Electronic And Optical Properties Of Two-dimensional Materials With Different Defects And Compositions

In the past few years,two-dimensional materials have captured unprecedented interests and extensive attention of researchers.The quantum effect and strong electronic interaction in two-dimensional materials make them have different electronic and optical properties from bulk materials,such as quantum spin Hall effect,superconductivity,exciton effect,etc.,which have broad application prospects in the fields of electronics,chemical industry,battery,energy,environment and other industries.However,the defect can not be avoided in the two-dimensional materials during experimental synthesis.Under the influence of different compositions and defects,the properties of these materials will have some distinctive changes.In this paper,several two-dimensional materials and their electronic and optical properties with different defects and compositions are studied.Based on density-functional theory,first-principles calculations are carried out to investigate the electronic structure and optical properties of these systems.The experimental synthesis is predicted and simulated.The application prospect of these two-dimensional materials is shown.The main contents are as follows:(1)The electronic and optical properties of two-dimensional H2-Ga(In)Bi systems are studied,including phonon spectrum,band structure and absorbance.The results show that hydrogenation give the system dynamic stability with enhanced bandgap.The electronic structure of the system undergoes band-inversion and spin-splitting effects under the influence of SOC effect.The optical properties of the system along the direction of AC and ZZ are basically the same.The light absorption is located in the visible and ultraviolet regions.The strongest absorption of visible light locate at 445 nm.(2)The influence of point defects on the electronic and optical properties of two-dimensional H2-Ga(In)Bi system is studied.VGa(In)vacancy and Ga(In)Bi antisite defect make the system metallic,while Ga(In)(?)Bi antisite defect increases the bandgap.The results of formation energy show that VGa(In),BiGa(In),Ga(In)(?)Bi and SW defects are more likely to occur in the system.SW-OP1 defect enhance the optical absorption effect in the visible region,while VGa(In),Ga(In)(?)Bi and SW defects enhance the optical absorption effect in the infrared region.(3)By designing the monolayer 4-8 defect structure,the electronic and optical properties of 1S-MX2 are studied.The conduction band and valence band of the system are in contact with each other at the ? point,showing a semi-metal characteristic,and the bandgap is opened under the effect of SOC.When the tensile strain is applied,the energy splitting effect of the conduction band will gradually disappear;when the compressive strain is applied,the bandgap will gradually decrease except for 1S-WTe2 system.Too large strain will lead to the disappearance of semi-metal characteristic and the topological phase transition of electronic structure.The light absorption of the system is located at the ultraviolet region,and it will produce a significant red shift with the increase of the period of chalcogen elements.(4)By introducing Janus structure,the influence of compositions on the electronic and optical properties of group VI TMDs was studied.Except 1S-WSTe and 1S-WSeTe,the structures of 1S-MXY systems have dynamic stability.The trend of formation energy shows that there is a potential barrier to replace light atom with heavy atom,otherwise it is not.Janus structure breaks the out-of-plane symmetry,which makes the valence band of the system with spin-splitting effect.This effect can be controlled by applying strain,it decreases under tensile strain and increases exponentially under compressive strain.The optical absorption coefficient of the system attenuates slightly in the visible and ultraviolet regions,and Janus structure makes the optical absorption coefficient decrease and there is no obvious absorption peak except in the infrared region.(5)The structure,electronic and magnetic properties of 1H phase Janus NbSSe monolayer were studied.The system is metallic and it is magnetic with a stable antiferromagnetic state in a 4×4 supercell.The transition of antiferromagnetic state to ferromagnetic state occurs in the system under tensile strain.Three CDW phases in a 3×3 supercell of Janus NbSSe system were found by random displacement and structural re-optimization.The results of electronic structure and density of states show that the CDW phases make the system appear a new electronic states peak near the Fermi level,and the electron may transmission as a whole in the form of CDW.The main innovations of this paper are as follows:(1)The formation of energy and electronic structure provides theoretical support for the experimental growth of H2-Ga(In)Bi materials.The bandgap of the system is enhanced by introducing point defects,which ensures that the materials have the conditions of quantum spin Hall effect at room temperature.(2)Due to the large bandgap,semi-metal characteristic and spin-splitting effects,group VI TMDs with 1S phase have good application prospects in spintronics devices and topological insulators.(3)Because of the existence of CDW phase and the characteristics of its magnetism suppression,Janus NbSSe is expected to become a new type of superconducting material,which has great potential in the fields of magnetic controlled pressure sensors and superconductors.