Graphene and its use in flexible electronics

Wang, Xiaohan, Ph. D.
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Graphene, a single layer of sp2 hybridized carbon atoms, was first isolated from graphite in 2004. It is the thinnest material known, but it is exceedingly strong, light and flexible. It conducts heat better than diamond, and may conduct electricity better than silver. This unique combination of properties makes graphene an ideal platform for flexible electronics. In the last decade, much effort has been devoted to synthesize graphene and then place (also known as “transfer”) it onto a flexible substrate for device applications. However, a large-scale and cost-effective method to accomplish this is missing, which limits the use of graphene in high-performance electronics. This dissertation reports an improved graphene synthesis. Oxygen on the catalytic copper surface was found to play an important role in graphene nucleation and growth during the chemical vapor deposition (CVD). Control of the surface oxygen enables repeatable growth of single-crystal graphene, the quality of which is among the best of reported for CVD graphene. The electrochemical reduction of graphene oxide was also explored as an alternative graphene synthesis. This method eliminates the high-temperature treatment and is more compatible with high-volume production. After CVD synthesis, graphene needs to be transferred from the copper surface onto a target substrate for device applications. Instead of using a chemical etching method to dissolve the copper, a rapid and nondestructive method was developed to delaminate the graphene from the copper surface. This produced isolated graphene films, which can be transferred onto dielectric substrates and patterned with lithography. In the latter process, poly(methyl methacrylate) (PMMA) is usually used as an electron-beam resist. The influence of PMMA residues on graphene properties was studied and a method to remove these residues from graphene surface was developed. This cleaned graphene surface demonstrated low surface friction and improved contact with the metal electrodes, which is desirable for coating and electronic applications. Finally, the possibility for scaling up graphene production in a roll-to-roll (R2R) system was explored. The CVD graphene cracks when a relatively low applied strain (~0.44%) is applied to the copper substrate. This provides a guideline for R2R system design and ultimately helps to achieve cheaper, faster, and more powerful graphene-based flexible electronics.