Y. Ishizawa

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.

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.

Surface characterization of α-Mo2C (0001)

Abstract The (0001) surface of α-Mo 2 C prepared by Ar ion bombardment and annealing was characterized using X-ray photoelectron spectroscopy and low energy electron diffraction (LEED). An ion-bombarded surface annealed to 1000xa0K exhibits an Mo/C ratio of 2.2, characteristic of an Mo-terminated surface. The Mo-terminated surface exhibits a complex, but predominantly ‘(1×1)’ hexagonal LEED pattern on a diffuse background. This pattern is composed of at least three periodicities: a (1×1) hexagonal periodicity, a faint 3 × 3 R 30° periodicity, and spots of a third, unidentified periodicity. A bulk-terminated, unrelaxed surface is expected to exhibit a (1×1) orthorhombic (i.e. rectangular) periodicity. The observed hexagonal periodicity is explained by LEED simulations as arising from the extinction of certain orthorhombic spots. An ion-bombarded surface annealed to 1500xa0K exhibits an Mo/C ratio of 1.8–1.9 and a (4×4) hexagonal LEED periodicity. The decrease in Mo/C ratio is consistent with the change from an Mo to a C termination. The (4×4) hexagonal periodicity is equivalent to three domains of C termination with (2×1) orthorhombic symmetry rotationally misaligned by 120°. The presence of rotationally misaligned domains is confirmed by scanning tunneling microscopy. Whereas changes in both the composition and periodicity are associated with the transformation from an Mo to a C-terminated surface, the details of the associated surface structures are not known.

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.

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.

Structure analysis of Ag overlayers on Si(111) by low-energy Li+ ion scattering

The structures of various Ag overlayers on the Si(111) surface have been analyzed by impact-collision ion scattering spectroscopy (ICISS) using a beam of Li+ ions of ≈ 1 keV. The Ag overlayers include (i) Ag crystallites deposited at room temperature, (ii) Ag atoms at the Si(111)−(√3×√3)Ag surface, (iii) Ag atoms at the Si(111)−(3×1)Ag surface, and (iv) a (111) epitaxial film of Ag with a thickness of ≈ 20 A.

Low energy D+ scattering from clean and alkalated TiO2(110) surfaces

Low energy D+ scattering (E0 = 100 eV) is employed to explore the bond nature of the TiO2(110) surface with and without alkali-metal (AM) adatoms. The D+ ions are found to be neutralized almost completely in the course of scattering from Ti, while D+ survives neutralization during scattering from Na, K and Cs adatoms in the low-coverage regime (< 0.5 ML). On the basis of the hole diffusion picture of resonance neutralization, the absence of the Ti surface peak is indicative of covalency in the Ti-O bond, while the presence of the marked surface peaks shows that the AMs are perfectly ionized. With increasing coverage, an ionic-to-neutral transition of the AM overlayer takes place at 0.5 ML coverage. Above this coverage, the AM surface peak markedly decreases in intensity because partial filling of the AM s state gives rise to resonance neutralization of D+.

Structure analysis of oxygen adsorbed TiC(111) by impact collision ion scattering spectroscopy

Abstract The atomic structures of the oxygen chemisorbed TiC(111) surface exhibiting both 1 × 1 and √3 × √3 R30° structures have been studied using impact collision He + ion scattering spectroscopy (ICISS). At the TiC(111)1 × 1-O surface, oxygen atoms are in two states on the three-fold hollow site which is characterized by the absence of carbon beneath it in the second layer. The two states arise from the difference in the position of the oxygen atom: one located 1.0 A above the topmost Ti layer and the other 0.8 A above it. The latter disappears at the TiC(111)√3 × √3 R30°-O surface which is obtained after heating of the TiC(111)1 × 1-O surface at about 1000°C.

Shadowing and focusing effects in the angular distributions of low-energy rare-gas ions scattered from solid surfaces

Abstract Low-energy rare-gas ion scattering spectroscopy (ISS) has been improved by the use of neutral beams as projectiles, which we call neutral beam incident ion scattering spectroscopy (NBISS). Features of the two techniques (NBISS and ISS) have been discussed on the basis of electron exchange between projectiles and solid surfaces. The shadowing and focusing effects are clearly observed in the NBISS results, while these effects are sometimes suppressed in the ISS results. It is found that the occurrence of the ionization of neutralized rare-gas atoms is necessary for the appearance of these effects. On the basis of such inspections, the guidelines for applying NBISS and ISS to surface structure analysis are given.

Trajectory-dependent neutralization probability of low-energy He + scattered from solid surfaces

Abstract It is demonstrated that information on the local spatial distribution of valence electrons at solid surfaces can be obtained by three-dimensional angle resolving ion scattering spectroscopy (3D-ARISS) using a beam of low-energy He + . The demonstration is made for the (001) surfaces of NaCl, MgO, NiO, and TiC with a rocksalt structure.