Controlled Growth Of Uniform Few-layer Graphene By Chemical Vapor Deposition And Their Physical Properties

Few-layer graphene(FLG,2-10 layers)shows many unique physical properties that are different from those of single-layer graphene since the number of layers and stacking order provide two important additional degrees of freedom to regulate its properties,which greatly expand the applications of graphene.CVD method shows distinct advantages over other methods in the preparation of large-area uniform graphene films.The current studies on the synthesis of FLG by CVD mainly focus on using solid metals as the growth substrate.Thus-obtained FLG typically suffers from the non-uniform stacking order and number of layers.In this thesis,a CVD method using a liquid-solid shell-core structured composite substrate was proposed to synthesize FLG by combining the advantages of solid and liquid metal substrates.The growth mechanism,microstructure,electrical and mechanical properties of the asgrown FLG were systematically investigated.The main results are shown as follows:Pt3Si/Pt composite substrate with liquid-solid shell-core structure at high temperature was designed and prepared,which combines the capability of carbon storage of solid metal with the high catalytic activity,high carbon diffusion capacity and atomically smooth surface of liquid metal.It solved the problems of non-uniform stacking order and number of layers as well as the low crystalline quality of FLG that are caused by the strong potential field and inevitable surface defects of solid substrates in traditional CVD methods.A two-step CVD method was developed to realize the growth of wafer-scale AB stacked BLG films on the Pt3Si/Pt composite substrate.They show crystalline quality,mechanical and electrical properties comparable to those of the BLG obtained by mechanical exfoliation.Its Young’s modulus and fracture strength reach 1.02 TPa and 122 GPa,respectively.The carrier mobility reaches 2100 cm2 V-1 s1 at room temperature,and the band gap can be tuned by external electric fields.The unique interlayer epitaxy mechanism of AB-BLG film on Pt3Si/Pt composite substrate surface was discovered.In this composite substrate,the Pt nucleus with high carbon solubility provides carbon atoms for the growth of the second layer of graphene.Theoretical calculations show that it only requires a small amount of energy for translating and rotating graphene on the isotropic and atomically smooth liquid Pt3Si surface.Therefore,in the second step CVD,the second layer of graphene will take the first layer graphene formed earlier in the first step CVD as the template to achieve the interlayer epitaxy growth of the lowest energy AB stacking structure.In addition,Pd5Si/Pd composite substrate with similar shell-core structure was also prepared,and high-quality BLG films with uniform AB stacking order were further synthesized by using the two-step CVD method,which demonstrates the universality of the interlayer epitaxy growth of graphene on the shell-core structured composite substrate.A novel growth mode of graphene,which features the inverse transition from layer-island growth to layer-by-layer growth,was discovered on the Pt3Si/Pt composite substrate under a constant growth temperature.Isotope labeling experiments indicate that the non-uniform multilayer islands decompose and transform into uniform FLG during the inverse transition process.Based on these understandings,a new CVD growth strategy was proposed,which involves storing carbon and transition at a constant temperature followed by decomposition of additional graphene island at a higher growth temperature.As a result,controlled interlayer epitaxial growth of 1-3 layers uniform high-quality graphene films free of islands was realized,in which BLG and trilayer graphene has AB and ABA stacking,respectively.This strategy solves the problems encountered in synthesizing FLG by the traditional CVD methods with solid substrates,such as inevitable multilayer graphene islands,non-uniform stacking orders,narrow growth windows and low scalability,and paves the way for the controlled growth of uniform FLG with designed number of layers.