The Investigation Of Optical And Electrochemical Properties Of Two-dimensional Transition Metal Dichalcogenides

Two-dimensional transition metal dichalcogenides have attracted considerable at-tentions due to their distinctive electronic structure and the consequent physical and chemical properties. In this doctoral dissertation, by employing first-principle density functional theory in combination with many-body pertubative GW method and Bethe-Salpeter equation we discuss respectively the electronic structure and optical response of both bulk and monolayer molybdenum disulfide(MoS2) and electronic structure of vanadium disulfide(VS2). Based on the above discussions we explore the potential ap-plications of these two-dimensional layered transition metal dichalcogenides. The basic contents of the dissertation are proclaimed as follows:(1) We concern the optical properties of bulk MoS2. By using the GW method and the Bethe-Salpeter equation we calculate the dielectric function and refractive index of the material. Our results confirm bulk MoS2 as a typical uniaxial crystal:a tremendous electromagnetic anisotropism exists in the low energy regime including the entire visi-ble region, whose averaged anisotropy parameter exceeds 2.5. By solving the Maxwell equations, we analyse the optical response of bulk MoS2 to external electromagnetic waves, and find that the propagation of electromagnetic waves is strongly modulated by the huge anisotropism of bulk MoS2. Owing to this strong modulation, the critical in-cident angle to trigger a negative refraction in the bulk MoS2 is calculated up to 90°. To study the efficiency of the negative refraction in bulk MoS2, we use the finite-difference time-domain simulations to analyse the propagation of light in MoS2 slabs. For a MoS2 slab with thickness of 0.1 μm,59.5 percent of incident light can be negatively refracted. Our findings guide a new way for the utilisation of MoS2-like materials in the novel field of optical integration, which substantially extends the capabilities of MoS2 photonics in the future.(2) We explore and propose a type of strain sensing technique based on the optical properties of monolayer MoS2. By employing the GW method and the Bethe-Salpeter equation we systematically study the variation of the optical response of monolayer MoS2 when external strain is applied, Upon applying linear strain, quasi-linear red-shifted reflectance spectral can be observed; We design and fabricate a new type of tunable monolayer MoS2 reflective grating in terms of this linearity. The applied in-plane strain on the grating causes the change of the reflectivity of monolayer MoS2, which ultimately lead to the intensity redistribution of the first-order diffracted light on the diffraction screen. We successfully calculate the redistributed intensities and through analyzing the distribution of the first-order diffracted light intensities the strain can thus be determined. Our research indicates that the proposed tunable monolayer MoS2 diffraction grating can serve as a simple and remote strain detection device.(3) We preliminarily discuss the application of another kind two-dimensional lay-ered transition metal dichalcogenides, namely VS2 in the field of ion batteries. By employing density functional theory we investigate the adsorption energy and diffusion coefficient of the lithium ion and sodium ion in monolayer and bulk VS2 for lithium ion and sodium ion batteries respectively. Our calculations show that the capacity of ion batteries using VS2 as anode materials can approach 466 mAh/g and a low output voltage can be achieved. The calculated lithium ion and sodium ion diffusion coeffi-cients in bulk VS2 are enhanced by three and seven orders of magnitude respectively compared to that reported in bulk MoS2. Our results also show that VS2 exhibits better electrochemical performance when used as anode material in the sodium ion battery than in the lithium ion battery. Our studies suggest that both monolayer and bulk VS2 are promising candidates for anode materials in the lithium and sodium ion batteries.