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

We present first-principles total-energy calculations that allow us to predict two new properties of carbon-based nanostructures. First,it is shown that carbon nanotubes (CNTs) encapsulating C 60; now commonly called carbon peapods,are one-dimensional conductors with different types of carriers. Second,CNTs and carbon nanoflakes with zigzag edges are predicted to have ferromagnetic ordering. It is also shown that hetero-nanotubes and hetero-sheets consisting of boron and nitrogen in addition to C are also nano-scale ferromagnets. r 2002 Elsevier Science B.V. All rights reserved.

Prediction of electronic properties of carbon-based nanostructures

We have done a study of graphene/hexagonal boron nitride ($h$-BN) thin films within the framework of density functional theory, and find that the interlayer interaction energy of a graphene/$h$-BN monolayer thin film is inversely proportional to the layer number. This analysis based on a method which simulates the interlayer interactions in lattice-mismatched thin films shows that thin films with four or more layers can have stable lattice-matched stacking geometries. We find that the maximum value of the band gap of the lattice-matched thin films having the same layer number, but different stacking sequences, decreases with respect to the layer number, even though one can consider several different stable stacking sequences of these feasible lattice-matched thin films. In addition, the band gap can be tuned by using an external electric field. We also propose six-layer graphene/$h$-BN bilayer thin films with 99-meV band gap or graphenelike linear dispersion depending on the stacking sequences.

Lattice matching and electronic structure of finite-layer graphene/ h -BN thin films

Gas adsorption, energetics and electronic properties of boron- and nitrogen-doped bilayer graphenes

We study electronic configurations of superheavy elements around one of islands of stabilities, from element 121 to 131, using the relativistic density functional theory with quantum electrodynamical corrections (Breit interaction) by MacDonald and Vosko. It is found that these corrections are important for studying electronic configurations of superheavy elements, and that g electrons in the atom first appear from element 126. We draw a table for the electronic configuration of elements including these superheavy elements.

Electronic Configurations of Superheavy Elements

We report on relationships between interlayer distances and electronic structures of hexagonal boron-nitride (h-BN) bilayers based on first-principles density-functional calculations. The energy-band structures and the band-gap values are calculated under uniaxial compressive strains along the direction perpendicular to the h-BN bilayer sheets. It is found that the band gap can be tuned by applying compressive strains uniaxially. More specifically, the band-gap value decreases with decreasing the interlayer distances between two layers of the h-BN bilayer. The behaviors of the band gaps under uniaxial strains are explained in terms of the energy-band structures.

Susumu Saito

Papers

Prediction of electronic properties of carbon-based nanostructures

Lattice matching and electronic structure of finite-layer graphene/ h -BN thin films

Gas adsorption, energetics and electronic properties of boron- and nitrogen-doped bilayer graphenes

Electronic Configurations of Superheavy Elements

Interlayer distances and band-gap tuning of hexagonal boron-nitride bilayers