Removing contaminants from transferred CVD graphene

Chemical vapor deposition (CVD) is among the most utilized techniques to fabricate single-layer graphene on a large substrate. However, the substrate is limited to very few transition metals like copper. On the other hand, many applications involving graphene require technologically relevant substrates like semiconductors and metal oxide, and therefore, a subsequent process is often needed to transfer CVD to the new substrate. As graphene is fragile, a supporting material such as a polymer film, is introduced during the transfer process. This brings unexpected challenges, the biggest of which is the complete removal of this support material without contaminating graphene. Numerous methods have been developed, each having advantages and drawbacks. This review will first introduce the classic transfer method using poly(methyl methacrylate) (PMMA) as the support material. The operating procedure and issues of PMMA residuals will be discussed. Methods to minimize/eliminate contamination will be presented, together with alternative approaches that do not require the use of PMMA.