Growth And Strain Of CVD Graphene And Its Oxidation Resistance For Cu Foils

Graphene, a single flat layer of carbon atoms arranged in a honeycomb lattice with sp2 hybridized, resulting in a σ bond between carbon atoms and a π bond perpendicular to the planar structure. Its unique structure brings so many unique properties and make it great potential applications in optoelectronic devices, FET, transparent conductive electrode, sensor, SERS and energy storage devices.Graphene can be prepared by mechanical exfoliation, chemical reduction, CVD, thermal decomposition on SiC and some other methods, wherein the CVD is the most effective way to grow single layer or few layer graphene. At the same time, CVD, the most popular way to grow graphene, can be compatible with the semiconductor technology and expected to achieve the goal for the large-scale production of graphene. In order to reduce the grain boundary scattering and maintain the electrical properties of the entire film, it is essential to grow large single crystal of graphene on metal foils with controlled conditions and various pretreated substrates. In addition, due to the strong interaction between graphene and the substrate, it is difficult to grow graphene on insulating substrate directly. Understanding its growth mechanism can provide more feasibility of graphene applications in the semiconductor device. Therefore, in this dissertation, we mainly study the growth of graphene on metal and insulating substrate, and discuss the protection of graphene to Cu foil.In chapter one, we briefly introduce the structure, properties and preparation methods of graphene and give the background of our research, the problems to be solved and the research content.In chapter two, we prepare graphene flakes with various sizes directly on SiO2/Si substrate using a remote catalytic method. The flakes are characterized by Raman spectra, and the peak of the 2D band is shown a obvious blue shift due to the in-plane compressive strain induced by the different coefficients of thermal expansion between the graphene and the substrate. More importantly, the compressive strain of the flake is found to increase with the flake size. The behavior can be understood by the strain releasing through the defects and the edge of graphene flakes. Additionally, we find that the defects contribute more than 75% of strain relaxation for all graphene samples, and the width of the edge region for the strain relaxation increases from ～50 to ～110 nm for 2 and 5 h grown flakes. Our finding indicates that the compressive strain inevitably exists in the as-grown graphene on SiO2/Si, which may be eliminated through a new preparation method. Finally, aiming at the problem of high defects on the graphene grown on SiO2/ Si substrate, we realize the decrease of defect density through the reduction of the carbon source and a series of controlled experimental conditions by two step growth. At the same time, the self-repairing growth mechanism of graphene on insulating substrate is also suggested.In chapter three, we investigate the anti-oxidation of copper foil covered by the graphene. We first prepare monolayer together with multilayer graphenes on Cu substrate and then anneal the sample in the diluted oxygen ambience. It is found that in the oxidation process, the monolayer and bilayer graphene show different oxidation stripes and rate. The underlying mechansim can be attributed to their different channels for the oxidation. Due to the existed compressive strain, the stripes perpendicular to the strain are preferred oxidized and etched for the monolayer graphene. However, for the bilayer graphene, the oxidation can occur only through the spots where the oxidized stripes of the two stacked layers intersect, and consequently the oxidation rate is apparently reduced. The explanation can be identified by the Arrhenius equation fitting to the relationship between oxidation rate of different layers and the reciprocal of temperature. Moreover, the importance of defects in the oxidation protection of graphene is further supported by the controlled experiment of Ar+ bombardment. Our results show that the growth of multilayer graphene over the Cu foil is a simple and effective way to protect Cu from oxidation.In chapter four, we focus on the problem of the formation of new nuclei during the CVD growth of graphene. It is found that the nucleation density can be reduced effectively by using the secondary growth method. Moreover, our investigation results show that, to obtain the large-size single crystals of graphene, the suppression of nucleation density should be considered and handled not only just in the initial nucleation stage but also in the following growth stage. By carefully controlling the nucleation density with the oxidized Cu substrate, we successfully prepare the single crystal graphene with the size as large as ～2mm.In chapter five, as for the application requirements of graphene and the unsolved problems, we prospect the future challenges for the graphene research.