First - Principles Study On The Electronic Structure Of Two - Dimensional Transition Metal Chalcogenides

Two-dimensional (2D) materials also called2D atomic membranes, with from zero to several nanometers thichness (atomic or molecular thickness) and infinite planar lengths, are the thinnest function nanomaterials. In these systems, electrons are confined to a plane, as the result these materials have some unique mechanical, thermotics, optical, electrical and magnetic properties. Then2D materials will be widely researched in physical, chemical and biologic fields and can be applied in energy conversion, biocatalysis, flexible electronic devices, sensors and other fields.In recent years, with the development of graphene, other layered2D crystals analogous to graphite such as hexagonal boron-nitride (h-BN), metal dichalcogenides, and transition metal oxides have attracted extensive attention from scientists for their promising practical applications and theoretical values. In the process, due to its unique layered structure, transition metal dichalcogenides (TMDs) have aroused much interest and are investigated for new generation high performance nano photoelectric device. Now,2D layered TMDs nanomembranes with high purity and quality have been obtained in the experiment, which also make their practical applications to be possible. The research of mechanical, optical and magnetic properties of2D layered TMDs have been carried out. Some unique properties were found and some valuable results were obtained. In addition, as semiconductor materials, the band engineering of2D layered TMDs has been studied widely. However, as new materials, the investigation of the band gap tuning is worth further developing. In this paper, using the first principles calculations, we study electronic structures of TMDs nanosheets subjected to mechanical strains, assembled superlattices and under the interface interaction.In prevenient literatures, though the electronic properties of monolayer and bilayer TMDs nanomembranes tuned by in-plane strains had been studied, no reports about the effect of the out-plane pressure on the multilayer are open published. Furthermore, the effect of mechanical strains on heterojunction has not been studied. In this paper, we investigate the electronic properties of bilayer and multilayer (4and6layers) MX2(M=Mo, W; X=S, Se) and MoS2/MX2(MX2=WS2, MoSe2, WSe2) heterojunction. Moreover, the variation of the band gap of these2D atomic membranes subjected to the out-plane normal compressive strain and the in-plane bi-axial strain was also studied. Results indicate that the band gaps of these2D atomic membranes can be tuned in a large energy range by in-plane strain and the out-plane pressure. Furthermore, a universal reversible semiconductor-metal transition was observed for all the semiconducting TMDs. The physical mechanism of the band gap variation was studied. The physical mechanism of the band gap variation under the compressive and tensile bi-axial strain is slightly different but we found that the physical mechanism of the band gap variation of bilayer and multilayer (4and6layers) MX2and MoS2/MX2heterojunction under mechanical strains was similar.Next, we investigated the geometric and electronic structures of MoS2/MX2(MX2=MoSe2, WS2, and WSe2) nanosheets assembled superlattices (SLs). These SLs can be easliy formed due to the same crystal structure and similar in-plane lattice constants of the TMD nanosheets. By calculating the interfacial binding energies the stability of the three SLs was studied and the most stable atomic configuration was found. By calculating the band offsets in the SLs, it was found that MoS2/WSe2and MoS2/MoSe2SLs can form multiple quantum wells (QW), and the depth of the QW are0.17eV and0.02eV, respectively. In QW MoS2is the potential well and WSe2and MoSe2are barrier materials. Taking MoS2/WSe2as an example, the influence of the mechanical strains on the SLs was investigated and the results show that the depth of the QW can be tuned over a wide range by the mechanical strains. Moreover, the electronic properties of the2MoS2/2WSe2and the effect of the mechanical strains on its electronic properties were also studied.Finally, we investigated the interface interaction between monolayer MoS2and the SiO2(0001) surface and analyzed the effect of the SiO2surface on the electronic properties of monolayer MoS2. We found that the band gap of monolayer MoS2does not change due to the interface interaction being very weak. The interface interaction between monolayer and bilayer MoSe2and LiNbO3(0001) surface was also investigated. The calculated results showed that, when putting monolayer and bilayer MoSe2on a clean surface, a large amount charge transfer happens at the interface and the band gaps of monolayer and bilayer MoSe2were significantly affected. If the clean surface is passivated with hydrogen, the charge transfer does not happen at the interface as the result the electronic properties of MoSe2nanosheets were almost unaffected. Our investigations enriched the understanding of this series of new materials of2D LTMDs. Our conclusions provide the necessary theoretical basis for extending the2D LTMDs nanomembranes application in electronics, optoelectronics and other nanodevices and help to facilitate new nanodevices.