Studies on the Synthesis of Bilayer Graphene by Chemical Vapor Deposition

Abstract

Bilayer graphene is a two-dimensional carbon allotrope consisting of two graphene layers with diverse stacking types including AA, AB, and twisted configurations. Depending on the stacking configuration, significant changes occur in the electronic structure of bilayer graphene, which lead to unique physical properties such as superconductivity, band gap opening, and van Hove singularity. To deeply investigate these properties, numerous synthetic methods have been proposed for the production of large-scale bilayer graphene with the desired stacking configuration. In general, a layer-by-layer transfer method has been employed to fabricate bilayer graphene in the desired form. However, the occurrence of interlayer contamination during the transfer process has emerged as a critical impediment, obstructing the observation of intrinsic properties. Therefore, it is essential to develop novel approaches for high-quality and large-scale bilayer graphene, possessing the desired stacking configuration without the interlayer contamination. This dissertation introduces novel strategies for controlling stacking configurations and describes the synthetic methodology of large-scale bilayer graphene with controlled stacking configuration. Part I provides fundamental information including structure, synthesis methods, and characterization of bilayer graphene, which is necessary to understanding Part II and III. Part I.1 describes the atomic and electronic structure of bilayer graphene with its physical properties and applications. Part I.2 summarizes the synthesis methods of bilayer graphene and introduces the synthetic strategies of bilayer graphene with AB-stacking and twisted stacking. In part I.3, main analysis methods for bilayer graphene are categorized into structural (atomic force microscopy, scanning electron microscopy, and transmission electron microscopy) and spectroscopic analysis (Raman spectroscopy, ultraviolet-visible spectroscopy, and X-ray photoelectron spectroscopy). Each method is used for analyze the structure, number of layers, and stacking configuration. Lastly, Part I.4 deals with several problems that need to be solved for develop the well-established technology to control the stacking configuration and the synthesis method of large-scale bilayer graphene with controlled stacking configuration. Part II presents strategies for controlling the stacking configuration and twist angle of bilayer graphene on a Cu catalyst. In Part II.1, the effect of the atomic morphology of Cu catalyst surface on the stacking configuration of bilayer graphene was introduced. Atomic-flat and -stepped surfaces were obtained by controlling the crystal plane of the Cu catalyst and bilayer graphene was synthesized on each surface. On the atomic-stepped surface, bilayer graphene preferred to form small twist angles of less than 5°, while AB-stacking or 30°-twisted bilayer graphene was favored on the atomic-flat surface. Theoretical calculations support these experimental results. This study holds academic significance as it successfully synthesizes bilayer graphene with small twist angles, despite this being an energetically unstable state. Furthermore, it demonstrates the influence of the atomic morphology of the catalyst on the stacking configuration of bilayer graphene. Part II.2 introduces the highly selective growth of AB-stacked and 30°-twisted bilayer graphene by controlling the rotation behavior of the graphene layer on the Cu(111) surface. Additionally, determination mechanism of stacking configuration during chemical vapor deposition was revealed. By adjusting the programmed growth temperature and engineering the edge state of the bottom layer's attachment to the Cu surface, the rotation behavior was accelerated or blocked. The edge state engineering leads to the production of bilayer graphene with AB-stacking (93%) and 30°-twisted stacking (54.5%), respectively. This study demonstrates the potential for controlling the stacking configurations and achieving a high selectivity in obtaining specific bilayer graphene. Part III introduces the large-scale synthesis of single-crystalline 10°-twisted bilayer graphene and investigates the mechanism behind twist angle formation. The key step in twist angle formation is producing the nickel carbide phase on the sub-surface of the catalyst. Methane was introduced during the Ni and Cu alloying process to create nickel carbide phase. With increasing growth temperature, the one of the nickel carbide layer transforms into graphene, forming the first layer of twisted bilayer graphene. Subsequently, the second layer is created through the precipitation of carbon dissolved in the catalyst, resulting in the formation of twisted angle of 10°. These findings highlight a novel synthetic approach for producing single-crystalline twisted bilayer graphene with a strong emphasis on high selectivity for specific twist angles. Also, this advancement contributes significantly to our understanding of the intrinsic physical properties and controlling of twist angle.