First-principles Investigation Of Exciton And Related Issues In Some Two Dimensional Materials

The civilization of human society is strongly related to the material industry.With progress in science and technology,more and more advanced materials are made and applied to all aspects of production and daily life.Among these new materials,two-dimenional?2D?materials are able to not only reduce the spatial scale,but also improve the performance of the devices.Starting from graphene,the family of 2D materials are getting bigger and bigger.As two typical examples,black phosphorus?BP?and MoS2 overcome the drawback of gapless in graphene,and still have extraordinary mechanics properties,high carrier mobility and strongly optical absorption of near-infrared or vis-ible light.Stacking two different 2D materials along the vertical direction,two layers are bound by van del Waals interaction,and forms a so-called van del Waals hetero-junction?vdWH?.Up to now,multiple 2D vdWHs are prepared and designed,in order to improve the performance of an individual 2D material,which makes 2D materials easier to be tuned.In addition,new function can also be realized by flexibly construct-ing different vdW heterostructures.Therefore,the investigations of basic theory and practical application of 2D materials become one of the hottest recearch fields.Among the numbers of research aspects of 2D materials,one of the main categories is the interaction between light and 2D semiconductors.It is strongly related to the topics of optoelectronics,photocatalysis and other applications.Accordingly,the many-body effects of light-matter interactions in 2D materials draw much attention.One of the most significant effect is that the electronic coulomb screening is dramatically reduced in 2D systems.This leads to the non-negligible influence on the light-matter interaction,by the leading actor in this thesis:the excitons.In fact,the topic of excitons is not new,which already exists with the rise of condensed matter physics.The concept of excitons was proposed at 1960s.They become more and more attractive with the tide of 2D materials and development of experimental tools.The excitons not only play a critical role in optical absorption,but also have deeply influence on the motion of photo-generated carriers.The theoretical investigation of excitons in 2D materials is of important meaning,which offer better understanding of the experiments and forward guidance for design of new materials.The theoretical study of excitons is the same as a lot of condensed matter sys-tems,which needs a description on the level of atomic scale.The developments of computer industry and computational methods enable us to realize the first-principles computational methods,which directly solves the Schrodinger equation with hamilto-nian of atomic systems.Nowadays,the properties of materials can be well understood and predicted by using quantum chemistry methods and density functional theory.By using first-principles calculations,not only the cost of experimental measurement can be reduced,but also more accurate results can be obtained compared to the theoretical model.At the same time,more reliable methods are developed for the excitation prop-erties of practical systems.In the present thesis,the first-principles many-body green's function methods are applied to study the excitonic effect in 2D materials.We mainly focus on the controlling of excitons in 2D BP and the understanding of photocatalysis in 2D carbon nitrides.The dissertation includes four chapters.In Chapter 1,we briefly introduce the ba-sic theory for first-principles calculations.The first-principles calculations are origined from the ab initio quantum chemistry methods,which based on the wave functions.This is a direct way to solve the many-body Schrodinger equation in the scheme of self-consistent interations.Limited by the huge computational cost for periodic systems,quantum chemistry methods are replaced by the density functional theory?DFT?meth-ods.By applying DFT calculations,we are able to obtain a reasonable description of electronic structure of ground state,mechanics properties and some properties of ex-ctied state for a certain system.Although DFT have achieved remarkable success and it is still moving forward,its incorrect result can not be resolved fundamentally when dealing with many excited systems.The most famous shortages of DFT are the seri-ous underestimation of electronic band gaps,and the helplessness in determination of many-body interactions in excitation process.In order to overcome the problem,the framwork of many-body green's functional?GW method?plus Bethe-Salpeter equation?BSE?was rapidly developed in the past thirty years.The GW method correctly works out the electronic band structures in solids,and the many-body interactions in excitation process are taken into account by the BSE.So far,GW+BSE method has become the state-of-art for calculating electronic structures and optical properties in periodic sys-tems.The framework of GW+BSE is the main computational methology to investigate excitons in the present thesis.In Chapter 2,we make a brief introduction to the basic concepts and theory of excitons,as well as the recent progress of excitons in 2D materials.As the develop-ment of science and technology,systems made from low dimensional materials and their heterostructures are extensively investigated.They are regarded as one of the most potential candiates as the next generation optoelectronics.One of the most sig-nificantly differences between low dimensional systems and 3D materials are the re-markably reduced coulomb screening effect.As a result,the photo-excited the inter-actions between electrons and holes in 2D materials are much stronger than traditional 3D materials.The strong coulomb interactions between single electron and hole can be described by a quasiparticle model called the exciton.In traditional 3D materials,the excitons are classified as weak-binding excitons and tight-binding excitons.In com-mon 2D semiconductors,the excitons have both large binding energy like tight-binding excitons and large excitonic radii like weak-binding excitons.For examples,we intro-duce the progress of researches in excitonic effects in two typtical 2D materials,BP and MoS2.The excitons can be investigated via the photoluminescence spectrum or the ultrafast laser dynamics in experiment.On the other hand,the progress of model for excitons in 2D materials are also introduced.Recently,most investigations about excitons focus on the topics such as native properties,charged excitons,valley exci-tons,ultrafast charge transition and so on.The excitons can also be tuned by electric,mechanic,magnetic and optical approaches.The excitons in 2D materials are studied for applications in many fields.Except for optoelectronics and photocatalysis,they are also strongly related to the exciton LED,optical modulator,exciton laser and so on.In Chapter 3,we used first-principles GW+BSE theory to theoretically investigate the native excitonic properties in several 2D materials.As the state-of-art for calculating excited states,the GW+BSE calculations offer the information of excitons,which is in good agreement with the experiment.In the first section,we perform a GW+BSE study on four allotropes of 2D BP.Although different excitonic properties are found in these four,they all have excitonic absorptions in the range of visible light.Within the study of electonic structures and optical properties in these four 2D materials,we also introduce some technical details of GW+BSE calculations.In the second section of this chapter,we investigated the excitonic effects in layered C2N,as well as water adsorbed monolayer.The electronic band gap of C2N was determined to be ranging from 3.75 to 1.89 eV from monolayer to bulk.The increasing number of layers makes the coulomb screening effect become stronger.As a result,we found large binding energies of greater than 0.6 eV for bound excitons in few-layer C2N,while it is only 0.04 eV in bulk C2N.Strong excitonic effects play a crucial role in optical properties for few layered C2N.All the structures exhibit strong and broad optical absorption in the visible light region.In Chapter 4,we show our investigation of excitons and related topics in photo-catalysis systems.The exciton plays a crucial role in two-dimensional materials in-volved photocatalytic water splitting,whose properties are determined not only by the material itself,but also the surrounding water environment.By the framework of many-body green's function theory,we investigated the excitonic effects in pure and water ad-sorbed g-C₃N₄.It shows the excitonic properties are very sensitive to the geometry of g-C₃N₄ and the adsorption of water molecules.Firstly,the optical band gap,i.e.the first bright excitonic energy of pure g-C₃N₄ could decrease remarkably from high symmetry planar structure?3.8 eV?to P1 buckled configuration?2.7 eV?.Secondly,the hydrogen bonds between water and g-C₃N₄ induce the generation of interface excitons.With the reduced binding energy?at least 0.2 eV?,interface excitons could contribute to a more efficient separation of electrons and holes.For C2N monolayer with water adsorbed,the VBM of the whole system is mainly contributed by the O atom in water,which is different to g-C₃N₄.As a result,the first bright exciton is a typical interlayer exciton.Our work provides an insight into the excitation mechanism of 2D photocatalysts in a real environment.At last,we focus on electron-hole separation?EHS?,which is also important in photocatalysis.We designed the rippled supercells of g-C₃N₄,in order to enhance the EHS.In rippled structures,a built-in Type ? band-alignment heterojunc-tion is formed.This makes the VBM and CBM distribute separately in real space,which promotes the EHS.In addtion,we found the rippled structures are more stable than the flat one,which makes it easer to be prepared in experiment.In Chapter 5,we present our results about how the excitonic effects in BP sys-tems are controlled.Firstly,we briefly investigated the response property of excitons in monolayer BP under a homogeneous uniaxial strain.Our calculations show the quasi-particle band gap and optical gap?first bright excitonic energy?become greater under a compressive strain,and get smaller under a tensile strain.The binding energy of the first bright exciton is not significantly changed.However,neither tensile nor compres-sive homogeneous strain is able to tune the spatial distribution and motion of excitons.In order to achieve an effective controlling of excitonic migration,we then investigated the electronic structures and excitonic properties in bending BP nanoribbons under an inhomogeneous strain.Mutiple excitonic funnels are found in bending BP nanorib-bons.Our results show the excitonic spatial distribution and migration direction can be tuned with different effectiveness by several meanings,including the direction and intensity of the inhomogeneous strain,number of layers and the periodic direction of nanoribbons.In addition,the strong optical absorption is maintained in bending BP ribbons.Our work presents a promising future of flexible control of excitons,which can be applied to advanced solar cells,photodectation,photo-communication and pho-tonic computation devices.At last,we studied the excitonic effects of water adsorbed BP.We calculated different models to consider the thickness,coverage and configura-tions of water molecules on monolayer BP.The adsorption of water makes the band gap greater,and the optical absorption of excitons blue shifted.The binding energies of excitons are slightly reduced because of the weakened screening effect.