Wataru Oohara

Basic studies of the generation and collective motion of pair-ion plasmas

A fullerene pair-ion plasma without electrons is generated and electrostatic modes propagating along magnetic-field lines are externally excited in the range of low frequencies. It is found that four kinds of wave modes, including theoretically unexpected ones, exist in the plasma, and the phase lag between the density fluctuations of positive and negative ions strongly depends on the frequency. In order to illuminate further collective motion of pair-ion plasmas in the range of high frequencies, a concept of a hydrogen pair-ion plasma consisting of only H+ and H− is proposed and an experimental configuration is presented. On the basis of the production of a hydrogen plasma by Penning ionization gauge discharge, the principles of ion cyclotron resonance and E×B drift motion are shown to be effective for ion-species analysis/selection and separated electron detection from negative ions in the generation of pure hydrogen pair-ion plasmas.

P-N junction with donor and acceptor encapsulated single-walled carbon nanotubes

Ultimate one-dimensional heterojunctions of electron donor and acceptor materials have been realized within the inner hollow space of a single-walled carbon nanotube (SWNT). The heterojunction structures of Cs/I and Cs/C60 inside SWNTs (Cs/I@SWNTs, Cs/C60@SWNTs) yield the air-stable rectifying performance. Clear tunneling currents through the p-n junction barrier could be also detected only for Cs/I@SWNTs, which is explained by the difference of depletion layer structures. Based on a potential calculation, symmetrical and asymmetrical depletion layers were found to be formed in Cs/I@SWNTs and Cs/C60@SWNTs, respectively. Low temperature measurements also supply evidence of asymmetric depletion layer formation in Cs/C60@SWNTs.Ultimate one-dimensional heterojunctions of electron donor and acceptor materials have been realized within the inner hollow space of a single-walled carbon nanotube (SWNT). The heterojunction structures of Cs/I and Cs/C60 inside SWNTs (Cs/I@SWNTs, Cs/C60@SWNTs) yield the air-stable rectifying performance. Clear tunneling currents through the p-n junction barrier could be also detected only for Cs/I@SWNTs, which is explained by the difference of depletion layer structures. Based on a potential calculation, symmetrical and asymmetrical depletion layers were found to be formed in Cs/I@SWNTs and Cs/C60@SWNTs, respectively. Low temperature measurements also supply evidence of asymmetric depletion layer formation in Cs/C60@SWNTs.

Novel-structured carbon nanotubes creation by nanoscopic plasma control

Original approaches to the nanoscopic plasma process control enable us to succeed in producing individually freestanding pristine single-walled carbon nanotubes (SWNTs) on a flat-surface substrate, and creating alkali metals, halogen-elements, ferromagnetic-atoms, fullerene molecules and DNA molecules encapsulated SWNTs or double-walled carbon nanotubes (DWNTs). Investigations of their electronic and magnetic properties result in realizing the continuous transition from p-type to n-type air-stable transport property by adjusting an amount of dosed atoms or molecules inside SWNTs and DWNTs, and in forming nanostructures of ferromagnetic semiconductor, nano-p–n junctions with rectifying characteristic and nanostructures with distinct negative differential resistance.

Alkali-halogen plasma generation by dc magnetron discharge

An alkali-halogen plasma is generated by a dc magnetron discharge using thermal cathodes under a uniform magnetic field. Alkali-salt vapor is dissociated and ionized by E×B-drift electron impact, and alkali positive ions and halogen negative ions are produced. A magnetic-filter region is situated at an exit of the discharge region and electrons are removed from the plasma. The electron emission and E∕B fields are optimized, resulting in the alkali-halogen plasma with the ion density of 3×108cm−3 at B=0.2T.

Experimental characterization of a density peak at low magnetic fields in a helicon plasma source

We investigated characteristics of a density peak observed in a magnetic field B0 lower than 100 G in the case of using helicon plasma sources with particular wavelength. For B0 > 30 G, the antenna launches an electromagnetic wave as a slow wave, the phase velocity of which becomes close to the electron thermal velocity under the density-peak condition for various gas pressures. The Landau damping frequency is higher than the electron–neutral and the electron–ion collision frequencies, which indicates that the wave produces the plasma via Landau damping at low B0. The wavelengths estimated from the density ne and B0 for the density peaks agree with those of the electromagnetic field generated by helicon antennas of various lengths. The measured density is found to vary under the condition of the agreement between wavelengths of the propagating wave and the antenna-excited field during the density increase. One of the causes of the density peak appearing as a function of B0 is considered to be the wavelength variation and the corresponding change of phase velocity of the slow wave which is enabled to propagate in the plasma by the introduction of B0.

Separation of ion components produced by plasma-assisted catalytic ionization

Positive and negative hydrogen ions are produced by plasma-assisted catalytic ionization using a porous nickel plate, where the irradiation current density and energy of positive ions produced by discharge to the porous plate are controlled. The ion energy distributions are analyzed from the properties of current densities of positive and negative ions extracted from the porous surface. Positive ions passing through fine pores of the porous plate and positive and negative ions produced on the porous surface are observed. It is clarified that the produced fluxes of positive and negative ions and the flux balance between them are controlled by the irradiation current density and energy, respectively.

Hydrogen atomic pair-ion production on catalyst surface.

To generate a hydrogen pair-ion plasma consisting of only hydrogen atomic pair ions, i.e., H(+) and H(-) ions, the efficient production of pair ions is required. When discharged hydrogen plasma is irradiated to a Ni catalyst, pair ions are produced on the catalyst surface. It is clarified that hydrogen chemisorption on the catalyst affects pair-ion production.

Plasma-assisted catalytic ionization using porous nickel plate

Hydrogen atomic pair ions, i.e., H(+) and H(-) ions, are produced by plasma-assisted catalytic ionization using a porous nickel plate. Positive ions in a hydrogen plasma generated by dc arc discharge are irradiated to the porous plate, and pair ions are produced from the back of the irradiation plane. It becomes clear that the production quantity of pair ions mainly depends on the irradiation current of positive ions and the irradiation energy affects the production efficiency of H(-) ions.

Hydrogen ions produced by plasma-assisted catalytic ionization using nickel grid

Positive and negative hydrogen ions are produced by plasma-assisted catalytic ionization using a nickel grid, where the irradiation current density of positive ions onto the grid can be controlled by the discharge power. The irradiation energy can be controlled by both the grid potential and the discharge plasma potential. Extraction properties and energy distributions of positive and negative ions produced in the cases of using the grid and a porous nickel plate are compared. Two production mechanisms of negative ions are found in the process of plasma-assisted catalytic ionization.

Formation of p–n Junction in Double-Walled Carbon Nanotubes Based on Heteromaterial Encapsulation

The formation of p–n junction in double-walled carbon nanotubes (DWNTs) is successfully investigated for the first time through encapsulating heteromolecules/atoms via a plasma–ion irradiation method. DWNTs filled with both electron donor atoms and electron acceptor molecules are synthesized during the plasma ion-irradiation process. It is found that the electrical transport properties of DWNTs after encapsulating either Cs–C60 or Cs–I are significantly different from those of unipolar p- or n-type DWNTs encapsulating one kind of molecules or atoms. The p–n junctions with excellent rectifying characteristics are successfully realized in many of DWNT-based devices, suggesting a new way in making functional DWNTs.