Tomoshige Sato

A stable binary quasicrystal.

All stable quasicrystals known so far are composed of at least three metallic elements. Sixteen years after the discovery of the quasicrystal, we describe a stable binary quasicrystalline alloy in a cadmium–ytterbium (Cd–Yb) system. The structure of this alloy represents a new class of packing of 66-atom icosahedral clusters whose internal structure breaks the icosahedral symmetry. The binary quasicrystal offers a new opportunity to investigate the relation between thermodynamic stability and quasiperiodic structure, as well as providing a basis for the construction of crystallographic models.

Surface diffusion of adsorbed Si atoms on the Si(111)7×7 surface studied by atom-tracking scanning tunneling microscopy

The initial adsorption process of Si atoms deposited on a Si(111)7×7 surface has been investigated at 80 to 500 K using a variable-temperature scanning tunneling microscopy. At room temperature, adsorbed Si atoms spontaneously formed tetramers over the center dimers in the dimers adatoms and stacking fault model of a 7×7 structure. Many other adsorbed Si atoms, which were not used for the formation of tetramers, were observed to diffuse within each half of the 7×7 unit cell. The diffusion of Si atoms across the surface was examined directly by using an atom-tracking technique. At low temperatures, the adsorption position of the Si atom was found to depend only on the potential energy. At high temperatures, the activation energy of an Si atom beyond the boundary between the half-unit cells was measured as Ea=1.14 eV.

Observation of SrTiO3 step edge dynamics by real-time high-temperature STM

Abstract We have used high-temperature scanning tunneling microscopy (STM) to study in real time SrTiO 3 step edge dynamics in the 600°C to 800°C temperature range. We observed a dramatic transformation of the step edge structure above 690°C, involving step edge straightening due to rapid migration of surface atoms. The STM study shows that if a NH 4 F-HF-etched substrate is annealed at 800°C, a well-ordered surface, suitable for high-T c superconductor thin film deposition, can be produced.

Scanning tunneling microscopy observation of hydrogen‐terminated Si(111) surfaces at room temperature

Scanning tunneling microscopy has been applied to observe hydrogen‐terminated Si(111) surfaces at room temperature. A clear image was easily observed for a Si surface prepared by rinsing in pure water with very low dissolved oxygen after removal of native oxide by 1% HF solution dipping. A smooth surface in an atomic scale was exhibited in a 50×50 nm area. Completely triangular‐shaped holes were observed on the surface. The holes were surrounded by steps which were very likely directed toward 〈112〉. The treatment of the surface was remarkably stable even after a 3 h air exposure. Furthermore, nm size pits were found at the bottom part of the triangular‐shaped holes. The results imply that the nm size pits appeared to be due to microdefects and that the pits might be the origin of surface etching at the Si surface.

Surface structure and structural transition of decagonal Al-Ni-Co quasicrystal

Abstract X-ray photoelectron diffraction (XPD) and reflection high-energy electron diffraction (RHEED) measurements have been performed to investigate the surface structure of the decagonal Al 72 Ni 12 Co 16 quasicrystal, which is known as a two dimensional quasicrystal. RHEED patterns for the surface parallel to the quasiperiodic planes are almost identical to the selected area electron diffraction (SAD) patterns taken along the twofold axes of the bulk sample. The XPD images show clear tenfold-symmetric patterns consistent with the long range orientational order of the decagonal quasicrystal. These facts suggest that the surface has the same quasiperiodic structure as in the bulk. The surface phase transition induced by Ar + ion bombardment is also investigated.

Real-time observation of the Si(111):(7×7)−(1×1) phase transition by scanning tunneling microscopy

Abstract The initial stage of the (1×1)−(7×7) phase transition of the Si(111) surface was directly observed by scanning tunneling microscopy. On cooling down to a transition temperature Tc, the (7×7) domain nucleated from the step edges and expanded towards the inner region of the terraces continuously, and the steps became straight [ 1 1 2] steps. The (7×7) domains took the shapes of triangles with acute top angles and fluctuated in size around Tc. The sizes of the observed domains were more than 6 units of (7×7) on the terraces. Based on free-energy considerations just at Tc as well as the energy difference between faulted and unfaulted triangles of (7×7) units, we have found that the real shapes of the (7×7) domains are equilateral triangles which are surrounded by unfaulted triangles and there exists a critical size around 6 units of (7×7) below which the domains disappear rapidly.

Scanning tunneling microscopy study on the initial adsorption behavior of C60 molecules on a reconstructed Au(111)-(23 × √3) surface at various temperatures

Abstract We have clarified the behavior of initial stage adsorption of C 60 molecules on a reconstructed Au(111)-(23 × √3) surface at the three temperatures (295, ∼80 and ∼30 K) using low-temperature scanning tunneling microscopy. At room temperature, C 60 molecules were found to adsorb only at the step edges and form one-dimensional C 60 nano structures. At ∼80 K, C 60 molecules were found to adsorb on terraces, but were not stable. The nucleation and growth of two-dimensional C 60 clusters from the one-dimensional C 60 nuclei at step edges were clarified at this temperature. At ∼30 K, C 60 molecules were found to be trapped, especially at the elbow positions of the herringbone structure of the Au(111) reconstructed surface, which have dislocations and protruded atoms.

Outgassing characteristics and microstructure of an electropolished stainless steel surface

Outgassing characteristics of an electropolished stainless steel (304) pipe wall were investigated by an isolation method. The free outgassing rate after an in situ bakeout (150 °C, 20 h) was estimated as low as 1.6×10−12 Pa l s−1cm−2. After the in situ bakeout, H2 molecules were steadily evolved from the pipe wall, whereas most of CO, C, CH4, and CO2 molecules were emitted from the operating mass spectrometer and Bayard–Alpert gauge with incandescent filaments. Auger depth profile analysis revealed that the oxide layer of an electropolished surface was cleaner, thinner, and finer in microstructure than that of a buff‐polished surface. This is the reason why an electropolished surface showed a very low outgassing rate.

Dynamic observation of In adsorption on Si(111) surfaces by UHV high-temperature scanning tunneling microscopy

Indium adsorption and desorption processes on Si(l 11) surfaces were observed in situ by UHV high-temperature STM. By the deposition of In on the Si(111)7 X 7 surfaces at 380°C, the surface structure changed successively to √3 x √3, √31 X √31 and 4 X 1. The number densities of silicon atoms in the restructuring layers for the √3 X √3, √31 X √31, and 4 X I structures were evaluated to be about 0, 1 and 2 ML, respectively. High-resolution STM images were also taken after the deposition.

STM studies of Si(5 5 12) 2 × 1 surfaces

Abstract We carried out STM observations of Si(5 5 12) surfaces. STM images showed that the surface has a long period of about 5 nm along the 〈665〉 direction that coincides with a period of the bulk terminated structure, and a period of about 0.7 nm along the 〈110〉 direction that coincides with a 2-fold period of the bulk terminated structure; the 2 × 1 structure. The long period along the 〈665〉 direction is composed of 7 arrays of bright dots. These bright dots appear at the adatom positions of a surface structure model of the (5 5 12) surface proposed previously from RHEED pattern analysis. The phase shifts of the long period which are equivalent to steps are observed on the surface. A surface structural phase transition found in REM-RHEED studies at about 900°C was also observed.