Norie Hirao

Contracted Interlayer Distance in Graphene/Sapphire Heterostructure

Direct growth of graphene on insulators is expected to yield significant improvements in performance of graphene-based electronic and spintronic devices. In this study, we successfully reveal the atomic arrangement and electronic properties of a coherent heterostructure of single-layer graphene and α-Al2O3(0001). The analysis of the atomic arrangement of single-layer graphene on α-Al2O3(0001) revealed an apparentcontradiction. The in-plane analysis shows that single-layer graphene grows not in a single-crystalline epitaxial manner, but rather in polycrystalline form, with two strongly pronounced preferred orientations. This suggests relatively weak interfacial interactions are operative. However, we demonstrate that unusually strong physical interactions between graphene and α-Al2O3(0001) exist, as evidenced by the small separation between the graphene and the α-Al2O3(0001) surface. The interfacial interaction is shown to be dominated by the electrostatic forces involved in the graphene π-system and the unsaturated electrons of the topmost O layer of α-Al2O3(0001), rather than the van der Waals interactions. Such features causes graphene hole doping and enable the graphene to slide on the α-Al2O3(0001) surface with only a small energy barrier despite the strong interfacial interactions.

Effect of substrates on the molecular orientation of silicon phthalocyanine dichloride thin films

Molecular orientations of silicon phthalocyanine dichloride (SiPcCl2) thin films deposited on three different substrates have been measured by near-edge x-ray absorption fine structure (NEXAFS) spectroscopy using linearly polarized synchrotron radiation. The substrates investigated were highly oriented pyrolitic graphite (HOPG), polycrystalline gold and indium tin oxide (ITO). For thin films of about five monolayers, the polarization dependences of the Si K-edge NEXAFS spectra showed that the molecular planes of SiPcCl2 on three substrates were nearly parallel to the surface. Quantitative analyses of the polarization dependences revealed that the tilted angle on HOPG was only 2?, which is interpreted by the perfect flatness of the HOPG surface. On the other hand, the tilted angle on ITO was 26?. Atomic force microscopy (AFM) observation of the ITO surface showed that the periodicity of the horizontal roughness is of the order of a few nanometres, which is larger than the molecular size of SiPcCl2. It is concluded that the morphology of the top surface layer of the substrate affects the molecular orientation of SiPcCl2 molecules not only for mono-layered adsorbates but also for multi-layered thin films.

Orientation of one-dimensional silicon polymer films studied by X-ray absorption spectroscopy

Molecular orientations for thin films of one-dimensional silicon polymers grown by vacuum evaporation have been assigned by near-edge X-ray absorption fine structure (NEXAFS) using linearly polarized synchrotron radiation. The polymer investigated was polydimethylsilane (PDMS) which is the simplest stable silicon polymer, and one of the candidate materials for one-dimensional molecular wire. For PDMS films deposited on highly oriented pyrolytic graphite (HOPG), four resonance peaks have been identified in the Si K-edge NEXAFS spectra. Among these peaks, the intensities of the two peaks lower-energy at 1842.0 eV and 1843.2 eV were found to be strongly polarization dependent. The peaks are assigned to the resonance excitations from the Si 1s to σ* pyz and σ* px orbitals localized at the Si-C and Si-Si bonds, respectively. Quantitative evaluation of the polarization dependence of the NEXAFS spectra revealed that the molecules are self-assembled on HOPG surface, and the backbones of the PDMS are oriented nearly parallel to the surface. The observed orientation is opposite to the previously observed results for PDMS on the other surfaces such as oxide (indium tin oxide) and metal (polycrystalline copper). The flat-lying feature of PDMS observed only on HOPG surface is attributed to the interaction between CH bonds in PDMS and p orbitals in HOPG surface.

Effect of oxygen partial pressure on the structural and magnetic properties of Ba(Fe0.5Mn0.5)O3−δ epitaxial thin films

Ba(Fe0.5Mn0.5)O3−δ (BFMO) single crystalline films with various amounts of the oxygen deficiencies on SrTiO3(001) substrates have been successfully synthesized by a pulsed laser deposition method. The lattice constant of the films significantly decreased with an increase in the oxygen partial pressure during the deposition process. This suggests that the radius of the Fe and Mn ions that is strongly correlated with their valence state systematically changed. In addition, the oxygen partial pressure has a considerable influence on the magnetic properties of the films: the saturation magnetization of the samples increased with increasing oxygen partial pressure, i.e., with decrease in the amount of the oxygen deficiencies. The maximum saturation magnetization of 43 emu/cc for the epitaxial BFMO films was obtained at 300 K. The results of the x-ray photoelectron spectroscopic analysis revealed that the relative amount of the Fe3+ (3d5) and Mn4+ (3d3) ions increased with decreasing amount of the oxygen defici...

Metal-molecular interface of sulfur-containing amino acid and thiophene on gold surface

Chemical-bonding states of metal-molecular interface have been investigated for L-cysteine and thiophene on gold by x-ray photoelectron spectroscopy (XPS) and near edge x-ray adsorption fine structure (NEXAFS). A remarkable difference in Au-S bonding states was found between L-cysteine and thiophene. For mono-layered L-cysteine on gold, the binding energy of S 1s in XPS and the resonance energy at the S K-edge in NEXAFS are higher by 8–9 eV than those for multi-layered film (molecular L-cysteine). In contrast, the S K-edge resonance energy for mono-layered thiophene on gold was 2475.0 eV, which is the same as that for molecular L-cysteine. In S 1s XPS for mono-layered thiophene, two peaks were observed. The higher binging-energy and more intense peak at 2473.4 eV are identified as gold sulfide. The binding energy of smaller peak, whose intensity is less than 1/3 of the higher binding energy peak, is 2472.2 eV, which is the same as that for molecular thiophene. These observations indicate that Au-S interface behavior shows characteristic chemical bond only for the Au-S interface of L-cysteine monolayer on gold substrate.

Effect of film thickness on structural and magnetic properties of single crystalline Ba(Fe0.2Zr0.8)O3−δ thin films on (001)SrTiO3 substrates

The epitaxially grown Ba(Fe0.2Zr0.8)O3−δ (BFZO) films with various thicknesses ranging from 30 to 170 nm were successfully synthesized on (001)SrTiO3 (STO) substrates by a pulsed laser-beam deposition method. The results of x-ray diffraction and transmission electron microscopy revealed that all the films exhibited the epitaxial relationship of (100)BFZO∥(100)STO and [100]BFZO∥[100]STO. In addition, 0.7% expansion of the a-axis lattice constant in the films with over 100 nm thickness was measured. Although all the BFZO films showed ferromagnetic and dielectric natures even at room temperature, the magnetization values were found to be significantly dependent upon the film thickness. The magnetization values of the films with a thickness over 100 nm films were fairly decreased rather than those of the smaller thickness films. In accordance with the x-ray photoelectron spectroscopy measurement, the decrease in the magnetization was mainly ascribed to the decreased number of the tetravalent Fe ions, which wa...

Magnetic Compton scattering studies of magneto-dielectric Ba(Co0.85Mn0.15)O3−δ

We revealed that the Ba(Co0.85Mn0.15)O3-δ ceramic samples exhibited ferromagnetic-dielectric behavior below the magnetic transition temperature of about 35 K. The origin of their magnetic ordering was expected to super-exchange coupling of Co4+(d5)-O2--Mn4+(d3) with bonding angle of 180° and/or Mn4+(d3)-O2--Mn4+(d3) with bonding angle of 90°. The magnetic spin momentum estimated by the magnetic Compton profiles (MCP) of the samples had similar temperature dependence as that determined by the temperature dependence of magnetic moment by superconducting quantum interference device, which meant that the observed magnetic moments could be ascribed to the spin moment. The shapes of the MCPs of the samples were completely same regardless of the temperature measured. This result indicates that there are no changes of the momentum space distribution of spin density between ferromagnetic and paramagnetic states. So, this magnetic transition is simply caused by a thermal fluctuation of the spin.

Photoluminescence and X-ray Absorption Fine Structure Analysis of Sm-Doped TiO2 Thin Films

The local structure of samarium-doped titanium dioxide (TiO2:Sm) thin films fabricated by laser ablation and post annealing has been investigated by Sm L III-edge X-ray absorption fine structure (XAFS) analysis using a synchrotron radiation. The TiO2:Sm samples at annealing temperatures lower than 1000 °C showed an anatase (A-)TiO2:Sm crystal structure and transferred into a rutile (R-)TiO2:Sm phase at annealing temperature of 1100 °C. All the A-TiO2:Sm phase samples showed intense Sm-related photoluminescence (PL) at room temperature. It was shown that the samples which showed intense PL have shorter coordination distance for the first nearest neighbor of Sm than that of the samples which did not show PL after the crystal phase transition.

Argon retentivity of carbonaceous materials: feasibility of kerogen as a carrier phase of Q-noble gases in primitive meteorites

Extremely large amounts of heavy noble gases are concentrated in phase Q, which seems to be a carbonaceous phase analogous to terrestrial Type III kerogen. Phase Q must have very high noble gas retentivity based on the presence of such extremely large amounts of heavy noble gases in a very minor fraction of the meteorite. To verify that kerogen is a carrier phase of Q-noble gases, X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) using synchrotron radiation were carried on for kerogens (coals) and carbon allotropes that had been bombarded by 3-keV Ar ions, and the Ar retentivities of the two materials were compared. This comparison of the estimated Ar concentrations in the target materials revealed that carbon allotropes (graphite, fullerene, carbon nanotube, and diamond) have a much higher Ar retentivity than kerogens. This unexpected result clearly shows that the terrestrial kerogens tested in our study are not suitable as a carrier phase of Ar and, consequently, that phase Q may not be similar to the terrestrial kerogen tested. If heavy noble gases are really concentrated in carbonaceous components of primitive meteorites, phase Q may have a more ordered structure than terrestrial kerogen based on the fact that the greatest difference between terrestrial kerogen and carbon allotropes is the degree of order of the molecular structure.

Orientation of Thin Films Synthesized from Silicon Phthalocyanine Dichloride on a Highly Oriented Pyrolytic Graphite Investigated Using Near Edge X-ray Absorption Fine Structure

Molecular orientation of thin films of silicon phthalocyanine (SiPc) compounds on highly oriented pyrolytic graphite (HOPG) was investigated by near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). The films were prepared by a casting method using solution of SiPc dichloride. XPS results showed that the chlorine atoms in SiPc dichloride were substituted by oxygen atoms when the film was heated in the air. The orientation of the molecules with respect to the substrate plane was investigated by the polarization dependences of the Si K edge NEXAFS spectra. For the sample heated in the air, two clear peaks appeared in the NEXAFS spectra at around 1847.2 and 1852.4 eV, which were assigned to Si 1s→σSi–N* and Si 1s→σSi–O*, respectively. The intensities of the resonance peaks showed strong polarization dependence. A quantitative analysis of the polarization dependence revealed that the Si–N bond was lying down while the Si–O bond was out of the plane.