Theoretical Studies And Structural Design On Novel Two-dimensional Photo-absorbing And Magnetic Semiconductors

Two-dimensional（2D）materials,which have only one or several layers of atoms,have received extensive attention due to their unique and outstanding properties since the successful experimental exfoliation of graphene in 2004.However,up to now,the understanding of two-dimensional materials is not yet mature.Numbers of potential physical phenomena in two-dimensional systems are not found.In addition,the preparation of new 2D materials is still not perfect,and only a few 2D materials can be prepared and synthesized experimentally.Benefiting from the development of computational materials science,we can explain,predict and tune the properties of materials and statistical mechanics,and design new and more desirable materials utilizing first-principles calculations and statistic mechanics.At the time,the first principle calculation has become an important instrument in the field of material science.In this thesis,we apply density functional theory to the investigation of catalytic reactions,magnetism,involving the study of electronic structure,lattice vibrations,optics,and magnetism of the materials,based on the methods such as exfoliation,atomic substitution to design two-dimensional layered nano semiconductor crystalline materials with different functionalities.The main contents of this thesis are shown as follows.1.We systematically designed and studied the structural and optoelectronic properties of two-dimensional silicon carbide semiconductor crystal materials.It is known that graphene and silicene are both Dirac semimetallic materials with no bandgap.While their intermediate state,2D Si C,is a large bandgap semiconductor,which makes the application of 2D Si C in optoelectronic devices possible.Based on the particle swarm optimization,four two-dimensional Si C₇ structures are predicted,namely,α-,β-,γ-and g-Si C₇.,g-Si C₇ is theoretical work that has been previously reported.However,Calculations reveal that g-Si C₇ processes the highest energy compared to the other three structures and is not the ground-state structure.Lattice dynamics analysis shows that the structure with the lowest energy,γ-Si C₇has large imaginary frequencies in the phonon spectrum,indicating structural instability.Lattice dynamics and molecular dynamics calculations show thatα-andβ-Si C₇ are stable structures.Electronic structure calculations show thatα-Si C₇ exhibits metallic properties,The main reason isα-Si C₇ consists of pentagonal,hexagonal,and octagon shapes.According to the crystal field theory,the inhomogeneous distribution of the compound will broaden the band of the crystal,which leads the electron and hole pockets in the band structure,and resulting in the metallic properties macroscopically.On the other hand,β-Si C₇processes a direct bandgap of 1.01 e V,whose hole mobility can reach～10 000 cm² V^-1 s^-1,moreover,β-Si C₇/g-Si C₇ hetero-bilayers can achieve a maximum power conversion efficiency as high as 20.7%,showing great potential for applications in nanoelectronic devices and optical devices.2.Based on the understanding of g-C3N4 and its analogs polymerized heptazinimide（PHI）,we designed a new series of two-dimensional boron chalcogenides,B₂X₃（X=S,Se,Te）,and explained the intrinsic mechanism of visible light absorption using transition dipole moment analysis and band symmetry.The structural stability,electronic structure,and optical properties are systematically investigated by the first principle calculation.The calculation shows that B₂Te₃ and B₂Se₃ monolayers have direct band gaps of～1.7 and～2.5 e V respectively,which makes them suitable for photovoltaic materials and photocatalysts under visible light conditions.Moreover,the valence band maximum of B₂Se₃ and the conduction band minimum of B₂Te₃ coincide with the redox potentials of the water,which demonstrates that B₂Te₃ and B₂Se₃ monolayers are suitable for hydrogen and oxygen evolution reaction,respectively.3.We simulate exfoliated the two-dimensional Al₂Te₅ structure from the van der Waals layered material Al₂Te₅ and studied its geometry,stability,mechanical,electrical and photovoltaic properties.The results show that the monolayered Al₂Te₅crystal possesses an indirect band gap of 1.91 e V（HSE06）and ultrahigh visible light harvesting with an absorption coefficient up to 10⁸cm^-1in the spectral range of visible light that from～380 to 800 nm.Besides,the indirect band gap can be alternated into a direct one under both compressive and tensile biaxial strain,which will further enhance the light-absorbing properties.To this end,the outstanding performance enables the Al2Te5 monolayer to be promising for nanoscale electronic and photovoltaic applications.4.Based on our previous research,we propose that the semiconducting ferromagnetism of 2D Mn S₂ can be significantly enhanced with Curie temperature（T_C）improved higher than room temperature,by switching the Mn atoms with Re atoms and forming an isovalent alloying Mn_xRe_1-xS₂ system.It was found that the ferromagnetic superexchange in magnetic semiconducting materials can be effectively increased by the method of alloying.By sequentially calculating the Mn_xRe_1-xS₂ system,the ferromagnetic properties of the system at different concentrations of Re ions and the effect of atomic ordering on the magnetic exchange were studied in detail.In the system,Mn₂Re S₆ was found to have the highest ferromagnetic stability,whose T_C reaches 360K without any strain,which substantially increases the Curie temperature of the Mn S₂.Monolayer,when the tensile strain is applied,its T_C will reach～500K.This approach greatly enhances the application potential of intrinsic magnetic semiconductors and provides a theoretical basis for the design of future magnetic semiconductors.Our studies,on the one hand,enrich the research of new two-dimensional semiconductor materials;on the other hand,provide new ideas and theoretical support for the design and synthesis of materials with these excellent properties.