| In recent years, carbon and silicon materials play an important role in the high-tech industries and condensed physics. Researchers have been studying on the novel types of carbon and silicon or other advanced materials. In particular, the semiconductivity of sil-icon and low-dimensional carbon are hot spots of materials science and physics research. We are interested in how to make them can get a wide range of applications such as molec-ular sieves, electro-optical devices, new type solar cells, nanoelectronics, spintronics de-vices and aerospace materials. In this paper, based on first-principles calculations and through micro design, we are predicted the allotropes of silicon, carbon, boron and iron sulfide including the case of three-dimension or two-dimension. Their properties, such as band structure, density of states segment, and optical absorption spectra were simulated, their characteristics and sources were discussed also. This paper is divided into six chap-ters; In the first chapter, we introduces the results of research and some properties of the allotropes of pure carbon and silicon from other researchers; In second chapter, the Den-sity functional theory, Born-Oppenheimer approximation, Hartree-Fock approximation, Kohn-Sham equation and some histories were presented; In the third chapter, we predict a novel silicon crystal, which have a new stable structure, is a direct band gap semicon-ductor; In the fourth chapter, we predicted a two-dimensional structure, the pentagon and octagon composed of graphene, is stable. its density is higher than common hexagonal graphene and the Dirac-like cone exist in the bands structure, the Dirac point is3.6eV higher than Fermi energy; In Chapter5, we predicted three kinds two-dimensional pentag-onal crystal:boron nitride and carbon compose a kind of convex surface and iron sulfide compose a pure flat, the pentagonal iron sulfide crystal has antiferromagnetic structure and d-electron spin polarized Dirac cone etc.; In the last chapter, we have made a sum-mary and outlook.The main results are as follows:1. On basis of the first principle calculation we show that a crystalline structure of silicon, as a novel allotrope is stable. The calculations reveal that this allotrope possesses a direct band gap wider by0.5eV than the indirect one of silicon with diamond structure. The crystal belongs to141/AMD space group, showing anisotropic optical properties and Young modulus. The bulk modulus is64.4GPa and the density is1.9g/cm3, lower than that of the diamond silicon. It is hopeful that the allotrope may widely expand applications of silicon in many fields due to its direct band gap and specific nanotubular structure.2. We report a possible stable structure of graphene. This possible two-dimensional (2D) structure consists of pentagons and octagons (PO), and likely be formed from or-dinary graphene by periodically inserting specific defects. Its density is2.78Atom/A2and the cohesive energy per atom is-8.96eV, slightly higher than that of graphene. The calculation indicates that PO-graphene behaves like a2D anisotropic metal. The disper-sion relation of electrons near the Fermi surface shows a significant flat segment along a direction and linear behavior in different regions of the Brillouin zone. If the growth of samples is successful, the PO-graphene not only be used as anisotropy conductor and other practical application, but also can be served as a good sample for experiments which need2D anisotropic materials.3. we show that the two-dimensional pentagonal (pt) structures, the compositions of pt-BN2, pt-C, and pt-Fe2S, are stable. As a common feature, they are composed of3com-ponents:2stretched honeycomb sublattices and1square sublattice, conferring flexibility of tailoring the properties peculiar to the graphene. Although the Dirac dispersion relation is removed in metallic pt-BN2and insulating pt-C due to the hybridization of two honey-comb sublattices, it survives in pt-Fe2S because of the suppression of such hybridization between different spins. As a result, in the dispersion relation of pt-Fe2S spin-polarized and anisotropic Dirac cones occur. We suggest that such type of dispersion relation could be used to produce spin-filter effect by applying electric bias in a specific direction.Based on first-principles calculations, we predicted the allotropes of Silicon, Car-bon, Boron Nitride and Iron Sulfide, and discussed and analyzed their structure and char-acteristics. These new materials materials are unique. For example, ntc-Si is a direct bandgap semiconductor, may be used in the solar cell; PO-graphene is a two-dimensional anisotropic metal, may be used as an anisotropic conductor, and the dispersion relation of pt-Fe2S may become a spin filter control through the gate voltage. They have different characteristics, so whether the study of experiments or theories are worth exploring. |
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