Graphene, a sheet of carbon just one atom thick, is a promising material for making molecular electronic devices of the future thanks to its unique electronic and mechanical properties that include extremely high electrical conductivity and exceptional strength. However, researchers need to better understand the interactions between the graphene surface and small molecules if efficient, real-world devices are to see the light of day.

Previous electrochemical studies looked at powders of graphene flakes attached to electrodes. It was therefore unclear as to whether the results from these experiments described the intrinsic properties of graphene, or simply an increased microscopic surface area.

"By carefully studying the electrochemical behaviour of individual monolayer graphene sheets with well-defined areas, our work has removed these uncertainties and we have now been able to elucidate the intrinsic electrochemical properties of graphene," team leader Daniel Ralph told nanotechweb.org.

Increased electron transfer rates
The Cornell researchers studied graphene samples made by mechanical exfoliation (the so-called "sticky-tape" technique) and those grown by chemical vapour deposition. In both cases, they found that the rate at which electrons from the chemical ferrocenemethanol were transferred onto the surface of the graphene was more than 10 times faster than the rate at which they transfer onto bulk graphite (the material from which graphene is "shaved off").

As an added bonus, Ralph and colleagues were also able to detect molecules desorbing from the surfaces of individual monolayer graphene sheets in real time. "These results provide new insights into the properties of graphene and show that electrochemistry is a powerful way to investigate the interactions between molecules and graphene surfaces," said Ralph.

The researchers were able to achieve these results because they succeeded in fabricating individual high-quality graphene sheets and use these sheets as electrodes. Because they were looking at the surface properties of graphene, the fabrication steps were optimized to minimize contamination on the material's surface. "We also carefully designed our device geometry to ensure that the metal electrodes contacting the graphene were not exposed to the electrochemical solution used in the experiment," explained Ralph. "This meant that the graphene surface was the only electrochemically active area during measurements."

The team confirmed that electrochemical reactions occurred on clean, well-defined areas of the graphene surface using surface characterisation and cyclic voltammetric measurements.

Graphene more reactive
The scientists reckon that the enhanced electron transfer rate is most likely related to the fact that graphene is more reactive than bulk graphite – something that has already been suggested. This could be because graphene sheets contain corrugations that are not present on the atomically flat surface of bulk graphite. "These corrugations lead to strain in graphene sheets at the atomic scale, which, in turn, activates the graphene surface to chemical reactions," said Ralph. "Our results also indicate that graphene, when compared with bulk graphite, is more efficient at transferring electrons to molecules in solution."

The methods employed in this study could be extended to understand the interactions between graphene and molecules other than ferrocenemethanol. And because higher electron transfer rates mean quicker electrochemical reactions, the higher electron transfer rates observed for graphene could lead to practical applications such as batteries and sensors, he adds.

The researchers would now like to use graphene as the electrode material for studying the electrical properties of a wide range of molecules. "For example, we would like to be able to 'sandwich' different molecules between two different graphene sheets and measure the electrical conductance of the molecules as the distance between the graphene sheets is varied," added Ralph. The idea behind such a measurement is that graphene might serve as an almost pure, atomically well-defined electrode – something that would eliminate the uncontrolled atomic-scale variations present in ordinary "disordered" metal electrodes.

The work was detailed in ACS Nano .