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Scientists pave way for carbon-based computers

This is a scanning tunneling microscope image of wide-band metallic graphene nanoribbon, the first of its kind. Photo by UC Berkeley / Daniel Rizzo
This is a scanning tunneling microscope image of wide-band metallic graphene nanoribbon, the first of its kind. Photo by UC Berkeley / Daniel Rizzo

Sept. 25 (UPI) -- Today's transistors, the building blocks of modern electronics, are mostly composed of silicon, but scientists theorize carbon-based transistors eventually could power faster, more efficient computers.

Testing that theory has proven difficult, but thanks to a recent breakthrough, detailed Friday in the journal Science, researchers now are closer to finally building a transistor entirely from carbon.

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A team of chemists and physicists at the University of California-Berkeley succeeded in building a metallic wire out of carbon.

"Our breakthrough was to make metallic nanoribbons," study co-author Michael Crommie, professor of physics at Berkeley, told UPI in an email.

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Previously, scientists theorized that graphene nanoribbons could only form insulators or semiconductors. But to build transistors, scientists needed wires, the conductive metal channels that link semiconducting elements and connect the thousands of transistors that form a computer chip.

Though the most recent breakthrough, the ability to turn graphene nanowires metallic, materialized within the last two years, the advance followed more than a decade's worth of work on the assembly of graphene nanowires from molecular building blocks.

"The advantages of carbon over silicon become greatest when transistors are made very small, down to the size scale of individual molecules," Crommie said.

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In other words, to build carbon computer components, researchers first had to figure out how to build graphene from the ground up -- and with atomic-scale precision.

Most material scientists working with graphene at nano scales use what are called graphene nanotubes. Graphene nanotubes are versatile and practical, but their electronic properties aren't easily manipulated.

"You can make graphene nanotubes from virtually any carbon source -- ethanol, methane, wood -- but you cannot control their critical parameters like diameter and chirality with the same precision and reproducibility," Felix Fischer, a study co-author and Berkeley physics professor, told UPI in an email.

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Enter graphene nanoribbons.

In 2010, a different team of researchers -- led by Roman Fasel, a material scientist in Switzerland, and German chemist Klaus Mullen -- found a way to stitch together graphene molecules using standard chemistry techniques to form narrow strips of graphene, or graphene nanoribbons.

"Specific -- and useful -- properties can be engineered into the molecules even before they are stitched together into a graphene nanoribbon," Crommie said.

"You can think about this as a LEGO-like approach where the shape and size of your 'molecular' LEGO brick predetermines all critical dimensions like width, or edge structure that are responsible for the electronic properties of the resulting material, be it an insulator, semiconductor or like in our case, a topological metal," Fischer added.

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What Crommie, Fischer and their research partners realized was that the chemical techniques developed by Fasel and Mullen could be adapted to synthesize graphene building blocks with localized electronic states.

Crommie and company were able synthesize the building blocks in such a way that molecules symmetrically stitched themselves together, creating the electronic state of a metal.

"We also came up with a novel method for tuning how metallic our graphene nanoribbons can be by slightly changing their chemical bonding structure," Crommie said.

The next step is to use their now-complete graphene toolbox -- featuring semiconducting, insulating and metallic graphene nanoribbons -- to build a working transistor.

Such a feat could have a significant impact on everyday electronics.

"Think about the possible impact of a mobile phone with comparable performance to the fastest desktop computers but with a power consumption that requires you only to charge it every other month," Fischer said.

Carbon-based circuity is probably still several years away, but Fischer thinks the ability to synthesize metallic graphene nanoribbons was one the last major hurdles.

"The translation of this technology into an integrated circuit is still a steep and windy road ahead but we are confident that following a proof of concept that validates the predicted performance, the field will attract many more expert research groups and an increased federal and industrial backing to support this effort." Fischer said.

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