C. Oshima

Ultra-thin epitaxial films of graphite and hexagonal boron nitride on solid surfaces

In this article, we have reviewed the recent progress of the experimental studies on ultra-thin films of graphite and hexagonal boron nitride (h-BN) by using angle-resolved electron spectroscopy together with other techniques. The fundamental properties of these high-quality films are discussed on the basis of the data on dispersion relations of valence electrons, phonon dispersion etc. The interfacial orbital mixing of the -state of the monolayer graphite (MG) with the d states of the reactive substrates is the origin for the phonon softening, expansion of the nearest-neighbour C - C distance, modification of the -band, low work function, and two-dimensional plasmons with high electron density, etc. In the cases of weak mixing at the interface between the MG and relatively inert substrates, the observed properties of the MG are very close to the bulk ones. In contrast to the case for MG, the interfacial interaction between the h-BN monolayer and the substrate is weak.

Atomic structure of monolayer graphite formed on Ni(111)

The atomic structure of monolayer graphite formed on a Ni(111) surface was investigated by means of LEED intensity analysis. We measured the I-V curves of the (1,0), (0,1) and (1,1) diffraction spots from a (1 × 1) atomic structure, and analyzed them using Van Hoves analytical program based on dynamical theory. Three different atomic structures meeting the experimental requirement of the 3m symmetry were evaluated with Pendrys reliability factor. The final best-fit structure characterized by the minimum Pendrys reliability factor of 0.22 is as follows: one carbon atom in a unit cell of the graphite overlayer is located at the on-top site of the topmost Ni atoms, while another carbon atom exists at the fcc hollow site. The spacing between the flat overlayer and the topmost Ni layer is 2.11 ± 0.07 A, which is much narrower than the interlayer spacing in bulk graphite (3.35 A).

Quantitative Surface Atomic Geometry and Two-Dimensional Surface Electron Distribution Analysis by a New Technique in Low-Energy Ion Scattering

A new technique for analyzing the quantitative surface atomic geometry and the two-dimensional surface electron distribution is reported. The remarkable effectiveness of this new technique is demonstrated for TiC(111).

Surface phonon dispersion curves of graphite (0001) over the entire energy region

The full surface phonon dispersion curves of graphite (0001) over the entire energy region and the entire Brillouin zone along the ΓM direction have been determined for the first time by using electron energy loss spectroscopy. These dispersion curves agree well in the appropriate limits with bulk values measured by infrared spectroscopy and neutron scattering. The intraplanar interactions in the topmost basal plane appear to be the same as those in the bulk.

Electronic states of monolayer graphite formed on TiC(111) surface

Abstract Electronic states of a monolayer graphite formed on TiC(111) surface have been investigated by means of XPS, UPS, and work-function measurement. The chemical shift of C1s peaks in XPS spectra has inhibited a large electron transfer from the substrate to the monolayer graphite. On the other hand, the band structure of the graphite overlayer has altered from that of bulk graphite. The work-function measurement has suggested an electron redistribution in the graphite layer. These results indicate that the electronic states of the graphite monolayer are modified not by the charge transfer, but mainly by the orbital hybridization between the graphite monolayer and the substrate, which differs from graphite intercalation compounds.

Atomic chemical composition and reactivity of the TiC(111) surface

Abstract The properties of both the clean and oxygen covered TiC(111) surfaces have been studied by electron spectroscopy and ion scattering spectroscopy. It has been confirmed that the topmost layer of the clean surface consists of Ti atoms, which are arranged in a 1 × 1 surface unit cell. The work function has been measured to be 4.7 ± 0.1 eV, which is much higher than those of the neutral surfaces such as (100). This may be attributed to the compensational charge, which is induced in front of the Ti topmost layer so as to eliminate the electric field of the polar surface.

Clean TiC(001) surface and oxygen chemisorption studied by work function measurement, angle-resolved X-RAY photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and ion scattering spectroscopy

Abstract The clean surface of TiC(001) prepared by flash-heating at ~ 1500°C shows a 1 × 1 ordered atomic structure. No gradient of chemical composition exists over the shallow surface region of ~15 A. The work function of the clean surface is 3.8 ± 0.1 eV at room temperature. The features of oxygen chemisorption are similar to those of noble metals such as Ag and Cu; the average sticking probability is about two order of magnitude less than those of Ti and W metals, and the work-function change is also small. The oxygen atoms are preferentially chemisorbed on the carbon sites at the topmost layer, which is responsible for the small change in work function.

Phonon dispersion in monolayer graphite formed on Ni(111) and Ni(001)

Abstract The phonon dispersion relations of monolayer graphite on Ni(111) and on Ni(001) were measured by using electron energy loss spectroscopy. Both samples gave almost the same results. The data were analysed with a force constant model in a slab geometry, and it was revealed that the vertical bond bending force constant and the bond twisting force constant were greatly softened. These force-constant changes are comparable with the previously reported case of monolayer graphite on (111) surfaces of transition-metal carbides. However, the interaction between the Ni substrate and the graphite overlayer is not so strong as that on the carbide (111) surface. The similar phonon structures between graphitic layers on Ni(111) and on Ni(001) suggest that both substrates have similar charge transfer capability.

A hetero-epitaxial-double-atomic-layer system of monolayer graphene/monolayer h-BN on Ni(111)

A double-atomic-layer system of monolayer graphene/monolayer h-BN has been prepared in an epitaxial manner on Ni(111) surface. The graphene overlayer formation changes drastically the pristine interface between the h-BN layer and Ni(111). As a result, a peculiar property of the pristine monolayer h-BN on Ni(111) changes to a bulk-like one. The π–d orbital hybridization at the interface disappears. Accordingly, a metallic character of monolayer h-BN disappears and softening TO phonons return to the normal one. The reduced lattice constant of h-BN presumably returns to the bulk one. The same phenomenon occurs by surface contaminations instead of monolayer graphene formation.

Mechanism of electron exchange between low energy He+ and solid surfaces

Abstract A general picture on the mechanism of electron exchange between low energy He + ions and solid surfaces is proposed on the basis of experiments on three-dimensional angle resolving ion scattering spectroscopy in which not only He + but neutral He is used as a projectile.