Engineering The Electronic Structures Of The Novel Two-dimensional Layered Materials From First-principles Calculations

With the stripping of laboratory layered graphene,the theoretical prediction that the two-dimensional structure cannot be stably existed due to the thermodynamic fluctuation effect is broken.Two-dimensional layered materials have entered people's sights with their remarkable thermal,mechanical and electrical properties,and have become a hot spot in scientific research today.Although at present,there are still long distance for the applications in the industry,with the improvement of experimental technique,people are full of confidence for the future application to industrial life.Especially for graphene,ultra-high carrier mobility,superb flexibility,and superhard strength show a wide range of applications.But the electronic structure of the zero-band gap limits its application,so people are looking for ways to open the graphene band gap or to discover new two-dimensional materials instead of graphene.In the course of the research,people also found quite interesting properties in the bulk form of the corresponding two-dimensional materials.Therefore,in this paper,we use density functional theory and quasi-particle approximation to study the recently emerged two-dimensional layered materials with very interesting properties,including:1.Engineering the electronic structure of low-temperature phase of SnSe in IV-VI compounds to improve its thermoelectric properties;2.A complex strain combination is applied to the layered tin oxide(SnO)to modulate the electronic structure and compared the results with that under high pressure;3.Highly ordered nitrogen doping graphene C3N was systematically studied and four different stacking of bi-ayer and bulk phases are also investigated.The full thesis is divided into six chapters,the main contents are as follows:In chapter one,we simply introduce the important meaning of the research on two dimensional layered materials,the basic features of the layered compounds comprised with IV-VI elements,the effect on the electronic structures with nitrogen doping in graphene,and the important role of strain played in the engineering the crystal and electronic structures.In the meantime,we briefly clarify the background and motivation of the thsisi,and the mainly content and object in our research.In chapter two,the first principle methods used in our theoretical simulation are mainly discussed,including the density functional theory and quasiparticle methods.In chapter three,based on the density functional theory and quasiparticle calculations,the engineering of electronic structure for SnSe are studied.The discovery of the unprece-dented figure of merit ZT of SnSe has sparked a large number of studies on the fundamental physics of this material and further improvement through guided materials design and opti-mization.Motivated by its rich chemical bonding characters,unusual multi-valley electronic structure,and the sensitivity of the band edge states to lattice strains,we carry out accurate quasiparticle calculations for the low temperature phase SnSe under strains.We illustrate how the band edge states can be engineered by lattice strains,including the size and the na-ture of the band gap,the positions of the band extrema in the Brillouin zone,and the control of the number of electron and/or hole valleys.The distinct atomic origin and orientation of the wave functions of the different band edge states dictates the relative shift in their band energy,enabling active control of the near-edge electronic structure of this material.Our work demonstrates that strain engineering is a promising way to manipulate the low-energy electronic structure of SnSe,which can have profound influences on the optical and transport properties of this material.In chapter four,the electronic structure engineering are systematically investigated forthe layered SnO in IV-VI compounds.Tunable band gap of semiconductors has attracted great scientific attention due to the different application requirement in electronic devices.Here we systematically investigate the strain effects on the structural and electronic properties of the bulk ?-SnO(litharge type)by using density functional theory and quasi-particle approximation.SnO exhibits an indirect quasi-particle band gap of 0.75 eV,which is ingreat agreement with the experimental band gap result.The size of the band gap is very sensitive to the strain along out-of-plane direction,and even closes when a combined in.plane strain is applied,inducing the semiconductor-metal transition.We reveal that both of the inter-layer Sn-Sn interaction and intra-layer Sn-O interaction play very important role on the band gap evolution under strains.Our results enrich the methods of tuning the electronic structures of materials and widen the application of SnO on various area.In chapter five,layer-dependent quasiparticle band structures of the newly emerged honeycomb C3N are systematically studied using both density functional theory and GW methods.The calculated GW band gap for monolayer C3N is about 1.5 eV.This moderate band gap may be ideal for future electronics applications.Our result is in marked contrast with a recent experimental report of 0.39 eV and calls for future experimental verifications.Interlayer chemical coupling effects on the electronic structure of C3N are investigated using several bilayer models.The electronic structure of bilayer C3N depends sensitively on the layer stacking pattern with the calculated quasiparticle band gap ranging from 0.87 to 1.35 eV.Finally,we illustrate the effects of interlayer chemical interaction and bulk dielectric screening on the electronic properties of C3N.Depending on the specific bulk stacking,C3N may be metallic or semiconducting with a narrow gap of about 0.6 eV,even though different bulk phases are essentially degenerate energetically.As a result,it may be challenging to prepare single-phase semiconducting bulk C3N unless synthetic kinetics can somehow prefer or prohibit certain stacking patterns.This issue deserves further investigations.