Aug 11, 2009
New form of carbon created
Researchers in the US and France have found what they claim is a new form of carbon. The new material is made from layers of graphene – sheets of carbon atoms just one atom thick – stacked on top of one another in such a way that each layer is electronically independent. The researchers claim that the material, dubbed multilayer epitaxial graphene (MEG), could be used in carbon electronics instead of costly single and double layer graphene sheets.
Graphene was first isolated only five years ago by exfoliating individual atomic layers from graphite by using transparent sticky tape. Since then graphene has amazed physicists with its high conductance at room temperature and with a breaking strength 200 times greater than steel making it one of the strongest materials ever tested.
Edward Conrad from the Georgia Institute of Technology and colleagues have now grown graphene layers from a silicon carbide substrate in such a way that each layer is rotated by 30 degrees with respect to the lower layers. This MEG differs from naturally occurring graphite where each layer is rotated by 60° with respect to the lower layers.
"With the stacking we see in natural graphite, which results in a special coupling between the layers, it seems like nature has managed to stack graphene sheets in a way that is not useful for electronics at all," says Conrad.
The researchers performed X-ray scattering and Angle Resolved Photoemission (ARPES) spectroscopy on a sample of MEG with 11 layers of graphene to measure its electronic structure. They found that the electron energy in a certain part of the band structure is proportional to its momentum giving the indication that the electrons behave like massless particles.
"This perfectly linear band structure, known as a Dirac cone, has not been so clearly measured before on other samples of graphene," says Conrad. Using ARPES, Conrad and colleagues saw no distortion of the Dirac cone, which they conclude shows there is no electron coupling to other layers in the sample and thus each layer is electronically isolated.
Although the researchers are just beginning to understand why the layers in a MEG sample do not couple with each other given that they are chemically bonded, they claim that these unique properties could be used as a basis for carbon electronics instead of single or double-layer graphene films, which are tricky and costly to isolate.
Another advantage of epitaxial graphene is that it can be manufactured on a large scale. Graphene is relatively inexpensive to grow epitaxially, as in the case of MEG, being dominated by the price of silicon carbide, which costs about $100 per centimetre square to manufacture. This, according to Conrad, is still a million times less than making exfoliated graphene. The researchers are now attempting to make high speed transistors from MEG.
This research has been published on the arXiv preprint server.
About the author
Michael Banks is news editor of Physics World.