Tetsuroh Shirasawa

A method for measuring the specular X-ray reflectivity with millisecond time resolution

A method for quick measurement of the specular X-ray reflectivity using a tapered undulator source is described. It uses a convergent X-ray beam with a one-to-one correspondence between X-ray energy and direction, which is produced by diffraction of a white X-ray beam at a curved silicon crystal. To increase the momentum transfer range, the sample is rotated 45? from the horizontal around the incident beam direction, so that both the X-ray energy and the incident angle change continuously with direction. The specularly reflected beam is observed with a two-dimensional detector. The X-ray reflectivity curve in a wide momentum transfer range can be observed in a single detector exposure with a time resolution in the millisecond range. Test measurements were done for a commercial silicon wafer and a gold thin film on silicon.

Nonvortical Rashba Spin Structure on a Surface with C1h Symmetry

A totally anisotropic peculiar Rashba-Bychkov (RB) splitting of electronic bands was found on the Tl/Si(110)-(1×1) surface with C_{1h} symmetry by angle- and spin-resolved photoelectron spectroscopy and first-principles theoretical calculation. The constant energy contour of the upper branch of the RB split band has a warped elliptical shape centered at a k point located between Γ[over ¯] and the edge of the surface Brillouin zone, i.e., at a point without time-reversal symmetry. The spin-polarization vector of this state is in-plane and points almost the same direction along the whole elliptic contour. This novel nonvortical RB spin structure is confirmed as a general phenomenon originating from the C_{1h} symmetry of the surface.

Si 2p core level shifts of the epitaxial SiON layer on a SiC(0001), studied by photoemissin spectroscopy

The epitaxial silicon oxynitride (SiON) layer grown on a 6H-SiC(0001) surface is studied with core level photoemission spectroscopy. Si 2p spectra show three spectral components other than the bulk one. Chemical shifts and emission angle dependence of these components are well explained within a framework of a determined structure model of the SiON layer.

Interface electronic structure at the topological insulator–ferrimagnetic insulator junction

An interface electron state at the junction between a three-dimensional topological insulator film, Bi2Se3, and a ferrimagnetic insulator film, Y3Fe5O12 (YIG), was investigated by measurements of angle-resolved photoelectron spectroscopy and x-ray absorption magnetic circular dichroism. The surface state of the Bi2Se3 film was directly observed and localized 3d spin states of the Fe3+ in the YIG film were confirmed. The proximity effect is likely described in terms of the exchange interaction between the localized Fe 3d electrons in the YIG film and delocalized electrons of the surface and bulk states in the Bi2Se3 film.

Structure determination of the clean (001) surface of strained Si on Si1−xGex

The surface structure of the strained Si(001) (thickness of 20 nm) on Si1−xGex (x = 0.1, 0.2, and 0.3) was studied by low-energy electron diffraction (LEED). LEED intensity-energy spectra of the 2 × 1 reconstructed clean surfaces showed a systematic change that indicates the lattice contraction along the [001] direction remains even at the surfaces. The atomic structures were quantitatively determined, and they were compared with the unstrained pristine Si. The differences in the atomic position almost follow the difference in the bulk lattice constant determined by X-ray diffraction measurements. The results indicate that the strain produced at the Si/Si1−xGex interface remains unchanged up to the surface layer.

Surface X-ray Diffraction

A brief introduction to surface X-ray diffraction for studying surface and interface structures is presented. The basic principle and the practical aspect of experiment and analysis for the structure determination are reviewed. Finally, its advanced applications are shortly illustrated.

Scanning tunneling microscopic and spectroscopic studies on a crystalline silica monolayer epitaxially formed on hexagonal SiC(0001¯) surfaces

An epitaxial silicon-oxide monolayer of chemical composition of Si2O3 (the Si2O3 layer) formed on hexagonal SiC(0001¯) surfaces has been observed by scanning tunneling microscopy (STM). Filled- and empty-state STM images with atomic resolution support the previously reported model. Typical structural defects in the Si2O3 layer are found to be missing SiOn (n = 1, 2, 3) molecules. The band gap of the Si2O3 layer obtained by point tunneling spectroscopy is 5.5±0.5 eV, exhibiting considerable narrowing from that of bulk SiO2, 8.9 eV. It is proposed that the Si2O3 layer is suitable as a relevant interface material for formation of SiC-based metal-oxide-semiconductor devices.

Gemstones and Salts as Light Emitters for Learning X-ray Detectors

The scintillation counter is a widely-used X-ray detector. It contains a scintillator as a luminescent material that converts X-rays into visible light, which is detected with a sensor. A well-known scintillator in the X-ray region is sodium iodide, NaI, an ionic crystal. Before use, it is important to understand how the detector works. For students, the material name and the chemical formula of the scintillator are not familiar, however. In addition, students cannot watch or touch the key element in the detector, because the scintillator is installed inside the housing. Many jewels emit visible light or change their colors under ultraviolet light irradiation. Under X-ray irradiation, the same jewels exhibit similar responses as well. If popular jewels instead of special ionic crystals were used as scintillators, students might show interest in these materials. We propose that photographs of beautiful, brightly shining gemstones and salts could be used as visual educational materials for students to learn the principles of X-ray detectors. Different gemstones and salts were irradiated by intense white synchrotron X-ray radiation at beamline NE7A1 of the PF-AR synchrotron radiation facility at KEK, Japan. Photographs of fluorescence and phosphorescence from the gemstones, and of color changes due to the irradiation, were taken with a remote controlled digital camera. It should be noted that the experimental setup of this study is an easily understood handmade X-ray detector. We will present photographs of exciting gemstones such as Fluorite from the US, Hackmanite from Afghanistan, Mangano Calcite from China, Ruby from Brazil, Selenite from Canada, and Black Opal from Australia. We also irradiated different kinds of colored Himalayan Rock Salt from India or Pakistan, shown in Fig. 1. We will explain basic concepts of X-ray detectors, such as photon counting, dead time, recording, and quantum efficiency, with these photographs.

Structural change of the TiO2(110) surface in the hydrophilic reaction

Photocatalysis of titanium dioxides has been extensively studied in the past. Especially, after the discovery of the UV-light induced hydrophobic-hydrophilic transition of the rutile-TiO2(110) surface in the late of 1990’s [1], the number of photochemistry-related publications increased dramatically over the last decade to the extent that ~2400 related papers were published in 2008, in which ~80% of the papers involve the TiO2-related materials. The remarkable research activity arises from the potential applications of the photo-induced wettability control such as anti-fog coatings or self-cleaning coatings. However, despite the intensive study, the mechanism of the hydrophilic reaction is not completely clarified yet, mainly due to the lack of the detailed information of the atomicscale surface structure. We have studied the surface structural change by means of surface X-ray diffraction. By using the recently developed time-resolved x-ray crystal truncation rod (CTR) scattering measurement [2] and the static measurement for the hydrophobic and hydrophilic surfaces, we confirmed that (i) the surface roughness increases during the reaction probably due to the desorption of the surface oxygen atoms and (ii) an ordered water molecular layer formed on the hydrophobic surface disappears in the hydrophilic surface. Considering the previous reports which show the increase of hydrogen bond density in the hydrophilic surface, we suggest that the ill-ordered surface of the hydrophilic phase allows a larger number of water molecules to adsorb by making a hydrogen-bond network.

Photoresponsive LB Films under Light Irradiation Observed by Time-Resolved XRR

Langmuir-Blodgett (LB) films containing azobenzene undergo reversible structural change under light irradiation because of trans-tocis or cis-to-trans transformations of azobenzene molecules. Such films are a candidate for molecular machines. Time resolved measurements of specular X-ray reflectivity (XRR) curves were carried out for polymer specimens of azobenzene-containing polyvinil alcohol (6Az10-PVA) monolayer LB films on quartz substrates during light (365 nm or 436 nm) irradiation (1 mW/cm2). Measurements were performed with a time-resolution of 10 s using an X-ray reflectometer [1, 2], which can simultaneously measure the whole XRR curves with no need to rotate the specimen, detector or monochromator crystal. Profiles of XRR curves changed as a function of the elapsed time after initiation of the light irradiation reflecting the structural change of the film. Despite of a common belief that the photo-induced structural change occurs directly between the initial and final states, we found an evidence of the existence of the intermediate third structure. We also found that the time needed for changes in XRR curves was several times longer than for optical absorption spectroscopy (OAS) spectra reported with the same irradiation power. Details of such changes of XRR curves and structures of the film will be discussed and compared with the changes of OAS spectra. An XRR curve for the intermediate state of the 6Az10-PVA monolayer LB film specimen separated from the XRR curves measured under 365 nm light irradiation is shown in the figure together with curves for the initial and final states of the same specimen.