Chemical vapor deposition graphene on polycrystalline copper foil
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Graphene, a single atomic layer of sp²-bonded carbon, has been of significant interest to basic sciences and engineering. Among its unique properties are exceptional mechanical strength, from the strong carbon-carbon bond; high in-plane thermal conductivity; high carrier mobilities, since electrons and holes travel through graphene as mass-less Dirac fermions; and quantum effects (such as the quantum Hall effect), which can be observed at room temperature. In 2009, Li et al., of Professor Ruoff's research group at the University of Texas at Austin, published a seminal paper detailing the production of fairly high quality graphene grown on copper foils using chemical vapor deposition (CVD). The potential for scalability of graphene CVD processing is extremely attractive, and this is currently the most promising method for its commercial viability, particularly for transparent conductive electrodes (TCEs). Here, graphene-based TCEs are compared with TCEs made with multi-walled carbon nanotubes (MWCNTs). A novel technique to reduce the sheet resistance of MWCNT-based TCEs in half is described in detail. Even with these improvements, graphene-based TCEs outperform MWCNT-based TCEs. The decomposition of copper oxides at high temperatures in an oxygen deficient environment is characterized. The ability for the oxygen evolved from the copper foil during this decomposition to react with carbon on the surface of the copper substrate is verified. This phenomenon was used to develop a technique for getting clean pre-graphene growth copper substrates and allowing repeatable graphene nucleation results. A technique for growing large graphene domains inside a copper vapor trapping 'copper enclosure' is described. The quality of the graphene grown inside the copper enclosure is characterized and shown to be of very high quality. This technique can grow graphene domains over 0.5 mm across. Finally, a possible cause of graphene ad-layer growth on the copper surface is suggested. It is proposed that gas diffusing through the copper substrate at high temperature delaminates the graphene from the copper surface in some regions. This then allows carbon containing molecules to diffuse under the graphene and grow new graphene layers. The increased ad-layer growth in the presence of helium supports this.