Yutaka Maniwa

Diameter control of single-walled carbon nanotubes

Laser furnace technique has been used for diameter selective formation of single-walled carbon nanotubes (SWNTs) using NiCo, RhPt and RhPd catalysts. Particularly RhPd catalyst can produce very thin SWNTs in high yield. In this work, we investigated the furnace temperature dependence for each catalyst to control the diameter of SWNTs more strictly. Further, we investigated the effect of the gas flow velocity to see the growth time of SWNTs. When the flow velocity was changed from 1.2 to 12 mm/s, the diameter distributions of the SWNTs obtained were changed significantly. This result suggests that the nucleation and the growth of SWNTs are much slower than the fullerene formation. Using the slow growth nature of SWNTs, there is a possibility of further diameter control of SWNTs.

Superconductivity in alkali-metal-doped picene

Efforts to identify and develop new superconducting materials continue apace, motivated by both fundamental science and the prospects for application. For example, several new superconducting material systems have been developed in the recent past, including calcium-intercalated graphite compounds, boron-doped diamond and—most prominently—iron arsenides such as LaO1–xFxFeAs (ref. 3). In the case of organic superconductors, however, no new material system with a high superconducting transition temperature (Tc) has been discovered in the past decade. Here we report that intercalating an alkali metal into picene, a wide-bandgap semiconducting solid hydrocarbon, produces metallic behaviour and superconductivity. Solid potassium-intercalated picene (Kxpicene) shows Tc values of 7 K and 18 K, depending on the metal content. The drop of magnetization in Kxpicene solids at the transition temperature is sharp (<2 K), similar to the behaviour of Ca-intercalated graphite. The Tc of 18 K is comparable to that of K-intercalated C60 (ref. 4). This discovery of superconductivity in Kxpicene shows that organic hydrocarbons are promising candidates for improved Tc values.

Phase Transition in Confined Water Inside Carbon Nanotubes.

In materials confined within nanometer channels in single-walled carbon nanotube (SWNT) bundles, interesting properties which are not observed in bulk materials are expected. In the present paper, we report an X-ray diffraction (XRD) study on water adsorption in SWNT bundles. It was found that a substantial amount of water is absorbed inside SWNTs at room temperature (RT). The desorption-adsorption of water molecules occurred reversibly above RT. We found that the liquid-like water is transformed into a new solid form, i.e., ice nanotubes, at 235 K.

Transport mechanisms in metallic and semiconducting single-wall carbon nanotube networks.

A fundamental understanding of the conduction mechanisms in single-wall carbon nanotube (SWCNT) networks is crucial for their use in thin-film transistors and conducting films. However, the uncontrollable mixture state of metallic and semiconducting SWCNTs has always been an obstacle in this regard. In the present study, we revealed that the conduction mechanisms in nanotube networks formed by high-purity metallic and semiconducting SWCNTs are completely different. Quantum transport was observed in macroscopic networks of pure metallic SWCNTs. However, for semiconducting SWCNT networks, Coulomb-gap-type conduction was observed, due to Coulomb interactions between localized electrons. Crossovers among a weakly localized state and strongly localized states with and without Coulomb interactions were observed for transport electrons by varying the relative content of metallic and semiconducting SWCNTs. It was found that hopping barriers, which always exist in normal SWCNT networks and are serious obstacles to achieving high conductivity, were not present in pure metallic SWCNT networks.

Photoconductivity in Semiconducting Single-Walled Carbon Nanotubes

We have observed the photoconductive response of film samples of single-walled carbon nanotubes for the first time. Two peaks in the photoconductivity excitation spectra around 0.7 and 1.2 eV are observed at room temperature, which can be interpreted as a photocurrent in semiconducting nanotubes. At a low temperature, we found a marked change in the intensity of the spectrum. In this paper, we discuss this temperature dependence and the mechanism of photoconductivity.

Confined water inside single-walled carbon nanotubes: Global phase diagram and effect of finite length

Studies on confined water are important not only from the viewpoint of scientific interest but also for the development of new nanoscale devices. In this work, we aimed to clarify the properties of confined water in the cylindrical pores of single-walled carbon nanotubes (SWCNTs) that had diameters in the range of 1.46 to 2.40 nm. A combination of x-ray diffraction (XRD), nuclear magnetic resonance, and electrical resistance measurements revealed that water inside SWCNTs with diameters between 1.68 and 2.40 nm undergoes a wet-dry type transition with the lowering of temperature; below the transition temperature T(wd), water was ejected from the SWCNTs. T(wd) increased with increasing SWCNT diameter D. For the SWCNTs with D = 1.68, 2.00, 2.18, and 2.40 nm, T(wd) obtained by the XRD measurements were 218, 225, 236, and 237 K, respectively. We performed a systematic study on finite length SWCNT systems using classical molecular dynamics calculations to clarify the effect of open ends of the SWCNTs and water content on the water structure. It was found that ice structures that were formed at low temperatures were strongly affected by the bore diameter, a = D - σ(OC), where σ(OC) is gap distance between the SWCNT and oxygen atom in water, and the number of water molecules in the system. In small pores (a < 1.02 nm), tubule ices or the so-called ice nanotubes (ice NTs) were formed irrespective of the water content. On the other hand, in larger pores (a > 1.10 nm) with small water content, filled water clusters were formed leaving some empty space in the SWCNT pore, which grew to fill the pore with increasing water content. For pores with sizes in between these two regimes (1.02 < a < 1.10 nm), tubule ice also appeared with small water content and grew with increasing water content. However, once the tubule ice filled the entire SWCNT pore, further increase in the water content resulted in encapsulation of the additional water molecules inside the tubule ice. Corresponding XRD measurements on SWCNTs with a mean diameter of 1.46 nm strongly suggested the presence of such a filled structure.

Radial breathing modes of multiwalled carbon nanotubes

Abstract A number of Raman-active peaks in the low-frequency region (100–600 cm−1) were observed for multiwalled carbon nanotubes (MWNTs). They were confirmed to be the radial breathing modes (RBMs) by using polarized Raman scattering. These RBMs originate from very thin innermost tubes (diameter, d=∼1 nm ) included in MWNTs, and the RBM peak of the smallest carbon nanotubes ( d=0.4 nm ) appears at 570 cm −1 . It has also been found that the innermost-diameter distribution calculated from the RBM frequencies agrees well with the observations of high-resolution transmission electron microscopy. This provides a new Raman-spectroscopy-based method for the determination of the innermost diameter of MWNTs.

Atomic structure and electronic properties of single-wall carbon nanotubes probed by scanning tunneling microscope at room temperature

A detailed three-dimensional structural analysis of single-walled carbon nanotubes was carried out using a scanning tunneling microscope (STM) operated at room temperature in ambient conditions. On a microscopic scale, the images show tubes condensed in ropes as well as tubes which are separated from each other. For a single-wall nanotube rope, the outer portion is composed of highly oriented nanotubes with nearly uniform diameter and chirality. On separated nanotubes, atomically resolved images show variable chirality ranges between 0°–30°, and variable diameter (1–3 nm), with no one type dominant. From STM and scanning tunneling spectroscopy measurements we confirmed the correlation between chirality and the electronic properties, namely the tuning from metallic to semiconducting. We also observed a rectifying behavior correlated with the chiral angle of 25°, an important observation for nanodevices application.

Giant Seebeck coefficient in semiconducting single-wall carbon nanotube film

We found a giant Seebeck effect in semiconducting single-wall carbon nanotube (SWCNT) films, which exhibited a performance comparable to that of commercial Bi2Te3 alloys. Carrier doping of semiconducting SWCNT films further improved the thermoelectric performance. These results were reproduced well by first-principles transport simulations based on a simple SWCNT junction model. These findings suggest strategies that pave the way for emerging printed, all-carbon, flexible thermoelectric devices.

Helicity and packing of single-walled carbon nanotubes studied by electron nanodiffraction

Abstract Electron nanodiffraction patterns from raft-like bundles of single-walled carbon nanotubes have been obtained to analyze the helicity distribution and the packing of the tubules. In most cases, the helical angles were found to be quite evenly distributed, ranging from the zigzag structure of 0° helicity to the armchair structure of 30° helicity. Examples of preferred helicities were encountered occasionally, such as bundles of tubules of almost single-helicity, corresponding to the armchair structure. The lattice constant of the hexagonally packed bundles varied and two example values were determined to be 1.64 nm and 1.71 nm, respectively.