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A study on intrinsic graphene defects of different origins in CVD-grown graphene

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Title
A study on intrinsic graphene defects of different origins in CVD-grown graphene
Other Titles
화학기상증착법을 통해 길러진 그래핀의 다양한 원인에 의해 형성된 고유 결함에 관한 연구
Author
Jo, Yongsu
Advisor
Kwon, Soon-Yong
Keywords
CVD; Graphene; Defects; Detection; Copper; Oxidation; Etching; Water impermeability
Issue Date
2020-08
Publisher
Graduate School of UNIST
Abstract
Chemical vapor deposition method has been spotlighted as a tool growing large-area, good quality, mass-producible and uniform graphene. In particular, the copper that grows a uniform monolayer of graphene has attracted researchers’ attentions as a graphene growth substrate. However, CVD graphene growth has a nature that graphene has various intrinsic defects such as point defects, grain boundaries, and wrinkles, undermining the properties of graphene. So, it is important to visualize various intrinsic defects in terms of optimizing graphene growth. Firstly, we examined selective oxygen permeation through atomic structural defects in CVD-grown graphene/copper composites measured by optical and electron microscopies. Using an simple air oxidation of graphene coated copper foils with heating of 200 °C, we developed a simple and large area characterization tool to visualize intrinsic graphene defects induced by different origins such as nucleation sites, inter grain boundaries and intra grain boundaries from CVD-grown graphene. The oxidation conduct of copper can vary with disorder of graphene structures according to sources of various graphene intrinsic defects such as growth condition, crystallographic orientations of copper substrates and growth rate. From the experimental and computational experiments, we found that oxygen atoms were dissociated from the water vapor of air are main origins oxidizing copper surface under graphene and selective oxygen permeation occurred at Stone–Wales defects into graphene membrane promoted by other accumulated oxygen atoms as catalyst. Secondly, we use HCl etching method to systemically investigate the water permeable origins on the graphene grown on copper by CVD observing copper etching pits through OM and SEM. When we observed the distribution of these etching pits through SE and BSE mode of SEM, it was found the copper etching pits were observed at the intersection of wrinkles of CVD-grown graphene and copper step bunches. In addition, it was also confirmed that nanosized ruptures, cracks, and holes were formed in folded graphene wrinkles through TEM. The HCl etching method can determine where the water permeations occur in the CVD-grown graphene, and these results will contribute to optimizing the graphene growth as production of water impermeable layer. Finally, we described a large-area technique to enhance impermeability of graphene by amorphous carbon layer deposited by an electron beam, sputtered carbon, and CH4 plasma. It was utilized to passivate the graphene nanosized ruptures, cracks and holes less than 3nm, developed on intersecting points of wrinkle and copper step bunch. In addition, the wrinkles on the part of multilayer graphene also had excellent water impermeability to copper etchants. Applying these carbon-based layers, flexible, transparent, and thin barrier films will be developed for water impermeable electrodes and substrates to safely protect future ubiquitous electronics from ambient air being exposed to everyday life
Description
Department of Materials Science and Engineering
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