Kazuhiko Mase

Active Surface Oxygen for Catalytic CO Oxidation on Pd(100) Proceeding under Near Ambient Pressure Conditions

Catalytic CO oxidation reaction on a Pd(100) single-crystal surface under several hundred mTorr pressure conditions has been studied by ambient pressure X-ray photoelectron spectroscopy and mass spectroscopy. In-situ observation of the reaction reveals that two reaction pathways switch over alternatively depending on the surface temperature. At lower temperatures, the Pd(100) surface is covered by CO molecules and the CO2 formation rate is low, indicating CO poisoning. At higher temperatures above 190 °C, an O-Pd-O trilayer surface oxide phase is formed on the surface and the CO2 formation rate drastically increases. It is likely that the enhanced rate of CO2 formation is associated with an active oxygen species that is located at the surface of the trilayer oxide.

Performance of PF BL-13A, a vacuum ultraviolet and soft X-ray undulator beamline for studying organic thin films adsorbed on surfaces

We report on the present status of a vacuum ultraviolet and soft X-ray undulator beamline, BL-13A, located at the Photon Factory. BL-13A is mainly dedicated to the study of organic thin films adsorbed on well-defined surfaces, using angle-resolved photoelectron spectroscopy (ARPES), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS). The photon-energy resolution (E/ΔE) is estimated to be about 10000 at a photon energy of 64 eV with an exit-slit width of 30 μm. The photon intensity is estimated to be 2.9 × 1012 to 5.6 × 108 photons/s for photon energies of 30-1600 eV with an exit-slit width of 100 μm at the ring current of 450 mA. An ultrahigh vacuum (UHV) chamber equipped with an electron-energy analyzer (Gamma Data/Scienta, SES 200) is used as the main end station for ARPES, XPS, and XAS measurements. A sample can be transferred from a UHV chamber for sample preparation or from a UHV chamber for the evaporation of organic materials. The sample-holder acceptors are equipped with a heating and cooling system. The overall electron-energy resolution is estimated to be about 12 meV at a photon energy of 30 eV.

Utilizing carbon nanotube electrodes to improve charge injection and transport in bis(trifluoromethyl)-dimethyl-rubrene ambipolar single crystal transistors

We have examined the significant enhancement of ambipolar charge injection and transport properties of bottom-contact single crystal field-effect transistors (SC-FETs) based on a new rubrene derivative, bis(trifluoromethyl)-dimethyl-rubrene (fm-rubrene), by employing carbon nanotube (CNT) electrodes. The fundamental challenge associated with fm-rubrene crystals is their deep-lying HOMO and LUMO energy levels, resulting in inefficient hole injection and suboptimal electron injection from conventional Au electrodes due to large Schottky barriers. Applying thin layers of CNT network at the charge injection interface of fm-rubrene crystals substantially reduces the contact resistance for both holes and electrons; consequently, benchmark ambipolar mobilities have been achieved, reaching 4.8 cm(2) V(-1) s(-1) for hole transport and 4.2 cm(2) V(-1) s(-1) for electron transport. We find that such improved injection efficiency in fm-rubrene is beneficial for ultimately unveiling its intrinsic charge transport properties so as to exceed those of its parent molecule, rubrene, in the current device architecture. Our studies suggest that CNT electrodes may provide a universal approach to ameliorate the charge injection obstacles in organic electronic devices regardless of charge carrier type, likely due to the electric field enhancement along the nanotube located at the crystal/electrode interface.

Electronic density tailing outside π-conjugated polymer surface

Penning ionization electron spectroscopy (PIES) was adopted to examine surface electronic property of conjugated poly(3-hexylthiophene) (P3HT) aiming to detect the electronic density tailing outside a polymer surface. The electronic wave function of the highest occupied molecular orbital (HOMO) state is shielded by side hexyl chain for a P3HT film with edge-on conformation and was not detected by PIES, while it is tailing outside the polymer surface for face-on conformation and was observed clearly by PIES. The presence of HOMO electronic wave function outside the polymer surface makes it possible to form HOMO-HOMO overlapping with overlayer materials, and therefore more efficient charge transfer is expected in a heterojunction structure for device application.

In situ removal of carbon contamination from optics in a vacuum ultraviolet and soft X-ray undulator beamline using oxygen activated by zeroth-order synchrotron radiation

Carbon contamination of optics is a serious issue in all soft X-ray beamlines because it decreases the quality of experimental data, such as near-edge X-ray absorption fine structure, resonant photoemission and resonant soft X-ray emission spectra in the carbon K-edge region. Here an in situ method involving the use of oxygen activated by zeroth-order synchrotron radiation was used to clean the optics in a vacuum ultraviolet and soft X-ray undulator beamline, BL-13A at the Photon Factory in Tsukuba, Japan. The carbon contamination of the optics was removed by exposing them to oxygen at a pressure of 10(-1)-10(-4)u2005Pa for 17-20u2005h and simultaneously irradiating them with zeroth-order synchrotron radiation. After the cleaning, the decrease in the photon intensity in the carbon K-edge region reduced to 2-5%. The base pressure of the beamline recovered to 10(-7)-10(-8)u2005Pa in one day without baking. The beamline can be used without additional commissioning.

Recent progress in coincidence studies on ion desorption induced by core excitation

This paper reports on recent studies of photostimulated ion desorption (PSID) using electron ion coincidence (EICO) spectroscopy combined with synchrotron radiation. H+ desorption from H2O dissociatively adsorbed on Si(111) and SiO2/Si(111) surfaces (H2O/Si(111) and H2O/SiO2/Si(111)) was studied for Si L-edge excitation. The Si 2p–H+ photoelectron photoion coincidence (PEPICO) and Si 2p photoelectron spectra of H2O/Si(111) and H2O/SiO2/Si(111) show that H+ desorption probability increases as the number of positive charges at the Si site increases. The H+ desorption probability per Si 2p ionization for the Si4+ site was estimated and found to be 5–7 × 10−5. We proposed a mechanism that H+ desorption is induced by Si 2p photoionization accompanied by a Si LVVV double-Auger transition. This article also reviews recent EICO work on site-specific ion desorption of 1,1,1-trifluoro-2-propanol-d1 (CF3CD(OH)CH3) adsorbed on Si(100) surfaces, and on the mechanisms of PSID of poly(tetrafluoroethylene) (PTFE) and TiO2(110). Clear site-specific ion desorption was observed for the C 1s core excitation of a CF3CD(OH)CH3 sub-monolayer on Si(100). A spectator-Auger-stimulated ion-desorption mechanism was proposed for F+ desorption induced by a transition from F 1s to σ(C–F)* of PTFE. O+ desorption induced by O 1s excitation of TiO2(110) was attributed mainly to three-hole final states resulting from multi-electron excitation/decay. For O+ desorption induced by Ti core excitation of TiO2(110), on the other hand, charge transfer from an O 2p orbital to a Ti 3d orbital, instead of the interatomic Auger transition, was proposed to be responsible for the desorption. These investigations demonstrate that EICO spectroscopy combined with synchrotron radiation is a useful tool for studying PSID.

Design and performance of a compact scanning transmission X-ray microscope at the Photon Factory

We present a new compact instrument designed for scanning transmission X-ray microscopy. It has piezo-driven linear stages, making it small and light. Optical components from the virtual source point to the detector are located on a single optical table, resulting in a portable instrument that can be operated at a general-purpose spectroscopy beamline without requiring any major reconstruction. Careful consideration has been given to solving the vibration problem common to high-resolution microscopy, so as not to affect the spatial resolution determined by the Fresnel zone plate. Results on bacteriogenic iron oxides, single particle aerosols, and rare-earth permanent magnets are presented as examples of its performance under diverse applications.

Full Picture of Valence Band Structure of Rubrene Single Crystals Probed by Angle-Resolved and Excitation-Energy-Dependent Photoelectron Spectroscopy

The valence band structure of rubrene single crystals was experimentally determined by high-resolution angle-resolved and excitation-energy-dependent photoelectron spectroscopy at room temperature. The energy position of the peak derived from the highest occupied molecular orbital did not depend on the excitation energy, reflecting an absence of energy dispersion along the surface normal direction. A two-dimensional valence band dispersion relation over the surface Brillouin zone obtained by angle-resolved photoemission to three critical points was reproduced excellently by a two-dimensional tight binding approximation. Highly anisotropic values of intermolecular transfer integrals to four adjacent molecules were obtained from the present results.

High-resolution core-level photoemission measurements on the pentacene single crystal surface assisted by photoconduction.

Upon charge carrier transport behaviors of high-mobility organic field effect transistors of pentacene single crystal, effects of ambient gases and resultant probable impurities at the crystal surface have been controversial. Definite knowledge on the surface stoichiometry and chemical composites is indispensable to solve this question. In the present study, high-resolution x-ray photoelectron spectroscopy (XPS) measurements on the pentacene single crystal samples successfully demonstrated a presence of a few atomic-percent of (photo-)oxidized species at the first molecular layer of the crystal surface through accurate analyses of the excitation energy (i.e. probing depth) dependence of the C1s peak profiles. Particular methodologies to conduct XPS on organic single crystal samples, without any charging nor damage of the sample in spite of its electric insulating character and fragility against x-ray irradiation, is also described in detail.

Molecular mixing in donor and acceptor domains as investigated by scanning transmission X-ray microscopy

The nanolevel molecular structure of a bulk heterojunction (BHJ) with a donor (D) polymer and acceptor (A) fullerene derivative is indispensable for true comprehension of highly efficient organic photovoltaic devices. Here, we performed scanning transmission X-ray microscopy of a poly(9,9-dioctylfluorene-co-bithiophene) (F8T2)/[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) blend film with periodic nanostructure. The spatially resolved carbon K-edge absorption spectra revealed that the nanostructure consists of two types of domains with considerable molecular mixing. The fullerene mass fraction is 71 ± 1 and 33 ± 2 wt % for the PC71BM- and F8T2-rich domains, respectively.