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Chemists Synthesize Narrow Ribbons of Graphene with Heat and Light

08 December 2017

Illustration of the molecular structure of the graphene nanoribbons prepared by UCLA chemistry professor Yves Rubin and colleagues. Source: Yves RubinIllustration of the molecular structure of the graphene nanoribbons prepared by UCLA chemistry professor Yves Rubin and colleagues. Source: Yves RubinSilicon is an essential ingredient of modern-day electronics. As electronic devices have become smaller and smaller, creating tiny silicon components that fit inside them has become more challenging and more expensive.

Now, UCLA chemists have developed a new method to produce nanoribbons of graphene, next-generation structures that many scientists believe will one day power electronic devices.

The nanoribbons are extremely narrow strips of graphene, the width of just a few carbon atoms. They are useful because they possess a bandgap, which means the electrons must be “pushed” to glow through them to create electrical current, said Yves Rubin, a professor of chemistry at UCLA and the lead author of the research.

"A material that has no bandgap lets electrons flow through unhindered and cannot be used to build logic circuits," he said.

Rubin and his research team constructed graphene nanoribbons molecule by molecule using a simple reaction based on ultraviolet light and exposure to 600-degree heat.

"Nobody else has been able to do that, but it will be important if one wants to build these molecules on an industrial scale," said Rubin, who also is a member of the California NanoSystems Institute at UCLA.

The process improves upon other existing methods for creating graphene nanoribbons. One of these methods involves snipping open tubes of graphene as carbon nanotubes. That approach is imprecise and produces ribbons of inconsistent sizes. This is a problem because the value of a nanoribbon’s bandgap depends on its width.

To create the nanoribbons, the scientists started by growing crystals of four different colorless molecules. The crystals locked the molecules into the perfect orientation to react. The team then used light to stitch the molecules into polymers, which are large structures made of repeating units of carbon and hydrogen atoms.

The scientists then placed the shiny, deep blue polymers in an oven containing only argon gas, and heated them to 600 degrees Celsius. The heat provided the necessary boost of energy for the polymers to form the final bonds that gave the nanoribbons their final shape: hexagonal rings composed of carbon atoms and hydrogen atoms along the edges of the ribbons.

"We're essentially charring the polymers, but we're doing it in a controlled way," Rubin said.

The process took about an hour and yielded graphene nanoribbons just eight carbon atoms wide but thousands of atoms long. The scientists verified the molecular structure of the nanoribbons, which were deep black in color and lustrous, by shining light of different wavelengths at them.

"We looked at what wavelengths of light were absorbed," Rubin said. "This reveals signatures of the structure and composition of the ribbons."

Rubin and his team are now focusing on how to better manipulate the nanoribbons. This is a challenge because nanoribbons tend to stick together.

"Right now, they are bundles of fibers," Rubin said. "The next step will be able to handle each nanoribbon one by one."

A paper on this research was published in the Journal of the American Chemical Society.

To contact the author of this article, email Siobhan.Treacy@ieeeglobalspec.com


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