A 2D metastable carbon allotrope, penta-graphene, composed entirely of carbon pentagons and resembling the Cairo pentagonal tiling, is proposed. State-of-the-art theoretical calculations confirm that the new carbon polymorph is not only dynamically and mechanically stable, but also can withstand temperatures as high as 1000 K. Due to its unique atomic configuration, penta-graphene has an unusual negative Poisson’s ratio and ultrahigh ideal strength that can even outperform graphene. Furthermore, unlike graphene that needs to be functionalized for opening a band gap, penta-graphene possesses an intrinsic quasi-direct band gap as large as 3.25 eV, close to that of ZnO and GaN. Equally important, penta-graphene can be exfoliated from T12-carbon. When rolled up, it can form pentagon-based nanotubes which are semiconducting, regardless of their chirality. When stacked in different patterns, stable 3D twin structures of T12-carbon are generated with band gaps even larger than that of T12-carbon. The versatility of penta-graphene and its derivatives are expected to have broad applications in nanoelectronics and nanomechanics.

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Penta-graphene: A new carbon allotrope

Flat carbon (sp(2) and sp) networks endow the graphdiyne and graphyne families with high degrees of π-conjunction, uniformly distributed pores, and tunable electronic properties; therefore, these materials are attracting much attention from structural, theoretical, and synthetic scientists wishing to take advantage of their promising electronic, optical, and mechanical properties. In this Review, we summarize a state-of-the-art research into graphdiynes and graphynes, with a focus on the latest theoretical and experimental results. In addition to the many theoretical predictions of the potential properties of graphdiynes and graphynes, we also discuss experimental attempts to synthesize and apply graphdiynes in the areas of electronics, photovoltaics, and catalysis.

Graphdiyne and graphyne: from theoretical predictions to practical construction

Using density functional theory coupled with Boltzmann transport equation with relaxation time approximation, we investigate the electronic structure and predict the charge mobility for a new carbon allotrope, the graphdiyne for both the sheet and nanoribbons. It is shown that the graphdiyne sheet is a semiconductor with a band gap of 0.46 eV. The calculated in-plane intrinsic electron mobility can reach the order of 10(5) cm(2)/(V s) at room temperature, while the hole mobility is about an order of magnitude lower.

Electronic structure and carrier mobility in graphdiyne sheet and nanoribbons: theoretical predictions

Graphynes (GYs) are carbon allotropes with single-atom thickness that feature layered 2D structure assembled by carbon atoms with sp- and sp2- hybridization form. Various functional theories have predicted GYs to have natural band gap with Dirac cones structure, presumably originating from inhomogeneous π-bonding between those carbon atoms with different hybridization and overlap of the carbon 2pz orbitals. Among all the GYs, graphdiyne (GDY) was the first reported to be prepared practically and, hence, attracted the attention of many researchers toward this new planar, layered material, as well as other GYs. Several approaches have been reported to be able to modify the band gap of GDY, containing invoking strain, boron/nitrogen doping, nanoribbon architectures, hydrogenation, and so on. GDY has been well-prepared in many different morphologies, like nanowires, nanotube arrays, nanowalls, nanosheets, ordered stripe arrays, and 3D framwork. The fascinating structure and electronic properties of GDY make i...

Progress in Research into 2D Graphdiyne-Based Materials

In light of the considerable impact synthetic 2D polymers are expected to have on many fundamental and applied aspects of the natural and engineering sciences, it is surprising that little research has been carried out on these intriguing macromolecules. Although numerous approaches have been reported over the last several decades, the synthesis of a one monomer unit thick, covalently bonded molecular sheet with a long-range ordered (periodic) internal structure has yet to be achieved. This Review provides an overview of these approaches and an analysis of how to synthesize 2D polymers. This analysis compares polymerizations in (initially) a homogeneous phase with those at interfaces and considers structural aspects of monomers as well as possibly preferred connection modes. It also addresses issues such as shrinkage as well as domain and crack formation, and briefly touches upon how the chances for a successful structural analysis of the final product can possibly be increased.

Two‐Dimensional Polymers: Just a Dream of Synthetic Chemists?

The electronic band structure of layered semiconductor α -In 2 Se 3 is investigated by using the self-consistent numerical-basis-set LCAO method with a potential based on the local density approximation.The resulting energy band gap occurs between Γ 5 - and Γ 1 + and its value is 0.99 eV. The effective masses in conduction and valence are 0.13 m 0 and 3.2 m 0 , respectively. The self-consistent valence of each ion is as follows: In 0.95+ In 1.23+ Se 1.52- Se 0.54- Se 0.12- . Thus the compound has a considerably ionic character though the interlayer bonding is found to be partly covalent from the valence electron charge distribution.

Band Structure of Layered Semiconductor α-In 2Se 3 by the Numerical-Basis-Set LCAO Method

The electronic band structure of layered semiconductor α-In 2 Se 3 is calculated by using the KKR method. For the muffin-tin potential constructed from the ionized atoms In +1.30 and Se -0.86 the band gap is obtained to be about 1.45 eV which is consistent with the observed value. All of the bands corresponding to Se-4s, Se-4p, In-5s and In-5p states are separated completely one another and are comparatively flat reflecting the general characteristics of layered materials with complex structure. The calculated effective masses in conduction and valence bands are considerably large, whose values are m c =1.1 m 0 and m v =1.9 m 0 , respectively. Further it is demonstrated that the calculation of structure constants is very serious for materials with complex crystal structure such as α-In 2 Se 3 .

Band structure of layered semiconductor α-In2Se3

A theoretical study is made on the shift of the O 17 nuclear resonance signal in MnO crystal. The shift is explained by the hyperfine interaction accompanying overlap effects between the manganese 3 d orbitals and the oxygen 2 s and 2 p orbitals. Calculations are carried out on the basis of the molecular orbital method. The calculated values of the fractions of unpaired spins in an oxygen ion are f s =0.0016 and f σ - f π =0.0010.

Hyperfine Interaction in MnO Crystal

The scattering cross section of X-rays inelastically scattered by an electron gas is calculated in the random phase approximation, for various scattering angles, making use of the formula previously derived by Ohmura and Matsudaira. When the angle is small, the profile deviates from the inverse parabolla due to the pauli exclusion principle and to the electron-electron interaction. Especially when the angle is near θ c , corresponding to the plasma cut-off wave number, a pronounced peak appears, reflecting plasmon emission. It is also shown that if a scattering angle is a little larger than θ c , yet the “plasmon peak” survives. These features are in excellent agreement with the experiments of Suzuki and Tanokura, and Alexandropoulos.

Inelastic scattering of X-rays by light metals

In order to understand consistently the origin of the helical spin ordering of the bcc Eu observed below TN = 91 K, we have calculated the band structure and investigated the detailed shape of the Fermi surface, by the KKR method. The present results are compared with those of the previous calculations.

Sumiaki Nagai

Papers

Band Structure of Layered Semiconductor α-In 2Se 3 by the Numerical-Basis-Set LCAO Method

Band structure of layered semiconductor α-In2Se3

Hyperfine Interaction in MnO Crystal

Inelastic scattering of X-rays by light metals

Band structure and the fermi surface of rare earth metal europium