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Atomically precise graphene nanoribbons become semiconductors

Atomically precise graphene nanoribbons (APGNRs) could be integrated onto nonmetallic substrates according to researchers from the Beckman Institute at the University of Illinois and from the University of Nebraska-Lincoln.

"APGNRs are serious candidates for the post-silicon era when conventional silicon transistor scaling fails," said Professor Joseph Lying from Beckman. "This demonstrates the first important step toward integrating APGNRs with technologically relevant silicon substrates."

Graphene stays much cooler and can conduct much faster than typical silicon crystals used as transistors. It needs to be in nanoribbon form, however, to act as a semiconductor. Despite much progress in the fabrication and characterisation of nanoribbons, cleanly transferring them onto surfaces used for chip manufacturing is said to have been a significant challenge.

"When you're going from the top-down, it's hard to get control over the width. It turns out that if the width modulates by just an atom or two, the properties change significantly," said Beckman doctoral student Adrian Radocea.

As a result, the nanoribbons must be made from ‘the bottom up’, from smaller molecules to create atomically precise nanoribbons with highly uniform electronic properties.

"The previously demonstrated synthesis on metallic substrates yields graphene nanoribbons of high quality, but their number is rather small, as the growth is limited to the precious metal's surface," said Alexander Sinitskii, associate professor at the University of Nebraska-Lincoln.

"It is difficult to scale this synthesis up. In contrast, when nanoribbons are synthesised in an unrestricted 3D solution environment, they can be produced in large quantities."

The difficulty lies in cleanly transferring nanoribbons stems. Both solution-synthesised and surface-grown nanoribbons are exposed to chemicals during the transfer process that can affect the performance of graphene nanoribbon devices. To overcome this challenge, the interdisciplinary team used a dry transfer in an ultra-high vacuum environment.

"I find the project exciting because you are building things with atomic level control, so you try to put every atom exactly where you want it to go," said Radocea. "There aren't many materials out there where you can say you have that ability."

Peggy Lee

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