Chemical Vapor Deposition Synthesis, Hydrogen Etching And Homoepitaxial Growth Of Single-crystal Graphene

Graphene is a two-dimensional (2D) crystal of sp2hybridized carbon atoms arranged in a hexagonal honeycomb lattice. In2004, Novoselov et al. prepared mono-and few-layer graphene and studied their electrical properties. Since then, graphene’s novel properties have been demonstrated experimentally one by one, and have motivated great research interest in this area. On the other hand, graphene have shown great promise in applications such as transparent conductive films, high frequency electronic and energy storage devices. The realization of novel physics and applications relies on high-quality materials. Accordingly, the preparation of graphene has emerged as an important issue in graphene research. In2009, chemical vapor deposition (CVD) was developed as a method for large-scale fabrication of monolayer graphene.Though it has already made considerable progress on CVD graphene synthesis, the preparation of high-quality single-crystal graphene still remains a significant challenge. This dissertation is about CVD synthesis and hydrogen etching of large-size single-crystal graphene grains, and lateral homoepitaxial growth of graphene. There are five chapters in this dissertation:the first chapter is an introduction of fundamental concepts, some recent advances in graphene synthesis and applications, and the motivation of this dissertation; the second chapter is controllable synthesis of submillimeter single-crystal monolayer and multilayer graphene grains on copper foils by pregrowth longtime annealing; the third chapter is about three types of hydrogen etching behavior of large-size graphene grains on copper foils; the fourth chapter is a demonstration of lateral homoepitaxial growth of graphene, and synthesis of large graphene grains by a secondary/multiple growth method; the fifth chapter is the conclusion and prospect. The main content of chapter two to five is given below.1. In chapter two, we successfully synthesized submillimeter single-crystal graphene grains on copper foils by chemical vapor deposition. In this study, we first developed a method via oxidation of copper substrate to observe the nucleation density and morphology of graphene grains on copper. Optical microscopy and SEM results showed that suppression of graphene nucleation can be achieved with decrease in methane concentration or increase in pregrowth annealing time of copper substrates. We proposed that the decrease of graphene nucleation density was caused by annealing inducing reduction of volatile impurities and defects on a copper surface. We synthesized submillimeter-size square and hexagonal shaped graphene grains on copper foils by methane CVD at1045℃with3h pregrowth annealing. AFM, Raman spectra, SAED, and POM showed that the submillimeter graphene grains were monolayer single crystals. Raman spectra indicated that monolayer graphene grains can only be obtained with a cooling rate of about0.1℃/s, while higher cooling rates resulted in multilayer graphene grains.2. In chapter three, we studied hydrogen etching of large-size graphene grains synthesized by the method proposed in chapter two. We observed the formation of regular-shaped pits in graphene grains under constant cooling rates. High quality graphene was retained at the edge of the etching pits, which was confirmed by Raman spectra. The resultant increase in hydrogen concentration or decrease in cooling rate produced an expansion of the size and density of etching pits. We also observed etching of graphene grains from the edge, which tended to destroy graphene structure and produce irregular shaped pits as confirmed by Raman spectra and optical microscopy. At the last part, we demonstrated a new kind of hydrogen etching of graphene at1045℃which cutting2D graphene into1D graphene nanoribbons with different widths down to100nm.3. In chapter four, we focused on demonstration of lateral homoepitaxial growth mechanism of graphene. We showed that exfoliated graphite/graphene flakes as well as CVD synthesized graphene grains can sever as seed crystals for lateral homoepitaxial single-crystal graphene growth. The epitaxial relationship was confirmed by electron diffraction and HRTEM lattice fringe images. Based on these findings, we developed a secondary/multiple growth process to epitaxially synthesize large-size single-crystal graphene films. Using optical microscopy and Raman mapping, we observed regular distribution of small multilayer grains in the large size graphene grains, and explained the formation mechanism of these microstructures. We also fabricated FET device using large graphene grains prepared by secondary growth method, and measured its electrical properties.