Akihito Kumamoto

Atomically ordered solute segregation behaviour in an oxide grain boundary

Grain boundary segregation is a critical issue in materials science because it determines the properties of individual grain boundaries and thus governs the macroscopic properties of materials. Recent progress in electron microscopy has greatly improved our understanding of grain boundary segregation phenomena down to atomistic dimensions, but solute segregation is still extremely challenging to experimentally identify at the atomic scale. Here, we report direct observations of atomic-scale yttrium solute segregation behaviours in an yttria-stabilized-zirconia grain boundary using atomic-resolution energy-dispersive X-ray spectroscopy analysis. We found that yttrium solute atoms preferentially segregate to specific atomic sites at the core of the grain boundary, forming a unique chemically-ordered structure across the grain boundary.

Reversible Diameter Modulation of Single-Walled Carbon Nanotubes by Acetonitrile-Containing Feedstock

Changing the carbon feedstock from pure ethanol to a 5 vol % mixture of acetonitrile in ethanol during the growth of vertically aligned single-walled carbon nanotubes (SWNTs) reduces the mean diameter of the emerging SWNTs from approximately 2 to 1 nm. We show this feedstock-dependent change is reversible and repeatable, as demonstrated by multilayered vertically aligned SWNT structures. The reversibility of this process and lack of necessity for catalyst modification provides insight into the role of nitrogen in reducing the SWNT diameter.

EXOTHERMIC CHEMICAL REACTIONS CAN DRIVE NONTHERMAL CRYSTALLIZATION OF AMORPHOUS SILICATE GRAINS

To explain how cometary silicates crystallize yet still preserve volatile interstellar ices in their parent comets, we experimentally demonstrate the possibility of chemical-reaction-driven crystallization, which is called nonthermal crystallization, using laboratory-synthesized amorphous Mg-bearing silicate grains. Analog silicate grains ~50-100 nm in diameter covered with a carbonaceous layer consisting of amorphous carbon, CH 4, and other organics to a thickness of ~10-30 nm were used as models. The analog silicate grains crystallized via the direct flow of surface reaction energy, which is produced by the graphitization of the carbonaceous layer due to oxidation at room temperature in air, into the silicates. The experimental results imply that amorphous silicates are transformed into crystalline silicates as the grains leave the comets surface, rather than as the comet was accreted 4.5 billion years ago. Thus, primordial ices and amorphous silicate grains are predicted to reside in most comets until they approach the Sun.

Effect of local coordination of Mn on Mn-L2,3 edge electron energy loss spectrum

The effects of the local coordination environment of Mn ions in perovskite manganese oxides on the Mn-L2,3 edge electron energy loss (EEL) spectra was experimentally and theoretically investigated. The Mn-L2,3 edge EEL spectra were observed for various perovskite manganese oxides, including YMnO3, LaMnO3, BaMnO3, SrMnO3, and CaMnO3, in which the Mn ions have different valence states and local coordination. The experiment revealed that the Mn L3/L2 ratio is influenced not only by the valence state but also by the local environment of the Mn ions. Furthermore, compared to the Mn L3/L2 ratios of Mn3+ compounds, the Mn L3/L2 ratios of the Mn4+ compounds are found to be much more sensitive to local distortions. The ab-initio multiplet calculation of the Mn-L2,3 edge EEL spectra revealed that the effects of local coordination on the spectral features are dependent on the local electronic structures of the Mn ions. These findings indicate that the valence state as well as the local environments of the Mn ions can be unraveled by combining experimental and theoretical investigations of Mn-L2,3 edge EEL spectra.

Chain-like nanostructures from anisotropic self-assembly of semiconducting metal oxide nanoparticles with a block copolymer

A facile method is reported for the preparation of chain-like nanostructures by anisotropic self-assembly of TiO(2) and SnO(2) nanoparticles with the aid of a block copolymer in an aqueous medium. Well-defined crystallographic orientations between neighbouring nanoparticles are observed in TiO(2) nanochains, which is important for tailoring the grain boundaries and thus enhancing charge transport.

Electron Microscopic Observation of Selective Excitation of Conformational Change of a Single Organic Molecule

Atomic resolution transmission electron microscopic observations at different electron acceleration voltages enabled us to observe visually the energy relaxation process of one conformer into another via rotation of various parts of the molecule. Cross-correlation analysis of sequential transmission electron microscopy (TEM) images or of the difference between experimental and simulated TEM images has been utilized for investigation of the conformational mobility and for structure identification of conformers.

Atomic structures of a liquid-phase bonded metal/nitride heterointerface

Liquid-phase bonding is a technologically important method to fabricate high-performance metal/ceramic heterostructures used for power electronic devices. However, the atomic-scale mechanisms of how these two dissimilar crystals specifically bond at the interfaces are still not well understood. Here we analyse the atomically-resolved structure of a liquid-phase bonded heterointerface between Al alloy and AlN single crystal using aberration corrected scanning transmission electron microscopy (STEM). In addition, energy-dispersive X-ray microanalysis, using dual silicon drift X-ray detectors in STEM, was performed to analyze the local chemistry of the interface. We find that a monolayer of MgO is spontaneously formed on the AlN substrate surface and that a polarity-inverted monolayer of AlN is grown on top of it. Thus, the Al alloy is bonded with the polarity-inverted AlN monolayer, creating a complex atomic-scale layered structure, facilitating the bonding between the two dissimilar crystals during liquid-phase bonding processes. Density-functional-theory calculations confirm that the bonding stability is strongly dependent on the polarity and stacking of AlN and MgO monolayers. Understanding the spontaneous formation of layered transition structures at the heterointerface will be key in fabricating very stable Al alloy/AlN heterointerface required for high reliability power electronic devices.

Subgrain boundary analyses in deformed orthopyroxene by TEM/STEM with EBSD-FIB sample preparation technique

High-resolution structure analyses using electron beam techniques have been performed for the investigation of subgrain boundaries (SGBs) in deformed orthopyroxene (Opx) in mylonite from Hidaka Metamorphic Belt, Hokkaido, Japan, to understand ductile deformation mechanism of silicate minerals in shear zones. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) analysis of Opx porphyroclasts in the mylonitic rock indicated that the crystal orientation inside the Opx crystals gradually changes by rotation about the b-axis by SGBs and crystal folding. In order to observe the SGBs along the b-axis by transmission electron microscopy (TEM) or scanning TEM (STEM), the following sample preparation protocol was adopted. First, petrographic thin sections were slightly etched with hydrofluoric acid to identify SGBs in SEM. The Opx crystals whose b-axes were oriented close to the normal of the surface were identified by EBSD, and the areas containing SGBs were picked and thinned for (S) TEM analysis with a focused ion beam instrument with micro-sampling system. High-resolution TEM imaging of the SGBs in Opx revealed various boundary structures from a periodic array of dissociated (100) [001] edge dislocations to partially or completely incoherent crystals, depending on the misorientation angle. Atomic-resolution STEM imaging clearly confirmed the formation of clinopyroxene (Cpx) structure between the dissociated partial dislocations. Moreover, X-ray microanalysis in STEM revealed that the Cpx contains a considerable amount of calcium replacing iron. Such chemical inhomogeneity may limit glide motion of the dislocation and eventually the plastic deformation of the Opx porphyroclasts at a low temperature. Chemical profiles across the high-angle incoherent SGB also showed an enrichment of the latter in calcium at the boundary, suggesting that SGBs are an efficient diffusion pathway of calcium out of host Opx grain during cooling.

Misalignment Induced Artifacts in Quantitative Annular Bright-Field Imaging

Local structural distortion at defects such as grain boundaries, hetero-interfaces, dislocations and surfaces can significantly influence on a broad variety of physical properties in complex oxide materials. Precise measurement of localized structure distortion at these defects can provide new insights into mechanistic understanding as to how materials properties quantitatively depend on the microstructures and subsequently how we can engineer defects to optimize the materials/devices. The recent advances in aberration corrected (scanning) transmission electron microscopy (Cs-S/TEM) have made it possible to measure the distance between atom positions with picometer-precision [1-3]. In particular, the annular bright field (ABF) imaging [4] allows us to simultaneous visualize both heavy and light element columns over a wide range of thickness, enabling determination of positions of full atomic species from a single image for quantitative measurements.

Characteristic Low-Temperature Crystallization of Amorphous Mg-bearing Silicate Grains under Electron Irradiation

Amorphous Mg-bearing silicate grains, which were produced by the coalescence between MgO and SiOx smoke particles, were crystallized to forsterite (Mg2SiO4) by electron-beam irradiation in a transmission electron microscope at room temperature. The crystallization induced by electron beams was accelerated by the presence of CH4 adsorbed on the surface and incorporated interior of the grains. This experimental result implies the possibility of low-temperature crystallization in a silicate carbon star. In the case of binary stars, since the materials that flow from the stars stationarily exist around the star, the formed amorphous silicate grains will be irradiated by electrons from the star for a long duration. As a result, a significant amount of crystalline silicates can be produced.