MODEL OF MOLYBDENUM TUNGSTEN DISULFIDE ALLOY STRUCTURE USING SPECIAL QUASIRANDOM STRUCTURE METHOD

Abstract

Substitutianal alloys are disorder crystalline structure. First-principles study of alloys is taken a large supercell that equivalent to a pure random structure. Structural models used in calculations of properties of substitutional random A1-xB​x alloys are usually constructed by randomly occupying each of the N sites of a periodic supercell by A or B. However, this method is not efficient. It is possible to design “Special Quasirandom Structures” (SQS) that simulate the small periodic supercell. It can be compared to structures that have large number of configurations or large cell sizes. The proposed method optimizes the supercell with the occupation of the atomic sites (A or B). This technique uses the language of Ising models to define the product of spin variable for each atomic site. Then calculate a lattice average to construct special periodic quasirandom structures. The SQS can be used in the calculation of optical and thermodynamic properties. This thesis uses the SQS method integrated with Density Functional Theory (DFT) to investigate alloy Molybdenum Tungsten Disulfide (Mo1-xWxS2​) monolayer. We found that SQS with 27 atoms to model 3x3x1 supercell structure is suitable for calculation of energy and band structure. Then the SQS of Mo1-x​Wx​S2 are studied by DFT calculations to evaluate band structures. We found tunability of band edges and band gaps by alteration of the W concentration. The results of the electronic structure studied by combination of DFT and SQS are in good agreement with experiment and Monte Carlo simulations from other works. We conclude that the model of alloy generated by SQS can be used to studies crystal structure behavior. This model uses low computational time but can achieve good result which agree well with the experiment report.