Kuniyoshi Yokoo

A layered ionic crystal of polar Li@C 60 superatoms

If the physical properties of C(60) fullerene molecules can be controlled in C(60) products already in use in various applications, the potential for industrial development will be significant. Encapsulation of a metal atom in the C(60) fullerene molecule is a promising way to control its physical properties. However, the isolation of C(60)-based metallofullerenes has been difficult due to their insolubility. Here, we report the complete isolation and determination of the molecular and crystal structure of polar cationic Li@C(60) metallofullerene. The physical and chemical properties of Li@C(60) cation are compared with those of pristine C(60). It is found that the lithium cation is located at off-centre positions in the C(60)-I(h) cage interior and that the [Li(+)@C(60)] salt has a unique two-dimensional structure. The present method of purification and crystallization of C(60)-based metallofullerenes provides a new C(60) fullerene material that contains a metal atom.

Active control of the emission current of field emitter arrays

The current control and the stabilization in field emission of gated field emitter arrays (FEAs) are the highest demand for applications to a flat panel display and other beam devices. The concept of the field‐effect‐controlled field emission cathode is very useful for these purposes. We carried out preliminary experiments of the idea and showed that the controllability and the stability of emission current of FEA were significantly improved by an actively controlled FEA. Additionally, we discussed beam focusing of FEA for a flat panel display application.

Emission characteristics of metal-oxide-semiconductor electron tunneling cathode

We have fabricated a metal–oxide–semiconductor (MOS) electron tunneling cathode with ultrathin SiO2 and examined the emission characteristics. We found that the emission occurred from an entire gate area by electron tunneling through the potential barrier in the MOS diode and the emission current was 0.7% of the total current flowing through the diode. The emission was also found to be nearly independent of pressure.

Highly efficient operation of space harmonic peniotron at cyclotron high harmonics

The paper describes the oscillation characteristics of cyclotron high-harmonic peniotrons based on space harmonic interactions in magnetron waveguide resonators. The experimental results showed extremely high electronic efficiencies of 75% for the cyclotron third harmonic and 6% for the tenth harmonic at 30 and 100 GHz, respectively. It was proven that almost all of the rotational energy of the beam is transferred to that of an electromagnetic wave and the efficiency of 75% at the cyclotron third harmonic corresponds to the effective efficiency of 92% by the energy recovery of the spent electrons at the depressed potential collector.

Fabrication of a Si1-xGex Channel Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) Containing High Ge Fraction Layer by Low-Pressure Chemical Vapor Deposition

A method for growing the high-quality strained epitaxial heterostructure of Si/Si1-xGex/Si by low-pressure chemical vapor deposition (CVD) and the fabrication of Si1-xGex-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) with a high Ge fraction layer have been investigated. It is found that lowering of the deposition temperature of the Si1-xGex and Si capping layers is necessary with increasing Ge fraction in order to prevent island growth of the layers. With the use of the optimized fabrication process, Si/Si1-xGex/Si heterostructures with flat surfaces and interfaces were realized, and a high-performance Si0.5Ge0.5-channel MOSFET has been achieved with a large mobility enhancement of about 70% at 300 K and over 150% at 77 K compared with that of a MOSFET without a Si1-xGex channel.

Preparation of endohedral fullerene containing lithium (Li@C60) and isolation as pure hexafluorophosphate salt ([Li+@C60][PF6−])

A synthetic protocol for endohedral fullerene containing lithium (Li@C60), as well as the synthesis, characterization, and properties of its chemically oxidized derivative [Li+@C60][PF6−] are reported in full. Simultaneous deposition of Li plasma and C60 produces a mixture containing Li@C60 and C60. Strong intermolecular attraction between Li@C60 and C60 has prevented their separation, but this obstacle was surmounted by selective chemical oxidation of Li@C60, which led to the isolation of a substantial amount of pure [Li+@C60][PF6−]. For this purpose, an HPLC technique using an electrolyte in the mobile phase was effective in purifying the ionic metallofullerene. [Li+@C60][PF6−] was stable in air and, compared with [Li+@C60][SbCl6−], was more stable to moisture and various chemicals. [Li+@C60][PF6−] holds promise for practical applications. The spectroscopic data (MS, 7Li and 13C NMR, UV-vis, and IR) and electrochemical data for [Li+@C60][PF6−] are disclosed. In addition, the high electron affinity of [Li+@C60][PF6−] is discussed.

Growth of single crystalline γ-Al2O3 layers on silicon by metalorganic molecular beam epitaxy

Single crystalline γ‐Al2O3 layers have been successfully grown on silicon substrates by the metalorganic molecular beam epitaxy using aluminum alkoxide gas. Single crystalline γ‐Al2O3 layers are grown only on crystalline silicon substrates, while polycrystalline Al2O3 layers are grown on amorphous oxidized silicon surface. By x‐ray photoemission spectroscopy measurements, the stoichiometry of the single crystalline layers were confirmed to be identical to Al2O3 bulk crystals and carbon contamination was not detected within the sensitivity of the measurements.

ENERGY DISTRIBUTION OF TUNNELING EMISSION FROM SI-GATE METAL-OXIDE-SEMICONDUCTOR CATHODE

An Si‐gate metal–oxide–semiconductor (MOS) electron tunneling cathode was fabricated and its emission characteristics were examined. The emission occurred at a gate voltage higher than the work function of the Si gate by electron tunneling through the potential barrier in the MOS cathode and was stable in the Si‐gate cathode. The energy distribution of emitted electrons was measured and was confirmed to be mainly determined by the scattering process of hot electrons in the oxide. The emission current from the Si‐gate MOS cathode was nearly independent of pressure.

Third-harmonic frequency multiplication of a two-stage tapered gyrotron TWT amplifier

The frequency multiplication of a two-stage tapered gyrotron traveling wave tube amplifier is experimentally verified in low-voltage and low-current operation. By modulating an axis-encircling electron beam at the fundamental harmonic cyclotron frequency in the input stage, a frequency-tripled signal induced by the third-harmonic component of the modulated beam current is chosen to be extracted from the output stage. Both interaction stages are linearly tapered to improve stability and bandwidth. The third-harmonic frequency multiplication is predicted theoretically and investigated using a self-consistent large-signal theory and a particle-in-cell code simulation, which estimate a pure third-harmonic generation and a power-scaling law. In the experiment, X-band drive signals from 10.6-12 GHz are multiplied by a factor of three to produce Ka-band output frequencies from 31.8-36 GHz, showing reasonable agreement with theoretical predictions for a 30-kV, 160-mA axis-encircling electron beam.

Infrared study of adsorption and thermal decomposition of Si2H6 on Si(100)

Sumario We have investigated the adsorption and thermal decomposition of disilane on Si(100) (2×1) using infrared absorption spectroscopy. We demonstrate that at room temperature, disilane dissociatively adsorbs onto the surface with the dimer bond unbroken, to produce mono-, di-, and tri-hydride species. At low coverages, the disilane molecule adsorbs on the surface without breaking the Si–Si bond of the molecule. Thermal annealing following room-temperature adsorption of disilane produces a hydrogen-terminated adatom dimer (HSi–SiH) and an isolated monohydride species. We suggest that these hydride species are generated through the rupture of the substrate dimer bonds. Hydrogen desorption from the isolated monohydride occurs at much lower temperature than that from the adatom dimer.