Polyanionic Hexagons: X6n– (X = Si, Ge)

Polysilyne chains bridged with beryllium lead to flat 2D Dirac materials

Polysilyne with repeating disilyne units, a silicon analogue of polyacetylene, has a high potential for application to various novel silicon-based electronic devices because of the unique properties of Si=Si units with a smaller HOMO-LUMO energy gap than that of C=C units. However, one-dimensional (1D) polysilyne has not been synthesized yet. Here we propose a planar and air-stable two-dimensional (2D) silicon-based material with one-atom thickness consisting of beryllium-bridged 1D all-trans polysilyne, based on the first-principles calculations. The flat structure of 1D polysilyne, which is essential for the air stability of silicon π-electron conjugated systems, is realized by embedding polysilyne in a planar sheet. It was found that the 2D crystal optimized at the rhombus unit cell with the D2h group symmetry is a silicon-based Dirac semimetal with linear dispersion at the Fermi energy and hosts anisotropic Dirac fermions.

Flat Zigzag Silicene Nanoribbon with Be Bridge

The emergence of flat one- and two-dimensional materials, such as graphene and its nanoribbons, has promoted the rapid advance of the current nanotechnology. Silicene, a silicon analogue of graphene, has the great advantage of its compatibility with the present industrial processes based on silicon nanotechnology. The most significant issue for silicene is instability in the air due to the nonplanar puckered (buckled) structure. Another critical problem is that silicene is usually synthesized by epitaxial growth on a substrate, which strongly affects the π conjugated system of silicene. The fabrication of free-standing silicene with a planar configuration has long been pursued. Here, we report the strategy and design to realize the flat zigzag silicene nanoribbon. We theoretically investigated the stability of various silicene nanoribbons with substituents at the zigzag edges and found that zigzag silicene nanoribbons with beryllium (Be) bridges are very stable in a planar configuration. The obtained zigzag silicene nanoribbon has an indirect negative band gap and is nonmagnetic unlike the magnetic buckled silicene nanoribbons with zigzag edges. The linearly dispersive behavior of the π and π* bands associated with the out-of-plane 3p_si and 2p_Be orbitals is clearly observed, showing the existence of a Dirac point slightly above the Fermi level. We also observed that spin-orbit coupling induces a gap opening at the Dirac point.

Designing the Backbone of Hexasilabenzene Derivatives with a High Unimolecular Kinetic Stability

It is an important subject to theoretically predict the kinetic stability of transient species. In this study, we have studied the kinetic stability of hexasilabenzene Si₆ H₆ and its derivatives, that is, decasilanaphthalene Si₁₀ H₈ and Li-substituted hexasilabenzene Si₆ Li₆ , theoretically by the artificial force induced reaction (AFIR) method combined with the rate constant matrix contraction (RCMC) method. Molecular design was further conducted to extend the unimolecular lifetime of hexasilabenzene derivatives. Although both Si₁₀ H₈ and Si₆ Li₆ were shown to possess shorter lifetimes than Si₆ H₆ , we found that the lifetimes of Si₆ Li₆ changed depending on arrangements of Li atoms around the monocyclic Si₆ backbone. Based on this knowledge, we found that a compound of an atomic composition Si₆ H₄ Li₂ with a planar, monocyclic Si₆ backbone has a relatively long unimolecular lifetime. Moreover, substitution of the two Li atoms by Na atoms further increased the lifetime.

Flat building blocks for flat silicene

Silicene is the silicon equivalent of graphene, which is composed of a honeycomb carbon structure with one atom thickness and has attractive characteristics of a perfect two-dimensional π-conjugated sheet. However, unlike flat and highly stable graphene, silicene is relatively sticky and thus unstable due to its puckered or crinkled structure. Flatness is important for stability, and to obtain perfect π-conjugation, electron-donating atoms and molecules should not interact with the π electrons. The structural differences between silicene and graphene result from the differences in their building blocks, flat benzene and chair-form hexasilabenzene. It is crucial to design flat building blocks for silicene with no interactions between the electron donor and π-orbitals. Here, we report the successful design of such building blocks with the aid of density functional theory calculations. Our fundamental concept is to attach substituents that have sp-hybrid orbitals and act as electron donors in a manner that it does not interact with the π orbitals. The honeycomb silicon molecule with BeH at the edge designed according to our concept, clearly shows the same structural, charge distribution and molecular orbital characteristics as the corresponding carbon-based molecule.

Synthesis and structural characterization of RE7Zn21Tt2 (RE = La-Nd; Tt = Ge, Sn, and Pb): new structure type among the polar intermetallic phases

Reported are 11 new ternary phases with the general formula RE7Zn21Tt2 (RE = La-Nd; Tt = Ge, Sn, and Pb), synthesized from the respective elements by reactions at high temperature. Their structures, established on the basis of single-crystal and powder X-ray diffraction work, are shown to be a new structure type with the orthorhombic space group Pbam (No. 55, Pearson symbol oP60). This complex atomic arrangement features condensed polyhedra made up of Zn atoms, interspersed by Ge, Sn, or Pb atoms in trigonal-planar coordination. The structure bears resemblance with the La3Al11 and the LaRhSn2 structure types, which are compared and discussed. Temperature dependent dc magnetization measurements confirm RE(3+) ground states for all rare-earth elements, and the expected local-moment magnetism due to the partial filling of their 4f states for RE(3+) = Ce(3+), Pr(3+), and Nd(3+). Theoretical considerations of the electronic structure based on the tight-binding linear muffin-tin orbital (TB-LMTO-ASA) method are also presented: the calculations support the experimental observation of a small, but not negligible, homogeneity range in RE7Zn(21+x)Tt(2-x) (x < 0.5). The partial substitution of the tetrel atoms by the electron-poorer Zn appears to be an important attribute, leading to an optimal valence electron concentration and, thereby, to the overall electronic stability of the crystal structure of this family of polar intermetallics.