| Janus MoSSe is a two-dimensional material with a Se-Mo-S arrangement structure formed by replacing the S atoms on one side of the monolayer MoS2 with Se.Its unique optoelectronic properties have attracted widespread attention in the field of semiconductor devices.In recent years,some researchers have proposed the liquid phase exfoliation method to synthesize monolayer MoS2-xSex with different concentration ratios.The study found that the material has unique optical modulation and saturable absorption characteristics,and has been applied to solid-state lasers and other directions.In the past decade,research on monolayer MoS2-xSex has been mainly based on experiments,which have shown that many properties of the material are regulated by the concentration of atomic components.However,due to the difficulty in controlling the concentration ratio of S and Se in the experiment,and the difficulty and high cost of experimental synthesis,people do not have a consistent and in-depth understanding of the relationship between the concentration of material components and properties.Therefore,theoretical research on such materials is very urgent.This topic mainly studies the supercell construction method of such alloy materials,so as to construct monolayer Janus MoS2-xSex alloys with different component concentrations,and use first-principles methods to calculate and analyze the photonic and electronic properties of materials with different concentrations.The content of the thesis is divided into the following three parts.The first part studies the supercell generation theory and algorithm realization of alloys.Firstly,based on the exhaustive method,a comprehensive search is performed in all possible configurations,and all configurations are processed based on the occupancy probability;secondly,the obtained configurations are subjected to symmetry merge processing;finally,based on a special quasi-random structure（SQS）,calculate the interatomic correlation coefficient,and screen out the configuration that satisfies the SQS standard correlation function.The algorithm flow mainly includes:inputting the original cell structure,copying the original cell to generate the supercell,occupancy correction,merge symmetry processing,structural analysis of the configuration,and storing the output structure.In conclusion,a scheme to generate alloy supercells with different component concentrations is realized in this paper.In the second part,the monolayer Janus MoS2-xSex alloys with x=0.25,0.5,0.75,1.25,1.5,and 1.75 were constructed by using the supercell generation algorithm in the previous part.The specific method is to replace 75%,50%,and 25%of the atoms on the side of the monolayer Janus MoSSe Se atoms with S,and replace 25%,50%,and 75%of the atoms on the side of the S atom with Se.In the third part,the optoelectronic properties of materials with different component concentrations are studied through first-principles calculations.Studies have shown that properties such as energy band structure,density of states,and light absorption can be tuned as the concentration of material components changes.The detailed research results are as follows,with the reduction of the atomic ratio of S and Se:1:The band gap of the material decreases accordingly.The monolayer Janus MoS0.5Se1.5and MoS0.25Se1.75 are indirect band gap semiconductors,and the rest are direct band gap semiconductors;The density of states at the bottom of the conduction band and the top of the valence band is mainly determined by the Mo-4d state,in which the contribution of the Se-3p state gradually increases,while the contribution of the S-3p state gradually decreases.2:The effective mass of the carrier shows a W-shaped change trend,and the effective mass determines the carrier mobility,indicating that the carrier mobility can be adjusted;3:The absorption bandwidth of the material for visible light increases,and the absorption capacity is enhanced,especially for Janus MoS0.25Se1.75,the absorption curve in the visible light range basically shows an upward trend,and the maximum value can be reached4.4×105 cm-1 at 414nm;The reflectivity and energy loss of such materials to visible light are always small. |
