Topological Band Engineering Graphene Nanoribbons*

Graphene nanoribbons (GNRs) are 1-dimensional semiconducting strips of graphene that possess novel electronic and magnetic properties. The atomic precision afforded by recent bottom-up synthetic techniques has led to a surge in tailor-made GNR structures whose physical properties can be controllably tuned through modification of width, edge chirality, and dopant incorporation. Recent theoretical work reveals the presence of non-trivial topological phases in GNRs, and correspondingly, the existence of zero-energy symmetry-protected topological interface states. Using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), we investigate the first experimental realization of controlled heterojunctions composed of two topologically distinct GNR classes. Our approach enables us to create a periodic superlattice of topological interface states, whose bonding/anti-bonding interactions lead to the creation of two new frontier bands. This yields a drastically reduced band gap compared to either of the two constituent GNRs, revealing the powerful role topology will play in engineering new GNR electronic structures.