Graphene nanoribbons for emerging quantum technologies

In the last decades, a mathematical description of symmetries in nature called topology has been applied to describe and predict new electronic and magnetic properties of materials. A very simple aspect of topology connects a symmetry in the atomic structure of a crystal with a class of materials. Many materials that we know or use in current technology (silicon, diamond, gallium arsenide, etc.) belong to a topological class called trivial, meaning standard, and behave as normal semiconductors or insulators.

Novel materials with “anomalous” topology (technically called non-trivial) can be fabricated with advanced techniques of material science, which achieve control of their structure with atomic precision. For such materials, mathematical models predict “exotic” properties that can be utilized in future technology, such as that they are insulating inside and metallic at their surfaces.

In a recent article published in the journal Nature Communications, a multidisciplinary group of Spanish research teams reported that certain stripes of graphene called graphene nanoribbons (GNRs) acquire the anomalous topological state of matter when narrowed down to just a few nanometres in width.

GNRs are atomically thin, planar carbon nanostructures that can be obtained from a sheet of graphene (carbon atoms arranged in a hexagonal lattice) by cutting in different directions. Conceptually, they can be thought of as stripes of graphene aligned along different directions, i.e,  as nanoscale wires that may be used to transport an electronic current.

The scientists fabricated with atomic precision narrow GNRs of different width and orientation, like in the figure below, and demonstrated that all types convert from a metallic into an insulating state when the width is reduced below a few nanometers. Unexpectedly, they found that this new state corresponds to a non-trivial topological class.

Because of their anomalous Topology, electronic states were found localized at the ends of the ribbons (as shown in the figure). These states represent a novel source of non-conventional magnetism with promising applications in quantum technologies.

Within the framework of the FET OPEN Project SPRING (https://www.springfetopen.eu/), this study was achieved through a multidisciplinary collaboration combining tools and methods of chemistry and physics. First, organic chemists at the CIQUS institute in the University of Santiago de Compostela synthesized molecular precursors for GNRs using solution chemistry. Physicist at CIC nanoGUNE and at the Centro de Física de Materiales (CFM), in San Sebastian, did the assembling reaction on metal surfaces to produce the desired GNRs with atomic precision and investigated their anomalous electronic properties with scanning tunnelling microscopy. The physicist at the Donostia International Physics Centre (DIPC) did theoretical simulations that demonstrated the anomalous topology of the narrow GNRs. This result widens the scope for the use of graphene nanostructures in emerging quantum technologies.

Figure: Scanning tunneling microscopy image resolving the hexagonal structure (schematically indicated by black lines) of one of the studied GNRs. The bright, localized feature at the right side reveals the emergent “exotic” state at the end of the ribbon.

SPRING FET Open project #863098