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Epitaxial Single Crystal Graphene Growth through Cu Foil Secondary Recrystallization

Epitaxial Single Crystal Graphene Growth through Cu Foil Secondary Recrystallization
Alternative Author(s)
Min Yong Lee
Issue Date
This thesis is to investigate a synthesis of easy and economic implementation of high quality graphene. Secondary recrystallization occurred and reorientation was converted into (111) plane by heat treatment of commercial Cu foil, and on the foil grain boundary-less single crystalline graphene was confirmed and continuously large monolayer was implemented. The size evolution of grain boundary-less graphene layer was determined by the grain size and crystallographic orientation of underlying catalytic metal film under reasonable chemical vapor deposition conditions. Wafer-scale single crystal graphene has been facilitated by CVD along with single-crystal Cu (111). Without employing a supported single crystal substrate for epitaxial growth, it is not possible to obtain epitaxial single Cu (111) film. Here we report the repeatable synthesis of large (centimeter-scale) GB-less graphene monolayer on commercial polycrystalline Cu foils without supporting substrate. Control of pre-annealing, foil thickness and foil manufacturing method for (111) recrystallization and abnormal grain growth of Cu foils enables conversion to graphene hexagonal symmetry matched (111) orientation and grain migration without significant GBs simultaneously. Also, continuously GB-less graphene monolayer was completed without the need to add pre-treatment process for Cu surface. Hydrogen enriched Cu (111) surface during annealing also promoted the growth of low defective graphene layer. This study discussed especially about secondary recrystallization mechanism of single crystal Cu foil and single crystal graphene growth mechanism showing epitaxial growth on the Cu foil. This thesis consists of five chapters based on recent research trend of high quality graphene synthesis, conversion and properties of single crystal of (111) plane of Cu foil used for metallic catalytic substrate, mechanism of its secondary recrystallization (abnormal grain growth), properties of graphene grown as CVD on the Cu foil, and epitaxial single crystal graphene growth synthesis mechanism. It was considered that epitaxial growth of graphene without grain boundary using in-situ chemical vapor deposition on the Cu foil converted into (111) plane single crystal, which is the most thermodynamically stable state by heat treatment can be implemented as large as foil size. Conversion of Cu foil used for metal catalyst into SAED (Selected Area Electron Diffraction) XRD (X-ray Diffraction), SEM (Secondary Electron Microscopy) and TEM (Transmission Electron Microscopy). (111) plane single crystal by heat treatment was implemented in chapter 2. Recrystallization and grain growth, microstructure, plane direction and orientation, crystallinity were observed using EBSD (Electron Back Scatter Diffraction), Especially, the wideness was confirmed under L/B/C/T/R (left, bottom, center, top and right) in EBSD scale bar 300 μm. Cu foil with various plane orientations was confirmed to be converted into single crystal as only (111) oriented GB-less single grain after heat treatment. Single crystal Cu foil of (111) plane in the area of 1.5*1.5 cm2 was confirmed, and Cu foil with (100) twin particles of 2% was confirmed in an area of 8*8 cm2. Also, cross section side of Cu foil was confirmed for a single plane in both front and back surface side and core of bulk. In chapter 3, the Cu foil secondary recrystallization mechanism through microstructure change and converted crystallinity was investigated. Abnormal grain growth rate increased as foil thickness decreased according to Thompson’s equation. Abnormal grain growth only appeared in 10 and 12.5 μm in the case of rolled Cu foil and its rate of appearance significantly increased. The critical thickness of pristine Cu foil was thin enough to be penetrated by the secondary recrystallized grains. After heat treatment, lattice structure in Cu FCC recrytallized to thermodynamically stable state and the lowest surface energy (111) plane direction and sequently grain rearranged and GB migrated to occur abnormal grain growth and then reached only one (111) single grain finally. Also, It was confirmed that the amount of hydrogen atom at Cu surface increased throughout detection of TOF-SIMS with increasing of hydrogen flow rate during heat treatment. Also, the highest hydrogen absorption rate in (111) compared to (100) and (110) could lower the melting temperature of Cu surface and this accelerated abnormal grain growth to (111) direction on grain crystal structure by increasing of Cu diffusivity and solubility. This specific surface direction of abnormal grain in this research was confirmed to be (111) based on EBSD. Recrystallization and grain growth is thermal activation process and therefore, Cu foil single crystal conversion process was investigated by thoroughly observed grain size, microstructure, plane direction, crystallinity of Cu in heat treatment temperature and time respectively, Prior enlarged grains through Cu foil thickness thinning and the amount increased of hydrogen induced secondary recrystallization had different rotation angles and therefore, included grain boundary due to lattice mismatch each other. It is found that big different angles (>11 degree) were detected between large to large grains, but small to large grains surrounding the small grains showed very small angles (< 1.5 degree) with proximately the same angle. Finally, grain migration from large to small grains was investigated using EBSD to obtain linear analysis during the abnormal grain growth. In chapter 4, through in-situ chemical vapor deposition on the Cu foil for single crystal graphene layer was continuously epitaxially grown without GBs and its properties were evaluated. Defects of graphene, especially regarding to be grain boundary or not, plane direction of grain, the number of layers were observed in nano-scale, micro-scale and macro-scale through OM after RAMAN, SEAD, STM, and UV treatment. Continuously GB-less graphene monolayer provided the possibility to be synthesized as large as the converted Cu foil area. This single crystal graphene was confirmed to have excellent carrier mobility of 26000 cm/Vs. Growth mechanism of graphene without GB was investigated. Graphene being composed of only one grain and the same orientation to the Cu plane orientation of single plane orientation by analyzing LEED (Low Energy Electron Diffraction) 1 mm beam size was observed, and nucleation of deposited graphene grain matched between lattices with hexagonal symmetry on single crystal of FCC (111) Cu. LEED analysis clearly shows an epitaxial relationship between graphene and Cu foil. At early stage of CVD, graphene grain of the same orientation was found to bind with other grains without GBs.
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GRADUATE SCHOOL[S](대학원) > ENERGY ENGINEERING(에너지공학과) > Theses (Master)
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