Atsushi Oshiyama

Bio: Atsushi Oshiyama is an academic researcher from Nagoya University. The author has contributed to research in topics: Density functional theory & Band gap. The author has an hindex of 46, co-authored 275 publications receiving 10096 citations. Previous affiliations of Atsushi Oshiyama include IBM & University of Strasbourg.

New one-dimensional conductors: Graphitic microtubules.

Abstract: On the basis of realistic tight-binding band-structure calculations, we predict that carbon microtubules exhibit striking variations in electronic transport, from metallic to semiconducting with narrow and moderate band gaps, depending on the diameter of the tubule and on the degree of helical arrangement of the carbon hexagons. The origin of this drastic variation in the band structure is explained in terms of the two-dimensional band structure of graphite.

Cohesive mechanism and energy bands of solid C60.

Abstract: We present microscopic total-energy calculations which provide a cohesive property and electronic structures of a new form of solid carbon, the face-centered-cubic ${\mathrm{C}}_{60}$ crystal (fcc ${\mathrm{C}}_{60}$). We find that ${\mathrm{C}}_{60}$ clusters are condensed by van der Waals force, and that the resulting fcc ${\mathrm{O}}_{60}$ is a novel semiconductor with direct energy gap of 1.5 eV at the Brillouin-zone boundary (X point). We also find that an ``impurity'' ${\mathrm{C}}_{60}$K cluster induces a shallow donor state in fcc ${\mathrm{C}}_{60}$.

Energetics and electronic structures of encapsulated C60 in a carbon nanotube.

Abstract: We report total-energy electronic structure calculations that provide energetics of encapsulation of ${\mathrm{C}}_{60}$ in the carbon nanotube and electronic structures of the resulting carbon peapods. We find that the encapsulating process is exothermic for the $(10,10)$ nanotube, whereas the processes are endothermic for the $(8,8)$ and $(9,9)$ nanotubes, indicative that the minimum radius of the nanotube for the encapsulation is 6.4 \AA{}. We also find that the ${\mathrm{C}}_{60}@(10,10)$ is a metal with multicarriers each of which distributes either along the nanotube or on the ${\mathrm{C}}_{60}$ chain. This unusual feature is due to the nearly free electron state that is inherent to hierarchical solids with sufficient space inside.

Magnetic ordering in hexagonally bonded sheets with first-row elements.

Abstract: We report first-principles total-energy electronic-structure calculations in the density-functional theory performed for hexagonally bonded honeycomb sheets consisting of B, N, and C atoms. We find that the ground state of BNC sheets with particular stoichiometry is ferromagnetic. Detailed analyses of energy bands and spin densities unequivocally reveal the nature of the ferromagnetic ordering, leading to an argument that the BNC sheet is a manifestation of the flat-band ferromagnetism.

Atomic corrugation and electron localization due to Moiré patterns in twisted bilayer graphenes

Abstract: We report on unprecedentedly large-scale density-functional calculations that clarify atomic and electronic structures of twisted bilayer graphene (BLG). We find the existence of the critical twist angle from either the AB or the AA stacking BLG, above which the two graphene layers are essentially decoupled and below which the atomic planes are corrugated and the Dirac electrons are localized. We also find a magic angle at which the Fermi velocity of the Dirac electron vanishes. We clarify that the Moir\'e pattern in tBLG with a tiny twist angle generates inhomogeneity for the electron systems and thus causes the drastic modification of the electronic properties, leading to flat bands at the Fermi level. Sensitivity to the Moir\'e of the valence-electron density and the electron state near the Fermi level is discussed.

Helical microtubules of graphitic carbon

Abstract: THE synthesis of molecular carbon structures in the form of C60 and other fullerenes1 has stimulated intense interest in the structures accessible to graphitic carbon sheets. Here I report the preparation of a new type of finite carbon structure consisting of needle-like tubes. Produced using an arc-discharge evaporation method similar to that used for fullerene synthesis, the needles grow at the negative end of the electrode used for the arc discharge. Electron microscopy reveals that each needle comprises coaxial tubes of graphitic sheets, ranging in number from 2 up to about 50. On each tube the carbon-atom hexagons are arranged in a helical fashion about the needle axis. The helical pitch varies from needle to needle and from tube to tube within a single needle. It appears that this helical structure may aid the growth process. The formation of these needles, ranging from a few to a few tens of nanometres in diameter, suggests that engineering of carbon structures should be possible on scales considerably greater than those relevant to the fullerenes. On 7 November 1991, Sumio Iijima announced in Nature the preparation of nanometre-size, needle-like tubes of carbon — now familiar as 'nanotubes'. Used in microelectronic circuitry and microscopy, and as a tool to test quantum mechanics and model biological systems, nanotubes seem to have unlimited potential.

Single-shell carbon nanotubes of 1-nm diameter

Abstract: CARBON nanotubes1 are expected to have a wide variety of interesting properties. Capillarity in open tubes has already been demonstrated2–5, while predictions regarding their electronic structure6–8 and mechanical strength9 remain to be tested. To examine the properties of these structures, one needs tubes with well defined morphologies, length, thickness and a number of concentric shells; but the normal carbon-arc synthesis10,11 yields a range of tube types. In particular, most calculations have been concerned with single-shell tubes, whereas the carbon-arc synthesis produces almost entirely multi-shell tubes. Here we report the synthesis of abundant single-shell tubes with diameters of about one nanometre. Whereas the multi-shell nanotubes are formed on the carbon cathode, these single-shell tubes grow in the gas phase. Electron diffraction from a single tube allows us to confirm the helical arrangement of carbon hexagons deduced previously for multi-shell tubes1.

Room-temperature transistor based on a single carbon nanotube

Abstract: The use of individual molecules as functional electronic devices was first proposed in the 1970s (ref 1) Since then, molecular electronics2,3 has attracted much interest, particularly because it could lead to conceptually new miniaturization strategies in the electronics and computer industry The realization of single-molecule devices has remained challenging, largely owing to difficulties in achieving electrical contact to individual molecules Recent advances in nanotechnology, however, have resulted in electrical measurements on single molecules4,5,6,7 Here we report the fabrication of a field-effect transistor—a three-terminal switching device—that consists of one semiconducting8,9,10 single-wall carbon nanotube11,12 connected to two metal electrodes By applying a voltage to a gate electrode, the nanotube can be switched from a conducting to an insulating state We have previously reported5 similar behaviour for a metallic single-wall carbon nanotube operated at extremely low temperatures The present device, in contrast, operates at room temperature, thereby meeting an important requirement for potential practical applications Electrical measurements on the nanotube transistor indicate that its operation characteristics can be qualitatively described by the semiclassical band-bending models currently used for traditional semiconductor devices The fabrication of the three-terminal switching device at the level of a single molecule represents an important step towards molecular electronics

Cobalt-catalysed growth of carbon nanotubes with single-atomic-layer walls

Abstract: CARBON exhibits a unique ability to form a wide range of structures. In an inert atmosphere it condenses to form hollow, spheroidal fullerenes. Carbon deposited on the hot tip of the cathode of the arc-discharge apparatus used for bulk fullerene synthesis will form nested graphitic tubes and polyhedral particles. Electron irradiation of these nanotubes and polyhedra transforms them into nearly spherical carbon 'onions'. We now report that covaporizing carbon and cobalt in an arc generator leads to the formation of carbon nanotubes which all have very small diameters (about 1.2 nm) and walls only a single atomic layer thick. The tubes form a web-like deposit woven through the fullerene-containing soot, giving it a rubbery texture. The uniformity and single-layer structure of these nanotubes should make it possible to test their properties against theoretical predictions.

Half-metallic graphene nanoribbons

Abstract: Electrical current can be completely spin polarized in a class of materials known as half-metals, as a result of the coexistence of metallic nature for electrons with one spin orientation and insulating nature for electrons with the other. Such asymmetric electronic states for the different spins have been predicted for some ferromagnetic metals--for example, the Heusler compounds--and were first observed in a manganese perovskite. In view of the potential for use of this property in realizing spin-based electronics, substantial efforts have been made to search for half-metallic materials. However, organic materials have hardly been investigated in this context even though carbon-based nanostructures hold significant promise for future electronic devices. Here we predict half-metallicity in nanometre-scale graphene ribbons by using first-principles calculations. We show that this phenomenon is realizable if in-plane homogeneous electric fields are applied across the zigzag-shaped edges of the graphene nanoribbons, and that their magnetic properties can be controlled by the external electric fields. The results are not only of scientific interest in the interplay between electric fields and electronic spin degree of freedom in solids but may also open a new path to explore spintronics at the nanometre scale, based on graphene.