Graphene nanoribbons are narrow strips cut from a graphene sheet — ideal systems for exploring one-dimensional edge effects. Originally a playground for theoretical condensed-matter physicists, progress in nanotechnological fabrication techniques has now made producing ribbons with tailored atomic structures a reality.
The two basic graphene edge types are known as armchair and zigzag. The graphene carbon atom network consists of two sub-lattices; for a zigzag edge, the outermost atoms belong to only one sub-lattice, whereas an armchair edge contains atoms of the two sub-lattices. Gábor Magda and colleagues employed a nanolithography technique based on scanning tunnelling microscopy to make cuts along well-defined crystallographic orientations in graphene deposited on an Au(111) surface. Room-temperature tunnelling spectra confirmed armchair ribbons to be semiconducting with a bandgap that decreases with strip width. For ribbons with zigzag edges, an unexpected semiconductor-to-metal transition was observed at a critical width of about 7 nm. Calculations show that the transition is a finite-temperature effect and reveal magnetic ordering: at the critical width, the configuration of electron spins at opposite edges changes from antiferromagnetic to ferromagnetic.