Takeru Okada

Synthesis and electronic properties of ferrocene-filled double-walled carbon nanotubes

Double-walled carbon nanotubes (DWNTs) are filled with ferrocene molecules by a vapour diffusion method for the first time. The as-synthesized ferrocene-filled DWNTs are characterized by transmission electron microscopy (TEM), energy-dispersive x-ray spectrometry (EDX) and Raman spectroscopy. Electronic properties of double-walled carbon nanotubes (DWNTs) filled with ferrocene molecules are studied by fabricating them as the channels of field-effect transistor (FET) devices. Our results reveal that electronic properties of ferrocene-filled DWNTs are greatly modified due to the charge transfer between ferrocene molecules and DWNTs. In addition, after ferrocene molecules are decomposed inside DWNTs, electronic properties of DWNTs exhibit a further change due to Fe encapsulation, and unipolar n-type semiconducting DWNTs are consequently obtained.

Nano Sized Magnetic Particles with Diameters Less than 1 nm Encapsulated in Single-Walled Carbon Nanotubes

The synthesis of magnetic nano sized Fe particle-encapsulated single-walled carbon nanotubes (SWNTs) has been realized for the first time by a two-step method. In this method, ferrocene is selected as a starting material and used in filling SWNTs. Then, the as-synthesized ferrocene-filled SWNTs are decomposed to release Fe atoms inside the SWNTs. Both ferrocene-filled SWNT and Fe-filled SWNT samples are characterized by transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDX) and Raman spectroscopy. Our results reveal that discrete or chain like Fe particles have successfully been filled in the SWNTs.

Electronic transport properties of Cs-encapsulated double-walled carbon nanotubes

Electronic transport properties of Cs-encapsulated double-walled carbon nanotubes (DWNTs) synthesized via a plasma irradiation method are investigated by fabricating them as field-effect transistor devices. The authors’ results indicate that Cs-encapsulated DWNTs exhibit a high performance n-type characteristic in contrast to ambipolar behavior of pristine DWNTs. Coulomb blockade oscillations are observed on the Cs-encapsulated DWNTs at low temperatures. In addition, it is found that the semiconducting characteristics of the as-synthesized Cs-encapsulated DWNTs can possibly be controllable by adjusting applied negative dc bias voltages during the plasma synthesis process.

Electrical properties of ferromagnetic semiconducting single-walled carbon nanotubes

Electrical properties of single-walled carbon nanotubes (SWCNTs) filled with Fe are studied by fabricating them as the channels of field-effect transistor devices. The synthesis of Fe-filled SWCNTs is realized by using ferrocene as the starting material. Our results reveal that ferrocene-filled SWCNTs show the interesting ambipolar behavior. In contrast, Fe-filled SWCNTs can exhibit high performance unipolar n-type semiconducting characteristics, suggesting the possibility of creating ferromagnetic semiconducting SWCNTs. Moreover, Coulomb blockade oscillations are significantly observed on Fe-filled SWCNTs, which indicates that they exhibit excellent single-electron transistor characteristics at low temperatures.

Negative differential resistance in tunneling transport through C60 encapsulated double-walled carbon nanotubes

The authors report electric transport properties of resonance tunneling field-effect transistors fabricated using C60-filled metallic double-walled carbon nanotubes. The devices exhibit strong resonance tunneling characteristics and the distinct negative differential resistance with high peak-to-valley current ratio about 1300 is observed at room temperature. In particular, at high bias voltages, the tunneling current is completely dominated by the Coulomb oscillation peaks with uniform conductance at room temperature, reflecting a strong single-electron tunneling effect.

Atmospheric Pressure Glow-Discharge Plasmas with Gas–Liquid Interface

An atmospheric pressure glow-discharge plasma in contact with liquid is generated using a capacitively coupled plasma (CCP) method, by which a boundary region between a plasma (gas-phase) and liquid paraffin (liquid-phase), i.e., gas–liquid interface is considered to be important. A stable atmospheric pressure plasma with liquid is achieved by selecting the appropriate mesh electrode and liquid paraffin. In addition, results of optical emission spectroscopy indicate that carbonic species come from paraffin in the interface region of the plasma. This plasma is accordingly expected to promote the use of an attractive plasma process for creating materials encapsulating various elements in liquids.

Selective in-plane nitrogen doping of graphene by an energy-controlled neutral beam.

Nitrogen-doped graphene promises to improve current electronic devices, sensors, and energy-based devices. To this end, the bonding states between carbon and nitrogen atoms can be manipulated to tailor the properties of the doped graphene. For example, graphitic nitrogen is known to promote desired catalytic activities in graphene fuel-cell systems, resulting from a four-electron reaction. However, established nitrogen-doping methods lack selectivity in dopant chemical identity and in dopant location; both are key factors in graphene property design because the properties depend on the chemical identity and location of the dopant. Here, we utilize a nitrogen neutral beam (NB) technique-with exquisite beam energy control-to dope graphene with nitrogen. Using x-ray photoelectron and Raman spectroscopy, we show that the energy of the nitrogen NB not only determines the chemistry of the nitrogen dopant introduced to graphene, but it also dictates the doping locations within graphene layers.

Single-Stranded DNA Insertion into Single-Walled Carbon Nanotubes by Ion Irradiation in an Electrolyte Plasma

The inside modification of single-walled carbon nanotubes using a single-stranded DNA is demonstrated. In this method, we regard DNA solution as a plasma, i.e., an electrolyte plasma. A direct current electric field is applied to the electrolyte plasma containing DNA negative ions in order to irradiate the single-walled carbon nanotubes with DNA ions. In addition, a radio frequency electric field is superimposed to the plasma to change the conformation of DNA ion molecules from random-coiled to stretched. DNA negative ion irradiation can be controlled by varying direct current electric field and irradiation time. In addition, transmission electron microscopy and Raman scattering spectrum analyses reveal that DNA is found to be encapsulated in the single-walled carbon nanotubes.

The effects of polymer side-chain structure on roughness formation of ArF photoresist in plasma etching processes

Low etching resistance and roughness formation of ArF photoresist during plasma etching are serious problems. We have previously found that decisive factors affecting the plasma resistance and roughness formation in an ArF photoresist are determined by ultraviolet/vacuum ultraviolet radiation and roughness formation is dominated by chemical reactions. In this paper, on the basis of our previous findings on the interaction between radiation species from plasma and ArF photoresist polymers, we investigated the polymer structural dependence for the degradation mechanism of ArF photoresist in the plasma etching processes. The etching resistance of ArF photoresist was improved by controlling the elemental ratio of oxygen atoms and ring structures in photoresist polymer. Furthermore, lactone C=O bond is found to be a key factor for roughness formation during the etching process. We have revealed the importance of the molecular structure of ArF photoresist for improving the surface roughness and etching resistance during the plasma etching process.

Estimation of activation energy and surface reaction mechanism of chlorine neutral beam etching of GaAs for nanostructure fabrication

To understand the etching mechanisms of GaAs materials through the neutral beam etching (NBE) process developed in our laboratory, we investigated the effect of substrate temperature on etching conditions. The etch rate as a function of wafer temperature was found to increase with temperature. The apparent activation energy was calculated to be an average of about 8.6 ± 1.4 kJ mol−1. However, as the vapour pressure of the etch product (GaClx) was above the chamber pressure, the etching mechanism was assumed to be temperature independent. It has been suggested that residual Ga2O3 oxide on the GaAs surface is responsible for the temperature dependence of the etch rate. A comparison with reactive ion etching (RIE) was done and a lower activation energy was calculated (4.0 ± 0.3 kJ mol−1). We argue that etching of residual oxide on the GaAs surface is more efficient with RIE because of the energetic ions and ultraviolet photons. It should be noted that when substrate temperature was increased, the etching rate ratio between NBE and RIE decreased, suggesting a stronger effect of chemical etching on the etching mechanism.