Study And Manipulation Of Electronic And Excited State Properties In Novel Transition Metal Compounds

Since the appearance of the first-generation semiconductor represented by silicon-based semiconductor in the 1950s,electronic devices become more and more integrated.While with downscaling of device dimensions,traditional silicon-based semiconducting materials are reaching the limits of physics and technology.The successful exfoliation of graphene makes two-dimensional(2D)materials come into people's sight.Subsequently,a considerable number of novel 2D materials have been found and synthesized in recent years,such as silicene,h-BN,transition metal dichalcogenides(TMDs)and black phosphorus.Even the 2D ferromagnetic semiconductors were discovered one after another,for example Cr2Ge2Te6,CrX3(X=I and Br)and VSe2,which cannot exist in theory.Due to the excellent physical and chemical properties resulting from the quantum confinement effect,2D materials show their own advantages in energy storage,environmental protection and optoelectronics devices compared to the bulk counterparts.Apart from studying the intrinsic properties,how to improve and manipulate the physical and chemical properties of 2D materials are worth of studing.Compared to monolayer structure,the interlayer interaction in bilayer structure can effectively alter its properties.By changing the stacking order,interlayer interaction can be easily modified.In addition,electrons in materials tend to be in excited states due to light irradiation,which has significant effect on the optoelectronic properties.By considering the electron-hole interaction and nonadiabatic molecular dynamics in excited states,we can gain deeper understanding of the nature and dynamics of excited states in 2D materials.During the processes of preparation and preservation,the defects and strain can be generated,which could affact the electronic and excitonic properties of 2D materials.Meanwile,by the artificial control of external conditions,the physical properties can be precisely manipulated in 2D materials,which provide theoretical foundation in design of novel optoelectronic devices.In this thesis,we investigate the electronic,magnetic,and optical properties of 2D materials through first-principles calculations based on the density functional theory and manipulate these properties by interlayer coupling and external stress.Furthermore,combining many body perturbation theory and nonadiabatic molecular dynamics,we explore the excition properties and dynamics of excited states in 2D materials.The thesis is divided into six chapters.The first chapter briefly describes the research status and development trends of 2D materials.The second chapter gives a simple introduction of theoretical and computational methodologies used in this paper.The third chapter systematically investigates the effect of build-in electric field on their excited states properties in heterostructure or homobilayer.The fourth chapter studies the electronic and excitonic properties in transition metal compunds(TMCs)tuned by defects and non-uniform stress.The fifth chapter explores some novel 2D TMCs as well as their electronic and magnetic properties.In the sixth chapter,we summarize the conclusion and innovations and give an outlook for the future development of 2D materials.The main content and results are listed as follows:(1)We studied the effect of existence and direction of build-in electric field on the photo-excited electron-hole pairs in WSe2-MoSe2 and WSe2-MoSSe heterostructures.For WSe2-MoSe2 heterostructure,the first excited state is dominated by interlayer exciton,while the first excited state is contributed by intralayer exciton in Se-Se interface WSe2-MoSSe heterostructure.However,there is a bright-to-dark transition occurs in the first excited state when changing the interface from Se-Se to S-Se.Through building a theoretical model,we find that the strong interlayer coupling leads to a coherence cancellation in the first excited state and eventually results in the bright-to-dark exciton transition.This unique nature provides a possible approach to engineering optical properties in heterostructure for potential optoelectronic and photovoltaic applications.(2)We investigate the electronic and excitonic properties of Janus-MoSSe bilayer.Due to the break of inverse symmetry,Janus-MoSSe exhibits intrinsic electric field.Under the effect of build-in electric field,type-? band alignment appears in Janus-MoSSe bilayer which leads to the spatial separation of photo-excited electrons and holes as well as the suppression of interlayer coupling.The separation of electron-hole pairs and suppression of interlayer interaction is robust that the influence of stacking order is negligible.Results of nonadiabatic molecular dynamics simulation show that the recombination time of photo-excited carriers is extended due to the existence of build-in electric field.The predicted 16.5 ns recombination time is even longer than that of the MoS2/WS2 vdW heterostructure.Our work provides a new way to prolong the lifetime of excitons in homogeneous bilayer.(3)We realize the manipulation of excitons in MoS2 and MoSSe monolayer with non-uniform strain and build-in electirc field by morphology contol.The excitonic funnel effect caused by local stress results in the localization of photo-excited electron-hole pairs which are fixed in the region with maximum stress gradient.When build-in electric field appears at the same time,the first excitons of rippled MoSSe monolayer translate from bright to dark and photo-excited electrons and holes are separated spatially.Our findings provide a way to tailor the excitonic properties in 2D materials and promote their performance in optoelectronic and photovoltaic devices.(4)Due to the limitation of preparation technology,vairous kinds of defects exist in 2D materials.We investigate the effect of four common point defects(VS,VSe,VMoS3,VMoSe3)on the optical absorption and exciton effect in Janus-MoSSe monolayer by employing the first-principles GW-Bethe-Salpter equation method.Results show that mono-chalcogen vacancies will introduce three localized states near Fermi level.Compared with pristine Janus-MoSSe,the first bright exciton(X1)is localized at defect region with larger exciton binding energy.The distribution range of excitons is about one unit cell.While for VMoS3 and VMoSe3 defects,the band structures are significantly changed and the light absoption in low energy level is enhanced.The generated insights highlight the diverse behavior of different types of defects,reveal unexpected features,and provide useful theoretical guidance for experments.(5)We investigate the mechanical,electronic,optical and excitonic properties of pentagon lattice structure PdSe2 monolayer.Due to the unique lattice structure,PdSe2 monolayer shows anisotropic character and has smaller Young's modulus.In addition,the poisson 's ratio is negative,indicating PdSe2 monolayer is a potential auxetic material.Results show that PdSe2 monolayer has suitable band gap and optical absorption efficiency as well as anisotropic carrier mobility.In the meantime,the absolute band edge position of PdSe2 monolayer can meet the requirement of overall water splitting.By molecular dynamics simulation,the stability in aquatic environment is proved and the estimated solar to hydrogen efficiency is relatively high.The above conclusions show that PdSe2 monolayer is a potential photocatalyst for overall water splitting.Combining with nonadiabatic molecular dynamics simulation,a subsequential study of carrier recombination kinetics is carried out.Due to the low symmetry,photo-excited electrons and holes are spatial separated which leads to the extremely long recombination time.Our finding provides a brand-new candidate for photocatalytic overall water splitting.(6)We explore the effect of stacking order on magnetic properties in ?3 bilayer and propose a possible application as spin valve.?3 monolayer and bulk are semiconducor,while the corresponding bilayer is half-metal.Results show that through-bond spin polarization effect guarantees the intralayer ferromagnetic state,while super-superexchange effect ensures the interlayer ferromagnetic state.Based on the half-metallic and ferromagnetic properties,we design a spin valve using ?3 bilayer.By calculating the transmission coefficients for spin parallel and spin antiparallel configuration,the spin-injection efficiency of the ?3 bilayer spin valve is predicted to be 100%.