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Register a new userKagome silicene: a novel exotic form of two-dimensional epitaxial silicon
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Since the discovery of graphene, intensive efforts have been made in search of novel two-dimensional (2D) materials. Decreasing the materials dimensionality to their ultimate thinness is a promising route to unveil new physical phenomena, and potentially improve the performance of devices. Among recent 2D materials, analogs of graphene, the group IV elements have attracted much attention for their unexpected and tunable physical properties. Depending on the growth conditions and substrates, several structures of silicene, germanene, and stanene can be formed. Here, we report the synthesis of a Kagome lattice of silicene on aluminum (111) substrates. We provide evidence of such an exotic 2D Si allotrope through scanning tunneling microscopy (STM) observations, high-resolution core-level (CL) and angle-resolved photoelectron spectroscopy (ARPES) measurements, along with Density Functional Theory calculations.
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Using a gold (111) surface as a substrate we have grown in situ by molecular beam epitaxy an atom-thin, ordered, two-dimensional multi-phase film. Its growth bears strong similarity with the formation of silicene layers on silver (111) templates. One of the phases, forming large domains, as observed in Scanning Tunneling Microscopy, shows a clear, nearly flat, honeycomb structure. Thanks to thorough synchrotron radiation core-level spectroscopy measurements and advanced Density Functional Theory calculations we can identify it to a $sqrt{3}$x$sqrt{3}$R(30{deg}) germanene layer in coincidence with a $sqrt{7}$x$sqrt{7}$R(19.1{deg}) Au(111) supercell, thence, presenting the first compelling evidence of the birth of a novel synthetic germanium-based cousin of graphene.
Isolated oxygen impurities and fully oxidized structures of four stable two-dimensional (2D) SiS structures are investigated by {em ab initio} density functional calculations. Binding energies of oxygen impurities for all the four 2D SiS structures are found larger than those for phosphorene, due to the lower electronegativity of Si atoms. The most stable configurations of isolated oxygen impurities for different 2D SiS structures are decided and the corresponding 2D structures with saturated oxidation (SiSO) are predicted. Among all the four fully oxidized structures, $alpha$-SiSO is demonstrated to be stable by phonon spectra calculations and molecular dynamics (MD) simulations. Electronic structure calculations indicate that $alpha$-SiSO monolayer is semiconducting with a direct band gap of ${approx}2.28$~eV, which can be effectively tuned by in-layer strain. The value of band gap and thermodynamic stability are found depending sensitively on the saturation level of oxygen.
Silicon dioxide or silica, normally existing in various bulk crystalline and amorphous forms, is recently found to possess a two-dimensional structure. In this work, we use ab initio calculation and evolutionary algorithm to unveil three new 2D silica structures whose themal, dynamical and mechanical stabilities are compared with many typical bulk silica. In particular, we find that all these three 2D silica have large in-plane negative Poissons ratios with the largest one being double of penta-graphene and three times of borophenes. The negative Poissons ratio originates from the interplay of lattice symmetry and Si-O tetrahedron symmetry. Slab silica is also an insulating 2D material, with the highest electronic band gap (> 7 eV) among reported 2D structures. These exotic 2D silica with in-plane negative Poissons ratios and widest band gaps are expected to have great potential applications in nanomechanics and nanoelectronics.
Ultrathin semiconductors present various novel electronic properties. The first experimental realized two-dimensional (2D) material is graphene. Searching 2D materials with heavy elements bring the attention to Si, Ge and Sn. 2D buckled Si-based silicene was realized by molecular beam epitaxy (MBE) growth1,2. Ge-based germanene was realized by mechanical exfoliation3. Sn-based stanene has its unique properties. Stanene and its derivatives can be 2D topological insulators (TI) with a very large band gap as proposed by first-principles calculations4, or can support enhanced thermoelectric performance5, topological superconductivity6 and the near-room-temperature quantum anomalous Hall (QAH) effect7. For the first time, in this work, we report a successful fabrication of 2D stanene by MBE. The atomic and electronic structures were determined by scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) in combination with first-principles calculations. This work will stimulate the experimental study and exploring the future application of stanene.
Recently, the concept of topological insulators has been generalized to topological semimetals, including three-dimensional (3D) Weyl semimetals, 3D Dirac semimetals, and 3D node-line semimetals. In particular, several compounds (e.g., certain three-dimensional graphene networks, Cu3PdN, Ca3P2) were discovered to be 3D node-line semimetals, in which the conduction and the valence bands cross at closed lines in the Brillouin zone. Except for the two-dimensional (2D) Dirac semimetal (e.g., in graphene), 2D topological semimetals are much less investigated. Here, we propose the new concept of a 2D node-line semimetal and suggest that this state could be realized in a new mixed lattice (we name it as HK lattice) composed by kagome and honeycomb lattices. We find that A3B2 (A is a group-IIB cation and B is a group-VA anion) compounds (such as Hg3As2) with the HK lattice are 2D node-line semimetals due to the band inversion between cation s orbital and anion pz orbital. In the presence of buckling or spin-orbit coupling, the 2D node-line semimetal state may turn into 2D Dirac semimetal state or 2D topological crystalline insulating state.
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