Fumihiko Matsui

Chemical vapor deposition and characterization of aligned and incommensurate graphene/hexagonal boron nitride heterostack on Cu(111).

Two limiting factors for a new technology of graphene-based electronic devices are the difficulty of growing large areas of defect-free material and the integration of graphene with an atomically flat and insulating substrate material. Chemical vapor deposition (CVD) on metal surfaces, in particular on copper, may offer a solution to the first problem, while hexagonal boron nitride (h-BN) has been identified as an ideal insulating substrate material. The bottom-up growth of graphene/h-BN stacks on copper surfaces appears therefore as a promising route for future device fabrication. As an important step, we demonstrate the consecutive growth of well-aligned graphene on h-BN, both as single layers, by low-pressure CVD on Cu(111) in an ultrahigh vacuum environment. The resulting films show a largely predominant orientation, defined by the substrate, where the graphene lattice aligns parallel to the h-BN lattice, while each layer maintains its own lattice constant. The lattice mismatch of 1.6% between h-BN and graphene leads to a moiré pattern with a periodicity of about 9 nm, as observed with scanning tunneling microscopy. Accordingly, angle-resolved photoemission data reveal two slightly different Brillouin zones for electronic states localized in graphene and in h-BN, reflecting the vertical decoupling of the two layers. The graphene appears n-doped and shows no gap opening at the K[overline] point of the two-dimensional Brillouin zone.

Adsorption of acetylene and ethylene on the Si(001)2×1 surface studied by NEXAFS and UPS

Abstract The adsorption of acetylene (C 2 H 2 ) and ethylene (C 2 H 4 ) on the Si(001)2×1 surface has been investigated by near edge X-ray adsorption fine structure (NEXAFS) and ultraviolet photoemission spectroscopy (UPS) using synchrotron radiation. Upon molecular adsorption, a drastic decrease of the π* resonance (C 2 H 4 ) and an energy shift of the σ* resonance (both C 2 H 2 and C 2 H 4 ) were observed in the NEXAFS spectra. The intensity of dangling bond states of Si in the UPS spectra diminished significantly by adsorption of both molecules. These results provide firm evidence for the structure model with Si dimers preserved and the di-σ bonding between the adsorbed molecules and Si dimers.

Structural studies of adsorbed alkanethiols on Cu(111) by use of S and C K-edge X-ray absorption fine structures

Surface structures of submonolayer alkanethiol CH3(CH2)n−1SH (n=6,12) adsorbed on Cu(111) at room temperature (0.3 ML) were investigated by means of S K-edge surface extended X-ray absorption fine structure (EXAFS) and S, C K-edge near-edge X-ray absorption fine structure (NEXAFS) spectroscopies. Polarization dependence of the C K-edge NEXAFS spectra revealed that alkanethiol molecules adsorb with the tilt angle of 12 ± 10° from the surface normal. The S K-edge surface EXAFS determined that the SCu distance is 2.31 ± 0.02 A and that the S atoms locate at a deep three-fold hollow site with significant lateral outward movements of the nearest neighbor Cu atoms.

Adsorption of SO2 on Cu(100) studied by X-ray absorption fine structure spectroscopy and scanning tunneling microscopy

Abstract The room temperature adsorption of SO2 on Cu(100) was studied using S K-edge X-ray absorption fine structure spectroscopy, Auger electron spectroscopy, low energy electron diffraction, X-ray photoelectron spectroscopy, and scanning tunneling microscopy (STM). It is known that the disproportionation reaction 3SO2(a)→S(a) + 2SO3(a) occurs on the Cu(100) surface. The surface coverages of the reaction product adsorbates as well as their respective geometric structures are reported. XAFS modeling calculation results indicate that the atomic S adsorbs in the fourfold hollow site and trigonal pyramidal SO3 adsorbs with the sulfur away from the surface. Thus, the three oxygen atoms are directly bonded to the Cu substrate. STM images show the coexistence of two different phases of the (2 × 2) and c(4 × 6) domains. The (2 × 2) unit cell contains one SO3 species but no S atoms. In contrast, the c(4 × 6) unit cell contains one SO3 and two S atoms. The presence of such domains implies a drastic migration of the reaction products on the surface.

Three-dimensional band mapping of graphite

We developed a method to visualize the curved surfaces of valence band dispersion by a display-type spherical mirror analyzer. The π and σ band “surfaces” of graphite measured and displayed three dimensionally contain fruitful information compared to the conventional band dispersion “curves” along a certain direction in k space. The slope and the curvature of local band structure correspond to the velocity and the mass of the valence electrons. By integrating band energy over the entire Brillouin zone, the electronic part of the condensation energy is obtained. Furthermore, the atomic orbitals composing each band are determined from two-dimensional photoelectron intensity distribution by using linearly polarized synchrotron radiation.

Surface Aligned Magnetic Moments and Hysteresis of an Endohedral Single-Molecule Magnet on a Metal

The interaction between the endohedral unit in the single-molecule magnet Dy_{2}ScN@C_{80} and a rhodium (111) substrate leads to alignment of the Dy 4f orbitals. The resulting orientation of the Dy_{2}ScN plane parallel to the surface is inferred from comparison of the angular anisotropy of x-ray absorption spectra and multiplet calculations in the corresponding ligand field. The x-ray magnetic circular dichroism is also angle dependent and signals strong magnetocrystalline anisotropy. This directly relates geometric and magnetic structure. Element specific magnetization curves from different coverages exhibit hysteresis at a sample temperature of ∼4  K. From the measured hysteresis curves, we estimate the zero field remanence lifetime during x-ray exposure of a submonolayer to be about 30 seconds.

Visualization of graphite atomic arrangement by stereo atomscope

The direct visualization of an atomic arrangement is essential for understanding the nature of materials. The rotation of forward focusing peaks in photoelectron angular distribution (PEAD) patterns excited by circularly polarized light with the opposite helicities are found to be the same as the parallax in stereo view. Taking advantage of this phenomenon of PEAD circular dichroism, the three-dimensional atomic arrangement image of graphite crystal was realized. Taking a stereo picture around carbon atom, which has been thought difficult due to a small angular momentum of photoelectron, proved that this method is applicable to all materials including biomolecules.

Dopant-site effect in superconducting diamond (111) studied by atomic stereophotography

We studied the cause of high TC of superconducting boron-doped diamond (111) when compared with that of (001) even at the same boron concentration. The atomic stereophotograph revealed that a disproportionate boron concentration is located at one of the two sites in (111). The distinction of the dopant site exists only in (111) surface and does not exist in (001) surface. Furthermore, there is a theoretical prediction of high density of state at Fermi level in ordered dopant diamonds. Therefore, this configuration should be the origin of the higher TC of (111) as compared with that of (001).

Site-Specific Orbital Angular Momentum Analysis of Graphite Valence Electron Using Photoelectron Forward Focusing Peaks

Two-dimensional photoelectron intensity angular distributions of the graphite valence band were measured using circularly polarized soft X-ray with a photon energy of 500 eV. Photoelectron intensit...

Development of High-Energy-Resolution Display-Type Photoelectron Spectrometer in the Ultraviolet Photoelectron Spectroscopy Region

We have built a newly designed 2D-photoelectron spectroscopy (PES) analyzer for the measurement of wide-angle photoelectron angular distribution patterns in the ultraviolet photoelectron spectroscopy (UPS) region with no distortion. The spherical electric field inside the analyzer is achieved by the 158 tin obstacle rings placed on the inner surface of the machinable ceramic outer hemisphere. The inner surface of the outer hemisphere is covered with graphite powder to avoid charging. Hence, the ideal shape and the smooth change of the potential on the surface of the hemisphere could be realized. The energy resolution (ΔE/ EP) is estimated to be 0.43% (73 meV) at a kinetic energy of 16.85 eV and 0.16% at a kinetic energy of 300 eV.