Heterojunctions formed with N=9 (wide) and N=7 (narrow) armchair graphene nanoribbons in two different geometric arrangements, resulting in two junctions of distinct electron topology – between topologically equivalent segments (upper panel) and topologically inequivalent segments (lower panel). The junction in the lower panel hosts a topological electronic junction state in the band gap. The carbon-carbon and carbon-hydrogen bonds are colored gray and silver, respectively. The color intensity plot shows the spatial distribution (charge density) of the topological junction state.Researchers at University of California, Berkeley, and the Lawrence Berkeley National Laboratory have used graphene nanoribbons (GNRs) to trap electrons, in a process that could find applications in quantum computing/information and as an alternative to silicon semiconductors. With much work going into producing new electronic devices using nanoribbons, this study combined two different types of GNR to produce a material that can trap single electrons at the junctions of ribbon segments.
This phenomenon originates from a 'topological' aspect of GNRs – the shape that propagating electron states take when moving quantum mechanically through the nanoribbon – leading to new quantum properties. As described in Nature [Rizzo et al. Nature (2018) DOI: 10.1038/s41586-018-0376-8], the team (co-led by Steven Louie, who predictd theoretically the phenomenon, and colleagues Michael Crommie and Felix Fischer, who did the measurements and synthesis) was able to demonstrate that the junctions of GNR strips of less than five nanometers wide had the right topology to host individual localized electrons, offering a platform that enables topological engineering strategies to be implemented in 1D nanostructures to construct flexible, modular systems that allow precise quantum engineering.
“This study opens a new avenue towards engineering the electronic properties of 1D material systems by means of topology. Such an approach dramatically increases the number and the complexity of electronic and magnetic systems attainable via bottom-up synthesis.”Steven Louie
A GNR with alternating ribbon strips of different widths to form a nanoribbon superlattice makes a line of junction electrons that interact quantum mechanically. Depending on the distance between the strips, the hybrid nanoribbon is either a metal, a semiconductor or chain of qubits, the basic unit of quantum information. The theory shows that GNRs are topological insulators – which, in 3D, are non-conducting in the interior, but metallic conductors along their surface.
A single 0D electron at a ribbon junction is confined in all directions. When another electron is similarly trapped close by, they can both tunnel along the nanoribbon and join up due to the rules of quantum mechanics. If spaced correctly, the spins of adjacent electrons can become entangled so that altering one will affect the others, which is essential for quantum computing. The length of each segment can be altered to affect the distance between trapped electrons, changing how they interact.
As Steven Louie told Materials Today, “This study opens a new avenue towards engineering the electronic properties of 1D material systems by means of topology. Such an approach dramatically increases the number and the complexity of electronic and magnetic systems attainable via bottom-up synthesis.”
Due to tunable band gaps and other properties, the GNRs could have great potential for electronics and optoelectronics, although new synthesis or transfer techniques are required so that nanoribbons can be made compatible with standard semiconductor device architectures. The team also hopes to look further into the topological phenomena to design 1D-based systems with interesting topological phases.