Yoshiteru Takagi

Phase control of magnetic state of graphite thin films by electric field

Based on first-principle total-energy calculations, we have found that by applying an external electric field it is possible to control the magnetic state of graphite thin film with the rhombohedral stacking arrangement. When exposed to a moderate electric field normal to the film, the surface of a thin film of rhombohedral graphite undergoes a magnetic phase transition from the antiferromagnetic state to the ferromagnetic state. The polarized electron spin is primarily distributed in the bottommost layer of the film, which forms the interface with the negative electrode. The amount of polarized electron spin is calculated to be 0.067 μB/nm2. The ferromagnetic ordering with the characteristic distribution of the polarized electron spin opens the possibility of using graphite thin films in electronic devices with spin degree of freedom.

Asymmetrical Electronic Structure of Folded Graphene

Using a model based on the generalized tight-binding approximation, we find that folded graphene possesses an asymmetrical energy band around the Fermi level. The interlayer interaction and the closed boundary condition imposed on one edge of the graphene layers induce the unusual asymmetrical electronic structure in folded graphene. Our calculations also show that the electronic structure of folded graphene strongly depends on the interlayer atomic displacement, with the resulting materials ranging from metals to semiconductors. The asymmetrical electronic structure in folded graphene causes a different distribution of occupied and unoccupied electron states around the Fermi level which should be corroborated by scanning tunneling microscope experiments.

Growth Mechanism of Single-Walled Carbon Nanotube from Catalytic Reaction Inside Carbon Nanotube Template

We report a numerical investigation on the catalytic growth mechanism of a single-walled carbon nanotube (SWNT) inside a template SWNT, that is, formation of a double-walled carbon nanotube (DWNT). The molecular dynamics simulations together with complementary ab inito calculations suggest that the DWNT formation from thermally annealed metallocene-encapsulating SWNT goes through formation of metal catalyst cluster, followed by SWNT precipitation at the root. The diameter of the inner SWNT is determined by the carbon/metal layered structure of the catalyst cluster, which gives rise to a DWNT interlayer distance significantly different from the van der Waals distance.

Electronic Structure and Surface-Localized State of Hyper-Graphite Network

Abstract We study electronic structures of an extended three-dimensional graphite network and its slab performing of the tight binding model. The one of networks we study is bipartite and three-coordinated, and also found in some materials like α-ThSi 2 . The electronic structures of this networks have characters which are peculiar to 2D graphite. One of these characters is that these networks are a semi metal and the other is that these networks which are cut in proper direction have a surface localized states (Edge State), which originate from the topological feature of the network as well as the zigzag edge in 2D graphite. We call these networks which are made by concept extending graphite network for higher dimensions ‘the Hyper Graphite’.

Electronic Structure Modulation of Graphene by Metal Electrodes

We investigated electronic structure modulation of graphene partially covered by Pd, Ag, Pt, and Au pillars by using first-principles total-energy calculations to simulate the effects of metal electrodes on the electronic structure of graphene. Our calculations show that the electrostatic potential in graphene strongly depends not only on the metal species but also on its displacement from the electrodes. This potential modulation leads to an energy shift of the Dirac point of graphene, resulting in carrier transfer between graphene and the metal pillars depending on the metal species and spatial position.

Modulation of Electron-States of Graphite Thin Films by the Nearly Free Electron States of Metal Surfaces

We have used density functional theory to study the electronic structure of the interfaces between graphite thin films and metal substrates. We found that unusual electron transfer from Ag to the graphite thin film occurs as a result of the spatial modulation of the Dirac point of the interfacial region of graphene layers. Detailed analysis of the modulation of electron-states revealed a mechanism where the surface states of the Ag substrate, which have nearly free electron (NFE) character, induce an electric field in the graphite thin film.

Formation of Multi-Walled Nanotubes from Diamond Nanowires

Based on classical molecular dynamics simulation, we show the possibility of the formation of multi-walled carbon nanotubes from diamond nanowires at elevated temperatures. Since the outermost shell of a diamond nanowire can be regarded as a corrugated graphene sheet bound via elongated intershell bonds, thermal annealing of the nanowire causes successive peeling of the outermost shell and results in a structural transformation from an sp3 structure into an sp2 rich nanostructure. The resultant structures could be classified as multi-walled carbon nanotubes with some stacking faults.

A possible superstructure: Hyper graphite

Abstract We have shown a theoretical method to construct structures called “hyper-graphite” which can be regarded as a generalization of the graphite structure. Based on the method, we propose a possible 3-dimensional network, which may be realized as a π-electron system of hydro-carbon. Naturally, localized edge states emerge at the Fermi level similar to the zigag-edged graphite sheet.

A theoretical study for mechanical contact between carbon nanotubes

We have theoretically investigated motions of single-walled carbon nanotubes (SWNTs) which are mounted on a flat substrate layer of SWNTs by tight-binding molecular dynamics simulations. One of the most interesting motions is the conversion of force and torque, where the force and torque acting initially on the mounted tube finally results in the lateral motion and rolling of the supporting tubes in the substrate. This motion is well understood in terms of the total energy surface of the SWNT/SWNT system. It is suggested that an undulation of the total energy surface plays a role as an atomic-scale gear tooth in the field of nanomechanics, in spite of the atomically smooth surface of SWNT.

The Optical Properties of Single-Walled Carbon Nanotubes in the Ultraviolet Region

The absorption properties of single-walled nanotubes in the ultraviolet (UV) region have been studied using a generalized tight-binding approximation. Calculations indicate that the absorption spectrum in the UV region strongly depends not only on nanotube diameter but also on chiral index. We found a substantial diameter dependence of the UV absorption peaks of the nanotubes with chiral indices of (even, even) and (odd, even). The nanotubes with chiral indices of (even, odd) and (odd, odd) exhibit weak diameter dependence. These spectral features are characteristic of inter-band transitions near the M point of the hexagonal Brillouin zone of graphene.