Control Of Geometric Configuration In Graphene By Chemical Vapor Deposition

Since graphene was exfoliated from the HOPG,many pioneering studies have been done to explore its physics properties.Series of novel physical phenomena were discovered,such as carriers in graphene show two-dimensional(2D)Dirac fermion characteristic,graphene exhibits the half-integer quantum Hall effect,and roomtemperature quantum Hall effect,and so on.Finding unique properties of graphene drive the research of other 2D crystals,opens a scientific field of 2D materials,and creates a new research direction for van der Waals heterostructure materials based on 2D materials.This article focuses on controlling the geometry configuration of graphene during the chemical vapor deposition(CVD)process.During the CVD process,transition metals were employed to modify the crystalline quality and edge structure.The process of annealing was used to control the spatial symmetry(crystal lattice rotation)of bilayer graphene.Using Raman and other types of characterization equipment,we explore the mechanism in detail.Finally,we found that graphene can also be used to control the spatial configuration of 2D transition metal carbides(MXene).By analyzing experimental data,we gave a method for preparing ultra-thin 2D molybdenum carbide(Mo2C)crystals and Graphene/Mo2C heterostructure and explain the mechanism in detail.The main contents of this thesis are as follows:The first,the avoidance of growing dendritic graphene on the copper substrate during the chemical vapor deposition process is greatly desired.We demonstrate that not only can transition metal plates suppress graphene growth rates to synthesize highquality single-crystalline graphene domains,but also transition metals and a minute portion of atoms that spit out from these plates can catalytically etch graphene edges under the hydrogen environment to rectify graphene-domain shapes from being dendritic to zigzag-edged hexagonal.Furthermore,this method can be utilized to etch graphene into hexagonal openings and can be applied to modulating topography and elevating the graphene crystalline quality.We have identified a mechanism,in which(ⅰ)transition metal plates placed inside the copper pockets reduce the majority of active carbon atoms to eventually suppress the graphene growth rate,and(ⅱ)transition metals and a minute portion of atoms that originate from these plates etch graphene C-C bonds along defective edges to grow into zigzag-edge ending domains with higher priorities.Via isotopic labeling of the methane method,we have observed bright-dark-alternating hexagonal-shaped rings,which are exhibited in Raman mapping images.Under the hydrogen atmosphere,we can acquire hexagonal openings within graphene domains employing transition-metal-atom-driven catalytic etching.This methodology may work as a simple and convenient way to determine graphene size and crystal orientation and may enable the etching of graphene into smooth and ordered zigzag edges nanoribbons without compromising the quality of graphene.Second,it has been rarely reported that stacking orientations of bilayer graphene(BLG)can be manipulated by the annealing process.Here,we discover that,at approximately 600℃,called the critical annealing temperature(CAT),most stacking orientations collapse into strongly-coupled or AB-stacked states.This phenomenon is governed(ⅰ)macroscopically by the stress generation and release in top graphene domains,evolving from mild ripples to sharp billows in certain local areas,and(ⅱ)microscopically by the principle of minimal potential obeyed by carbon atoms that have acquired sufficient thermal energy at CAT.Conspicuously,we can observe evolutions of stacking orientations in Raman mappings under various annealing temperatures.Furthermore,MoS2 synthesized on BLG is used to directly observe crystal orientations of top and bottom graphene layers.The finding of CAT provides a guide for the fabrication of strongly coupled or AB-stacked BLG and can be applied to aligning other 2D heterostructures.Third,this paper proposes an effective method to control the thickness of twodimensional transition metal carbides by graphene.Though compared experimental results of 2D-Mo2C crystals grown on Cu/Mo substrates by low pressure and atmospheric pressure CVD,we found a good method that can synthesize high crystalline quality graphene/Mo2C heterostructures.We also proposed the growth mechanism of synthesizing Mo2C and graphene/Mo2C heterostructures.The morphology,size,thickness,and crystalline quality of Mo2C and graphene/Mo2C heterostructures were characterized,especially introducing Raman characteristic peaks in detail,for further controllable growth and application.