Excited-State Properties Of Two-Dimensional Materials From Green’s Function And Density Functional Theory

The successful realization of graphene has aroused the enthusiasm of researchers for the exploration and research on emergent two-dimensional(2D)materials.With the continuous development and advancement in theory and experiment,many new 2D materials have been synthesized and studied.The applications of 2D materials in electronics,optoelectronics,and photocatalysis have also become the focus of research.The optical properties of 2D materials exhibit unique phenomena and characteristics that do not exist in conventional bulk semiconductors,such as enhanced light-matter coupling caused by two-particle excited states.Therefore,2D materials are ideal platform for exploring excited states and light-matter interactions.Actually,the interesting light-matter interactions in 2D materials have always been the spotlight of optical investigations.On the other hand,in order to achieve sustainable development,noval light-energy conversion systems based on 2D materials have been designed and proposed to improve the solar energy utilization through photoelectric conversion,such as photocatalytic and photovoltaic systems that convert sunlight into chemical energy.Generally,the prerequisite for light-energy conversion system design is to understand the behavior of light-induced charge carriers,such as optical absorption that involves a two-particle progress.In the light of quantum confinement effect and less efficient electronic screening in 2D structures,the enhanced Coulomb interaction causes the photoexcited electrons in the conduction band and the holes in the valence band to form bound electron-hole pairs,i.e.,excitons.The excitonic effects play a crucial role in the optical and charge transport properties of 2D materials,which should be correctly considered for the photoelectronic applications of 2D materials.However,current first-principles calculations based on density functional theory usually fail to give correct results when describing the properties of excited states.It requires an efficient two-body method that goes beyond the single-particle picture while investigating the optical excitations,and the corresponding spectra are significantly influenced by electronhole Coulomb interactions.Consequently,the Green’s function method is a highly available strategy including the many-body effects to obtain the quasiparticle properties and optical absorption properties of 2D materials.Correctly understanding the excited-state properties of 2D systems is of great significance for electronic excitation and related basic fundamental scientific research,and lays a theoretical foundation for the potential applications.In this thesis,we apply the many-body Green’s function method combined with the firstprinciples calculations to explore the excited-state properties of 2D materials.The thesis is mainly divided into five chapters:Chapter 1 briefly introduces the advances in 2D materials and excited-state properties of 2D materials;Chapter 2 summarizes the theoretical background and related calculation softwares used in this thesis;Chapter 3 studies the excited-state properties of CuInP2S6 monolayer and reveals the factors affecting photocatalytic behavior;Chapter 4 investigates the characteristic excitonic absorption of MoSi2N4 and WSi2N4 monolayers;Chapter 5 summarizes the content of this thesis and provides the outlook for further research on excited-state properties of low-dimensional materials.The main research contents and conclusions of this thesis are as follows:(1)2D materials with vertical ferroelectricity have great potential in solar energy conversion processes such as photocatalytic water splitting,though the optical properties of such materials are rarely discussed.In the present thesis,CuInP2S6 of room-temperature ferroelectricity is considered as a prototype.Combined with the first-principles calculations,we apply the many-body Green’s function method to obtain the excited-state properties of CuInP2S6 to unravel the factors affecting the photocatalytic behavior.According to GW+BSE calculation results,large quasiparticle correction(1.25/1.38 eV)and excitons with large binding energy(0.93/0.87 eV)are obtained for PE(paraelectric)/FE(ferroelectric)CuInP2S6.In addition to promoted charge carrier recombination,we propose that large exciton binding reduces the reduction potential of photoexcited electrons.After considering the exciton binding energy,the hydrogen evolution reaction probably cannot occur on FE CuInP2S6 monolayer.As for bilayer structures of 2D CuInP2S6,the improvement of photocatalytic performance should be attributed to the type-Ⅱ band arrangement and large band edge offsets(0.44 and 0.33 eV),rather than the increased light absorption due to the reduced band gap.(2)As new members to the 2D material family,MoSi2N4 and WSi2N4 monolayers exhibit unique physical properties.However,their optical properties have not been discussed yet,especially in the consideration of spin-orbit coupling(SOC).Based on the many-body perturbation theory,we study the excited-state properties of MoSi2N4 and WSi2N4 monolayers and find that the quasiparticle correction leads to large band gap renormalization(more than 1 eV).The characteristic A and B excitons at K valley owing to SOC characterize their optical absorption with large exciton binding energies of about 1 eV.Interestingly,MoSi2N4 shows more abundant excitons(A’,B’ and C excitons),turning out to be a promising candidate to explore intra-and inter-exciton transitions.The exciton wave function indicates that low-energy excitons in MoSi2N4 and WSi2N4 monolayers are confined in the middle MoN2/WN2 layer,which is unfavorable for excitonic photocatalysis.On the other hand,the valley states based on excitons in MoSi2N4 and WSi2N4 monolayers can be protected from the environment by the SiN layers from both sides.