A method for forming graphene nanoribbons (GNRs) might enable their properties to be tuned, widening their potential applications in electronic and spintronic devices.

Nanoribbons of this type are one dimensional nanostructures – one atom thick, a few atoms wide and effectively infinite in length – as opposed to parent graphene’s 2D construction. This gives GNRs important advantages over graphene. Electrons are free to travel along their length but are severely confined in the other two dimensions, which means GNRs’ electronic properties depend strongly on the width and structure of their edges.

A GNR can have one of two types of edge – armchair or zigzag. Those with a zigzag edge – as produced here – develop an electronic band gap that graphene does not have, making them more promising for transistors and other electronic devices.

The experimental method, investigated by researchers at the University of Nottingham, UK, and Ulm University in Germany, relies on producing a GNR inside a pre-formed nanotube. The nanotube’s tips are cut open and the tube loaded with reactant molecules (nanoreactors) containing carbon to build the GNR and sulphur to stabilise its edges.

The host nanotube is then encapsulated and a chemical reaction triggered using either heat or high-energy radiation. This stimulates the breaking of the reactant molecules into atoms or atomic fragments such as C2, which spontaneously re-assemble as nanoribbons. Raman spectroscopy or direct real-time imaging is used to prove nanoribbon formation within the nanotube.

A time-series of graphene
nanoribbons in a carbon
nanotubeLead Researcher at University of Nottingham, Dr Andrei Khlobystov, says this approach improves on existing methods of nanoribbon production. ‘Our method uses cheap, readily available molecules as precursors for nanoribbons. The exact structure and composition of the starting molecules, their concentration or relative ratio are unimportant, as the nanoribbons essentially self-assemble, picking just the right amount of carbon and sulphur from the broth of atoms within the nanotube,’ he says.

Tuning a GNR’s functional properties is partly a consequence of forming the ribbon inside a nanotube, he adds. For example, the ribbon’s width is largely determined by the nanotube’s diameter. Also, imaging has shown that a GNR is not flat, but helically twisted, and that this twist changes over time. This alters the ribbon’s properties, such as the magnitude of the electronic band gap, which could be exploited in nanoelectromechanical systems.

Dr Khlobystov says that, once the process is optimised, the cost should be far lower than current methods and that environmental implications should be minimal. ‘As far as I can see, the production process does not generate any waste and can be carried out in a solventfree environment.’ The team also claims it should be easy for nanotube manufacturers to adapt it for their existing production process.

Dr Mohammad Al Hakim of the Nano Research Group at the University of Southampton, UK, greets this method as an exciting development, but says some issues still need to be resolved before it is practicable. For example, it does not exclude the lack of control over nanotube reactor locations and nanotube lengths and diameters are random, yielding GNRs with the same randomness. But Dr Khlobystov concedes that there are many such fundamental questions that need to be answered.