Synthesis Of Two-dimensional Metal Dichalcogenides By Chemical Vapor Transport And Investigation Of Their Properties

Two-dimensional?2D?metal dichalcogenides?MDs?are a family of emerging materials with extensive chemical compositions and crystal structures,which endow them with rich properties and broad applications.As the properties of 2D MDs are highly depended on their compositions and structures,it is essential to achieve the controlled synthesis of these materials in the scope of atomic scale for exploring their fundamental properties and applications.Chemical vapor transport?CVT?is currently one of the most widely used method for the synthesis of MDs bulk crystals which provide high quality parent materials for preparing 2D crystals by mechanical exfoliation.The intrinsic properties of materials can be maintained through mechanical exfoliation,but it is not feasible to control the shape,the layer numbers and the size of the exfoliated 2D crystals,which hinders large-scale applications.This dissertation innovatively proposed a new idea of synthesizing 2D crystals by CVT in one step.Starting with the control of the transport rate,and the reduction of the thickness of the synthesized crystals from millimeter scale to nanometer scale,this dissertation aims to establish a new method for the synthesis of 2D crystals by CVT,and achieved the controlled synthesis of some typical 2D crystals including Mo S2,Ptand so on.This approach holds the advantages of high-quality growth,universality and good controllability,and therefore,opens up a new avenue for the controlled synthesis and properties investigations of 2D MDs.The main contents of this dissertation are as follows:1.The CVT synthesis and characterizations of 2D Mo S₂.We started with the controlled synthesis of the most representative 2D MDs,Mo S2,and investigated the effects of several key parameters on the growth results with the analysis of the growth procedure and mechanism.Combining various microscopic and spectroscopic characterizations with electrical measurements,we confirmed the high quality and good controllability of the CVT-grown Mo S2,and realized the CVT synthesis of monolayer 2D crystals for the first time.2.The CVT synthesis of 2D Pt Se₂ and investigation of its layer effect.2D Ptis with very strong interlayer interaction,which makes it more challenging to achieve the layer-controlled synthesis of this 2D crystal.Through the careful screening of the growth system and the detailed optimization of the growth conditions,we pictured the relationships between the layer numbers and the growth onditions,and realized the layer controlled synthesis of Ptfrom monolayer to more than 20 layers,which demonstrates high controllability of CVT method for synthesizing 2D materials.Based on the layer controlled synthesis of Pt,for the first time,the semiconductor to metal transition of 2D Ptwas confirmed to be between 3L and 4L,and the layer effect for the electrochemical hydrogen evolution reaction performance of Ptwas revealed.We also provided fundamental explanations of the layer effect through theoretical calculations,which provides new insights on the rational design of 2D electronics and catalysts.3.The CVT synthesis of other 2D MDs.We applied CVT method to the synthesis of more 2D MDs,including,Mo,W1-x S2 alloys,Nband In Te,by screening suitable reactants and transport agents and optimizing the suitable temperatures of the reaction zone and the growth zone.Based on the understandings on the growth procedure and mechanism of CVT,we analyzed the similarities and differences between the growth of bulk crystals and 2D materials,and proposed a general idea of CVT for the synthesis of 2D MDs,providing an important foundation on the exploration of new materials,properties and applications.