Masahiko Shimoda

Tsai-type icosahedral quasicrystals (i-QCs) are present in numerous alloy systems. Among them, the icosahedral Ag–In–Yb quasicrystal (i-Ag–In–Yb) is regarded as a prototype for studying various features of QCs, such as crystalline structure and physical properties, due to its good average structural order, good stability both in air and in ultra-high vacuum, amenability to growth of large single grains, and lack of elements with high vapor pressure or toxicity. Here we present a review on recent studies of i-Ag–In–Yb, including studies on bulk structure, crystal growth, physical properties, surface structure and thin film growth on quasicrystalline surface. We believe the results will encourage studies on other QCs.

We report a study of the atomic structure of the threefold icosahedral (i-)Ag-In-Yb quasicrystal surface using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). The LEED confirms that the surface exhibits quasicrystalline long-range order with the threefold symmetry expected from the bulk. The STM reveals large atomically flat terraces separated by steps of different heights. A comparison of atomically resolved STM images for the terraces and the step-height distribution with the bulk structure of isostructural i-Cd-Yb shows that the terraces are formed at bulk planes intersecting the centers of the rhombic triacontahedral clusters that make up the bulk structure of the system. However, the stability of particular terraces may be influenced by the density of atoms in the interstices (glue atoms that bind the clusters) in the terraces and also by the chemical environment in the underlying atomic plane. The surface exhibits screw dislocations, which is explained in terms of a continuous atomic density along the threefold axis.

We present a hard x-ray photoelectron spectroscopy study on the preparation process of palladium catalyst immobilized on an S-terminated GaAs(100) surface. It is revealed that Pd(II) species are reduced on the GaAs surface and yield Pd nanoparticles during the process of Pd immobilization and the subsequent heat treatment. A comparison with the results on GaAs without S-termination suggests that the reduction of Pd is promoted by hydroxy groups during the Pd immobilization and by S during the heat treatment.

A novel use of GaAs, namely, as a scaffold for a heterogeneous palladium catalyst, is proposed. Hydroxy groups on the GaAs surface play important roles. During the adsorption of Pd(OAc)₂ on the GaAs surface, the hydroxy groups attract Pd(II) species by anion exchange. A subsequent redox reaction proceeds to generate Pd(0) nanoparticles, which are stabilized on the GaAs surface. This process is confirmed by surface-sensitive measurements: diffuse reflection IR spectroscopy and X-ray photoelectron spectroscopy. Moreover, a more bulk-sensitive measurement, hard X-ray photoemission spectroscopy with synchrotron radiation, also supported our considerations. The amounts of Pd(0) nanoparticles on the surface were evaluated by catalytic activity, yield, and recyclability in the Heck reaction, in addition to the deposit test.

The surfaces of quasicrystals have proved to be a very interesting playground for thin film growth. They offer a complex potential energy surface where heterogeneous nucleation of islands at specific quasilattice sites is frequently observed. These islands tend to locally adopt the symmetry of the quasicrystalline substrate. For some specific adsorbates, a complete 2‐dimensional quasiperiodic metal overlayer is even formed. Other interesting phenomena are also observed in the multilayer regime. This includes the formation of novel structures, like 1‐dimensional quasiperiodic Cu films or Bi allotropes, the formation of nanoscale crystalline domains with 5‐fold rotational epitaxy, or the occurrence of quantum size effects influencing the film morphology. This article presents a short review of some of the achievements in thin film growth on quasicrystalline surfaces that the discovery of quasicrystals by Dan Shechtman has enabled.

We have synthesized nitrogen-doped (N-doped) TiO2 (rutile) single crystals by annealing in a 1.0 MPa N2 atmosphere after reduction treatment. Transmittance, electrical resistivity, and sample weight decreased with the reduction treatment, but recovered after the subsequent N-doping process. The N 1s X-ray photoelectron spectroscopy (XPS) signal disappeared after 8 min of Ar sputtering. Diodelike behaviors were observed in the voltage?current (V?I) characteristics of the TiO2?metal contact for the as-received TiO2 and N-doped TiO2. They demonstrated that the as-received TiO2 is an n-type semiconductor and the N-doped TiO2 is a p-type semiconductor. Photoinduced voltage was measured under irradiation with a Xe lamp. The sign of the photoinduced voltage in the N-doped TiO2 was found to be opposite to that of the as-received or reduced TiO2, which also indicates that the N-doped TiO2 is a p-type semiconductor. We discuss the electronic states of dopant nitrogen atoms.

We have synthesized nitrogen-doped (N-doped) TiO₂ (rutile) single crystals by annealing in a 1.0 MPa N₂ atmosphere after reduction treatment. Transmittance, electrical resistivity, and sample weight decreased with the reduction treatment, but recovered after the subsequent N-doping process. The N 1s X-ray photoelectron spectroscopy (XPS) signal disappeared after 8 min of Ar sputtering. Diodelike behaviors were observed in the voltage–current (V–I) characteristics of the TiO₂–metal contact for the as-received TiO₂ and N-doped TiO₂. They demonstrated that the as-received TiO₂ is an n-type semiconductor and the N-doped TiO₂ is a p-type semiconductor. Photoinduced voltage was measured under irradiation with a Xe lamp. The sign of the photoinduced voltage in the N-doped TiO₂ was found to be opposite to that of the as-received or reduced TiO₂, which also indicates that the N-doped TiO₂ is a p-type semiconductor. We discuss the electronic states of dopant nitrogen atoms.