beam by a silicon wafer was well explained by the simple Snells law when the position of the detector (solid-state detector) was changed. On the other hand, two refracted X-ray beams were observed from the organic thin film. The energy variation of one beam which propagated through the silicon substrate followed Snells law, while that of another beam did not and changed little when the detector position was varied.

Magnetization, Curie temperature and perpendicular magnetic anisotropy of evaporated Co-rare earth amorphous alloy films

Abstract Magnetization, Curie temperature and perpendicular magnetic anisotropy have been investigated for Fe100-xRx (R=Pr, Nd, Sm, Gd, Dy, Er) amorphous alloy films prepared by evaporation method. It is found that the Curie temperature of these films is much lower than that of amorphous alloys prepared by rapid quenching technique. The reason may be a difference of compositional short range ordering owing to the different sample preparation. The dependence of perpendicular magnetic anisotropy on temperature and composition is discussed based on one ion magnetic anisotropy of rare earth ions. The main part of the perpendicular magnetic anisotropy can be considered due to the large one ion magnetic anisotropy of rare earths.

Additive color mixture of emission from InGaN∕GaN quantum wells on structure-controlled GaN microfacets

Altering the mask geometry controls the apparent emission color from InGaN∕GaN quantum wells (QWs) grown on GaN microfacets formed by regrowth on SiO2 mask stripes over a wide spectral range, including white. The mask stripes are along the ⟨11¯00⟩ direction and the microfacet structure is composed of the (0001) and {112¯2} planes. With a large occupancy of the mask opening within a period, both facets simultaneously appear and emit different colors. For example, the {112¯2} facet QWs emit blue and the (0001) facet QWs emit green. On the other hand, with a small occupancy of the mask opening, the {112¯2} facets become dominant and a greenish-blue light is emitted. To synthesize these spectra, the mask patterns are designed so that two different microfacet structures are included within a period. Hence, the macroscopically observed emission color, which depends on the pattern design, can change from green to purple through white due to the additive color mixture.

Photoelectron spectra enhanced by x‐ray total reflection and diffraction from periodic multilayer

Photoelectron spectra from a periodic multilayer of (Mo/B4C)30/Si were measured by changing the x‐ray glancing angle. The angular dependencies of the photoelectron intensities of O 1s and Mo 3d revealed the atomic depth profiles of the Mo oxide overlayer, and the compositions of the MoO3 upper layer and MoO2 lower layer. Furthermore, we observed enhancement of photoelectron emission by means of x‐ray standing waves generated near the multilayer surface during x‐ray diffraction from the superlattice.

Structural evaluation of epitaxially grown organic evaporated films by total reflection x‐ray diffractometer

An in‐plane‐type total reflection x‐ray diffractometer was newly constructed, and the epitaxial growth of paraffin molecules (n‐C33H68) vacuum‐evaporated onto a KCl (001) surface was investigated. The molecular chains in as‐evaporated films were shown to arrange themselves parallel to the 〈110〉 direction of the KCl substrate. After the evaporation, moreover, some molecules were revealed to reorient and tend to deviate their c axis by about ±5° from the 〈110〉 direction of the KCl crystal, probably because of the lattice mismatchings between the paraffin and substrate crystals.

Extended X-Ray Absorption Fine Structure (EXAFS) in X-ray Fluorescence Spectra

Aluminum K - L L radiative Auger X-ray fluorescence spectra of AlPO 4 and Al 2 O 3 powders are measured using an X-ray fluorescence spectrometer for elemental analysis. We demonstrate that the Fourier transform of the oscillation in the radiative Auger spectra has information similar to that of the extended X-ray absorption fine structure (EXAFS). The Al–O distances in AlPO 4 and Al 2 O 3 are determined by the radiative Auger X-ray fluorescence spectra. The accuracy of the Al–O bond length is within 0.03 A.

Effect of applied electric field on the molecular orientation of epitaxially grown organic films

In order to control and obtain desired molecular orientations, we investigated the effects of applying an electric field during the epitaxial growth of organic crystals in this study. We deposited interdigitated gold electrodes on polished KCl (001) substrates, and then grew copper phthalocyanine (CuPc) thin films, applying the voltages during evaporation process. Using an energy dispersive grazing incidence x-ray diffraction system, we evaluated the orientations of α-CuPc crystals relative to the KCl substrates. The CuPc crystals with applied electric fields were found to orient in different directions from those of the usual epitaxially grown films without applied electric fields. These facts suggest that the combination of applying an electric field and epitaxial growth provide us a new method to control the molecular orientation in the organic thin films.