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 study the adsorption of NH3 and H2O molecules on pyridine-type nitrogen-doped graphene using a first-principles electronic-structure calculation. The adsorption energies of NH3 and H2O molecules on the pyridine-type defect are calculated and it is found that the adsorptions of NH3 as well as H2O molecules become energetically favorable. The pyridine-type defect in N-doped graphene is therefore expected to be highly reactive, and should be useful for reaction centers in chemical processes as well as for sensor applications.

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Adsorption of Molecules on Nitrogen-Doped Graphene: A First-Principles Study

We study the stabilities and geometric and electronic properties of hexagonal boron nitride $(h\text{\ensuremath{-}}\mathrm{BN})$ trilayers by using first-principles electronic-structure calculations within the framework of the density functional theory. From the results of total-energy calculations, we reveal the relative stabilities for various stacking sequences of $h$-BN trilayers. We also show that energy-band structures as well as spatial distributions of wave functions at the valence-band maximum (VBM) and the conduction-band minimum (CBM) strongly depend on the stacking sequences of the $h$-BN trilayers. We further investigate the effects of substitutional doping of a carbon atom on the electronic properties of the $h$-BN trilayers. In several stacking sequences of the C-doped $h$-BN trilayers, we find that the C-atom dopant can be spatially separated from the carrier transport layers associated with the VBM or the CBM, suggesting the possibility of realizing conduction channels only weakly disturbed by the C-atom impurity in $h$-BN trilayers. Interestingly, these donor states spatially separated from the CBM state are found to become rather shallow. This theoretical finding of ``atomically thin modulation doping'' using the $h$-BN layers may open an important way to design future layered electronic device materials.

Electronic states and modulation doping of hexagonal boron nitride trilayers

The electronic states of small‐diameter carbon nanotubes are investigated using the ab initio GW approximation It is found that quasiparticle excitation energies are increased significantly in semiconducting nanotube due to the many‐body correction Lifetimes of the quasiparticles are also calculated and turned out to be much longer in semiconducting tube than in metallic tube

Quasiparticle Band Structure of Carbon Nanotubes

We study the electronic structures and scanning tunneling microscopy (STM) images of heterostructures consisting of pristine h-BN, graphene, and C-doped h-BN layers using first-principles total-energy calculations within the framework of the density functional theory. We find that substitutional doping of a C atom at B and N sites in the h-BN layer induces the different charge carrier types and asymmetric carrier densities on the graphene layer. We also find that C-atom impurity state can be clearly observed in the STM image when a C atom is doped at the N site in the h-BN layer. We discuss the relationship between the STM images and the spatial distribution of the C-induced impurity state.

https://iopscience.iop.org/article/10.7567/1347-4065/ab0ff8/pdf

STM visualization of carbon impurities in sandwich structures consisting of hexagonal boron nitride and graphene

Relative stabilities and electronic structure of graphene/h-BN superlattices are discussed in the framework of the density functional theory. Most importantly, relative stabilities between commensurate and incommensurate superlattices are studied. Commensurate graphene/h-BN monolayer superlattices are found to be definitely more stable than incommensurate superlattices. In graphene/h-BN bilayer superlattices, commensurate superlattices are found to be slightly more stable than incommensurate superlattices. Results also imply that a finite pressure can induce transition from an incommensurate superlattice to a commensurate superlattice.

Susumu Saito

Papers

Adsorption of Molecules on Nitrogen-Doped Graphene: A First-Principles Study

Electronic states and modulation doping of hexagonal boron nitride trilayers

Quasiparticle Band Structure of Carbon Nanotubes

STM visualization of carbon impurities in sandwich structures consisting of hexagonal boron nitride and graphene

Geometries and Electronic Structure of Graphene and Hexagonal BN Superlattices