Graphene etched with an underlying silica substrate produces uneven edges (left), but forms precise edges when placed on boron nitride (right). Image: Guangyu Zhang.
Graphene etched with an underlying silica substrate produces uneven edges (left), but forms precise edges when placed on boron nitride (right). Image: Guangyu Zhang.

Researchers from China and Japan have developed a way to produce monolayer graphene nanoribbons with zigzag edges, a prized material for use in advanced spintronic devices and semiconductors. They report their work in a paper in Applied Physics Letters.

Miniscule ribbons of graphene are highly sought-after building blocks for semiconductor devices because of their predicted electronic properties. But making these nanostructures has remained a challenge. "Many studies have predicted the properties of graphene nanoribbons with zigzag edges," said Guangyu Zhang from the Beijing Key Laboratory for Nanomaterials and Nanodevices, and senior author of the paper. "But in experiments it's very hard to actually make this material."

Previously, researchers have tried to make graphene nanoribbons by placing sheets of graphene over a layer of silica and using atomic hydrogen to cut strips with zigzag edges, a process known as anisotropic etching. These edges are crucial to modulating the nanoribbon's properties.

But this method is only really able to produce ribbons made up of two or more graphene layers; irregularities created by electronic peaks and valleys on the surface of the silica layer make it difficult to etch precise zigzag edges into graphene monolayers. Zhang and his colleagues from the Chinese Academy of Sciences’ Beijing Key Laboratory for Nanomaterials and Nanodevices and the Collaborative Innovation Center of Quantum Matter teamed up with Japanese collaborators from the National Institute for Materials Science to solve this problem.

They replaced the underlying silica layer with boron nitride, a crystalline material that's chemically sluggish and has a smooth surface devoid of electronic bumps and pits. By using this substrate and the anisotropic etching technique, the group successfully made graphene nanoribbons that were only one-layer thick and had well-defined zigzag edges.

"This is the first time we have ever seen that graphene on a boron nitride surface can be fabricated in such a controllable way," Zhang explained.

The zigzag-edged nanoribbons showed high electron mobility in the range of 2000cm2/Vs, even at widths of less than 10nm – the highest value ever reported for these structures. They also possessed clean, narrow energy band gaps, which makes them promising materials for spintronic and nano-electronic devices.

"When you decrease the width of the nanoribbons, the mobility decreases drastically because of edge defects," said Zhang. "Using standard lithography fabrication techniques, studies have seen mobility of 100cm2/Vs or even lower, but our material still exceeds 2000cm2/Vs even at the sub-10nm scale, demonstrating that these nanoribbons are of very high quality."

In future studies, extending this method to other kinds of substrates could allow the quick, large-scale processing of monolayers of graphene to make high-quality nanoribbons with zigzag edges.

This story is adapted from material from the American Institute of Physics, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.