“Surface-Selective Synthesis of Graphene Nanoribbons on Nanowire Templates,” awarded to Yuegang Zhang

Yuegang Zhang came to the Materials Sciences Division from the Intel Corporation, where researchers have long studied the potential of carbon nanotubes for consumer electronics.

The “Discovery” track of Berkeley Lab’s Laboratory Directed Research and Development proposal review encourages bold, highly innovative concepts with strong potential for impact in their fields, independent of divisional programs and lab-wide initiatives. In a five-part series we describe the winning proposals for 2010. This is Part Five.

“The scientific possibilities were intriguing,” Zhang says, “but applications were a different story.” Industrial-scale processes require pure materials, tight control over quality, and a high yield. Carbon nanotubes fail on all counts.

“Just getting carbon nanotubes lined up is a problem,” Zhang says. “They have a tendency to get tangled like a garden hose.”

Moreover, the electrical properties of carbon nanotubes depend on their diameter and even on their helicity – the angle, known as the chiral angle, at which a sheet of carbon atoms is wound into a tube.

Yuegang Zhang (photo Roy Kaltschmidt)

Yuegang Zhang (photo Roy Kaltschmidt)

“A carbon nanotube and graphene are cousins,” Zhang says. “Both are a single layer of carbon atoms. So if you slice a sheet of graphene into ribbons, you should get similar properties, except for the edges. And for nanoelectronic applications, graphene actually looks more interesting.”

Industrial tools for two-dimensional processing are well advanced, but with standard silicon-chip techniques, controlling edge structures remains a challenge. With resolutions of at best 10 nanometers (10 billionths of a meter), even the most advanced electron-beam lithography method leaves nanoribbons with rough edges.

A more basic problem is that graphene is normally semimetallic. “That’s not good enough,” says Zhang. “To build switches and transistors we need a semiconductor with a band gap.”

This is possible, but only if the carbon nanoribbons are very narrow. “We need a ribbon less than five nanometers wide to make a semiconductor with a band gap between 0.1 and 0.5 eV” – from a tenth to a half an electron volt, narrow but wide enough to do the job. “But most of all we need smooth edges.”

Zhang believes the most promising path is to grow narrow nanoribbons of graphene on nanowire templates, perhaps silicon carbide, “which has already been used for thin films and nanowires. Although the wires are still too big, nevertheless this could prove the principle.”

There are other possible approaches, such as nanoribbons self-assembled from carbon vapor deposition on a template, but whatever the solution, says Zhang, “the race for carbon nanoribbons is a hot topic and the competition is severe. The Molecular Foundry is the perfect place to make fast progress.”

Zhang credits the Discovery LDRD program with offering a congenial path for a scientist with lots of industry experience “but who’s not used to applying for grants,” a scientist who’s just beginning his career in a DOE national laboratory.

The prospect of producing perfect carbon nanoribbons is just the kind of high-risk but potentially high-payoff project the Discovery-track LDRDs were designed to address.

Additonal information
A Novel Route to Discovery, Part One

A Novel Route to Discovery, Part Two

A Novel Route to Discovery, Part Three

A Novel Route to Discovery, Part Four

Berkeley Lab’s Laboratory Directed Research and Development page

The complete list of 2010 LDRDs

Call for 2011 proposals