Chemical Routes to Graphene for Applications

Graphene, a zero band gap semiconductor, is a promising candidate for the next generation of nano-electronic devices. While many theoretical studies of graphene promote exciting properties, experimental results have been far less forthcoming likely due to the difficulty in producing single layer specimens. Despite tremendous efforts made to develop a scalable production method, bulk processing has not yet been achieved. Single layer samples are currently made by either a laborious drawing method, highly oriented pyrolytic graphite (HOPG) is repeatedly pealed using Scotch tape and deposited onto a silicon substrate. Alternatively, the reduction of silicon carbide can be used to produce very small domains of single layer samples, however, temperatures of greater than 1100oC are needed and producing large domains remains a challenge. We suggest an alternative method for creating single sheets starting from graphite oxide (GO). Graphite can be oxidized to produce GO and then exfoliated to create stable aqueous dispersions of individual sheets. After deposition, GO may be reduced to graphene either chemically or via thermal annealing. The scale of single sheets produced allows for characterization by scanning electron and atomic force microscopies (SEM and AFM), Raman spectroscopy, and field effect measurements. It is through the combination of these diagnostics that we able to confirm the presence of single sheets. Such specimens show step heights of ~0.6 nm in AFM and exhibit strong G and 2D peaks in Raman spectroscopy, both good indications of their similarities to peeled graphene. Field effect measurements display p-type behavior, with currents in the mili-amp range with source-drain voltages of 1 V under ambient conditions. The lack of ambipolar effect is likely due to the presence of residual oxygen or sp3 hybridized carbons. With additional chemical preparation, reduced samples may indeed exhibit electrical properties approaching those of native graphene. A chemical approach toward graphene provides several advantages, including high scalability and solution processing. The method also allows for a variety of chemical modifications before deposition, something yet to be seriously investigated. As the demand for graphene increases, graphite oxide may well play an increasing role in production of samples.