Takanori Koitaya

Core level excitations—A fingerprint of structural and electronic properties of epitaxial silicene

From the analysis of high-resolution Si 2p photoelectron and near-edge x-ray absorption fine structure (NEXAFS) spectra, we show that core level excitations of epitaxial silicene on ZrB2(0001) thin films are characteristically different from those of sp(3)-hybridized silicon. In particular, it is revealed that the lower Si 2p binding energies and the low onset in the NEXAFS spectra as well as the occurrence of satellite features in the core level spectra are attributed to the screening by low-energy valence electrons and interband transitions between π bands, respectively. The analysis of observed Si 2p intensities related to chemically distinct Si atoms indicates the presence of at least one previously unidentified component. The presence of this component suggests that the observation of stress-related stripe domains in scanning tunnelling microscopy images is intrinsically linked to the relaxation of Si atoms away from energetically unfavourable positions.

Adsorption and reaction of NO on the clean and nitrogen modified Rh(111) surfaces

The adsorption states and thermal reactions of NO on the clean and nitrogen modified Rh(111) surfaces were investigated between 20 and 150 K using infrared reflection adsorption spectroscopy (IRAS) and temperature programmed desorption. On the clean surface, singleton species at atop and hollow sites were observed at 1816 and 1479 cm(-1), respectively. Using time-resolved IRAS, the activation energy and pre-exponential factor of the site change from atop to hollow sites on Rh(111) were estimated to be 117 meV and 1.7x10(10) s(-1), respectively. On the saturated monolayer, physisorbed NO dimers were formed. In the second layer, they were adsorbed with the N-N bond nearly parallel to the surface. In the multilayer formed at 20 K, the NO dimers were randomly oriented. On the nitrogen modified Rh(111) surface, a new adsorption state of chemisorbed monomer was observed as well as atop and hollow species. Physisorbed NO dimers were a precursor to N(2)O formation on the nitrogen modified Rh(111) surface. In the N(2)O formation reaction, three kinds of N(2)O species were identified. The first species desorbed from the surface immediately after the formation reaction, which is a reaction-limited process. The second species was physisorbed on the surface and desorbed at 86 K, which is a desorption-limited process. The third species was chemisorbed on the surface and decomposed above 100 K.

Kinetic and geometric isotope effects originating from different adsorption potential energy surfaces: Cyclohexane on Rh(111)

Novel isotope effects were observed in desorption kinetics and adsorption geometry of cyclohexane on Rh(111) by the use of infrared reflection absorption spectroscopy, temperature programmed desorption, photoelectron spectroscopy, and spot-profile-analysis low energy electron diffraction. The desorption energy of deuterated cyclohexane (C(6)D(12)) is lower than that of C(6)H(12). In addition, the work function change by adsorbed C(6)D(12) is smaller than that by adsorbed C(6)H(12). These results indicate that C(6)D(12) has a shallower adsorption potential than C(6)H(12) (vertical geometric isotope effect). The lateral geometric isotope effect was also observed in the two-dimensional cyclohexane superstructures as a result of the different repulsive interaction between interfacial dipoles. The observed isotope effects should be ascribed to the quantum nature of hydrogen involved in the C-H···metal interaction.

Two-dimensional superstructures and softened C–H stretching vibrations of cyclohexane on Rh(111): Effects of preadsorbed hydrogen

Adsorption structures and interaction of cyclohexane molecules on the clean and hydrogen-preadsorbed Rh(111) surfaces were investigated using scanning tunneling microscopy, spot-profile-analysis low-energy electron diffraction, temperature-programmed desorption, and infrared reflection absorption spectroscopy (IRAS). Various ordered structures of adsorbed cyclohexane were observed as a function of hydrogen and cyclohexane coverages. When the fractional coverage (θ(H)) of preadsorbed hydrogen was below 0.8, four different commensurate or higher-order commensurate superstructures were found as a function of θ(H); whereas more densely packed incommensurate overlayers became dominant at higher θ(H). IRAS measurements showed sharp softened C-H vibrational peaks at 20 K, which originate from the electronic interaction between adsorbed cyclohexane and the Rh surface. The multiple softened C-H stretching peaks in each phase are due to the variation in the adsorption distance from the substrate. At high hydrogen coverages they became attenuated in intensity and eventually diminished at θ(H) = 1. The gradual disappearance of the soft mode correlates well with the structural phase transition from commensurate structures to incommensurate structures with increasing hydrogen coverage. The superstructure of adsorbed cyclohexane is controlled by the delicate balance between adsorbate-adsorbate and adsorbate-substrate interactions which are affected by preadsorbed hydrogen.

The Quantum Nature of C–H···Metal Interaction: Vibrational Spectra and Kinetic and Geometric Isotope Effects of Adsorbed Cyclohexane

The nature of C-H···M (M = metal surface) interactions is reviewed based mainly on our recent investigations of cyclohexane on Rh(111). Infrared reflection-absorption spectroscopy measurements at low temperature (∼20 K) have shown that the softened CH stretching band consists of several sharp peaks. At temperatures above 80 K, each peak is broadened, most probably by anharmonic coupling with thermally excited low-energy frustrated translational modes. The origin of fine structure in this band and its similarity to that in hydrogen bond systems are discussed. In addition, novel isotope effects were observed in desorption kinetics and adsorption geometry of cyclohexane on Rh(111) using temperature programmed desorption, ultraviolet photoelectron spectroscopy, and spot profile analysis low-energy electron diffraction. The desorption energy of deuterated cyclohexane (C6 D12 ) is lower than that of C6 H12 (inverse kinetic isotope effect). In addition, the work function change by adsorbed C6 D12 is smaller than that by adsorbed C6 H12 . These results indicate that C6 D12 molecules are slightly more distant from the surface than C6 H12 molecules (vertical geometric isotope effect). A lateral geometric isotope effect was also observed for the two-dimensional cyclohexane superstructures as a result of the repulsive interaction between interfacial dipoles (= work function change). These isotope effects are ascribed to the quantum nature of hydrogen involved in the C-H···M interaction.

Energy level alignment of cyclohexane on Rh(111) surfaces: The importance of interfacial dipole and final-state screening

Adsorption states and electronic structure of cyclohexane on clean and hydrogen-saturated Rh(111) surfaces were investigated by scanning tunneling microscopy and photoelectron spectroscopy. Monolayer cyclohexane molecules form an ordered superstructure on the clean Rh(111) surface. The energy level alignment of adsorbed cyclohexane depends on each adsorption site; molecular orbitals of adsorbed cyclohexane on the atop site have lower binding energies than those on the other sites. In contrast, it becomes insensitive to adsorption sites on the hydrogen-saturated Rh(111) surface. By preadsorption of hydrogen, all cyclohexane molecular orbitals are uniformly shifted to lower binding energy compared to those on the clean Rh(111) surface. The observed energy level alignment of cyclohexane on the Rh(111) surfaces is determined by the vacuum level shift and the final-state screening effects.

Observation of Fano line shapes in infrared vibrational spectra of CO2 adsorbed on Cu(997) and Cu(111)

Adsorption states of carbon dioxide on the Cu(997) and Cu(111) surfaces were investigated by infrared reflection absorption spectroscopy, temperature programmed desorption, and X-ray photoelectron spectroscopy. CO2 molecules are physisorbed on the Cu(997) surface at temperatures below 70 K; neither chemisorption nor dissociation of CO2 occurs on the Cu(997) surface at this low temperature. However, the vibrational spectra of adsorbed CO2 depend significantly on the substrate temperature and coverage. IR spectra of CO2 vibrational modes at 70 K show asymmetric Fano line shapes, while only normal absorption bands are observed when CO2 is adsorbed at 20 K. Fano line shapes are also observed for CO2 on Cu(111) at 85 K. The observation of Fano effect indicates the coupling between the electronic continuum states of the Cu surface and the internal vibrational modes of CO2 even in such physisorbed system.

Configuration change of NO on Cu(110) as a function of temperature.

The bonding structure of nitric oxide (NO) on Cu(110) is studied by means of scanning tunneling microscopy, reflection absorption infrared spectroscopy, and electron energy loss spectroscopy at 6-160 K. At low temperatures, the NO molecule adsorbs at the short bridge site via the N end in an upright configuration. At around 50 K, this turns into a flat configuration, in which both the N and O atoms interact with the surface. The flat configuration is characterized by the low-frequency N-O stretching mode at 855 cm(-1). The flat-lying NO flips back and forth when the temperature increases to ~80 K, and eventually dissociates at ~160 K. We propose a potential energy diagram for the conversion of NO on the surface.

Quantitative analysis of chemical interaction and doping of the Si(111) native oxide surface with tetrafluorotetracyanoquinodimethane

The charge-transfer states and the carrier concentration of the native oxide Si(111) surface adsorbed with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) were investigated by X-ray photoelectron spectroscopy (XPS) and independently driven four-probe electrical conductivity measurements. The XPS results show that F4-TCNQ molecules are involved in charge transfer with the SiO2/Si(111) surface. The Si 2p XPS spectra and the surface photovoltage shift provide the evidences of (i) change in the oxidation states at the SiO2-Si(111) interface region and (ii) formation of a p-type space charge layer (SCL) with a hole concentration of 1.7 × 1010 cm−2, respectively. The four-probe I–V measurements also support the formation of the p-type SCL, and the estimated hole concentration of 2.0 × 1010 cm−2 agrees well with the XPS results. The estimated SCL hole concentrations were much smaller than the excess charge density in the F4-TCNQ layer, of the order of 1013 cm−2, suggesting that most of charges wer...

Dewetting growth of crystalline water ice on a hydrogen saturated Rh(111) surface at 135 K

We investigated the water (D(2)O) adsorption at 135 K on a hydrogen pre-adsorbed Rh(111) surface using temperature programmed desorption and infrared reflection absorption spectroscopy (IRAS) in ultrahigh vacuum. With increasing the hydrogen coverage, the desorption temperature of water decreases. At the saturation coverage of hydrogen, dewetting growth of water ice was observed: large three-dimensional ice grains are formed. The activation energy of water desorption from the hydrogen-saturated Rh(111) surface is estimated to be 51 kJ/mol. The initial sticking probability of water decreases from 0.46 on the clean surface to 0.35 on the hydrogen-saturated surface. In IRAS measurements, D-down species were not observed on the hydrogen saturated surface. The present experimental results clearly show that a hydrophilic Rh(111) clean surface changes into a hydrophobic surface as a result of hydrogen adsorption.