Masashi Shiraishi

Work function of carbon nanotubes

Abstract The work functions of multi- and single-walled carbon nanotubes are found to be 4.95 and 5.05 eV, respectively. The measurements were carried out using the photoelectron emission (PEE) method, which allows easy and precise measurements to be made in air. We have found that the work function of the nanotubes is 0.1–0.2 eV larger than that of highly oriented pyrolytic graphite (HOPG) of which the valence state is σ–π orthogonal. This result is ascribable to reflection of the σ–π mixed valence state in the case of carbon nanotubes. The experimental data were well reproduced in ab-initio calculations on planar and cylindrical conjugated states.

Gas–solid interactions in the hydrogen/single-walled carbon nanotube system

Abstract Gas–solid interactions between hydrogen and single-walled carbon nanotubes (SWNTs) were investigated using highly purified SWNTs. The activation energy of hydrogen desorption, measured to be 0.21 eV, indicates that hydrogen is physisorbed in the pores and that inter-tube pores have an adsorption potential of about −0.21 eV, which induces hydrogen physisorption at ambient temperature. The total amount of adsorbed hydrogen, about 0.3 wt% at 9 MPa, shows that 38% of the inter-tube sites are occupied. These findings are interpreted in terms of the chemical potential of hydrogen and the adsorption potential of the inter-tube pores.

Conduction mechanisms in single-walled carbon nanotubes

The transport properties of single-walled carbon nanotubes (SWNTs) were examined to investigate their conduction mechanisms. By measuring the temperature dependence of the resistance, it was clarified that the conduction mechanism in SWNT mats can be changed from the non-metallic to the coexistence of metallic and non-metallic modes as the thickness of the mats changed. This change is attributable to the quality and the quantity of metallic nanotubes.

Dense hydrogen adsorption on carbon subnanopores at 77 K

Hydrogen adsorption measurements on two types of different carbon nanomaterials were carried out at 77 K up to hydrogen pressures of 2 MPa using the volumetric method modified for low-temperature experiments. The adsorption property was concluded as dense hydrogen physisorption in subnanometer-sized pores because of the Langmuir-type isotherm, reversible adsorption/desorption, and large hydrogen uptake exceeding 2 wt %. The estimated density of adsorbed hydrogen was comparable to the density of bulk liquid hydrogen, indicating that hydrogen filling would be attainable when both the chemical potential of hydrogen and adsorption potential of carbon were optimized.

Hydrogen storage in C70 encapsulated single-walled carbon nanotube

Molecular hydrogen desorption from C 70 encapsulated single-walled carbon nanotube (C 70 -peapod) has been investigated by Temperature-Programmed-Desorption (TPD) method. The single-walled carbon nanotube (SWNT) is in the spotlight recently due to reports of high-density hydrogen storage capability at moderate temperature. Here, we find that C 70 -peapod loose hydrogen molecule at about 365 K and the NaOH treatment is crucial for the withdrawal of hydrogen at moderate temperature. This result fairly agrees with the previous our report in SWNT-bundles and C 60 -peapod.

Tomonaga–Luttinger-liquid behavior in single-walled carbon nanotube networks

Abstract The electrical conduction behavior of single-walled carbon nanotube networks was investigated. Although the nanotubes were entangled with each other and multiple junctions were formed within the network, they showed an apparently strong Tomonaga–Luttinger-liquid (TL-liquid) character, which seemed to be attributable to their quasi one-dimensional structures. The appearance of the TL-liquid behavior was induced by removing impurities in the junctions. This fact indicates that the TL-liquid can be observed in the entangled SWNTs after the purification.

Analysis of Degradation in Graphene-Based Spin Valves

The degradation mechanisms of multilayer graphene spin valves are investigated. The spin injection signals in graphene spin valves have been reported to be linearly dependent on the drain bias voltage, which indicates that the spin polarizations of injected spins in graphene are robust with respect to bias voltage. We report that the disappearance of this robustness can be due to two different degradation mechanisms of the spin valves. Our findings indicate that the disappearance of robustness is due to degradation rather than any intrinsic characteristic of graphene. Thus, the robustness can be greatly enhanced if degradation can be prevented.

The measurement of work function of carbon nanotubes

The work function of multi-walled carbon nanotubes (MWNTs) is found to be about 5 eV by the photo-electron emission method. For comparison, the work function of highly oriented pyrolytic graphite (HOPG) was also measured and it was determined that the work function of MWNT is larger than that of HOPG. MWNT has σ-π non-orthogonal valence state, while HOPG has an orthogonal state. Because of this non-orthogonality, HOMO level of MWNT is deeper, which gives a larger work function. To check the validity of this experimental result, the first principle ab-initio calculation was carried out and the result shows the validity of the experiment.

Observation of Magnetic‐Switching and Multiferroic‐Like Behavior of Co Nanoparticles in a C60 Matrix

Tunneling magnetoresistance (TMR) via oxides or molecules includes fruitful physics, such as spin filtering and hybridized interface states, in addition to various practical applications using large TMR ratio at room temperature. Then, a larger TMR effect with a new fundamental physics is awaited because further progress on spintronics can be realized. Here we report a discovery of a gigantic TMR ratio of 1,400,000% in a C60-Co nanocomposite spin device. The observed effect is induced by a combination of a Coulomb blockade effect and a novel magnetic switching effect. Theoretical investigation reveals that an electric field and a magnetic field control the magnetization and the electronic charging state, respectively, of the Co nanoparticles as in physics of multiferroicity.

Electrical investigation of the interface band structure in rubrene single-crystal/nickel junction

The electronic structure of the interface between rubrene (C42H28) single crystal and ferromagnetic Ni is studied using an electrical method from a viewpoint of spintronics applications of organic single crystals. The Schottky barrier height at the interface is estimated to be 0.56 eV, and our finding is compared with previous results in spectroscopic method. This study clarifies the importance of electrical investigations of the Schottky barriers for various ferromagnet/organic systems and suggests that functionalization and carrier doping to rubrene single crystals are potential for obtaining the thinner barriers and yielding conclusive electrical spin injection.