Two independent groups of physicists have shown that the topology of the electronic states of graphene nanoribbons can be controlled by adjusting the width of the material. Both teams made nanoribbons that alternated between wide and narrow sections and then showed that sections with different widths have different topologies.
Graphene is a sheet of carbon just one atom thick that was first isolated in 2004. It has since been shown to have a wide range of fascinating and potentially useful electronic properties. In this latest research Daniel Rizzo, Gregory Veber and Ting Cao and colleagues at the University of California, Berkeley – and an independent team including Oliver Gröning and Shiyong Wang of EMPA in Switzerland and Xuelin Yao of the Max Planck Institute for Polymer Research in Germany – created graphene nanoribbons that were nine or fewer atoms wide. Crucially, the nanoribbons were defect-free – particularly at the edges, where the carbon atoms had the distinctive “arm-chair” configuration. In both cases, the widths of the nanoribbons varied by just two atoms from a narrow section to a wide section (see figure).
The nanoribbons were made to investigate whether they behaved as topological insulators. Such materials are electrical insulators in the bulk but conduct electricity like metals on their surfaces. This property can arise from the interaction between the spin of an electron and its motion, making it impossible for electrons to scatter when moving on the surface of a material.
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