Since graphene sheets were first isolated using relatively crude mechanical techniques, production methods have advanced at a rapid rate. By using chemical vapour deposition (CVD) it is now possible to grow large sheets of graphene. However this technique is not ideal, as CVD is suited to producing only pure graphene, at very high temperatures. A group from Rice University have now demonstrated that large areas of pure and doped graphene can be grown using a simple one-step process [Sun et al., Nature, (2010) doi:10.1038/nature09579].
The new method, developed by Professor James Tour and colleagues, uses solid carbon sources heated under on a copper substrate to grow high quality sheets of graphene. Although the team began by using poly(methyl methacrylate) [PMMA, Plexiglass], they soon found that other carbon sources could be equally effective, including household sugar. In addition to being able to produce graphene using a simple process, the group also found that the growth could be performed at a relatively low temperature of 800 oC.
The revelation that both sugar and fluorene could be used to grow high quality graphene came as something of a surprise, as both materials contain “potential topological defect generators”. However at the temperatures involved in the growth, the atoms are able to reconfigure themselves to correct for prospective structural problems.
The source material is heated under a flow of hydrogen gas, which both carries the carbon and reduces the PMMA. Slowing the flow rate means that more carbon is present for the growth of graphene. It is therefore not only possible to grow single layer graphene, but simply by controlling the flow bilayer and multilayer graphene can be produced. Tour explains that “there is fine control to get very cleanly either monolayer vs. bilayer”, which means that the new method may be able to challenge the dominance of CVD. The significantly lower temperature of this process will also be of advantage to any manufacturers wishing to exploit the electrical properties of graphene, as high temperatures can damage heterogeneous substrate materials.
Nevertheless, pure graphene is perhaps not suited for use in electronics, as it has no bandgap and so is too metallic. By doping the material it is possible to produce a potentially more useful material. By adding nitrogen to the gas flow the researchers have demonstrated that it is possible to dope graphene in the same one-step process. At present the doped nitrogen is randomly positioned within the graphene, meaning that the bandgap can not be tuned, although Tour believes patterned doping will be “possible with some work”. This is part of the group’s strategy for the future, as they hope to grow “more elaborate structures of nanotubes/graphene hybrids” and look into “advanced doping”.
Stewart Bland