Tetsuya Aruga

Adsorption of Na atoms and oxygen-containing molecules on MgO(100) and (111) surfaces

The adsorption of Na atoms and oxygen-containing molecules such as H 2 O, CH 3 OH, CO 2 , HCOOH and HCOOCH 3 has been studied on the annealed MgO(100) and (111) surfaces by means of XPS, UPS and LEED. Annealing a (111) surface at 1000 K yielded microfacets ~ 20 A across. This faceted (111) surface showed a high adsorption activity compared with the flat (100) surface. Hence it is suggested that the coordinatively unsaturated edge-sites or the multicentered valley-sites on the microfaceted surface play important roles in the chemisorption.

Adsorption of CH3OH, HCOOH and SO2 on TiO2(110) and stepped TiO2(441) surfaces

Abstract A TiO2(441) surface was prepared whose electronic states and chemisorption properties for CH3OH, HCOOH and SO2 were compared with those of a TiO2(110) surface by means of XPS, UPS and LEED at 298 K. The (441) surface had a regular step structure which is indexed as [3(110) × (111)]. Its work function was smaller by 0.7 eV than that of the (110) surface due to the pinned Fermi level originated from a small amount of Ti3+ species. CH3OH and HCOOH were molecularly adsorbed on both the surfaces, giving a p(2×1) structure for HCOOH adsorbed on (110). It was suggested from the change of the work function that they were absorbed with their dipole axes normal to the local crystal plane on the step sites, thus the adsorbed species at step sites being more inclined as compared with those on the (110) terrace. SO2 was absorbed on (441) forming SO2-3 and also reacted with Ti3+ probably at the step to form S2-, while only SO2-3 was detected on the (110) surface.

Modification of surface electronic structure on TiO2(110) and TiO2(441) by Na deposition

Abstract The geometric and electronic properties of Na overlayers deposited on TiO 2 (rutile) (110) and stepped (441) surfaces have been examined by means of XPS, UPS, XAES (X-ray excited Auger electron spectroscopy), EELS and LEED. Na atoms form a smooth atomic layer interacting with surface oxygen atoms on TiO 2 (110). A surface model consisting of ordered Na 2 O dimers is proposed for a c(4×2) structure which appears at 0.5 ML Na coverage. At the first monolayer, the Na 2 O units are considered to complete an array along the [001] direction in (1×1) periodicity. Bonding of Na with oxygen causes charge transfer to the substrate, resulting in a downward band bending toward the surface. With the increase of the band bending, the Fermi level crosses an unfilled surface state localized in five-fold coordinated Ti 4+ ions, which leads to the reduction of these Ti 4+ to Ti 3+ . The strong interaction of the TiO 2 (110) surface toward the Na overlayer is ascribed to surface states just above the valence band maximum localized on protruded ridge oxygen atoms, from comparison with previous results for a flat MgO(100) surface. Almost the same picture is drawn for a stepped TiO 2 (441) surface.

Alkali-metal adsorption on metals

Abstract The interplay between microscopic electronic structure and ordering phenomena is surveyed for alkali-metal monolayers on metal surfaces. First, the electronic structure of alkali-metal atoms adsorbed on surfaces is discussed on the basis of experimental results obtained with a variety of surface science techniques as well as theoretical works including those using a jellium model, the Anderson-Newns formalism, and a local-density-functional theory. The focus of interest here will be brought into drastic changes in electronic properties observed when coverage increases from zero to one monolayer. Then we turn to a variety of structural phases and transitions in alkali-metal monolayers. The microscopic mechanism and driving force of the ordering is discussed in terms of the electronic structure of adatoms.

Large Rashba spin splitting of a metallic surface-state band on a semiconductor surface

The generation of spin-polarized electrons at room temperature is an essential step in developing semiconductor spintronic applications. To this end, we studied the electronic states of a Ge(111) surface, covered with a lead monolayer at a fractional coverage of 4/3, by angle-resolved photoelectron spectroscopy (ARPES), spin-resolved ARPES and first-principles electronic structure calculation. We demonstrate that a metallic surface-state band with a dominant Pb 6p character exhibits a large Rashba spin splitting of 200 meV and an effective mass of 0.028 me at the Fermi level. This finding provides a material basis for the novel field of spin transport/accumulation on semiconductor surfaces. Charge density analysis of the surface state indicated that large spin splitting was induced by asymmetric charge distribution in close proximity to the nuclei of Pb atoms.

H-atom relay reactions in real space

Hydrogen bonds are the path through which protons and hydrogen atoms can be transferred between molecules. The relay mechanism, in which H-atom transfer occurs in a sequential fashion along hydrogen bonds, plays an essential role in many functional compounds. Here we use the scanning tunnelling microscope to construct and operate a test-bed for real-space observation of H-atom relay reactions at a single-molecule level. We demonstrate that the transfer of H-atoms along hydrogen-bonded chains assembled on a Cu(110) surface is controllable and reversible, and is triggered by excitation of molecular vibrations induced by inelastic tunnelling electrons. The experimental findings are rationalized by ab initio calculations for adsorption geometry, active vibrational modes and reaction pathway, to reach a detailed microscopic picture of the elementary processes.

Photoelectron spectroscopic study of clean and CO adsorbed NI/TiO2(110) interfaces

Abstract Nickel deposited on a rutile TiO 2 (110) surface was examined by XPS, AES, UPS and LEED. The nickel deposits formed an atomic layer at 300 K with a density of 8 × 10 14 cm −2 . A three-dimensional agglomeration was detected above 1 ML, by analyzing the forward scattering of photoelectrons from the Ni 2p core level. The work function deduced from UPS decreased with Ni coverage with a minimum at 0.5 ML. From its initial decrease by 0.7 eV we evaluated the outward dipole to be 0.5 debye per Ni atom. This dipole was attributed to an electron transfer from the Ni deposits into the substrate. The charge transferred was estimated to be not more than 0.1 electron per Ni adatom. As the overlayer grew more dense, the lateral interaction between Ni adatoms predominated and inhibited the electron transfer through the interface. XPS and UPS data revealed that CO was adsorbed molecularly on the Ni/TiO 2 (110) surfaces at 300 K for all coverages. The uptake of CO increased proportionally to the Ni coverage below 1 ML.

Chemisorption of CO and H2 on clean and oxygen-modified Mo(112)

Chemisorption of CO and H2 on clean and oxygen-modified Mo(112) has been investigated by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and temperature-programmed desorption (TPD). Oxygen adatoms are expected to be adsorbed in trough sites between close-packed Mo rows and, hence, the first-layer Mo atoms remain accessible to gas-phase molecules. Oxygen adsorption on a clean Mo(112) surface at 300 K, followed by annealing to 600 K, has produced a series of LEED patterns with increasing oxygen coverage. Models for these structures are presented. These ordered oxygen adlayers are stable up to 1100 K and have been used to examine the effect of oxygen modification on the adsorption and dissociation of CO and H2. Both CO and H2 molecules are dissociated on clean Mo(112). The amount of dissociated species decreases linearly with oxygen coverage and is suppressed completely on a p(2 × 1)-O surface (θO = 0.5). The structure of the Mo ensemble required for the dissociation of CO and H2 is discussed. The desorption peaks of molecular CO show characteristic changes as a function of oxygen coverage. These observations made it possible to discuss the local environment of the first-layer Mo atoms bonded with CO. The spatial extent and the strength of the electronic modification effect of oxygen adatoms is also discussed. We also found that the Mo(112) surface undergoes an hydrogen-induced surface reconstruction to form a p(1 × 2) structure. The hydrogen coverage corresponding to the completion of p(1 × 2) has been determined to be 1.5.

Interaction between CO and NH3 coadsorbed on Ru(001): its effects on the ordering in mixed adlayers and the ammonia dissociation

The coadsorption of CO and ammonia on Ru(001) has been investigated by low-energy electron diffraction (LEED), temperature-programmed desorption (TPD) and high-resolution electron energy-loss spectroscopy (HREELS). The main focus has been on the interaction between different admolecules on the surface and its important role in surface reaction. Exposing CO-precovered Ru(001) to ammonia at 100 K leads to the formation of mixed ordered layers with a (2 × 2) periodicity. It was found that two types of (2 × 2) structures are formed depending on the CO precoverage. One of the (2 × 2) structures (α-phase) contains one CO and two ammonia molecules per (2 × 2) unit cell and the other (β-phase) contains two CO and one ammonia. Structure models for the two phases are proposed based on vibrational spectra measured for the coadsorbed phases of CO and ammonia (15NH3 or ND3). TPD results suggest that the ammonia dissociation takes place on clean and CO-precovered Ru(001). The amount of dissociated ammonia decreased initially with increasing CO precoverage, passed a minimum at θCO = 0.25, increased with a further increase of CO coverage, and eventually reached a saturation value above θCO = 0.5. The dissociation of ammonia in the β−(2 × 2) structure was found to be enhanced by a factor of 4–6 as compared with the dissociation in the α−(2 × 2) structure. The HREEL spectra indicated that the C3v molecular axis of ammonia is tilted in the coadsorbed layers, the tilting being most pronounced in the β−(2 × 2) phase with a high CO partial coverage. This observation suggests that the tilting of ammonia due to the interaction with CO facilitates electron donation from Ru 4d to LUMO of ammonia, leading to the N-H bond dissociation. The microscopic model for the CO-NH3 interaction on metal surfaces is presented.

Interaction of NO with CO on Pd(100): ordered coadsorption structures and explosive reaction

Abstract An ordered mixed structure of c(3 × 2) is formed for a (NO + CO) coadsorption layer. The c(3 × 2) islands are considered to consist of equimolar NO and CO. The local fractional coverage ( θ NO + θ CO ) in the domain is estimated to be 0.33. Explosive production of CO 2 takes place in the c(3 × 2) islands. The vacancy requirement model is considered to be valid for the autocatalytic reaction. Since the reaction is not accompanied with any substrate reconstruction, the autocatalytic behaviour is attributed only to the formation of mixed islands. The desorption of N 2 follows, however, the second-order kinetics on Pd(100). As a result of the competition between NO and CO for the surface electrons, the CO-metal bond is weakened by the coadsorbed NO, which influences the explosive reaction. On the other hand, strengthening of the NO-metal bond is observed. When NO is in excess of CO, a p(3 × 2) structure coexists with the c(3 × 2) structure. The local coverage in the p(3 × 2) islands is estimated to be 0.33. In this coverage region, another path for the CO 2 production is available.