The Electronic Structure And Elastic Property Of Two Dimensional Transition Metal Dichalcogenides MX2(M=Mo,W;X=O,S,Se,Te):A First-principles Study

Graphene is the most thin and tough nano-material in the world. Its perfect property, such as exceptionally high electron mobility and unique combination of high modulus and tensile strength has make it to be one of most hot topic at the beginning of 21 century. While the zero-band-gap pristine graphene is not perfect for everything,for example, graphene Field-effect transistors(FETs) have low on/off switching ratio.As a member of nano-materials, two dimensional transition metal dichalcogenides(TMDCs) can be a supplement for graphene, duo to their sizable direct band-gaps.Two dimensional transition metal dichalcogenide is a kind of graphene-like layered compounds with a hexagonal honeycomb structure. In many researches it is showed that there are band-gaps around 1~2eV in several kinds of monolayer TMDCs which make them be promising in electronic and optoelectronic device application.While, due to the atomic size and synthesis technology, we can hardly get the property for some 2D TMDCs though experiments. Theoretical arithmetic is an appropriate way to research the property of 2D TMDCs. Theoretical results can also give advice to the applications of 2D TMDCs. In this paper, we mainly study the structural,electronic and mechanical properties for 2D monolayer and bilayer TMDCs with the help of first principle calculations. The influence of bilayer stacking for monolayer TMDCs is also shown in the calculation. Besides, different calculation functionals are adopted in this research for comparison. The detailed context is as following.1. We get the optimized 2H monolayer structures for TMDCs, including eight kind of MX2(M=Mo,W;X=O,S,Se,Te),by first principle calculations with different functional. After that the band structure for those monolayers are work out. The band structures show that all those monolayer TMDCs are semiconductors with a direct band-gap in k point, except for the oxide. In order to analysis the underlying mechanism, we have performed Bader charge analysis, which is known to be able to get the charge transfer between different atomic species. And then, we get the Young modulus and Poisson ratio for those monolayer TMDCs with LDA,PBE,optB88 method. The calculated Young modulus and Poisson ratio show that all those monolayer TMDCs has excellent mechanical property which is comparable to graphene.2. The property of bilayer TMDCs was calculated out by using of VASP with first principle calculation. The influence of bilayer structures is shown in our calculation. Firstly, bilayer structures in variety of stacking order are optimized with different method. Secondly, the band structure of those bilayer TMDCs with AA1 configuration is calculated. We find that the stacking of monolayers will change the band structures. All those bilayers have indirect band-gaps except WTe2 which remain direct band-gap. A decrease of band-gap is found in bilayers structure. Finally we get the Young modulus and Poisson ratio for those bilayer structures with LDAand optB88 functional. The Young modulus for those bilayers shows a negligible interlayer mechanical coupling.