Hiroyuki Hirayama

Epitaxial growth of silicene on ultra-thin Ag(111) films

Epitaxial growth of silicene on atomically flat ultra-thin Ag(111) films was investigated using scanning tunneling microscopy. The Ag films were prepared by low-temperature deposition of Ag on Si substrates, followed by soft annealing at room temperature. Patchy 4 × 4, √13 × √13, √7 × √7, and 2√3 × 2√3 silicene domains were nucleated on regions that were one monolayer lower than the pristine Ag surface after the initial morphological change had been completed. On the first-layer silicene containing the above domains, two types of √3 × √3 silicene domains (√3 × √3-α and β) were formed in the second layer. A bias voltage-independent Moire pattern was observed for the √3 × √3-α silicene, and a bias voltage-dependent standing wave pattern for the √3 × √3-β silicene. The √3 × √3-β silicene had a dispersion relation that could be fitted using a linear function, although the resulting Fermi velocity was twice as large as the expected value.

Photodissociation and photodesorption of O2 adsorbed on Pd(111)

Abstract The dynamics of the UV-photochemistry of molecularly adsorbed oxygen on Pd(111) has been studied using pulsed laser light between 3.9 and 6.4 eV photon energy. Photodissociation to form atomic surface oxygen and photodesorption of O2 are the primary processes. The latter is characterized by a translational energy of about 800 K indicating non-thermal desorption. Since the photolytically formed atomic oxygen competes with the molecular adsorbate for binding sites some of the latter is displaced. This manifests itself in a conversion process between adsorbate states and in a desorption channel with a translational energy of 120 K indicating accommodation to the surface temperature. The cross sections for all processes rise exponentially from 3.9 to 6.4 eV by a factor of 38.

Growth of a single-wall carbon nanotube in the gap of scanning tunneling microscope

Single-wall carbon nanotubes (SWNTs) were grown in the tunneling gap of a scanning tunneling microscope (STM). We could observe their growth processes in situ by operating the STM in a transmission electron microscope. The STM tip and sample were covered by graphite layers. The tip was lightly touched to the sample and subsequently retracted. Occasionally, a carbon nanobridge was generated between the tip and the sample. The bridge had the shape of SWNT at the tip side.

Hydrogen desorption from 6H-SiC(0001) surfaces during graphitization

Desorption of hydrogen from 6H-SiC(0001) has been systematically studied during graphitization. The surface structure was controlled by thermal desorption of silicon at high temperatures in an ultrahigh vacuum and was characterized by low energy diffraction and Auger electron spectroscopy. The temperature of the dominant peak in the hydrogen desorption spectrum was found to shift from 670 to 490 K between 3×3 and 3×3R30° reconstructions. The shift can be assigned to a change of the adsorption sites from silicon to carbon.

B doping effect on reflection high‐energy electron diffraction intensity oscillation during gas source silicon molecular beam epitaxial growth

B doping effect on reflection high‐energy electron diffraction (RHEED) intensity oscillation was studied during gas source Si molecular beam epitaxial growth. During high doping of B above 1020 cm−3, no RHEED oscillation was observed on Si (100) surfaces. This is caused by surface B atoms which disturb surface migration of disilane (Si2H6) molecules. On the other hand, RHEED oscillation was observed on Si (111) √3×√3 B surfaces. At √3×√3 B surfaces, B exists in a subsurface substitutional site, directly underneath a Si adatom. This is the reason why surface migration was not disturbed by surface B atoms on Si (111) √3×√3 B surfaces.

Surfactant role of Ag atoms in the growth of Si layers on Si(111) √3×√3-Ag substrates

The growth of Si layers on Si(111) √3×√3-Ag substrates was studied for coverages of up to a few mono-layers. Atomically flat islands were observed to nucleate in the growth at 570 K. The top surfaces of the islands were covered in Ag atoms and exhibited a √3×√3 reconstruction with the same surface state dispersions as Si(111) √3×√3-Ag substrates. These results indicate that the Ag atoms on the substrate always hop up to the top of the Si layers.

Quantized electrical conductance of gold helical multishell nanowires

The conductance of helical multishell (HMS) gold nanowires was measured while simultaneously examining their structure using an ultrahigh-vacuum transmission electron microscope. It was found that thinning successively yielded 15-8-1, 14-7-1, 13-6, 12-5, and 11-4 HMS nanowires. The conductance of these nanowires was independent of their length. Conductance varied with the position of contact between the chiral nanowire and the crystalline electrode so that each nanowire had more than two conductance values. The channel transmissions were reduced at the nanowire-electrode contact. The observed conductance values for electrode-suspended chiral nanowires were compared with recent theoretical values.

Simultaneous observation of scanning tunneling microscopy and reflection electron microscopy image of the Si(111)7 × 7 surface

Abstract Atomic structures of scanning tunneling microscopy (STM) tips were characterized by reflection electron microscopy (REM). The Si(111)7×7 surface was observed simultaneously by REM and STM in an ultrahigh vacuum electron microscope. In high-resolution REM images of the specular beam reflection, a tungsten tip apex was observed with its mirror image. The distance between the true and the mirror images was used to estimate the tip–surface distance. As the tip scanned over the Si(111) surface in STM experiments, the tip apex was scraped to give a plateau of a (110) atomic surface. An atomic-resolution STM image of the Si(111)7×7 surface was obtained when the STM tip apex had a single atom on the (110) terrace.

Si growth at graphene surfaces on 6H-SiC(0001) substrates

We studied the growth of Si at the surface of epitaxial graphene on 6H-SiC(0001). Characteristic flower-like islands with a thickness of 2 to 3 nm nucleated during the growth from 290 to 420 K. The islands became featureless in growth at higher temperatures. The growth was predominantly governed by diffusion-limited aggregation. The diffusion energy was evaluated to be 0.21 eV from the temperature-dependent decrease in the density of the islands.

Analysis of oxide–nitride–oxide/Si(111) interfaces by optical second harmonic generation and secondary ion mass spectroscopy

We use optical second harmonic generation and secondary ion mass spectroscopy (SIMS) to study oxide–nitride–oxide (ONO)/Si(111) interfaces. We explain the relationship between the second harmonic (SH) intensity from the interfaces and the SIMS depth profiles. The intensity from the wet oxide/Si(111) interface decreases as the temperature during wet oxidation increases, because the number of interface dangling bonds is reduced as the wet temperature is increased. With subsequent nitridation, this SH intensity of the oxidized interface decreases. This is because hydrogen and nitrogen terminate the interface dangling bonds. However, the SH intensity increases again with reoxidation, because the dangling bonds grow again at the interface. The SH intensity from annealed nitridation of the oxide/Si(111) interface remains constant, although the number of hydrogen atoms decreases and the number of nitrogen atoms slightly increases. We propose two reasons for this. One is that most of the desorbed hydrogen atoms d...