Iwao Shimoyama

Study of electronic structure of graphite-like carbon nitride

Abstract The electronic structures of carbon nitride (CNx) films prepared by low energy ion implantation in graphite have been investigated by near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). NEXAFS and XPS spectra were drastically changed by annealing procedure. The polarization dependence of NEXAFS spectra shows that there are three types of local structure, i.e., pyridine-like structure, cyanic structure, and graphite-like structure. From the angular dependence analysis of NEXAFS spectra, the orientations of the local structures of CNx and the assignment of N 1s XPS spectra are discussed.

X-ray photoelectron spectroscopic observation on B–C–N hybrids synthesized by ion beam deposition of borazine

B–C–N hybrid thin films were grown from ion beam plasma of borazine (B3N3H6) on graphite substrate at room temperature, 600 °C, and 850 °C. The films were characterized in situ by x-ray photoelectron spectroscopy (XPS). XPS study suggested that B and N atoms in the deposited films are in a wide variety of chemical bonds, e.g., B–C, B–N, N–C, and B–C–N. The substrate temperature and ion fluence were shown to have a significant effect on the coordination and elemental binding states on the B–C–N hybrids. It was found that B–C–N hybrid formation is enhanced at high temperature, and this component is dominantly synthesized at low fluence. The results imply that it is possible to control the composition of B–C–N hybrid by changing the ion fluence and the temperature during ion implantation.B–C–N hybrid thin films were grown from ion beam plasma of borazine (B3N3H6) on graphite substrate at room temperature, 600 °C, and 850 °C. The films were characterized in situ by x-ray photoelectron spectroscopy (XPS). XPS study suggested that B and N atoms in the deposited films are in a wide variety of chemical bonds, e.g., B–C, B–N, N–C, and B–C–N. The substrate temperature and ion fluence were shown to have a significant effect on the coordination and elemental binding states on the B–C–N hybrids. It was found that B–C–N hybrid formation is enhanced at high temperature, and this component is dominantly synthesized at low fluence. The results imply that it is possible to control the composition of B–C–N hybrid by changing the ion fluence and the temperature during ion implantation.

Reconstruction of BL7B for UV, VIS and IR spectroscopy with a 3 m normal-incidence monochromator

The beamline BL7B at the UVSOR facility for solid-state spectroscopy is currently under reconstruction. This reconstruction mainly involves the replacement of the 1 m Seya-Namioka-type monochromator (50-600 nm) with a 3 m NIM (modified version of McPherson model 2253), which covers the 50-1000 nm range with three gratings. The deviation angle of the gratings is 15 degrees. For linear and circular polarization experiments, the beamline optics consist of a two-grazing-incidence (87.5 degrees ) pre-mirror system and a normal-incidence (15 degrees ) post-mirror.

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.

Selective adsorption of atomic hydrogen on a h-BN thin film

The adsorption of atomic hydrogen on hexagonal boron nitride (h-BN) is studied using two element-specific spectroscopies, i.e., near-edge x-ray absorption fine structure (NEXAFS) spectroscopy and x-ray photoelectron spectroscopy (XPS). B K-edge NEXAFS spectra show a clear change in the energy region of the π* band before and after reaction with atomic deuterium. On the other hand, N K-edge NEXAFS spectra show only a little change. B 1s XPS spectra show a distinct component at the low binding energy side of a main component, while N 1s XPS spectra show peak broadening at the high binding energy side. These experimental results are analyzed by the discrete variational Xα method with a core-hole effect and are explained by a model in which hydrogen atoms are preferentially adsorbed on the B sites of h-BN. Based on the experimental and theoretical results, we propose a site-selective property of BN material on adsorption of atomic hydrogen.

Chemical-state analysis for low-dimensional Si and Ge films on graphite

Synchrotron radiation photoemission spectroscopy has been used to study the chemical interaction and the electronic structures of low-dimensional semiconducting Si and Ge on a graphite surface. The core level spectra, such as Si 1s, Ge 2p, and C 1s, show that no chemical interaction occurs between adatoms and the substrate atoms, and that the electronic structures of graphite remain intact upon deposition of the adatoms at room temperature. The Si 1s and Ge 2p photoemission spectra reveal that Si and Ge films on graphite remain in their elemental form. However, the electronic structures of both Si and Ge films are found to be thickness dependent. In the thicker films, such as Si 5.5 A or Ge 4.2 A, a single bulk-like component is detected. In the case of thinner films, such as Si 2.7 A or Ge 0.3 A, some additional components are observed at binding energies higher than the bulk-like component. We attribute these peaks to semiconductor nanostructures, for example, nanowires, and individual or polymerized ch...

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.

X-Ray Photoelectron Spectroscopic Observation on the Formation of Carbon Nitride Thin Films Produced by Low-Energy Nitrogen Ion Implantation.

X-ray photoelectron spectroscopy (XPS) spectra of N 1s and C 1s were measured for carbon nitride thin films prepared by low-energy nitrogen ion implantation in graphite. To assign the XPS spectra, we also measured XPS spectra for some standard materials which have sp2 or sp3 C-N bond configurations. In N 1s XPS spectra for the ion-implanted graphite, we found three clear peaks at the binding energies of EB=398.3, 400.3, and 402.6 eV. However, in spite of the difference in bonding systems, all the carbon nitride compounds showed similar N 1s binding energies corresponding to the second peak (EB=400.3 eV). Furthermore, we found a broad structure which resulted from the formation of C-N bonds at a binding energy 2 eV higher than that of the graphite peak, by removing the damage effect from the C 1s XPS spectra for the ion-implanted graphite. Based on these results, we propose an assignment of XPS spectra for the ion-implanted graphite.

Mechanism of Cs Removal from Fukushima Weathered Biotite by Heat Treatment with a NaCl–CaCl2 Mixed Salt

An in situ extended X-ray absorption fine structure (in situ EXAFS) spectroscopic analysis at high temperature was conducted to investigate the mechanism of Cs removal from weathered biotite (WB) from Fukushima, induced by heating with a mixed salt of NaCl and CaCl2. This indicated that most Cs remained in WB during heating at 200–700 °C. In addition, the in situ EXAFS spectra gradually changed on heating with the mixed salt and a completely different spectrum was observed for the sample after cooling from 700 °C to room temperature (RT). Ex situ EXAFS measurements and X-ray fluorescence analyses were also conducted on samples after heat treatment and removal of the mixed salt to clarify the temperature dependence of the Cs removal ratio. On the basis of the results of radial structure function analysis obtained from in situ EXAFS, we concluded that almost all of the Cs was removed from WB by heating at 700 °C with the mixed salt, and that Cs formed Cs–Cl bonds after cooling to RT from 700 °C. In contrast, although more than half of the Cs present was removed from WB by heat treatment at 500 °C, most Cs was surrounded by silica tetrahedrons, maintained by Cs–O bonds. On the basis of these results, different Cs removal processes are suggested for the high-temperature (600–700 °C) and low-temperature (400–500 °C) regions.

Synthesis and characterization of oriented graphitelike B-C-N hybrid

In order to clarify the structure of graphitelike B–C–N hybrid thin films, borazine (B3N3H6) ion plasma was implanted in highly oriented pyrolytic graphite at 800°C and room temperature. The films were characterized in situ by x-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS) using linearly polarized synchrotron radiation. The XPS study suggested that B atoms in the deposited films display in a wide variety of chemical bonds, e.g., B–C, B–N, and B–C–N. In the polarization-dependent NEXAFS spectra at the B K edge, the resonance peaks from B 1s to both π*-like and σ*-like orbitals are clearly observed, which suggest the existence of sp2 configuration around the boron atoms. On the basis of the graphitelike polarization dependencies observed in the B 1s→π* resonance peaks for B–C–N films deposited at 800°C, we conclude that the highly oriented graphitelike B–C–N hybrids are stable at low boron content.