Personal perspectives on graphene: New graphene-related materials on the horizon
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While working at the Molecular Physics Laboratory at SRI International in 1991, I noted an article by Iijima in Nature on carbon nanotubes.1 Considering graphite (three-dimensional) along with this article on carbon nanotubes (one-dimensional), I started thinking: Why not large two-dimensional sheets of sp2bonded carbon (now referred to as graphene)? One could expect such a material to have exceptional mechanical, thermal, and possibly electronic properties, from the well-known in-plane properties of graphite. However, my search of the literature did not find any examples of scalable methods of exfoliating graphite based on liquid-phase processing, although there were some tantalizing discussions of achieving enormous expansion in the interlayer spacing of other layered materials such as vermiculite2 and montmorillonite3 in the early literature. Eventually, the expansion can reach a point where the layers are so far apart from each other that crystal integrity is lost and the gentlest agitation of the liquid leads to complete exfoliation. Indeed, vermiculite, a phyllosilicate layered material similar to mica in structure, is exfoliated on a very large scale, and the thin layers obtained are used in many applications, including as an additive to cement.4 To date, liquid-phase processing methods for converting graphite to pristine graphene platelets that are scalable to tens
or millions of tons have not been reported. Meeting this challenge would provide such graphene “flake” material for a host of important applications, for example, conductive inks; filler for polymer, rubber, ceramic, and possibly metal composites; electrode material for electrical energy storage in ultracapacitors and batteries; conductive thin films; barrier films; and many others. After moving to the Physics Department at Washington University in 1996, I conceived of a method to achieve individual layers of graphite (i.e., graphene, as defined by the International Union of Pure and Applied Chemistry in a 1995 publication5) of controlled shape and lateral dimensions, through top-down lithographic patterning of graphite, such as from high-quality samples of highly oriented pyrolytic graphite (HOPG).6,7 Figure 1 shows the patterned pillars obtained using this process, and Figure 2 shows what happened when the pillars were rubbed with a piece of silicon wafer, as observed by scanning electron microscopy. Quoting from Reference 7: Since graphite can be easily cleaved along the basal plane, the islands can be transferred to flat surfaces of other substrates, such as mica or Si, simply by
Rodney S. Ruoff, Mechanical Engineering Department and Materials Science and Engineering Program, The University of Texas at Austin; [email protected] DOI: 10.1557/mrs.2012.278
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MRS BULLETIN • VOLUME 37 • DECEMBER 2012 • www.mrs.org/bulletin
© 2012 Materials Research Society
PERSONAL PERSPECTIVES ON GRAPHENE: NEW GRAPHENE-RELATED MATERIALS ON THE HORIZON
Figure 2. SEM images of multilayer graphene platelets transferred by rubbing onto a Si(001) substrate.
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