Structure Design And Applications Of Graphene And Transition Metal Dichalcogenide:A First-principles Study

Two-dimentional（2D） materials with novel properties and various applications have been a promising family of new materials. By performing first-principles calculations, we studied and simulated the e?ects of di?erent configurations of graphene and its ralatives in the applications in Li-S battery. And we also investigated the unsolved problems from the process of rolling up transition metal dichalcogenide, another important 2D material,and we propose several structural design to facilitate experimental progress and explain the experimental observations.Motivated by the recent progresses and remaining technical challenges in Li-S battery, we employ defective graphene as a prototype cathode framework to illustrate how battery performance is influenced by the mesoporous carbon materials. We show that the immobilization of S unavoidably sacrifices its ability to further interact with Li, which leads to an enhanced cycle life but a decreased capacity. Based on our calculated results,we suggest a suitable S binding-energy range of ～4-5 eV to balance the battery stability and capability under thermodynamic equilibrium conditions. We also studied N-doped graphene and 2D transition metal carbide Ti₃C₂ in the application of absorb Li-S clusters.The former has greater ability than pure graphene in absorbing Sx Liy clusters due to the structural and electronic changes in the sheet. And the latter, which has bare Ti surface can greatly absorb Sx Liy clusters that the large clusters can be dissembled into small ones,which can decrease the loss of sulfur and lithium in the process of charge/discharge and then improve the cycle life. Our results may promote the understanding and architecture design of Li-S battery.Similar to graphene, 2D transition metal dichalcogenide（TMD） can be rolled into one dimensional（1D） nanotubes. While, owing to their three-atom-thick sandwich structure, the large energy penalty greatly hinders the synthesis of small diameter TMD nanotubes. So we propose a solution of synthesizing hybrid TMD nanotubes with different chalcogen on each side（X-TM-Y） by self-assembly rolling up. Our calculations indicate the tube formation can be driven by the relaxation of the intrinsic strain in X-TMY and the hybrid nanotubes as small as ～ 2.0 nm could be synthesized. The rich variety of polymorphs exhibit unique and tunable electronic properties. Our finding opens a door to synthesize hybrid small diameter TMD nanotubes for various applications.The tubes synthesized in experiments have large diameter and di?erent tube polymorphs with di?erent angles. Here we propose a model of MoS2 nanotubes with multiple line defects of inner S atoms since a nanoribbon with S line defect can realize selfbending. Our first-principles calculations verify the formation energy of the tubes are derived from curvature and line defects. The presence of line defects reduces the curvature of tube walls and help to synthesize small diameter tubes. The subsequent rich variety of polymorphs exhibit unique and tunable electronic properties. Our proposed S line defected nanotubes and nanocages are in agreement with the experimental observations. It can help to understand the local structure of TMD tubes and synthesize small diameter tubes for various applications.