Preparation And Properties Of Two-dimensional Layered Materials With Controllable Structure By Chemical Vapor Deposition

Two-dimensional（2D）layered metallic disulfides（LMD）materials possess the fascinating characteristics with adjustable band gap and plasticity structure,and endow great potential in the fields of energy storage,adsorption,catalysis and optoelectronics and so on,which have attracted widespread attention.These excellent properties are necessarily related to the abundant structure of 2D LMD materials.The controllable preparation of 2D LMD materials with specific structures is the basis of the property’s exploration and application research of 2D materials.According to the structure-activity relationship between the structure and properties of 2D LMD materials,the electronic,catalytic and optical properties of 2D LMD materials were studied.Hence,the controllable preparation of a series of 2D LMD materials with specific structures based on chemical vapor deposition（CVD）was conducted,and the physical-chemical properties of the corresponding structures were explored.The main innovative achievements are as follows:1.In the isotropic 2D LMD materials,the composition proportion in the growth atmosphere restricts the morphology of 2D LMD materials.Based on the ratio of Sn/S in the growth atmosphere by adjusting the growth factors such as growth temperature and molar ratio of precursor,under the assistant of potassium halide,a simple CVD method was used to investigate the shape engineering of single-layer 2D SnS₂.The influence mechanism of these growth factors on morphology of 2D SnS₂was analyzed,such as growth temperature,SnO₂:S ratio,gas flow rate,growth time,and the distance between S and SnO₂.These growth factors could effectively regulate the 2D SnS₂morphology from hexagonal,triangular,windmill,dendritic to coral-like.The change of growth conditions was related to the ratio of the concentration of Sn source to that of S source.The concentration of Sn source was the key restriction mechanism to control the fractal dimension of the various morphologies of SnS₂from 1.01 to 1.81.Compared with the catalytic performance of the different morphology SnS₂,more branched SnS₂exhibited better hydrogen evolution reaction（HER）performance.Meanwhile,the potassium halide could significantly change the migration barrier of 2D SnS₂on the substrate,and promoted the monolayer growth of SnS₂.The shape engineering based on fractal dimension provides a reliable method for the preparation of T-phase LMDs with different morphologies,which will have great potential applications in the fields of electrochemistry and electronics.2.In anisotropic 2D LMD materials,the atomic arrangements are diverse in different crystal axis directions,showing abundant crystal axis dependent physical-chemical properties.Regulation of in-plane growth along different crystal axes is helpful to explore the anisotropy properties and growth mechanism.The controlled anisotropic growth of 2D SnSe with few layers along a-axes,b-axes and c-axes was systematically studied from experiment and theory.The controlled anisotropic growth of 2D SnSe was attributed to the differences in edge atomic structure,the bonding characteristics of zigzag edges（along b-axis）and armchair edges（along c-axis）,and the chemical inertness of the base plane at a-axis.The growth rates along the b-axis and c-axis were precisely regulated by the H₂flow rate.The 2D SnSe morphologies changed from square nanosheets to linear nanowires,showing a significantly adjustable edge growth rate.By theoretical calculation,this regulation mechanism is closely related to the different binding energies of H₂at different edges.The a-axis of 2D SnSe could be controlled by temperature program adjustment to achieve few layers thickness.The thickness of 2D SnSe could be precisely adjusted during the growth process by a two-step method of rapid cooling or annealing etching.The controllable anisotropic growth of 2D SnSe demonstrate a facile strategy towards the controllable anisotropic growth of other 2D LMD materials with different edge structures.3.In the horizontal direction of 2D LMD material,based on the atomic structure similarity of the intrinsic 2D LMD semiconductor and the 2D alloy formed by heteroatom doping,and coupled with the transition of electrical behavior from semiconductor to metal by the doping of heteroatoms in the 2D LMD semiconductor,we accurately designed the epitaxial metal-semiconductor（M-S）Sn_xMo_1-xS₂/MoS₂heterostructure by the sequential in-situ growth of monolayer Sn-doped metal Sn_xMo_1-xS₂and semiconductor MoS₂.The epitaxial growth of Sn_xMo_1-xS₂covalently bonded at the MoS₂interface edge could effectively eliminate the pinning effect of Fermi level and reduce the contact resistance.The M-S Sn_xMo_1-xS₂/MoS₂heterostructure was more beneficial to improve the photocatalytic performance than MoS₂,which benefits from the separation of electron-holes and effectively promotes the charge transfer.Therefore,this precise design approach provides a general strategy for self-assembled M-S epitaxial heterostructures.The seamless assembly of 2D metal materials in interface engineering can provide effective strategy in the fields of electricity and catalysis.4.In the vertical direction of 2D LMD material,2D LMD materials with specific stacking angles are fabricated by regulating the interlayer stacking energy barrier based on heteroatom,and angle-dependent specific electrical/optical properties are studied.With the assistant of Sn atom,60^o-stacking WS₂with high order stack order and various stacking angles was designed accurately by CVD.Through adjusting the hydrogen flow rate and growth temperature,the layer number of 60^o-stacking WS₂could be effectively adjusted,and the self-assembly target structure with different layers could be realized.60^o-stacking WS₂with 2H phase structure revealed significantly different PL and nonlinear optical behaviors due to stronger interlayer electronic coupling and overall inversion symmetry.STEM characterization further revealed the60^o-stacking bilayer structure with 2H phase,as well as the unstable AB’and AB’’stacking structure caused by interlaminar slip.The heteroatom assisted approach provides a general strategy for self-assembling 2D LMD materials and heterostructures with different stacking angles,which is beneficial to the applications of moire excon,spintronics and spin-orbit coupling in grain electrons.