Susumu Okada

Electronic Properties of Graphite with Rotational Stacking Arrangement

The electronic structure of graphite is studied using the generalized tight-binding approximation. We investigate how the energy spectra around the Fermi energy of graphite depend on interlayer stacking arrangement. We find that graphite and bilayer graphene with rotational stacking arrangement are metals with a massless Dirac fermion at the Fermi level, as is the case for isolated graphene sheets. Due to the spatially separated K points on each layer, two pairs of linear dispersion curves cross at the Fermi energy and are expected to exhibit characteristics different from those of graphite with AB stacking.

Origin of the n-type transport behavior of azafullerene encapsulated single-walled carbon nanotubes

The transport properties of C59N encapsulated semiconducting single-walled carbon nanotubes (SWCNTs) (C59N-peapod) are investigated. Transport measurements of the peapods in field effect transistors (FETs) reveal that ∼14% of the C59N-peapod sample shows n-type behavior even though the electronic properties of the host SWCNTs are similar to those of C60-peapods that exhibit only p-type property. First-principles electronic-structure calculations reveal that the unique transport behavior originates from the monomer form of C59N encapsulated in SWCNTs. The singly occupied (SO) state of C59N lies in the energy gap of the SWCNT and the energy of this state increases substantially when electrons are injected. Because of this shift to higher energy, the SO state acts as a shallow donor state for the conduction band of the nanotube, which leads to n-type behavior in FET measurements.

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.

Method for probing the magnetic state of nanomaterials encapsulated in carbon nanotubes

We propose a method for optically probing the magnetic states of metallic atoms encapsulated in single-walled carbon nanotubes. The absorption spectrum is calculated by solving the Bethe–Salpeter equation, which includes the effects of magnetic atoms, under the tight-binding approximation. Due to the exchange interaction between excitons and polarized spins in ferromagnets, triplet excitons acquire a finite oscillator strength and can thus be excited by light. This mechanism is promising for detecting magnetic ordering of materials encapsulated in carbon nanotubes.