Mar 27, 2009
Smoothing out graphene ribbon edges
As-processed graphene nanoribbons are limited by their edges. This is because even minute deviations from the ideal edge shapes, "armchair" and "zigzag", can seriously degrade graphene's exceptional properties. Now, researchers in the US have developed a new method to smooth out nanoribbon edges using a heat treatment that makes most of the resulting edges either zigzag or armchair.
One of the most important challenges in graphene nanoribbon technology is controlling the edge morphology, says lead author Xiaoting Jia, a member of Mildred Dresselhaus' team at the Massachusetts Institute of Technology. This has proved difficult to do until now. The new technique is an efficient way to transform defective rough edges in the ribbons into atomically smooth ones using "Joule heating". Here, an electrical current is applied across a suspended graphene nanoribbon inside a high-resolution transmission electron microscope (HRTEM).
With enough heat, the carbon atoms at the edges start to move and reposition themselves either into zigzag or armchair configurations. The electronic properties of the nanowires depend on which configuration the edges are in – the wires are metallic when the edges are zigzag and less conducting when they are armchair-shaped. The researchers observe the transition from rough to smooth edges inside the HRTEM.
"This multidisciplinary work solves the edge roughness problem – the bane of many graphene researchers," Jia told nanotechweb.org. "Atomically smooth edges are essential for lots of applications, including electronic devices."
Smooth edges are also important for fundamental experimental physics – for example, when observing magnetic phenomena as the width of graphene ribbons goes down to just a few nanometres. Until now, previous methods to make graphene nanoribbons always produced rough edges that were difficult to study.
"Our method now makes graphene nanoribbon-based electronic devices possible, and also facilitates experimental work on certain properties of graphene that have only been predicted theoretically," said Jia.
The team, which includes researchers from the Terrones group at IPICyT in Mexico and computational scientists from the Oak Ridge National Lab (ORNL), now hopes to apply its findings to larger samples. It also plans to make real devices from graphene nanoribbons with smoothed edges. "This will allow us and our collaborators to measure further fundamental properties of this stunning material," added Jia.
The work was reported in Science.
About the author
Belle Dume is contributing editor at nanotechweb.org