Theoretical Study Of Electronic And Related Properties,Weak Interlayer Interactions In Two-dimensional Materials

Represented by BN,graphene,transition metal dichalcogenides,MXene,phosphorene;CrI3 and MoSi2N4,two-dimensional(2D)materials harbor flourish distinct electronic,optical and mechanical properties,has attracted enormous research interests among scientists from all disciplines.They also show great commercial application prospects in nanotechnologies.At present,the founded two-dimensional family has covered nearly all the research fields,such as transition metal dichalcogenides and phosphorene are star blocks in photovoltaics;MXene/graphene are widely used in anodes;CrI3/VS2 are important foundation for spintronics researches;and SnTe is considered as excellent system for two-dimensional ferroelectrics.However,the investigation and application of many two-dimensional materials are impeded by deficiencies such as low carrier mobility and vulnerable of oxygen.Apart from intrinsic two-dimensional materials,the successful fabrication of two-dimensional van der Waals multilayers furnish new possibilities for observation of various interesting physical phenomena and novel nanoelectronics design.Two-dimensional van der Waals multilayers can be divided into van der Waals hetero-structures and multilayers.For van der Waals hetero-structure,one can combine multiple two-dimensional monolayers with distinct features together through weak van der Waals forces,which makes the new systems being multifunctional carriers with integrated various physical characters.While for van der Waals multilayer,the properties of the multilayer system vary impressively depend on the relative lattice stacking and twist,etc.Setting graphene multilayer as an example,emergent new physical can even lack in single-layer form,including magnetism,Chern insulator states,topological flat band and so on.Unfortunately,the hidden properties of multilayer composed of different two-dimensional blocks has not been clearly understood.To this end,the design and study of electronic related properties of stable new two-dimensional monolayers and multilayers,as well as clear analyzation from theoretical manner,will be of great important for guiding the propel of fundamental research and specific application of two-dimensional materialsTo solve various research limitations in experimental and theoretical studies,many fresh two-dimensional materials and van der Waals multilayers are designed.The electronic properties,photoexcitation dynamics,ferroic orders and other properties,as well as the physical mechanism and functionalities,are investigated systematically and expounded deeply.The dissertation is consists of six parts:the research fields and current stage of two-dimensional monolayers and multilayers are sketched out in chapter one;The theoretical basis and calculation method that are used are introduced briefly in chapter two;In chapter three,some new two-dimensional monolayers are proposed and the electronic properties as well as the related properties are investigated,huge potential of their applications in photovoltaics and Chern insulators are discussed.In chapter four,we show detail study of two-dimensional heterostructures for light power-electricity transduction.In chapter five,the design principles of van der Waals ferroelectrics using multilayer engineering are proposed,the coupling between electronic properties and other related properties are revealed systematically.In chapter six,we summarize conclusions of the whole research in this dissertation,and provide the outlook for the future of two-dimensional materials and their assembly.The main research themes and results included in this dissertation are listed below:(1)Inspired by recent experiments,we design and report new two-dimensional GroupⅤ-Ⅵ compounds Sb2Te2X(X=S,Se),which are dynamically and thermally stabile.We find that two-dimensional Sb2Te2S and Sb2Te2Se possess moderate band gaps,isotropic high electron mobility and remarkable optical absorption over the whole visible region.These results illustrate that the great useage of Sb2Te2X for photovoltaic and nanoelectronics devices,and boost the new possibilities in research of 2D semiconductors based on group V-VI elements.(2)We investigate the stability,magnetism and topological properties of asymmetrically functionalized Bi(111)film.The results indicate that Bi(111)film is high temperature quantum anomalous insulator.The FM Curie temperature is as high as 420 K,while the nontrivial bulk gap reaches up to 454 meV,rendering that the quantum anomalous halleffect can readily be observed at room-temperature or even high temperature.Further analysis of the gapless chiral edge states,Chern number(C=1)and the quantized quantum anomalous hall conductivity offers solid evidences of its nontrivial feature.This investigation provides a new opportunity to realize high-temperature quantum anomalous hall effect and fabricate energy-efficient spintronics operating at room temperature.(3)Based on the first-principles simulations,we predict that strain tuned g-C3N4/InSe and bilayer g-C3N4/InSe hetrostructures can significantly promote the photocatalytic activity of isolated g-C3N4.The results demonstrate that the proposed heterostructures possess direct bandgaps,tunable electronic properties,type-II band alignment,and efficient optical absorption.In addition,the photoinduced current of the composites is evaluated using the non-equilibrium Green’s function method under the framework of DFT(NEGF-DFT).we found the predicted photocurrent under the visible light radiation is up to 2.9 μA/mm2 in our proposed heterostructures,which is far superior to other g-C3N4-based composites.These findings suggest that InSe sheet is a promising candidate as a co-catalyst for g-C3N4 water splitting photocatalyst,which provide valuable hints for experimental design of the visible-light-driven high-efficient photocatalysts for water splitting.(4)We investigate a family of realistic two-dimensional tetragonal-structuredmaterials TiNX,X=F,Cl,Br.By virtue of their direct bandgap,high optical absorption,good photoresponsivity together with small effective masses and exciton binding energy,making them plaussible platform for donor and acceptor materials for solar cells.Importantly,we find that,if they are superimposed with each other to form type-Ⅱheterostructures.The energy conversion of TiNF/TiNBr,TiNCl/TiNBr and TiNF/TiNCl bilayer solar cells can be as high as～18%,19%and 22%,respectively.These results suggest these heterobilayer systems are great two-dimensional solar cell candidates with high conversion efficiency.(5)On the basis of TD-DFT,we discuss the charge transfer in Janus-MoSSe/WS2 van der Waals hetero-structures.Ultrafast charge separation is calculated,due to the large overlapping region between the electronic states.While the electron-hole recombination is two much slower than the charge separation,which arises from the initial and final transfer states are seperated to different materials.Moreover,using DFT-NEGF,we discussed the photoresponsivity of the van der Waals hetero-structures.Such atomistic study provides useful information and facilitates the applications of future nano-devices.(6)We report for the first time that out-of-plane ferroelectricity can exist in bilayer phosphorene,arsenene and antimonene elemental two-dimensional lattices,which can be synthesized with in experiments.We revealed that their sizable out-of-plane ferroelectricity are attributed to the stacking-induced charge redistribution.Suitable hole doping making bilayer phosphorene and arsenene to present a strong coupling between ferroelectricity and ferromagnetism.And reversible spin texture caused by ferroelectric switching can be realized in these three bilayers.This study thereby not only broaden the scope of two-dimensional ferroelectric materials,but also endow designing future multi-functional minimized devices.(7)We propose a new scheme for achieving two-dimensional intercorrelated ferroelectrics using van der Waals interaction,and apply this scheme to a vast family of two-dimensional van der Waals materials.We demonstrate that two-dimensional van der Waals multilayers—for example,BN,MoS2,InSe,CdS,PtSe2,TI2O,SnS2,Ti2CO2 etc.—can exhibit coupled in-plane and out-of-plane ferroelectricity,thus realizing large family of two-dimensional intercorrelated ferroelectrics.We also predict that such intercorrelated ferroelectrics could coupled with many distinct properties,for example,electrical full control of spin textures and of valley-contrasting physics.This study deepens the understaning toward intercorrelated ferroelectrics,and also point out a new direction for two-dimensional intercorrelated ferroelectric research.(8)We explore and propose a general paradigm to design two-dimensional ferroelectric topological insulators by sliding topological multilayers,taking trilayer Bi2Te3 as a model system,we show that in the van der Waals multilayer based two-dimensional topological insulators,the in-plane and out-of-plane ferroelectricity can be induced through a specific interlayer sliding,to enable the coexistence of ferroelectric and topological orders.The strong coupling of the order parameters renders the topological states sensitive to polarization flip,realizing non-volatile ferroelectric control of topological properties.The revealed design-guideline and ferroelectric-topological coupling not only are useful for the fundamental research of the coupled ferroelectric and topological physics in two-dimensional lattices,but also inspire novel applications in nanodevices.