Rikizo Hatakeyama

Narrow-Chirality Distributed Single-Walled Carbon Nanotube Growth from Nonmagnetic Catalyst

We present the first demonstration of the nonmagnetic catalyzed synthesis of narrow-chirality distributed single-walled carbon nanotubes (SWNTs). Based on the systematic investigation using different combinations of catalyst types (magnetic or nonmagnetic) and chemical vapor deposition (CVD) methods (thermal CVD (TCVD) or plasma CVD (PCVD)), PCVD with the nonmagnetic catalyst under the appropriate H(2) concentration is found to be critical as the methodological element of realizing the narrow-chirality distribution. Electrical measurements of thin film SWNTs produced under the different combinations of catalyst types and CVD methods are also investigated, which reveals the SWNTs grown from the nonmagnetic catalyst with PCVD display the best device performance.

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.

Measurement of the energy distribution of trapped and free electrons in a current-free double layer

In the high potential plasma, upstream of the double layer, the measured electron energy distribution function EEDF shows a very clear change in slope at energies break corresponding to the double layer potential drop. Electrons with lower energy are Maxwellian with a temperature of 8 eV, whereas those with higher energy have a temperature of 5 eV. The EEDF in the downstream plasma has a temperature of 5 eV. Over the range of pressures wherein the double layer and accelerated ion beam are detected by analysis of a retarding field energy analyzer, the strength of the double layer corresponds to the energy where the slope changes in the EEDF break. We deduce that the downstream electrons come from upstream electrons that have sufficient energy to overcome the potential of the double layer, and that only a single upstream plasma source is required to maintain this phenomenon.

Direct growth of doping-density-controlled hexagonal graphene on SiO2 substrate by rapid-heating plasma CVD.

A transfer-free method for growing carrier-density-controlled graphene directly on a SiO(2) substrate has been realized for the first time by rapid-heating plasma chemical vapor deposition (RH-PCVD). Using this method, high-quality single-layer graphene sheets with a hexagonal domain can be selectively grown between a Ni film and a SiO(2) substrate. Systematic investigations reveal that the relatively thin Ni layer, rapid heating, and plasma CVD are critical to the success of this unique method of graphene growth. By applying this technique, an easy and scalable graphene-based field effect transistor (FET) fabrication is also demonstrated. The electrical transport type of the graphene-based FET can be precisely tuned by adjusting the NH(3) gas concentration during the RH-PCVD process.

Ion irradiation effects on ionic liquids interfaced with rf discharge plasmas

The availability of plasma ion irradiation toward a gas-liquid interface is investigated in a rf discharge system incorporating an ionic liquid. The introduction of the ionic liquid to the plasma causes the formation of a sheath electric field on the ionic liquid surface, resulting in the acceleration of the ions to the ionic liquid and the generation of secondary electrons from the ionic liquid by the ion irradiation. These effects are found to advance the discharge process and enhance the plasma production.

Direct growth of short single-walled carbon nanotubes with narrow-chirality distribution by time-programmed plasma chemical vapor deposition.

We have realized the direct growth of the short-length (<100 nm) single-walled carbon nanotubes (SWNTs) with a narrow-chirality distribution by time-programmed plasma chemical vapor deposition (TP-PCVD). Transmission electron microscope and atomic force microscope analyses reveal that the very short (<100 nm) SWNTs are selectively grown by precisely controlling their growth time on the order of a few seconds. Direct photoluminescence excitation measurements also show that the chirality distribution of the short SWNTs is fairly narrow, and (7, 6) and (8, 4) dominant short SWNTs are successfully synthesized by TP-PCVD.

Magnetron-type radio-frequency plasma control yielding vertically well-aligned carbon nanotube growth

In order to understand the effects of plasma parameters on the nanotube formation and further controlled growth, we have investigated the optimal growth condition using a rf plasma-enhanced chemical vapor deposition method. The magnetic field introduced for a magnetron discharge enhances the nanotube growth as a result of the plasma-density increment and the self-bias reduction of a rf electrode. It is also found that the optimum ion flux and ion bombardment energy is a key parameter for the uniform, well-aligned, and density-controlled nanotube growth.

Double layer dynamics in a collisionless magnetoplasma

Investigations of double layer dynamics are performed in a Q-machine plasma by applying a positive step potential to a cold end-plate collector. The double layer created at the grounded plasma source just after the pulse is applied propagates towards the collector with the plasma flow speed. Large oscillations occur in the plasma current which is related to a recurring formation and propagation of the double layer. The current is limited during the propagation by a growing negative potential dip formed on the low-potential tail. Similar phenomena appear on the low-potential tail of the stationary double layer formed by applying a potential difference between two plasma sources.

Air-stable p-n junction diodes based on single-walled carbon nanotubes encapsulating Fe nanoparticles

The authors report electrical transport properties of p-n junction based on semiconducting single-walled carbon nanotubes (SWCNTs). The formation of p-n junction is realized in SWCNTs, which are encapsulated with Fe nanoparticles at low filling fractions. The devices exhibit an excellent rectifying behavior, and no current down to 10−14A level flows when the device is biased in reverse. During measurements performed in the temperature range from 10to300K, the devices maintain high reproducibility. More importantly, even after exposure to air, the rectifying characteristic keeps stable, which strongly suggests that ideal p-n junction diodes can be fabricated by SWCNTs.

Room‐Temperature Edge Functionalization and Doping of Graphene by Mild Plasma

13.56 MHz Graphene has recently received a great deal of attention because it possesses unique properties such as a linear energy dispersion relation and high carrier mobility. [ 1–4 ] Graphene nanoribbons (GNRs), [ 5 ] strips of graphene, are the counterparts of carbon nanotubes (CNTs) in terms of their allsemiconducting nature, [ 6 ] and they could potentially solve the problem of the chirality dependence of the metal or semiconductor nature of CNTs in future nanoelectronics. The graphene edge structure is one of the most unique geometrical features of 2D graphene sheets; this is not seen in 1D CNTs. It has been theoretically predicted that, because of the narrow width of GNRs, their electronic state is strongly infl uenced by their edge structure. [ 7–9 ] Thus, to obtain desirable properties for devices using GNRs, it is essential to be able to precisely control the edge structure and chemical terminations of GNRs. Experimental investigations of edge functionalization and doping have been very limited because of the diffi culty in controlling the necessary reaction and in the analysis. Nitrogen doping of graphene by annealing under an NH 3 atmosphere has been reported; [ 9 , 11 ] however, the high reaction temperatures and the need to control the degree of doping by tuning the reaction conditions make it diffi cult. Very recently, the selective edge functionalization of graphene sheet by a wet chemical process was reported. [ 12 ]