First-Principle Study on Electronic Structures of Two-Dimensional Transition Metal Dichalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDs), because of their sizable direct band gap, ultrathin body, good electron mobility, and versatile electronic structures, have attracted a lot of attention. They have shown their potential in future electronic, optoelectronic, photovoltaic, phase change, and photocatalytic devices. To examine their applicability to these devices, understanding their fundamental electronic structure and structural stability is a prerequisite. A systematic study of the electronic structure of monolayer transition metal dichalcogenides (TMDs) is performed by the density functional theory (DFT) method. Their band alignments are summarized and several representative bilayer heterostructures of two types of monolayer TMDs are investigated to understand the band realignment in the process of stacking. A formula is developed to predict the band realignment in the TMD bilayer heterostructure. Different to graphene, there exist several polymorphs of the same TMDs (e.g., 2H-MoTe2 and 1T-MoTe2). The total energies of various phases of TMDs are compared to determine the most energetically favorable phase. The electronic structure of TMDs, depending on the phase, can vary from metallic to semiconducting. This characteristic correlation between atomic and electronic structures opens up the possibility of controlling the electrical property by phase engineering, external field or charging. The charge-driven phase transition in monolayer WxMo1-xTe2 alloy has been investigated. Critical charge concentration and composition to induce phase transition is investigated. Doping, as an important technique of modulating the electronic properties of semiconductors, has been also investigated by DFT in monolayer MoS2. Substitutional doping of halogen group elements and nitrogen group elements is found to introduce n-type and p-type doping, respectively. The reduced dielectric screening will affect the exciton binding energy in 2D TMDs which will further change the impurity levels. NO2 adsorption-doping at the surface of monolayer WSe2 is also investigated showing p-type doping characteristic on WSe2. These findings have enabled the utilization of two-dimensional TMDs in the realization of future electronic devices.