Controlled Growth And Characterization Of Monolayer Molybdenum Disulfide Crystal And Heterostructure

Carbon element is everywhere on earth,and human beings are carbon-based life forms.Since the advent of graphene in 2004,there has been an upsurge in the study of two-dimensional nanomaterials.The growth and properties of layered molybdenum disulfide and molybdenum disulfide/graphene heterostructures have attracted much attention.There are still many problems to be solved in the preparation of molybdenum disulfide crystals and molybdenum disulfide/graphene heterostructures with large-scale controlled growth.For example:large-scale batch growth preparation,controlled syntheses of in-plane or vertical heterostructures of molybdenum disulfide/graphene,and regulation of its physicochemical properties.This paper focuses on the controllable syntheses of molybdenum disulfide crystals,combines high vacuum vapor deposition epitaxy instrument and various characterization techniques,to explore the effects of precursors,substrates,and temperature on the growth process.The crystal orientation directly visualizes the grain boundaries of the graphene and annealing treatment causes bilayer graphene domains to rotate.The main research contents and conclusions of this paper are as follows:First,a lab-built high-vacuum vapor deposition epitaxial instrument is used to synthesize centimeter-scale,nearly single-crystal monolayer molybdenum disulfide film on C-axis sapphire substrate by adopting suitable sublimation precursor source materials.Because the growth chamber is under high vacuum,there is less pollution;the growth temperature is relatively low,and the thermal expansion effect is less obvious to reduce lattice mismatch rate during the cooling process;the slower growth rate is conducive to regular and precise arrangement of atoms.Self-limiting vapor deposition epitaxy mechanism is proposed:van der Waals force induces controllable epitaxial growth,and the larger saturated vapor pressure of precursor source materials limits the growth of the second layer.Uniform continuous monolayer films can be obtained with high repeatability,avoiding pyramidal or spiral structures of molybdenum disulfide grains.Molybdenum disulfide films grown by this method are uniform and continuous in thickness,and have high carrier mobility and strong photoluminescence effect.Second,using graphene,hexagonal boron nitride,graphene/silicon wafer,and graphene/sapphire as substrates,adsorption energy and diffusion barrier of the substrate surface affect the growth process of molybdenum disulfide crystals.On the surface of graphene and hexagonal boron nitride substrates,the adsorption energy and diffusion barrier are smaller,and the precursor has a shorter adsorption time and a longer diffusion distance,we can grow high-quality molybdenum disulfide crystals.The surface of sapphire substrates has larger adsorption energy and diffusion barrier.The precursor stays on the surface for a longer time and average diffusion distances are shorter.The growth rate is increased and it is easier to form a layer film.In addition,temperature is also an important factor affecting the morphology of molybdenum disulfide crystals.As the growth temperature increases,the morphology evolution of molybdenum disulfide crystals is visually displayed,providing a new method for controllable growth of special patterns of molybdenum disulfide crystals.Third,using full-layer graphene prepared by chemical vapor deposition as the substrate,we have epitaxially grown molybdenum disulfide crystals.A new method to directly visualize the crystal orientation and grain boundary of full-layer graphene has also been explored.By means of transmission electron microscope,diffraction spots of molybdenum disulfide match with the graphene substrate's diffraction spots,verifying that molybdenum disulfide do grow epitaxially along crystal orientations of graphene domains.Directions of triangular single-crystal molybdenum disulfide with a size of about 100 nanometers have been collected to depict the direction and size of graphene crystal domains,as well as the outline of grain boundaries.Based on van der Waals epitaxial growth of triangular molybdenum disulfide crystals,this method can also be used to detect grain boundaries of other large-area two-dimensional nanomaterials.Using this marking technique,we can study physical properties of grain boundaries and fabricate electronic devices.Fourth,full-layer layer and sub-layer graphene have been transferred to silicon wafer substrates to prepare randomly stacked bilayer graphene.Then the bilayer graphene is annealed at a suitable temperature.The prepared bilayer graphene as a substrate template,molybdenum disulfide crystals have been epitaxially grown in a high-vacuum chamber.Directions of triangular molybdenum disulfide have been collected and sorted,so that crystal orientations of bilayer graphene can be directly visualized,depicting the evolution of stacked patterns of bilayer graphene.The top sub-single layer of annealed bilayer graphene has more wrinkles,presumably because crystal orientations of top graphene match with the bottom graphene.Lattice mismatch and wrinkles in local area become more serious,so that crystal orientations of most areas top graphene can align with the bottom graphene.This method is advantageous for preparing small-angle strong coupling or AB stacking bilayer graphene,and can be used to construct other two-dimensional heterostructures with aligned crystal orientations.