Hidefumi Hiura

Raman studies of carbon nanotubes

First- and second-order Raman spectra of closed-carbon nanostructures (nanotubes and nanoparticles) have been measured and compared to those of glassy carbon and highly oriented pyrolitic graphite. It is shown that this novel material has unique Raman spectral features which should be useful in its identification. Furthermore, the results indicate that nanotubes possess a high degree of crystalline order.

Patterns in the bulk growth of carbon nanotubes

Abstract The growth of nanotubes in carbon arc plasma is described in detail. The structure and organization of the nanotube deposits observed by SEM and AFM reveal a fractal-like pattern of growth. One of the key units of growth appears to be the micro-bundle composed of neatly packed and aligned nanotubes. The micro-bundle together with the field and current might explain the high formation yield of nanotubes.

ANNEALING EFFECT ON CARBON NANOTUBES AN ESR STUDY

The effect of annealing on the electronic properties of nanotubes has been studied. The conduction ESR intensity is temperature-independent for both annealed and non-annealed nanotubes. However, the g-value of the nanotubes and its temperature dependence change significantly after annealing. These results suggest that some types of defects are present in the nanotubes which are then removed by the annealing process. The ESR studies of the annealed nanotubes indicate that the intrinsic electronic properties of nanotubes are quite different from those of graphite.

Electron spin resonance of carbon nanotubes

Abstract The electron spin resonance (ESR) of carbon nanotubes at various stages of purification have been measured between 4 and 296 K. The conduction electron spin resonance is identified and the results imply that metallic and/or narrow gap semiconducting nanotubes are actually present as predicted by theory. From the temperature dependences of the ESR intensity, linewidth and g -value, the electronic properties of the observed nanotubes are found to be similar to those of graphite.

Determination of the Number of Graphene Layers: Discrete Distribution of the Secondary Electron Intensity Stemming from Individual Graphene Layers

Using a scanning electron microscope, we observed a reproducible, discrete distribution of secondary electron intensity stemming from an atomically thick graphene film on a thick insulating substrate. We found a distinct linear relationship between the relative secondary electron intensity from graphene and the number of layers, provided that a low primary electron acceleration voltage was used. Based on these observations, we propose a practical method to determine the number of graphene layers in a sample. This method is superior to the conventional optical method in terms of its capability to characterize graphene samples with sub-micrometer squares in area on various insulating substrates.

Enhanced Logic Performance with Semiconducting Bilayer Graphene Channels

Realization of logic circuits in graphene with an energy gap (EG) remains one of the main challenges for graphene electronics. We found that large transport EGs (>100 meV) can be fulfilled in dual-gated bilayer graphene underneath a simple alumina passivation top gate stack, which directly contacts the graphene channels without an inserted buffer layer. With the presence of EGs, the electrical properties of the graphene transistors are significantly enhanced, as manifested by enhanced on/off current ratio, subthreshold slope, and current saturation. For the first time, complementary-like semiconducting logic graphene inverters are demonstrated that show a large improvement over their metallic counterparts. This result may open the way for logic applications of gap-engineered graphene.

Enhanced Raman Scattering from Vibro‐Polariton Hybrid States

Ground-state molecular vibrations can be hybridized through strong coupling with the vacuum field of a cavity optical mode in the infrared region, leading to the formation of two new coherent vibro-polariton states. The spontaneous Raman scattering from such hybridized light–matter states was studied, showing that the collective Rabi splitting occurs at the level of a single selected bond. Moreover, the coherent nature of the vibro-polariton states boosts the Raman scattering cross-section by two to three orders of magnitude, revealing a new enhancement mechanism as a result of vibrational strong coupling. This observation has fundamental consequences for the understanding of light-molecule strong coupling and for molecular science.

Direct Observation of C60 Exciton

Photoexcitation of pure C60 thin films by picosecond absorption spectroscopy reveals a transient species with absorption maxima at 540 nm and 660 nm which is assigned to the exciton. Considering the dynamic behaviour of this species and luminescence properties of solid C60, it is concluded that a singlet exciton is observed both in its free (τ1/2 < 15 ps) and self-trapped (τ1/2 = 250 ns) forms.

Role of atomic terraces and steps in the electron transport properties of epitaxial graphene grown on SiC

Thermal decomposition of vicinal SiC substrates with self-organized periodic nanofacets is a promising method to produce large graphene sheets toward the commercial exploitation of graphenes superior electronic properties. The epitaxial graphene films grown on vicinal SiC comprise two distinct regions of terrace and step; and typically exhibit anisotropic electron transport behavior, although limited areas in the graphene film showed ballistic transport. To evaluate the role of terraces and steps in electron transport properties, we compared graphene samples with terrace and step regions grown on 4H-SiC(0001). Arrays of field effect transistors were fabricated on comparable graphene samples with their channels parallel or perpendicular to the nanofacets to identify the source of measured reduced mobility. Minimum conductivity and electron mobility increased with the larger proportional terrace region area; therefore, the terrace region has superior transport properties to step regions. The measured elect...

Growth of hydrogenated silicon cluster ions using an ion trap

Abstract Medium-sized hydrogenated silicon cluster ions, Si n H x + (10 n 4 ) at a pressure of ∼10 −6 Torr. Time-resolved time-of-flight mass spectroscopic measurements revealed that there are two kinds of fast, efficient, cluster growth occurring via repetitive increments of eight Si atoms. One cluster growth continued beyond Si 54 H x + , which has a considerably larger n value than has been previously reported ( n ⩽10). Experimental evidence indicates that neutral Si n H x molecules, where n =6–8, formed from SiH 4 and accumulated on the chamber wall, serve as the source material for both successive reactions.