Kozo Mukai

Chemically homogeneous and thermally reversible oxidation of epitaxial graphene

With its exceptional charge mobility, graphene holds great promise for applications in next-generation electronics. In an effort to tailor its properties and interfacial characteristics, the chemical functionalization of graphene is being actively pursued. The oxidation of graphene via the Hummers method is most widely used in current studies, although the chemical inhomogeneity and irreversibility of the resulting graphene oxide compromises its use in high-performance devices. Here, we present an alternative approach for oxidizing epitaxial graphene using atomic oxygen in ultrahigh vacuum. Atomic-resolution characterization with scanning tunnelling microscopy is quantitatively compared to density functional theory, showing that ultrahigh-vacuum oxidization results in uniform epoxy functionalization. Furthermore, this oxidation is shown to be fully reversible at temperatures as low as 260 °C using scanning tunnelling microscopy and spectroscopic techniques. In this manner, ultrahigh-vacuum oxidation overcomes the limitations of Hummers-method graphene oxide, thus creating new opportunities for the study and application of chemically functionalized graphene.

Direct observation of site-specific valence electronic structure at the SiO2/Si interface

Atom specific valence electronic structures at interface are elucidated successfully using soft x-ray absorption and emission spectroscopy. In order to demonstrate the versatility of this method, we investigated SiO2/Si interface as a prototype and directly observed valence electronic states projected at the particular atoms of the SiO2/Si interface; local electronic structure strongly depends on the chemical states of each atom. In addition we compared the experimental results with first-principle calculations, which quantitatively revealed the interfacial properties in atomic-scale.

STM studies of oxygen-induced reconstruction on a PtRh(100) alloy surface

Oxygen-induced reconstruction of a PtRh(100) alloy surface was studied by scanning tunneling microscopy (STM). When a clean Pt-enriched PtRh(100) surface was exposed to O2 at more than 600 K, oxygen-induced reconstruction took place with segregation of Rh atoms to the surface. Depending on the coverage of oxygen, two types of reconstruction on the PtRh(100) alloy surface are induced. A p(3 × 1)-O structure appeared at relatively low oxygen coverage, and the TPD spectrum of the p(3 × 1)-O surface gave a desorption peak of O2 at ca. 930 K. The p(3 × 1)-O structure showed several different STM images depending on the tip condition, from which we could distinguish the two component metals, RhO rows and Pt rows. Based on this, a model structure was deduced. At a high oxygen coverage, more Rh atoms were segregated to the surface and the STM image suggests a c(2 × 20)-O structure. The c(2 × 20)-O surface changed into the p(3 × 1)-O by heating with the concomitant desorption of O2 at ca. 830 K. The STM images for this c(2 × 20)-O surface is analogous to the arrangement of Rh atoms on the top layer of a Rh(111) hexagonal lattice. Due to the lattice mismatch between the quasi-Rh(111) overlayer and PtRh(100) substrate, a moire pattern with 20 times periodicity along the 〈111〉 directions was observed. Both the p(3 × 1)-O and c(2 × 20)-O surfaces are easily reduced by H2 even at room temperature.

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.

A miniature effusion cell for the vacuum deposition of organic solids with low vapor pressures in surface science studies.

A miniature effusion cell for the vacuum deposition of volatile organic solids with low vapor pressures has been fabricated and used in surface science studies. The effusion cell is designed to operate at up to 200 degrees C under ultrahigh vacuum (UHV) conditions. The size of this cell is so small that it is attached to the top of a transfer rod and can be introduced from a subchamber into a main UHV chamber (retrievable). In addition, the small heat capacity of this cell means rapid heating and cooling rates. The advantages of this evaporator are its simplicity of design and ease of fabrication, assembly, and operation.

High resolution Si 2p photoelectron spectroscopy of unsaturated hydrocarbon molecules adsorbed on Si(100)c(4×2): the interface bonding and charge transfer between the molecule and the Si substrate

Abstract In order to elucidate the nature of the interface bonding between different molecules and a silicon surface, high resolution Si 2p photoelectron spectroscopy measurements were performed. After adsorption of unsaturated hydrocarbon molecules (ethylene, cyclopentene, and 1,4-cyclohexadiene), the peaks corresponding to the up and down atoms of surface asymmetric dimers vanished, while new peaks appeared between 215 and 398 meV relative to the bulk Si peak. These peaks are assigned to the di-σ Si–C bonds at the interface between the molecules and the surface. We can also estimate the amounts of reacted asymmetric dimers and the charge transfers from the peak intensities and the relative binding energies, respectively.

The growth process of first water layer and crystalline ice on the Rh(111) surface.

The adsorption states and growth process of the first layer and multilayer of water (D(2)O) on Rh(111) above 135 K were investigated using infrared reflection absorption spectroscopy (IRAS), temperature programed desorption, spot-profile-analysis low-energy electron diffraction, and scanning tunneling microscopy (STM). At the initial stage, water molecules form commensurate ( radical3x radical3)R30 degrees islands, whose size is limited for several hexagonal units; the average diameter is approximately 2.5 nm. This two-dimensional (2D) island includes D-down species, and free OD species exist at the island edge. With increasing coverage, the D-up species starts to appear in IRAS. At higher coverages, the 2D islands are connected in STM images. By the titration of Xe adsorption we estimated that the D-down domain occupies about 55% on Rh(111) at the saturation coverage. Further adsorption of water molecules forms three-dimensional ice crystallites on the first water layer; thus, the growth mode of crystalline water layers on Rh(111) is a Stranski-Krastanov type. We have found that an ice crystallite starts to grow on D-down domains and the D-down species do not reorient upon the formation of a crystalline ice.

Direct Evidence for Asymmetric Dimer on Si(100) at Low Temperature by Means of High-Resolution Si 2p Photoelectron Spectroscopy.

We have investigated the electronic states of the Si(100) surface at low temperature by means of high-resolution Si 2p photoelectron spectroscopy. The peak intensities of up and down atoms of the asymmetric dimer in Si 2p spectra do not change from 140 K to 55 K, showing that the number of asymmetric dimers is preserved. Therefore, we can conclude that the ground state of the dimer is asymmetric and the symmetric dimer images observed by scanning tunneling microscopy at this temperature range are due to extrinsic or dynamical intrinsic effects on the buckled dimer.

Reaction mechanism and adsorbed states of cyclohexene on Si(100)(2×1)

Abstract We have studied the adsorption states of cyclohexene on Si(100)(2×1) by means of photoelectron spectroscopy (PES) and scanning tunneling microscopy (STM). PES results indicate the interaction between π bond of cyclohexene and the dangling bond of the dimer. Two adsorbed state are observed in STM images; one shows a symmetric protrusion, and another one is asymmetric and consists of some small protrusions. The former is assigned to the boat conformation of adsorbed cyclohexene, and the latter is assigned to the twist-boat conformation.

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.