Study on Graphene Growth and Etching Behavior on Liquid Surface

Abstract

Graphene, a two-dimensional material with excellent mechanical strength, electrical and thermal conductivity, has attracted considerable research interest. The past decade has seen advances in the scalable synthesis of high-quality, low-cost graphene products. However, significant obstacles to achieving great performance applications have not been removed. As a relatively high-quality and low-cost method, bottom-up chemical vapor deposition(CVD) synthesis of graphene is considered the most promising method for industrial applications. The obstacle lies in the defects generated during the CVD synthesis process. A prerequisite for defect engineering to overcome this hurdle is basic knowledge of graphene growth. Basic knowledge includes (Ⅰ) An understanding of the growth and coalescence of graphene islands. (Ⅱ) The synthesis of graphene with specific grain boundaries can bring unique properties to graphene products. (III) Synthesis of graphene islands with specific edge structures. This thesis conducts experimental methods to study graphene's growth and etching behavior on liquid surfaces. Chapter 1 will discuss the background information and motivation for this study. Chapter 2 will discuss the experimental setup of the CVD furnace and the measurement methods used in this thesis. Chapter 3 will discuss research on the growth of graphene on the surface of liquid copper. An unexpected finding was that graphene polycrystals with only 30-degree twinned grain boundaries grew on liquid copper surfaces. Theoretical simulations demonstrate that the unique island arrangement in graphene polycrystal structure results from the free rotation of graphene islands on the liquid copper surface and the formation of energy-stable 30-degree grain boundaries (GBs) in graphene structure. The further discussion predicted 30 kinds of possible 30-degree twinned graphene polycrystal islands, 27 of which were observed experimentally. The revealed formation mechanism on liquid copper surfaces broadens the basic understanding of polycrystal growth and can guide the controlled synthesis of twinned 2D materials in the future. Chapter 4 describes how to synthesize the ultra-long straight twinned GBs. A model is reported to shed light on the mechanism of 2D materials GBs formation during the growth. Based on this model, twinned graphene's growth has been successfully demonstrated on liquid copper surfaces in experimental, where all GBs are ultralong straight twinned boundaries. Moreover, high-energy twins adsorb hydrogen atoms preferentially and can be selectively etched. Therefore, this study presents the mechanism of 2D materials GB formation during the growth and paves the way for the growing 2D nanostructures of various controlled GBs. Chapter 5 introduces edge structure engineering for CVD synthesis of graphene. The main finding is that the edge-etching rate of graphene is controlled by the nickel content of the liquid copper substrate. The etching edge of graphene islands will approach the armchair's structure by adding nickel to the liquid copper. This phenomenon can be explained by kinetic Wulff construction that the fastest etching-rate outer edge remains in the etching process. The AIMD simulation results show that a minor nickel element will preferentially attach to the graphene armchair edge and increase the etching rate. Therefore, this study provides a state-of-the-art engineering approach for synthesizing 2D materials with armchair edge structures. Chapter 6 will discuss the coalescence behaviors of graphene on liquid surfaces. Fundamental discoveries include a new model that will illustrate the self-limiting nature of graphene growth. Based on a proposed new self-limiting model, graphene merge behaviors on liquid surfaces can be simulated by phase-field theory. The mysteries of previous research on graphene coalescence may find an answer. High-order graphene islands are synthesized on liquid copper surfaces and demonstrate the robustness of new self-limiting modes. This study brought new insights into the coalescence behaviors of graphene and other 2D materials and facilitated high-quality product synthesis for applications.