Toshihisa Anazawa

Surface induced state on ZrC(111): Angle-resolved photoemission study

Abstract Angle-resolved photoemission spectroscopy (ARPES) has been used to study the electronic structure of a ZrC 0.9 (1 1 1) surface. A sharp emission is observed at just below the Fermi level in normal emission spectra. This peak is ascribed to the emission from a surface induced state formed through charge redistribution around the surface. The two-dimensional dispersion of the state is found to be small.

Hydrogen adsorption on a TiC(111) surface: angle-resolved photoemission study

Abstract Angle-resolved photoemission spectroscopy (ARPES) utilizing synchrotron radiation has been used to study the adsorption of hydrogen on a TiC(111) surface at 300 K. This system was studied by Bradshaw et al. [Solid State Commun. 37 (1980) 37] using ARPES and their results are well reproduced in this study. Furthermore, the two-dimensional band structure of the H 1s-induced state (which is found at 6.5 eV at the Γ point) along the Γ M and Γ K directions is directions is fully determined. The result shows that the bandwidth of this state is 1.1 eV. This band-width correlates well with those of metal-H adsorption systems. An H-induced surface state is also found at just below E F in off-normal-emission spectra.

Adsorption of K on NbC(100): photoemission and thermal desorption study

Abstract The adsorption state of K on NbC(100) has been studied with core-level photoemission spectroscopy (CLPES) and thermal desorption spectroscopy (TDS). The adsorption mechanism of K on the NbC(100) surface is found to be very unique as compared with those of many alkali-metal surface systems. The TDS study shows that the desorption energy of K increases with the increase of K coverage ( θ K ). The work function measurements show that the polarization of the NbC(100) surface after K adsorption is small as compared with those for many alkali-metal surface systems. The lineshape analysis of the K 3p photoemission spectra shows that the valence electronic density of states for adsorbed K is nearly independent on the coverage. Considering these results, it is proposed that the K-adsorbed layer is grown via the island formation mechanism.

Angle-resolved photoemission study of the surface electronic structure of TaC(111)

Abstract Angle-resolved photoemission spectroscopy utilizing synchrotron radiation has been used to study the electronic structure of the TaC0.95(111) surface. A strong emission from a surface induced state is found just below the Fermi level in the normal-emission spectra. Resonant photoemission study shows that the state is derived from Ta 5d orbitals. The two-dimensional band structure for the surface electronic state of the TaC(111) is mapped in a surface Brillouin zone using off-normal-emission spectra. The electronic structure of the surface is found to be very similar to those of NbC(111) and TiC(111) surfaces, though some surface induced states, which are not found for NbC(111) and TiC(111) surfaces, are newly found.

Interaction of hydrogen with ZrC(111) surface: angle-resolved photoemission study

Abstract Angle-resolved photoemission spectroscopy (ARPES) utilizing synchrotron radiation has been applied to study the adsorption of hydrogen on a ZrC(111) surface at room temperature. The electronic structure of the ZrC(111) clean surface is characterized by a surface-induced state at around the Fermi level. Hydrogen adsorption attenuates the surface-induced state and induces a H 1s-induced split-off state at 6.5 eV at the \ qG point, at saturation coverage. The two-dimensional electronic structure of the H saturated surface is investigated by off-normal-emission measurements and the H 1s-induced state is found to have a dispersion of (1 × 1) periodicity. The bandwidth of this state is determined to be 1.0∼1.2 eV, which correlates well with those of metal-H adsorption systems.

Chemisorption of CO on NbC(111) at 80 K: Angle‐resolved photoemission study

Angle‐resolved photoemission spectroscopy with synchrotron radiation has been used to study the adsorption of CO on a NbC(111) surface at 80 K. CO is adsorbed molecularly on the NbC(111) surface. Binding energies of the CO valence levels are in good agreement with previously published results for metal–CO systems. The adsorbed CO forms a (1×1) overlayer. The two‐dimensional band dispersions of CO valence levels in the (1×1) phase are measured in the ΓM direction. The considerable dispersions (∼0.35 eV for both 4σ and 5σ+1π levels) are observed as in the case for the periodic CO layer on the metal surfaces. A shape resonance of the 4σ level of the adsorbed CO is observed at ∼32 eV, which is nearly the same energy as that for the gas phase CO. The 4σ cross section at the resonance energy is measured as a function of incidence angles and the result is consistent with a molecular orientation normal to the surface.

Chemisorption state of atomic oxygen on TiC(100) surface: angle-resolved photoemission study

Abstract Angle-resolved photoemission spectroscopy with synchrotron radiation has been used to study oxygen adsorption on a TiC(100) surface at room temperature. Oxygen is adsorbed dissociatively, and the O 2p-derived states of atomic oxygen are found at 4.1 and 5.9 eV at the ΓgG point. From polarization-dependent measurements, these states are assigned to the 2p x,y - and 2p z -derived states, respectively. Two-dimensional band dispersions of these states in the ΓgGM direction are mapped from the off-normal-emission measurements. The measured band structure suggests that the 2p z orbital is relatively more stabilized through the chemisorption bonding. The cross section of the O 2p-derived states does not show a resonance at the energy region of the Ti3p-3d transition threshold, indicating that a hybridization of the Ti3d and O 2p orbitals contributes little to the chemisorption bonding. These results are compatible with the C on-top site adsorption model.

Electronic structure of the nitride layers formed on a Si(111) surface: angle-resolved photoemission study

Angle-resolved photoemission spectroscopy utilizing synchrotron radiation has been applied to study the electronic structure of the nitride overlayers formed on a Si(111) surface. As the Si(111) surface exposed to NO at saturation coverage is heated to 900°C, the (8 × 8)-N overlayer is formed. The NZnpπ non-bonding, N 2px,y + Si 3p bonding, and Si 3s-derived bonding bands are identified in the ARPE spectra using the polarization-dependent measurements. As the surface is further heated to 1050°C, the (8 × 8)-N overlayer is changed to the quadruplet-N overlayer, however, the spectral profile for the quadruplet-N overlayer is similar to that for the (8 × 8)-N overlayer. The effect of the “surface umklapp” process arising from the lattice mismatch between nitride overlayers and Si(111) (1 × 1) substrate are investigated with off-normal-emission measurements.

Angle-resolved photoemission study of the clean and oxygen-covered NbC(110) surface

Abstract Angle-resolved photoemission spectroscopy utilizing synchrotron radiation has been used to study the electronic structure of a NbC0.95(110) clean surface and the surface exposed to oxygen at room temperature. The spectra for the clean surface show that the bulk band emissions show small hv-dependent dispersions at hv = 27.5–65 eV, thus the density of state effect is dominant in the photoemission process at this photon energy range. A surface state is found to be formed on this surface at 1.7 eV at the \ gG point. The state is attributed to a Tamm state pulled off from the Σ3 bulk band. As the oxygen is adsorbed on the (110) surface, the O 2p-derived states are formed at 4.7 and 6.5 eV for 5–10 L exposure. The inner valence C 2s band emission is strongly affected by oxygen adsorption, indicating that the oxygen atom sits on the surface carbon atoms.