Studies On Growth And Properties Of Two-dimensional Van Der Waals Heterostructures

In recent years,two-dimensional(2D)layered materials,represented by graphene(Gr),have been widely applied in fundamental research and already show great potential in the industry due to their unique mechanical,thermal and electrical'properties.However,how to stack these 2D materials to form different types of van der Waals(vdWs)heterostructures with special properties still have many unsolved problems.For examples,conditions(substrate,temperature,etc.)that affect the dynamic growth process of 2D materials remain unclear,the introduction of interlayer impurities during the preparation of 2D vdWs heterostructures and unable to construct an atom-level clean interface,techniques need to be developed to alter the epitaxial growth and electronic properties of these heterostructures.Studying growth processes and intrinsic properties of 2D materials is the most basic and important part before their practical applications.Among these 2D materials,Gr and MoS2 have been widely studies due to the high mobility and wide range of optical absorption respectively.Additionally,heterostructures that comined them together may greatly extend their properties.Therefore,Gr and MoS2 are selected as research objects to explore the preparation of high-quality MoS2/Gr heterostructures,and also to study their related physical properties.From preparation,characterization and first-principles calculations,this thesis systematically and comprehensively states the structure,optical and electronic related properties of the MoS2/Gr vdWs heterostructures.This research is divided into six parts:The first chapter systematically introduces the basic characteristics of Gr and MoS2 crystals and their preparation methods.Additionally,the research progress of MoS2/Gr heterostructures is also introduced.Basing on current scientific problems we have proposed the research route of this thesis.The second chapter mainly introduces the materials preparation and characterization methods used in this thesis.In the third chapter,the growth of MoS2 crystals on multi-surface substrates and related kinetic mechanisms are studied.In the fourth chapter,the temperature-related crystal morphology and atomic terminations of MoS2 is investigated by using Gr transferred on silicon dioxide(SiO2)as the substrate.In the fifth chapter,the inert platinum foil(Pt)is used as substrate to directly grow MoS2/Gr heterostructures,which show atomic-level clean interface and strong inter-layer coupling.In the sixth chapter,the factors affecting the formation of large-area MoS2 films are explored by changing the surface stress of Gr/Pt,and related energy band alteration of MoS2 crystals are also studied by means of STM/STS and PL.The main work of this paper is shown as below:1.High-vacuum vapor deposition epitaxy can create an ideal molecular dynamic environment for the growth of vdWs 2D materials,which shows superior for the investigation of related mechanisms.We find that the distribution,morphology,size of MoS2 crystals can be changed when grown on different surfaces.The intrinsic mechanism of the difference relies on the adsorption energy and diffusion barriers of precursors on surfaces.Accordingly,the growth of MoS2 can be controlled by choosing different substrates,and the patterning growth of MoS2 can also be realized on multi-surface substrates.2.Temperature is also one of the most important factors that determine the growth of MoS2 crystals.Temperature-related competing sulfur-capture principle jointly with strict epitaxial mechanism is proposed for the initial topography evolution and the final intrinsic highly oriented growth of triangular MoS2 domains with Mo or S terminations on the Gr template.Additionally,potential distributions on MoS2 domains and bare Gr are presented to be different due to the charge transfer within heterostructures.The findings offer the mechanism of templated growth of 2D transition metal dichalcogenides,and provide general principles in syntheses of vertical 2D heterostructures that can be applied to electronics.3.The interlayer coupling of MoS2/Gr heterostructures on the surface of Pt is studied in detail.The MoS2/Gr heterostructures can be directly grown on inert metal substrates,which effectively avoid the introduction of impurities comparing with transfer methods.An atomic clean interface will greatly preserve the intrinsic properties of MoS2/Gr heterostructures.In addition,although the weak vdWs force can effectively assist the epitaxial growth between 2D materials which have large lattice differences,the interlayer tension can be inevitably generated and may also affect the interlayer coupling of heterostructures,and then in turn affect their physicochemical properties.By experimental characterizations and first-principles calculations,we find that there exists a strain effect and strong interlayer coupling between MoS2 and graphene resulting from the intrinsic crystal lattice mismatch,which could generate potential metallic behavior of the heterostructure.4.By means of scanning electronic microscope,scanning tunneling microscope/spectroscopy and photoluminescence measurements,we have discovered that considerably stressed Gr can strongly affect the epitaxy of MoS2 crystals and enhance the interlayer electronic coupling between monolayer MoS2 and its Gr-platinum(Pt)substrate,leading to the stack of MoS2 layers and a narrowing direct bandgap of monolayer MoS2.