Removing contaminants from transferred CVD graphene
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BSTRACT 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.
KEYWORDS chemical vapor deposition (CVD), graphene, poly(methyl methacrylate) (PMMA), transfer
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Introduction
Graphene is a single-layer two-dimensional (2D) material, consisting of exclusively sp2 carbon atoms in a gigantic aromatic structure [1]. Since the successful isolation of monolayer graphene by mechanical exfoliation of highly-oriented pyrolytic graphite (HOPG) using a Scotch tape in 2004 [1], extensive research has been conducted on the preparation, functionalization and applications of this fascinating 2D material [2]. Despite its tremendous potentials, the availability of high quality pristine graphene is still an ongoing challenge [3]. Graphene can be generally prepared by one of the following methods: mechanical exfoliation, liquid exfoliation, chemical exfoliation, thermal decomposition of silicon carbide (SiC), chemical vapor deposition (CVD), and chemical synthesis. The first successful graphene preparation, i.e., mechanical exfoliation using a Scotch tape, has been acknowledged to yield the highest quality graphene, as the starting material HOPG consists of many pristine graphene layers, and the peeling of which can give a single pristine graphene layer without contamination [1, 4]. The challenge, however, is to be able to produce single-layer graphene consistently and reproducibility, as peeling produces mostly multi-layer graphene and single-layer graphene requires multiple rounds of peeling. Additionally, the product is mostly irregular in shape and it is difficult to obtain samples larger than millimeters in size. Liquid exfoliation is another top-down fabrication method which involves reducing the size of graphite by subjecting graphite powders suspended in a solvent by mechanical forces like sonication or blending [5, 6]. This process is the most straightforward but it produces few-layer rather than the single-layer graphene. Also, the quality of the product is poor as mechanical agitation not only breaks the gr
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