Lab talk
Jun 28, 2011
Charge storage observed on mesoscopic graphitic islands
Graphite, an allotrope of carbon, is composed of stacks of two-dimensional graphene sheets weakly coupled together by van der Waals forces. The delocalization of the π electrons along the sp2 carbon network leads to metallic conduction in a graphene layer while the weak coupling between layers greatly reduces electrical conduction in the c-direction. Isolated graphene sheets have become a popular material due to their extra-ordinary physical and electrical properties. Spatially confined graphene and its derivatives are of great interest for devices such as field effect transistors, super capacitors and resistive switching memory.
Recently, researchers have reported local storage of charge in mesoscopic graphitic islands (MGIs) – few-layer graphene cut out in the form of islands but remaining embedded in highly oriented pyrolytic graphite (HOPG) substrates. The embedded MGIs are isolated from the surrounding graphite surface by nanometre-scale trenches formed by local electrochemical reactions using a biased atomic force microscope (AFM) tip. By applying a bias voltage between the tip and the HOPG substrate, the spontaneous formation of a water meniscus generates several oxidative species (O–, OH–), leading to the local oxidation and etching of the underlying graphite surface.
Interestingly and unexpectedly the MGIs are found to store electrical charge, as revealed by electrostatic force microscopy (EFM). The retention of charge seems to be nearly permanent as there is only a weak interaction between the MGI and the bottom graphitic layers. What makes it even more interesting is that the charge storage is right on the conducting surface.
These results may be exploited in graphene devices if MGI extraction and transfer onto desired substrates such as boron nitride can be achieved. In addition, the oxy-functional groups in the trenches offer a means of further functionalization with molecules carrying suitable mating groups of specific interest.
The work has resulted from collaboration between the Jawaharial Nehru Centre for Advanced Scientific Research, India, and the Birck Nanotechnology Center at Purdue University, US, and has been published in the journal Nanotechnology.
About the author
G U Kulkarni received his PhD in solid state chemistry (1992) from the Indian Institute of Science, Bangalore, India. He is currently professor at the Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research. His main research interests are direct write nanolithography, self assembly, optical and electronic properties of nanomaterials, molecular wires and crystals. He has published more than 170 research articles and coauthored a book on nanocrystals.
Ronald G Reifenberger received his PhD in physics (1976) from the University of Chicago, US. He is currently professor of physics at Purdue University and a member of Purdue's Center for Sensing Science and Technology. Since the 1980s his group has focused on research problems that emphasize the role of scanning probe microscopy (SPM) as one of the key enablers of nanotechnology. His current research is focused on non-linear dynamics of SPM cantilevers, micro patterning of substrates for the rapid detection of targeted bacteria, and fundamental measurements related to current flow in molecules, carbon nanotubes and gold nanocluster networks. He has published more than 180 refereed publications and co-authored four US patents.
