Studies on the Growth of Multilayer Graphene on Copper Surface by Chemical Vapor Deposition

Abstract

Graphene is a two-dimensional carbon allotrope exhibiting high charge-carrier mobility, thermal conductivity and Young’s modulus with flexibility. When graphene layers are stacked, distinctive electronic structure develops depending on the number of graphene layers and stacking orientation with preserving superior properties of graphene. High sheet resistance of single layer graphene (SLG) due to atomic thickness goes down as increasing the thickness of graphene film which makes it promising material for transparent electrode. The most intriguing property of multilayer graphene (MLG) is that its electronic structure is tunable by generating differences in the on-site energy between graphene layers; In contrast to SLG is zero bandgap semimetal, MLG exhibits bandgap under a vertical electric field. In this motivation, I have studied the synthesis method for the Bernal-stacked multilayer graphene on Cu surface by chemical vapor deposition (CVD). This objective was achieved by introducing heteroatom (Nickel, Sulfur, Phosphorus) in Cu foil. In Chapter 1, I introduce the background of the research field for MLG. First, I review the factors which determine the electronic structure of MLG. Then, various applications based on tunable optoelectronic properties of MLG is introduced. After that, previous CVD methods for the growth of MLG and post-growth processes for applying electric field in MLG are addressed. Finally, research objectives of thesis are introduced by defining research direction based on the limitations of previous approaches. In Chapter 2, the growth of Bernal-stacked graphene on Cu foil which has asymmetric-carbon-solubility is studied. For this, I deposited a thin Ni film on the back of Cu foil. C atoms are absorbed through back side instead of forming graphene and diffuse toward the front side. Gradient Ni profile facilitates the bulk diffusion of C atoms. As a result, Bernal-stacked graphene with low sheet resistance and high uniformity in large-scale is obtained. The number of graphene layers is easily controlled by adjusting the initial thickness of the Ni film. In Chapter 3, the growth of Bernal-like stacked graphene with vertical electric-field is investigated. Sulfur-dissolved Cu foil was used as a catalyst to grow MLG. There exists gradient S atom concentration in c-axis of synthesized graphene. Therefore, electric field inside of thick MLG could be applied. The reduction of S atoms in Cu foil affects the growth of MLG and doping at the same time. Thus, electronic structure of Bernal-stacked graphene can be modified at the synthesis stage. The photovoltaic effect of MLG signify that electric field inside of MLG is strong enough to separate photogenerated carriers. This approach could be expanded to other heteroatom, phosphorus. In Chapter 4, the growth of Bernal-stacked graphene with tunable doping types on eutectic P-Cu binary system by CVD was studied. I used P-dissolved Cu foil as a catalyst. Synthesized MLG is intrinsically n-type due to adsorption of Pcomplexes in MLG, and doping type can be overturned to p-type by adsorption of H2O. I controlled the doping type and doping level of single MLG sample by interface modification. Based on mechanistic studies, simultaneous control of graphene growth and doping were achieved as well as the elaborate patterned growth MLG by 1-step CVD.