Faster, more accurate method for producing nanoribbons
BY LOUIS BERGERON
A world of potential may lie tied up in graphene nanoribbons, particularly for electronics applications. But researchers have been hampered in their efforts to fully explore that potential because they had no reliable way of creating the large quantities of uniform nanoribbons needed to conduct extensive studies. Now a team at Stanford University under Hongjie Dai has developed a new method that will allow relatively precise production of mass quantities of the tiny ribbons by slicing open carbon nanotubes.
It is relatively easy to produce fairly uniform carbon nanotubes in large numbers. But being the tiny, delicate structures that they are, slicing open nanotubes requires a tender touch. "The key is to be able to open up the tubes without destroying the whole structure," Dai said. "I mean, it doesn't have any zipper on it, right?"
Dai's method effectively creates the needed zipper. Carbon nanotubes are placed on a substrate, then coated with a polymer film. The film covers the entire surface of each nanotube, save for a thin strip where the nanotube is in contact with the substrate. The film is easily peeled off from the substrate, taking along all the nanotubes and exposing the thin strip of polymer-free surface on each of them. A chemical etching process using plasma can then slice open each nanotube along that narrow strip. It's not unlike generating flat linguini noodles by slicing open bucatini, a long tubular pasta.
The process works not only on single-layer carbon nanotubes but also on nanotubes with concentric layers of nanotubes, allowing each layer to be sliced open along the same "dotted line." The work is detailed in a paper published in the April 16, 2009, issue of Nature. Dai, the J. G. Jackson and C. J. Wood Professor of Chemistry, is the senior author of the paper.
Given all the other methods of nanoribbon production that have been tried—lithography, chemical reactions and ultrasound-influenced chemistry—all of which failed to produce the needed quantity or quality of graphene nanoribbons, Dai's method is surprisingly simple. "Once we overcame the hurdle of how to unzip the nanotubes, everything seemed so obvious," he said. "It is one of those things where you go, 'Why didn't I think of that earlier?'"
In addition to being fairly straightforward and easy to do, the process can be extremely efficient. "We can open up every carbon nanotube at the same time and convert many nanotubes into ribbons at the same time," Dai said.
Depending on how large a surface they cover with nanotubes—anything from a chip to a wafer—Dai said his team can create anywhere from one to tens of thousands of graphene nanoribbons at a time. The ribbons can easily be removed from the polymer film and transferred onto any other substrate, making it easy to create items such as graphene transistors, which may hold promise as a way to possibly make high-performance electronic devices.
"How much better computer chips using graphene nanoribbons would be than silicon chips is an open question," Dai said. "But there is definite potential for them to give a very good performance."
Another advantage of Dai's method is that the edges of the nanoribbons produced are fairly smooth, which is critical to having them perform well in electronics applications.
The next step in the team's research is to better characterize the ribbons and try to refine their control of the production process. Dai said it is important to control the width of the ribbon and the edges of the structure of the ribbon, as those things could potentially affect the electrical properties of the ribbons and any device in which they are used.
Dai said that graphene nanoribbons have other uses in addition to potential electronics applications.
"It is a very nice system to study nanoscale phenomena, in general," he said. "This method now opens up all these things that we can explore."
Liying Jiao, a postdoctoral researcher in the chemistry department, and Li Zhang, a graduate student in chemistry, are co-first authors of the Nature paper and contributed equally to this work. Xinran Wang, a graduate student in physics, and Georgi Diankov, a graduate student in chemistry, also are authors of the paper.
The research was funded by Microelectronics Advanced Research Corporation—Materials, Structures and Devices Center, Intel and the U.S. Office of Naval Research.
Hongjie Dai is on the East Coast this week, so email or his cell phone number, (650) 714-3116, is the best way to reach him.