Structure and electronic properties of 2D transition metal dichalcogenides : first principles study

Abstract

Semiconducting transition metal dichalcogenides (TMDs) represent a class of layered materials with nonzero band gap, and as a result attract great attention. Density functional theory (DFT) is a method of choice for theoretical exploration of condensed matter. However, there exists well-known problem with band gap evaluation within DFT framework. Thus, general theoretical approach for an accurate band gap computation within DFT is in big demand, which can be used for exploration of new materials with the required properties. The thesis introduces recently proposed the general method for the band gap calculation (GVJ-2e). GVJ-2e method is implemented within DFT framework, is based solely on the total energy calculations and is adjustable parameter free. The GVJ-2e method was verified on wide range of materials from bulk semiconductors (ex. Si, C, Ge) to wide gap insulators (ex. Xe, Kr) and yielded the band gaps which correlate well with experimental ones. The errors of proposed method are smaller than errors of other widely used methods (GW, hybrid functional HSE, TB-mBJ functional). In the thesis the structural and electronic properties of bulk and monolayer TMDs (MoS2, MoSe2, WS2, WSe2) are studied. The calculated relaxed lattice parameters of bulk TMDs deviated from experimental values for less than 2%. The fundamental and optical band gaps are obtained with proposed GVJ-2e method and agree well with experimental results. From the analysis of Kohn-Sham band structures is shown that considered monolayer TMDs are direct gap semiconductors, while their bulk forms are indirect semiconductors. The structure, formation and electronic properties of monolayer MoS2(1-x)Se2x alloy are studied for entire range of substitution rates x from 0 to 1. The analysis of various structures of alloy demonstrated that the structures with almost symmetric occupation of top and bottom chalcogen planes by Se (S) atoms are most likely to occur during synthesis. The fundamental and optical band gaps are calculated with proposed GVJ-2e method and are found to be in range of corresponding band gaps of MoS2 and MoSe2 monolayers. It is shown that the dependency of the lattice parameter, fundamental and optical band gap on substitution rate (x) is nonlinear. The study of the electron and hole effective masses showed that both are smaller than a free electron mass and that in the alloy electrons are lighter than holes (similarly to MoS2 and MoSe2 monolayers) almost for all substitution rates. It was shown that the physics of the formation and properties of the MoS2(1-x)Se2x can be described in terms of percolation theory.