Graphene concept art

Graphene ribbons synthesised via ‘atomically precise’ method

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An international team of researchers has synthesised graphene nanoribbons using an extremely precise method which could remove a barrier to custom-designed carbon nanostructures necessary for quantum information applications.

Graphene nanoribbons are tiny strips of atom-thick carbon sheets, with potential applications in a range of nanoscale devices. However, the lack of atomic-scale precision associated with current top-down synthesis methods – cutting a graphene sheet into narrow strips – limits the practical applications of graphene ribbons.

Just one or two atoms difference in width can change the properties of the components dramatically, compromising the valuable properties of single layers of graphene.

The team of researchers, led by Department of Energy’s Oak Ridge National Laboratory, have developed a bottom-up method for building graphene nanoribbons which could have applications in building other nanoscale devices. This atomically precise method allows for the valuable properties of graphene to be retained as the sections of graphene are reduced further and further.

The laboratory made use of their expertise in scanning tunnelling microscopy in order to manipulate and explore the material: “These microscopes allow you to directly image and manipulate matter at the atomic scale,” said Dr Marek Kolmer, lead author of the Science paper describing the method.

“The tip of the needle is so fine that it is essentially the size of a single atom. The microscope is moving line by line and constantly measuring the interaction between the needle and the surface and rendering an atomically precise map of surface structure.”

In previous experiments, nanoribbons were synthesised on a metallic substrate, which suppresses the electronic properties of the nanoribbons.

“Having the electronic properties of these ribbons work as designed is the whole story. From an application point of view, using a metal substrate is not useful because it screens the properties,” Kolmer explained. “It is a big challenge in this field: how do we effectively decouple the network of molecules to transfer to a transistor?”

The standard decoupling approach involves removing the component from a vacuum and subjecting it to a multistep chemical process, which requires etching away the metal substrate. This compromises the extreme precision involved in creating the system.

Kolmer and his colleagues instead developed a design for a chemical precursor which would allow for synthesis directly on the surface of titanium dioxide (on-surface synthesis). Their synthesis process allows them to obtain certain properties which are “essentially programmed into the precursor”, giving them precise control over the system. The resulting system could be explored further; its wide bandgap could make it suitable as the basis of a nanoscale transistor, for instance.

This process also helped the researchers maintain an open-shell structure, which allows them atom-level access to explore molecules with unique quantum properties.

“It was particularly rewarding to find that these graphene ribbons have coupled magnetic states, also called quantum spin states, at their ends,” said An-Ping Li, who is also based at Oak Ridge National Laboratory. “These states provide us a platform to study magnetic interactions, with the hope of creating qubits for applications in quantum information science.”

As there is minimal disturbance to magnetic interactions in carbon-based molecular materials, this approach could allow for long-lasting magnetic states to be “programmed” from within the material.

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