Electronic excitations lie at the origin of most of the commonly measured spectra. However, the first-principles computation of excited states requires a larger effort than ground-state calculations, which can be very efficiently carried out within density-functional theory. On the other hand, two theoretical and computational tools have come to prominence for the description of electronic excitations. One of them, many-body perturbation theory, is based on a set of Green’s-function equations, starting with a one-electron propagator and considering the electron-hole Green’s function for the response. Key ingredients are the electron’s self-energy S and the electron-hole interaction. A good approximation for S is obtained with Hedin’s GW approach, using density-functional theory as a zero-order solution. First-principles GW calculations for real systems have been successfully carried out since the 1980s. Similarly, the electron-hole interaction is well described by the Bethe-Salpeter equation, via a functional derivative of S. An alternative approach to calculating electronic excitations is the time-dependent density-functional theory (TDDFT), which offers the important practical advantage of a dependence on density rather than on multivariable Green’s functions. This approach leads to a screening equation similar to the Bethe-Salpeter one, but with a two-point, rather than a four-point, interaction kernel. At present, the simple adiabatic local-density approximation has given promising results for finite systems, but has significant deficiencies in the description of absorption spectra in solids, leading to wrong excitation energies, the absence of bound excitonic states, and appreciable distortions of the spectral line shapes. The search for improved TDDFT potentials and kernels is hence a subject of increasing interest. It can be addressed within the framework of many-body perturbation theory: in fact, both the Green’s functions and the TDDFT approaches profit from mutual insight. This review compares the theoretical and practical aspects of the two approaches and their specific numerical implementations, and presents an overview of accomplishments and work in progress.

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Electronic excitations: density-functional versus many-body Green's-function approaches

Recently discovered carbon nanotubes have exhibited many unique material properties including very high thermal conductivity. Strong sp 2 bonding configurations in carbon network and nearly perfect self-supporting atomic structure in nanotubes give unusually high phonon-dominated thermal conductivity along the tube axis, possibly even surpassing that of other carbon-based materials such as diamond and graphite (in plane). In this chapter, we explore theoretical and experimental investigations for the thermal-transport properties of these materials.

Unusually High Thermal Conductivity of Carbon Nanotubes

Hexagonal boron nitride (h-BN) is a layered material with a graphite-like structure in which planar networks of BN hexagons are regularly stacked. As the structural analogue of a carbon nanotube (CNT), a BN nanotube (BNNT) was first predicted in 1994; since then, it has become one of the most intriguing non-carbon nanotubes. Compared with metallic or semiconducting CNTs, a BNNT is an electrical insulator with a band gap of ca. 5 eV, basically independent of tube geometry. In addition, BNNTs possess a high chemical stability, excellent mechanical properties, and high thermal conductivity. The same advantages are likely applicable to a graphene analogue-a monatomic layer of a hexagonal BN. Such unique properties make BN nanotubes and nanosheets a promising nanomaterial in a variety of potential fields such as optoelectronic nanodevices, functional composites, hydrogen accumulators, electrically insulating substrates perfectly matching the CNT, and graphene lattices. This review gives an introduction to the rich BN nanotube/nanosheet field, including the latest achievements in the synthesis, structural analyses, and property evaluations, and presents the purpose and significance of this direction in the light of the general nanotube/nanosheet developments.

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Boron Nitride Nanotubes and Nanosheets

Modeling hardness of polycrystalline materials and bulk metallic glasses

Carbon nanotubes are unique tubular structures of nanometer diameter and large length/diameter ratio. The nanotubes may consist of one up to tens and hundreds of concentric shells of carbons with adjacent shells separation of ∼0.34 nm. The carbon network of the shells is closely related to the honeycomb arrangement of the carbon atoms in the graphite sheets. The amazing mechanical and electronic properties of the nanotubes stem in their quasi-one-dimensional (1D) structure and the graphite-like arrangement of the carbon atoms in the shells. Thus, the nanotubes have high Young’s modulus and tensile strength, which makes them preferable for composite materials with improved mechanical properties. The nanotubes can be metallic or semiconducting depending on their structural parameters. This opens the ways for application of the nanotubes as central elements in electronic devices including field-effect transistors (FET), single-electron transistors and rectifying diodes. Possibilities for using of the nanotubes as high-capacity hydrogen storage media were also considered. This report is intended to summarize some of the major achievements in the field of the carbon nanotube research both experimental and theoretical in connection with the possible industrial applications of the nanotubes.

Carbon nanotubes: properties and application

The pressure-induced structural phase transition of carbon nanotubes is studied using the constant-pressure tight-binding molecular-dynamics method. We find that an interesting sp2–sp3 hybrid structure, which is considered as a graphitic nanoribbon solid, is obtained from the nanotube solid composed of armchair nanotubes. The sp3-rich phase with anisotropic atomic network is also obtained at elevated pressure and is predicted to have high hardness which is comparable to that of cubic diamond. In contrast, the bundle composed of both armchair and chiral nanotubes transforms into amorphous diamond phase at high pressure. From the electronic-structure study in the framework of the density-functional theory, the nanoribbon solid phase obtained is found to be metallic.

Constant-Pressure Molecular-Dynamics Study of Carbon Nanotubes and Electronic Structure of New Phases

http://absimage.aps.org/image/MAR08/MWS_MAR08-2007-002000.pdf

Electronic structures and electron-phonon interactions of boron-doped carbon nanotube

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Okada, Saito, and Oshiyama Reply:

We study the energetics, electronic structures, and electron-phonon couplings of a one-dimensional potassium-doped ${\mathrm{C}}_{60}$ chain encapsulated in a boron nitride nanotube using the framework of the density-functional theory. We demonstrate that the reaction of potassium doping is exothermic and the resulting material is one-dimensional metal where conducting electrons are only in the ${\mathrm{C}}_{60}$ chain. Interestingly, the Fermi-level density of states has a peculiar pressure dependence and can be larger than those in the three-dimensional alkali-doped fullerene compounds, indicating the possibility of various phase transitions. We also discuss the electron-phonon couplings and the possibility of superconductivity.

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One-dimensional alkali-doped C 60 chains encapsulated in BN nanotubes

The authors calculated the electronic structure of fullerenes, C60, C70, C76 and C84 by using the density-functional theory (DFT) or the realistic tight-binding model. C60K and C60Cl where K and Cl are trapped inside C60, are also studied. Results are discussed from the viewpoint of the fullerenes as 'artificial atoms', a spherical quantum system which one can modify and make use of to construct new materials. They also report the electronic structure of solid fullerenes: pristine FCC C60, FCC C60K, and FCC BC59 (one of the sixty C atoms in each C60 is replaced by B), and fullerides: KNC60(N=1-3) and C60Br (K and Br atoms are on the interstitial sites of the C60 lattice), obtained by the DFT.

Susumu Saito

Papers

Constant-Pressure Molecular-Dynamics Study of Carbon Nanotubes and Electronic Structure of New Phases

Electronic structures and electron-phonon interactions of boron-doped carbon nanotube

Okada, Saito, and Oshiyama Reply:

One-dimensional alkali-doped C 60 chains encapsulated in BN nanotubes

Electronic structure of fullerenes and fullerides: artificial atoms and their solids