Structure Design And Property Prediction On The Graphene-like Materials And Nanotubes

Low-dimensional materials have aroused general interests in the field of condensed-matter physics and materials science.The studies on the preparation and performance of graphene and carbon nanotubes have contributed to the development of low-dimensional material.Graphene exhibits various of unique physical properties,which are determined by its electronic structure,namely,gapless semimetal with massless Dirac fermions being the charge carriers.The most remarkable features of graphene is the thinnest crystal known to date with a combination of extremely high stiffness with elasticity and an abnormally high thermal conductivity.The unique properties give rise to enormous scientific and technological impacts in a wide range of areas,such as physics,chemistry,material,and information sciences.Carbon nanotubes can be considered as one-dimensional structure made of crimped graphene.It exhibits regularity adjustable electrical,thermal and mechanical properties depending on the chirality,diameter,stacking ways,such as more extraordinary thermal conductivity than diamonds,superior mechanical properties,armchair nanotubes with regular change in the bandgap and so on.Carbon nanotubes are regarded as ideal materials for the future electronic components.Graphene applications are still subject to many restrictions.It has a zero band gap,but the uses in logic circuits and other electronic components require a finite bandgap at the Fimi level.Spin-orbit coupling?SOC?in graphene is particularly weak,and making the achievement at quantum spin hall effect?QSHE?quite difficult.And there is no spin polarization in graphene.Graphene is not the end of the road.Currently,a novel materials called graphene-like ones are being developed intensively.These graphene-like materials and nanotubes have some properties similar to those of graphene,for example,both silicene and germanene are gapless semiconductors.Some of the graphene-like structures are even superior to graphene in specific aspects,e.g.graphene-like carbon nitrides exhibit spin gap-less semiconductor?SGS?properties.With the research progress of graphene-like materials and nanotubes,they will have a wide range of applications in the future of electronic technology,machinery manufacturing,environment protection,spintronic devices and other aspects.In this dissertation,several graphene-like nanomaterials with novel properties are proposed and studied using the first-principles calculations within the density functional theory?DFT?.Silicon germanium?SiGe?honeycomb lattice is a gapless semimetal.Oxide graphitic carbon nitride?g-C₂NO?has spin gap-less semiconductor property.A superhard carbon allotrope?bct-C8?is constructed via cold compression of T-graphene.Nanotube bundles of carbon monoxide a new phase of solid carbon monoxide at low pressure.Two-dimensional graphene-like silica and nanotubes has potential applications in the field of biopharmaceuticals,catalyst carriers,etc.The study on the graphene-like materials is a cutting edge in physics and material science,which is helpful for revealing the mechanisms of their excellent performance as well as improving the photocatalytic efficiency.The first chapter describes the related theoretical and experimental background on the graphene-like materials and nanotubes.The second chapter will introduce the first-principles research method and the computational details of mechanical properties,A brief description for the software packages used in these works is also made.From third to sixth chapter,a detailed introduction to the main research work and research results during my Ph.D.degree studies woll be given.Chapter eight is the summary and innovation.The end is to express my thanks and list my publications.The main contents and conclusions in this dissertation are listed as follows:1)A novel two-dimensional Dirac material with a buckling honeycomb network of Si and Ge atoms is proposed.Compared with germanene,the incorporation of Si atoms improves not only the energetic stability but also the kinetic stability of the monolayer.Linear dispersion relations of the valence and conduction bands around the K and K' points characterized by Dirac cones are preserved in SiGe monolayer.Because the difference of the electronegativity between the silicon element and the germanium element is very small,the origin of dirac cones is contributed by the pz orbit of the two atoms,the spin orbit coupling effect is very weak.Different from graphene,hydrogen atoms have higher binding preference to Si atoms than to Ge atoms in SiGe monolayer,which facilitates the achievement of half-hydrogenation of the monolayer.The half-hydrogenated SiGe monolayer?HSiGe?is spin-polarized semiconductors with local magnetic moments locating at the Ge atoms and stable ferromagnetic ordering of these magnetic moments.Monte Carlo?MC?simulations within a two-dimensional Ising model indicate that the Curie temperature of the HSiGe is about 110 K.The Dirac fermion feature,energetic and kinetic advantages over germanene,and easily-achieved ferromagnetism of SiGe monolayer may hold great promise for fundamental physics studies and future electronics technology.2)A stable two-dimensional metal-free SGS?g-C₂NO?composing of C,N and O atoms is proposed.We demonstrated that it has a spin-polarized ground state with magnetic moments of 1.0?b in one primitive cell which prefers to interact in a ferromagnetic way.The electronic bands of one spin channel exhibits zero-band-gap semiconductors,while another spin channel has a band gap of 1.15 eV.The valence band and the conduction band nearest to the Fermi level have parabolic energy-momentum dispersion relations with different effective masses which can be applied to spin filter and spin injection.These results offer an effective approach in design of metal-free SGSs which are promising for applications in environmentally-friendly spintronic devices.3)New superhard crystalline carbon allotrope?bct-C8?is constructed via cold compression of T-graphene with high symmetry,141/amd?141?space group.The structural stability is verified by phonon mode analysis and MDS.Though there is eight carbon atms rings,the hardness of bct-C8 is 74.18Gpa,so it is superhard material.Electronic band structure calculations reveal that it is a semiconductor with an indirect band gap of about 4.74 eV.As a new carbon allotrope,these results could broaden our understanding of carbon allotropes and provide new ideas for the design of superhard materials.4)The low-dimensional structure of silica is prposed.And The two-dimensional graphene-like silica structure?2D-silica?is composed of hexagon Si-O rings connected by Si-O-Si bond in the interlayer.2D-silica is more stable than other silica nanostructure.Through the AB stacking of 2D-silica,a stable multi-layer structure can be achieved.When the radius of silica nanotubes is larger than 12 A,the energy required for rolling up a 2D-silica to silica nanotubes is comparable to that required for rolling up a graphene to carbon nantube.It is noteworthy that the 2D-silica has a well-ordered porous structure with pore diameter of about 5.23 A which is twice as large as that of graphene?-2.48 A?.Small molecules,such as H₂ and O₂ can permeate the silica nanosheets.Therefore,these porous silica nanosheets may serve as a molecular sieve and find potential applications in many fields such as separation and purification,biological materials and catalysis.In addition,we theoretically predicted a new low-pressure phase of carbon monoxide,nanotube bundles.Enthalpy analysis and phonon spectroscopy have demonstrated the stability of the new phase.This work provids a new phase of carbon monoxide,as well as a possible strategy for carbon monoxide.