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 investigate the electronic properties of boron-carbon-nitrogen (B-C-N) analogues of a recently proposed carbon kagome lattice [Chen et al., Phys. Rev. Lett. 113, 085501 (2014)]. The B-C-N kagome lattices are constructed by replacing the carbon zigzag chains of the carbon kagome lattice with boron nitride zigzag chains. We use calculations of phonon dispersion curves to demonstrate the thermodynamic stabilities of the BCN and ${\mathrm{BC}}_{4}\mathrm{N}$ kagome lattices. The B-C-N kagome lattices are wide-band gap semiconductors although the band gaps of the BCN and ${\mathrm{BC}}_{4}\mathrm{N}$ kagome lattices are increased and reduced, respectively, compared with the carbon case. The reduction of the band gap is found to be caused by a direct to indirect gap transition in the ${\mathrm{BC}}_{4}\mathrm{N}$ kagome lattice.

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Electronic properties of B-C-N ternary kagome lattices

We study the energetics and the electronic structure of MgB 2 nanotubes in the framework of the density-functional theory. It is found that in the case of (4,4) nanotube the presence of the outer M...

Electronic Structure and Energetics of MgB2 Nanotube

We report the energetics and the electronic properties of twisted carbon nanotubes (CNTs). We use a real-space density functional theory with helical-symmetry operation, and apply it for several CNTs with the diameters of around 0.8 nm including the experimentally abundant (6,5) nanotube. By using this computational code, one can now obtain the total energies with enough accuracies to optimize the CNT geometries including quasi-continuous twisting levels for any type of nanotube in principle. As a result, it is found that chiral nanotubes possess twisted geometries at their ground states. The electronic structures of CNTs depend sensitively on twisting levels in this diameter region, and the twisting effects on their fundamental gap values can be judged by the value of mod(n – m, 3), where n and m are the chiral indices.

https://iopscience.iop.org/article/10.1088/1742-6596/302/1/012007/pdf

Twisting Effects on Carbon Nanotubes: A First-Principles Study with Helical Symmetry Operations

Design of solid C70 conductor: Na2CsC70

Graphene is a good candidate material for high-performance sensor devices due to its excellent conductivity and large surface-to-volume ratio. Bilayer of graphene exhibits rich electronic properties owing to a variety of its stacking patterns, which may provide novel sensors. In this article, we present adsorption effects of environmental pollutants or hazardous molecules (CO, CO2, NO, and NO2) and abundant molecules (O2 and N2) in air on stabilities of boron (B)-doped bilayer graphenes with Bernal (AB)-stacking as well as AA- and AA′-stacking patterns based on a first-principles density-functional study. We find that only NO and NO2 molecules in air are chemically adsorbed on B-doped bilayer graphenes with AB, AA and AA′ stackings. We further investigate, for the first time, electronic transport properties through pristine and B-doped bilayer graphenes with and without NO and NO2 molecules from our first-principles density-functional study. We find that electronic transport properties of AB-, AA-, and AA′-stacked bilayer graphenes exhibit specific behaviors depending on the stacking patterns and the presence of the NO/NO2 molecule. Therefore, NO and NO2 molecules should be individually detectable under low bias voltages by using B-doped bilayer graphene. In addition, we also reveal the electronic transport properties of the very thin pristine and doped carbon nanotubes (CNTs) with and without a hazardous molecule and discuss their curvature effects of the CNT. Our theoretical findings for bilayer graphenes and CNTs should provide new insight into designing graphene-based low-power sensor devices.

Stacking and curvature-dependent behaviors of electronic transport and molecular adsorptions of graphene: A comparative study of bilayer graphene and carbon nanotube

Susumu Saito

Papers

Electronic properties of B-C-N ternary kagome lattices

Electronic Structure and Energetics of MgB2 Nanotube

Twisting Effects on Carbon Nanotubes: A First-Principles Study with Helical Symmetry Operations

Design of solid C70 conductor: Na2CsC70

Stacking and curvature-dependent behaviors of electronic transport and molecular adsorptions of graphene: A comparative study of bilayer graphene and carbon nanotube