Silicon-based technology may soon reach its limit, and a promising alternative is to use individual sheets of carbon atoms called graphene. Researchers at the National Institute of Advanced Industrial Science and Technology in Tsukuba and the University of Reading, UK, have observed of graphene sheets with open bonds at their edges, which could provide some interesting electronic properties.

“We anticipate wide applications for graphene in condensed matter physics and electronics,” says project scientist Zheng Liu. “However very few studies have looked at its edge structures, which may play a key role.”

Previously studies have found graphene edges terminated with hydrogen, or folded, closed edges, but to-date, there has been no evidence of completely open edges. These open edges could have either a ‘zigzag’ or ‘armchair’ structure depending on the orientation of the carbon hexagons.

Liu, Kazu Suenaga and co-workers1 heated graphite powder to 2000 °C to produce graphene sheets with neat, straight edges, and examined their samples under transmission electron microscopy. They obtained very high resolution and contrast without damaging the specimen, by using a low electron dose to take several images of the same specimen, and then superimposing the frames.

The images showed most graphene sheets to exist in pairs—one on top of another. The paired-layers were joined together at their edges by a curved section similar to half of a carbon nanotube.

Fig. 1: Transmission electron microscope images and schematics of graphene sheets, which exist as bilayers joined by a closed folded edge. For edges with an ‘armchair’ structure (a,b,c) the open parts appear as loose carbon hexagons, while edges with the ‘zigzag’ structure (d,e,f) show dangling carbon bonds.Copyright © Liu & Suenaga 2008

However, the researchers also found areas where the closed edges were broken. On edges with the armchair structure, the broken parts took the form of exposed carbon hexagons, while zigzag edges showed two dangling carbon bonds (Fig. 1).

“We have not seen fully-open edges yet,” says Suenaga. “There is an intrinsic problem that the edge is likely to be curled and closed because open edges are energetically unstable. But now we have seen a partially open edge.”

The findings are very important for the development of graphene electronics, as Liu explains: “Graphene is a zero-gap semiconductor which precludes it from standard applications. However, band gaps can be introduced by confining electrons in narrow graphene ribbons. The presence of open edges will dominate the electronic properties of graphene ribbons.”