Preparation Of Two-dimensional Transition Metal Dichalcogenides And Manipulation Of Their Optical Properties

Two-dimensional(2D)materials have gained tremendous attentions from the researchers owing to their numerous excellent properties.As a typical representation of the 2D materials,Two-dimensional Transition Metal Dichalcogenides(TMDCs)have unique potentials for application in fields as sensor,logical electric device and photoelectric device,and the controllable preparation of large area and high quality TMDCs thin-film materials is the foundation for realization of their wide applications.At present,the TMDCs thin film materials are mainly prepared by Chemical Vapor Deposition(CVD),however,CVD method generally requires a relatively high temperature and restrict experiment conditions,and the thin film obtained usually has small area.It is fairly difficult to realize the preparation of continuous film in large area.On the other hand,TMDCs have the properties of weak light adsorption and weak light emission because of the atomic level of film thickness,and inevitably limits its application in photodetectors and light-emitting diodes,etc.Additionally,a whole new coding method could be realized by a stable control to the valley polarization because of the abundant physical properties relating to spin and energy valley in the monolayer Tungsten disulfide(WS₂)system.But at present,the control of valley polarization for WS₂was normally realized by traditional antiferromagnetic materials such as MnO,CoO,etc.While as a bulk material,the heterojunctions formed would destroy the atomic thickness and excellent bendable properties of TMDCs themself.In response to the three problems above,this dissertation carried out researches on the following three aspects.The specific works are as follows:(1)A two-step thermal decomposition method to prepare WS₂thin film materials was proposed through the analysis of thermal decomposition process of Ammonium tetrathiotungstate((NH₄)₂WS₄),and studies on the influences of various conditional parameters as precursor solvents and spin speeds on the surface topography of WS₂ were carried out.Based on optimized experimental parameters,the preparation of uniform and continuous WS₂ thin film materials with a wafer level large area was successfully realized on the Si O₂/Si substrate.The good electrical conductivity of the WS₂ thin films prepared was verified through the tests of volt-ampere characteristic curve.(2)Silver nanoarrays with periodic triangular prism structure of various side lengths and sizes were prepared by controlling the geometrical shapes and etching times of Polystyrene(PS)nanospheres which were used as masks.And then,Ag/WS₂ composite structure was formed by transfer the monolayer WS₂ thin film prepared by CVD method to the Silver nanoarrays with periodic triangular prism structure which had been optimized to have an optimal resonance matching.Succeed in realizing significant enhancements of photoluminescense(PL)and Raman scattering of the monolayer WS₂,especially a higher enhancement was achieved on its neutral excitons.Additionally,through the Finite Difference Time Domain(FDTD)method and first-principles theoretical calculations,it is revealed that the enhancement of plasma resonance is mainly achieved through the enhancement of the local electric field and the charge transfer.(3)Six heterogeneous stacking structures of 2D ferromagnetic metal material and monolayer WS₂ were constructed,energy band structures and densities of states simulated by the first principles calculations with a consideration of the spin orbit coupling,and the physical properties relating to spin and energy valley of the heterostructure of FeB₂/WS₂ was studied.This dissertation found that by introducing the 2D ferromagnetic metal FeB₂,the spin splits at the two energy valleys K and K'of monolayer WS₂ were separately suppressed and enhanced with the former of which enhancing to 642meV and the latter drop to 33 meV,causing two energy valley splits accordingly of 383 meV and 223meV at the same time.And the mechanism in changes of the spin splits were clarified by density of states,which suggested that neighboring magnetic moment coupling effect caused spin polarizations of different degrees in the monolayer WS₂.