Teruyasu Mizoguchi

Atomic Structure of a CeO2 Grain Boundary: The Role of Oxygen Vacancies

Determining both cation and oxygen sublattices of grain boundaries is essential to understand the properties of oxides. Here, with scanning transmission electron microscopy, electron energy-loss spectroscopy, and first-principles calculations, both the Ce and oxygen sublattices of a (210)Σ5 CeO(2) grain boundary were determined. Oxygen vacancies are shown to play a crucial role in the stable grain boundary structure. This finding paves the way for a comprehensive understanding of grain boundaries through the atomic scale determination of atom and defect locations.

Mechanism of incorporation of zinc into hydroxyapatite.

The atomic level mechanism of incorporation of Zn(2+) into hydroxyapatite (HAp), which is a potential dopant to promote bone formation, was investigated, based on first principles total energy calculations and experimental X-ray absorption near edge structure (XANES) analyses. It was found that Zn(2+)-doped HAp tends to have a Ca-deficient chemical composition and substitutional Zn(2+) ions are associated with a defect complex with a Ca(2+) vacancy and two charge compensating protons. Moreover, first principles calculations demonstrated that Zn(2+) incorporation into HAp can take place by occupying the Ca(2+) vacancy of the defect complex. The Ca(2+) vacancy complex is not only the origin of the calcium deficiency in HAp, but also plays a key role in the uptake of trace elements during mineralization.

Atomic-scale imaging of individual dopant atoms in a buried interface

Determining the atomic structure of internal interfaces in materials and devices is critical to understanding their functional properties. Interfacial doping is one promising technique for controlling interfacial properties at the atomic scale, but it is still a major challenge to directly characterize individual dopant atoms within buried crystalline interfaces. Here, we demonstrate atomic-scale plan-view observation of a buried crystalline interface (an yttrium-doped alumina high-angle grain boundary) using aberration-corrected Z-contrast scanning transmission electron microscopy. The focused electron beam transmitted through the off-axis crystals clearly highlights the individual yttrium atoms located on the monoatomic layer interface plane. Not only is their unique two-dimensional ordered positioning directly revealed with atomic precision, but local disordering at the single-atom level, which has never been detected by the conventional approaches, is also uncovered. The ability to directly probe individual atoms within buried interface structures adds new dimensions to the atomic-scale characterization of internal interfaces and other defect structures in many advanced materials and devices.

Direct Imaging of Reconstructed Atoms on TiO2 (110) Surfaces

Determining the atomic structures of oxide surfaces is critical for understanding their physical and chemical properties but also challenging because the breaking of atomic bonds in the formation of the surface termination can involve complex reconstructions. We used advanced transmission electron microscopy to directly observe the atomic structure of reduced titania (TiO2) (110) surfaces from directions parallel to the surface. In our direct atomic-resolution images, reconstructed titanium atoms at the top surface layer are clearly imaged and are found to occupy the interstitial sites of the TiO2 structure. Combining observations from two orthogonal directions, the three-dimensional positioning of the Ti interstitials is identified at atomic dimensions and allows a resolution of two previous models that differ in their oxygen stoichiometries.

Coordination and interface analysis of atomic-layer-deposition Al2O3 on Si(001) using energy-loss near-edge structures

The coordination and interface of Al2O3 formed on Si(001) by atomic layer deposition (ALD) were studied using electron energy-loss spectroscopy in a transmission electron microscope. Al energy-loss near-edge structures (ELNESs) were interpreted using first-principles calculations. The Al L23 ELNESs show two peaks at 78.2 and 79.7 eV, which originate from tetrahedrally and octahedrally coordinated aluminum, respectively. The depth profile of coordination in ALD Al2O3/Si was investigated. While both tetrahedrally and octahedrally coordinated Al atoms exist in the ALD Al2O3, the former is dominant near the interface. Aluminum silicate was detected near the interface, and it may cause the difference in aluminum coordination.

Enhanced Seebeck coefficient of quantum-confined electrons in SrTiO3∕SrTi0.8Nb0.2O3 superlattices

We report two-dimensional Seebeck coefficients (∣S∣2D) of [(SrTiO3)x∕(SrTi0.8Nb0.2O3)y]20 (x=1–60, y=1–20) superlattices, which were grown on the (100) face of insulating LaAlO3 substrates to clarify the origin of the giant ∣S∣2D values of the SrTiO3 superlattices [H. Ohta et al., Nat. Mater. 6, 129 (2007)]. The ∣S∣2D values of the [(SrTiO3)17∕(SrTi0.8Nb0.2O3)y]20 superlattices increased proportionally to y−0.5 and reached 320μVK−1 (y=1), which is approximately five times larger than that of the SrTi0.8Nb0.2O3 bulk (∣S∣3D=61μVK−1). The slope of the log∣S∣2D-logy plots was −0.5, proving that the density of states in the ground state for SrTiO3 increases inversely proportionally to y. The critical barrier thickness for quantum electron confinement was also clarified to be 6.25nm (16 unit cells of SrTiO3).

The Invar Problem

Anomalous thermal expansion and its relation to magnetic properties are surveyed for various Invar alloys. Saturation magnetization was measured for alloy systems (FexNi1−x)92M8, where M=Cu, Co, Mn, Cr, and V, and also for Fe70(RexPt1−x)30 at 4.2°K. Sharp drop of the saturation moment occurs at the average electron concentration, N=8.13 to 8.52, for all the Invar alloys including the above‐mentioned alloy systems and N=8.21 to 8.52 for all the Fe–Ni base alloys. When, however, it was plotted against the electron concentration of the matrix surrounding less electropositive impurity atoms such as Mn, Cr, or V, all the alloy systems were found to exhibit a sharp drop of the moment in narrower concentration range of 8.48 to 8.60 for all the Fe–Ni base alloys. Saturation moment and a reversible susceptibility in the saturation range were also measured at room temperature for (110) [001] and (001) [110] rolled single crystals of the composition 29 at.% Ni–Fe. It was found that saturation moment was reduced from...

Theoretical ELNES using one-particle and multi-particle calculations

One-, two-, and many-particle calculations for electron-energy-loss near-edge structures (ELNES) are reviewed. The most important point for the ELNES calculation is the proper introduction of the core-hole effect. By introducing the core-hole effect in a sufficiently large supercell, one-particle calculations are applicable to the ELNES of many edges. On the other hand, the two-particle interaction between the excited electron and the core-hole, namely the excitonic effect, is significant in the K edges of very light elements and the L(2,3) edges of Mg and Al. Many-particle interactions, including both electron-electron and electron-hole interactions, are indispensable for the L(2,3) edges of transition metals and the M(4,5) edges of lanthanides, namely white lines. In this review, we present the basics, methodologies, and some applications of one-, two-, and many-particle calculations. In addition, importance of momentum transfer vector in the ELNES calculations for comparison with the experiments is discussed.

Atomic structures of supersaturated ZnO–Al2O3 solid solutions

Supersaturated ZnO–Al2O3 (>20at.% Al) thin films are grown by pulsed laser deposition technique on silica glass substrates at 600°C. They are characterized by combining x-ray diffraction, Al-K edge x-ray absorption near edge structures (XANESs), high resolution transmission electron microscope (TEM) imaging, TEM analysis, and a series of first principles calculations. The films are composed of textured wurtzite grains with c planes parallel to the substrate. The distance between c planes expands significantly when the Al concentration is greater than 10at.%. The expansion disappears after annealing the films at above 800°C. High density of dislocationlike defects is found in the as deposited film. Any segregation of Al cannot be detected either at the grain boundaries or inside the grains. The lattice expansion toward c axis and the experimental XANES can be satisfactorily explained by taking a hypothetical homologous model with the composition of (ZnO)3(Al2O3) as the local environment of Al in the supers...

Growth and structure of PbVO3 thin films

Multifunctional materials promise to provide the foundation for a new class of devices in which functional properties are coupled to one another. Examples include magnetoelectric materials in which magnetic and ferroelectricproperties are coupled. Here the authors report the successful growth of single phase, fully epitaxialthin films of the multifunctional material, Pb V O 3 , using pulsed laser deposition. This growth offers an alternative means for the production of Pb V O 3 outside of high-temperature and high-pressure techniques through growth of epitaxialthin films on various substrates. The structure of this highly distorted perovskite is examined using x-ray diffraction, Raman spectroscopy, and transmission electron microscopy.