April 15, 2009

Rice University and Stanford University Researchers Have Unzipped Carbon Nanotubes into Thin Graphene Strips

Rice University researchers have unzipped carbon nanotubes to make graphene ribbons tens of nanometres wide. This is the cover story from the April 16, 2009 issue of the Journal Nature.

"Ribbon structures are very important structures and they're not easy to make," says James Tour, a chemist at Rice University in Houston, Texas. Early techniques used chemicals or ultrasound to chop graphene sheets into ribbons, but could not make ribbons in large amounts or with controlled widths.

As a solution, Tour and his co-workers, and a separate group led by Hongjie Dai of Stanford University in California, decided to try to generate ribbons from carbon nanotubes.

Dai and his colleagues opted to slice the tubes using an etching technique borrowed from the semiconductor industry. They stuck nanotubes onto a polymer film and then used ionized argon gas to etch away a strip of each tube. Once cleaned, the remaining ribbons were just 10–20 nanometres wide.

Tour's group, by contrast, used a combination of potassium permanganate and sulphuric acid to rip the tubes open along a single axis. The resulting ribbons are wider — around 100–500 nanometres — and not semiconducting, but easier to make in large amounts.

"The techniques complement each other," says Mauricio Terrones, a physicist at the Institute for Science and Technology Research of San Luis Potosi in Mexico, who was not involved in the work.

Nanowerk has coverage.

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. [There is separate recent MIT work in using heat to control the edges of graphite]

MIT Technology Review has coverage as well

Tour's unzipping method yields graphene in bulk, which is an advantage from a manufacturing perspective. But "[Dai]'s going to have better control," admits Tour. The width of the Rice group's nanoribbons is determined by the diameter of the nanotubes that they come from. In contrast, using the Stanford team's technique, it's possible to finely control the width of the nanoribbons. In today's publication, Dai and his colleagues describe nanoribbons six nanometers wide, but he says that they have subsequently made narrower and more semiconducting ones. "There might be an optimum width; that needs to be investigated," he says.

Tour's nanoribbons are easy to process because they are graphene oxide, which is soluble in water. "You can use sheer force to align them like logs in a river lining up in parallel," says Tour. "You can paint them down, and they will align." Tour adds that the nanoribbons can be made into devices using ink-jet printing. Once the ribbons are in place on a chip, they're treated with hydrogen at high heat to remove the oxygen at their edges and turn them into semiconductors. Without this step, the ribbons are insulators.

The Stanford research was funded by Intel, and Tour says that he is in talks with companies interested in licensing his manufacturing method as well as devices made with the nanoribbons.

Both techniques are likely to be useful to researchers, and both have a variety of potential applications. Tour believes that his larger ribbons could be used in solar panels and flexible touch displays, where cheap, transparent materials are in demand. They could even be spun into lightweight, conducting fibres that might replace bulky copper wiring on aircraft and spacecraft. Dai's narrower ribbons, meanwhile, might find uses in electronics because of their semiconducting properties

Dai says that his group has already used the ribbons to make basic transistors, but, he adds, it's too early to tell whether they will be commercially competitive. "It's very early in the game," he says.

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