Nucleation and growth studies of graphene synthesis on Cu surfaces

Abstract

In this dissertation, two major issues of graphene growth and solutions for two issues were discussed. First issue is developing transfer-free graphene growth process on target substrate (chapter 3 and 4) and the other issue is study on graphene growth and obtaining high-quality and large grain size graphene via understanding nucleation and growth kinetics (chapter 5 and 6). In introductory part (chapter 1 and 2), general overview on graphene and growth methods is presented. Literature survey on previous graphene growth work and motivation for following work in this dissertation are also covered in chapter 1. In the chapter 2, review on various approaches for obtaining high-quality graphene will be covered based on its fundamental aspects of nucleation and growth. The relationships between each parameters of growing graphene and properties of as-grown graphene are discussed. In chapter 3, a rapid graphene growth method that can be carried out on any desired substrate, including insulator, thus negating the need for the transfer from metal substrate is introduced. Rapid annealing of a bilayer of a-C and metal deposited on the surface leads to the formation of graphene film, and to subsequent breaking-up of the thin metal layer underneath the film, resulting in a formation of a graphene-metal hybrid film which is both transparent and electrically conducting. In chapter 4, graphene film on silicon substrates having various orientations by simple heating in the presence of carbon source gas is discussed. We observed that a 3C-SiC (111) film would form upon carburizing silicon with carbon deposited from a carbon source because it is well lattice-matched with Si (110) (less than 2 %). Graphene grew on the buffer layer of 3C-SiC (111). The surface consists of hexagonal arrays that can act as a template for graphene growth. In chapter 5, the effect of gas transport inside a micrometre-scale jig gap on the growth of graphene on Cu foil located in the gap is reported. Due to the small size of the gap, a boundary layer is fully developed inside the gap, and the gas molecule transport is controlled by the molecular flow. First, the Cu surface is protected from the sublimation and re-deposition of Cu during pre-annealing, which results from the relatively static gas environment of the molecular gas flow. Second, suppression of the gas conductance resulted in strongly reduced overall graphene coverage with a smaller average grain size but with almost the same density as that of the graphene nuclei. Furthermore, the suppression of gas conductance leads to the formation of well-bounded graphene morphology instead of a dendritic morphology. In chapter 6, continuous graphene layer was grown on top of liquid Cu surface and grain boundaries were revealed by SEM (Scanning electron microscopy) and optical characterization via Cu oxidation. Hydrogen etch revealed the grain boundaries of graphene on liquid Cu easily. Small gaps exist between graphene islands even after few hours of growth time, thus, CH4 flux was increased at the final step of chemical vapor deposition growth in order to confirm the stitching of graphene islands. Hydrogen etch and optical characterization clearly demonstrated that graphene islands are uniform before and even after the merging of its graphene grains and tiny gaps between graphene islands were fully stitched by two-step growth method. Transmission electron microscopy images and diffraction pattern study revealed the importance of self-assembly. The resistance was measured by TLM (Transfer length measurement) pattern. Grain boundary resistance was considered to be negligible if two grains meet in the same direction without rotation of atomic lattice on liquid Cu, which has a great impact compared to typical grain boundary. In conclusion, novel graphene growth methods for direct formation on target substrate were suggested. Also, based on the fundamental understanding of nucleation and growth of graphene synthesis, two approaches are studied which are enlarging grain size and self-assembly. Graphene growth on catalytic metal surfaces is governed by heteronuclei effect, and overcoming way is self-assembly via growth on liquid catalyst according to this study.