Teruaki Matsuba

Raman spectral mapping of self-aligned carbon nanowalls

Carbon nanowalls (CNWs) are a nano-carbon material constructed with a few layers of graphene and thus may exhibit similar properties to graphene. We developed a self-alignment process for the creation of carbon nanowalls using graphoepitaxy, which can be used for the structural fabrication of field-effect transistors. We used Raman spectral mapping to study the mechanism of the self-alignment process. The dependence of growth on temperature and the use of Raman spectra enabled the observation of geometrical catalyst effects and the induction of defects for self-aligned regions. The same effects were observed when analyzing the dependence of deposition on time. For low-temperature growth, which corresponds to the initial growth, the growth of CNW flakes can be observed only on the edge of processed patterns. On the other hand, the self-alignment process should change the shape of the flakes, thereby affecting grain structures and inducing carrier scattering. The use of graphoepitaxy can therefore help in the initial growth, and then mainly induces defects in CNW films. Finally, these defects are cured with the overgrowth.

In-situ Observation of Surface Graphitization of Gallium Droplet and Concentration of Carbon in Liquid Gallium

Although carbon has been recognized to be insoluble in gallium, we found that the outermost surface of gallium has unexpectedly high carbon solubility, particularly the limited region of about a few nanometers in depth. Our in-situ transmission electron microscope observations revealed that a graphene layer was precipitated at the surface of a gallium droplet simultaneously with gallium evaporation, and some of the droplets created an internal graphitic layer. On the basis of these experimental data, we evaluated a substantial carbon solubility that seemed to exceed about 50 at. %, but was realized in a very thin surface region of about 4 nm in depth. We believe that this high carbon solubility at the gallium surface is the key mechanism for the catalytic ability of gallium that was observed at the interface between liquid gallium and solid amorphous carbon.

Enormous Shrinkage of Carbon Nanotubes under Low-Energy Electron Beam Irradiation with Uniaxial Tensile Stress

We found that low-energy electron irradiation combined with uniaxial tensile stress strongly enhanced the shrinkage of carbon nanotubes (CNTs). The shrinking maximized at 5 keV, although some believe that the low-energy electron irradiation from 1 to 30 keV using a scanning electron microscope hardly induces structural deformation of CNTs. Such shrunk CNTs showed periodic and zigzag folding of their sidewalls, resulting in about 90% shrinkage from the original length. Such deformation could be induced in either single- or multi-walled nanotubes (MWNTs). The surface area per unit volume drastically increased up to 560% when a tube shrunk to 10% of its original length, a phenomenon observed in the shrinkage of MWNTs.

Noise spectroscopy of self-aligned carbon nanowalls

Nano-carbon materials are promising for use in future electronic devices. Their superior properties are derived from the low dimensional structure based on the two dimensional carbon (graphene) sheet. Carbon nanowalls (CNWs) are nano-carbon materials comprising a few layers of graphene. However, controlling the position of CNWs in the devices is difficult during the growth process. This is the limitation of CNWs in applications such as field-effect transistors (FETs). We have developed a self-alignment process for CNWs and investigated the growth control of a CNW channel in CNW-FETs. The CNW-FET shows 1/f-type noise in a drain-source current. There are two noise resources in the CNW channels. One relates to environmental noise such as molecular adsorption with large surface area, and the second relates to grain boundary scattering. Higher growth temperatures enlarge the grain size in CNWs, and this results in a decrease in carrier scattering at the grain boundary. Therefore, the drain-source current could increase. In this case, the noise derived from the grain boundary might be small and the total 1/f noise becomes small.

Growth of bridging carbon nanofibers in cracks formed by heat-treating iron oxide thin sheets in acetylene gas

We produced novel carbon nanofibers (CNFs) by oxidizing high-purity iron foil and then carburizing it in acetylene gas flow. This formed cracks in the heat-treated iron foil with CNFs bridging the two walls of each crack. The CNFs were drawn out from the walls as the crack opened during heat treatment. This will be a new method to grow and arrange carbon nanotubes and nanosheets without using metal nanoparticles or template substrates.

Enormous shrinkage of carbon nanotubes by supersonic stress and low-acceleration electron beam irradiation

The authors demonstrated a new method for inducing enormous shrinkage in single-walled carbon nanotube bundles by applying low energy electron beam irradiation along with supersonic vibration, and a maximum shrinkage rate of −100% cm2/C was obtained under electron acceleration of 1 keV. The characteristic feature of the shrunken single-walled carbon nanotubes was a wavy deformation that affected the entire bundle. The authors believe that a uniaxial stress induced by the supersonic vibration broke the equilibrium of the internal stress and allowed the uniform accumulation of defects under low energy electron beam excitation. The wavy deformation of the single-walled carbon nanotubes resulted in the enormous shrinkage of the bundle.

Effects of the plasma process for self-aligned nano-carbon field-effect transistors

We fabricated CNW-FETs using a self-alignment process. We will discuss the plasma processes which improve the device properties of CNW-FETs, such as the gate leak current by O2 plasma etching and internal electrode contact by H2 plasma. H2 plasma etching also reduces the noise power spectral density of the drain current, and noise spectroscopy is useful in examining device properties and improving the device fabrication process for Schottky devices such as nano-carbon FETs.