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Better ways to produce graphene nanoribbons for nanotechnology applications

In results that augur well for nanotech uses for graphene, two research groups have published in the same issue of Nature two different ways to unzip carbon nanotubes to create graphene ribbons, which heretofore have been more difficult than carbon nanotubes to produce in quantity. The Stanford method produces ribbons just 10-20 nm wide, which are semiconducting because of their narrow width and thus could prove important to the electronics industry. The Rice method produces ribbons 100-500 nm wide, which could be of use in solar panels and flexible displays.

From Stanford University, via AAAS EurekAlert “Nanoribbons from sliced open nanotubes: new, faster, more accurate method from Stanford“:

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 [abstract]. …

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 Stanford researchers expect that their method of making narrow graphene nanoribbons will lead to semiconductor device fabrication. From the abstract of their Nature paper:

Unzipping CNTs with well-defined structures in an array will allow the production of GNRs with controlled widths, edge structures, placement and alignment in a scalable fashion for device integration.

From Rice University, via AAAS EurekAlert “Rice researchers unzip the future: Simple process makes thin, conductive nanoribbons“:

Scientists at Rice University have found a simple way to create basic elements for aircraft, flat-screen TVs, electronics and other products that incorporate sheets of tough, electrically conductive material.

And the process begins with a zipper.

Research by the Rice University lab of Professor James Tour, featured on the cover of the April 16 issue of the journal Nature [abstract], has uncovered a room-temperature chemical process that splits, or unzips, carbon nanotubes to make flat nanoribbons. The technique makes it possible to produce the ultrathin ribbons in bulk quantities.

These ribbons are straight-edged sheets of graphene, the single-layer form of common graphite found in pencils. You’d have to place thousands of them side by side to equal the width of a human hair, but tests show graphene is 200 times stronger than steel.

“If you want to make conductive film, this is what you want,” said Tour, Rice’s Chao Professor of Chemistry and also a professor of mechanical engineering and materials science and computer science [and also 2008 winner of the Foresight Institute Feynman Prize in the Experimental category]. “As soon as we started talking about this process, we began getting calls from manufacturers that recognized the potential.”

The process involves sulfuric acid and potassium permanganate, which have been in common use since the 1890s. This chemical one-two punch attacks single and multiwalled carbon nanotubes, reacting with the carbon framework and unzipping them in a straight line.

The unzipping action can start on the end or in the middle, but the result is the same — the tubes turn into flat, straight-edged, water-soluble ribbons of graphene. When produced in bulk, these microscopic sheets can be “painted” onto a surface or combined with a polymer to let it conduct electricity.

Nanotubes have been used for that purpose already. “But when you stack two cylinders, the area that is touching is very small,” Tour said. “If you stack these ribbons into sheets, you have very large areas of overlap. As an additive for materials, it’s going to be very large, especially for conductive materials.” …

Nearly all of the nanotubes subjected to unzipping turn into graphene ribbons, Tour said, and the basic process is the same for single or multiwalled tubes. Single-walled carbon nanotubes convert to sheets at room temperature and are good for small electronic devices because the width of the unzipped sheet is highly controllable. But the multiwalled nanotubes are much cheaper starting materials, and the resulting nanoribbons would be useful in a host of applications.

That’s why Tour is banking on bulk, made possible by processing multiwalled tubes, which unzip in one hour at 130 to 158 degrees Fahrenheit. (Until now, making such material in more than microscopic quantities has involved a chemical vapor deposition process at more than 1,500 F.) “Multiwalled carbon nanotubes are concentric tubes, like Russian nesting dolls,” he said. “We cut through 20 walls, one at a time, during the reaction process.”

It seems increasingly likely that graphene will play a major role in the development of nanotechnology—unusual and useful properties, progress in creating atomically precise nanostructures, and now, ways to make enough to consider practical applications.
—Jim

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