STM Characterization of Metallic Graphene Nanoribbons*

Graphene nanoribbons (GNRs) are narrow strips of graphene that can host a wide range of 1-dimensional phenomena, including non-trivial topology and magnetism. The advent of molecule-based bottom-up synthesis techniques has enabled GNR properties to be tuned via the precise placement of individual carbon atoms. Realization of robust metallicity in bottom-up GNRs, however, remains elusive. The ability to engineer the frontier band structure of GNRs via hybridization of topologically-protected interface states suggests a strategy for inducing GNR metallicity by symmetrizing the hopping between adjacent interface states. Using this approach we have realized metallic GNR superlattices that are gapless by virtue of the symmetric placement of π-radical states along the GNR backbone. Scanning tunneling microscopy and spectroscopy (STM/STS) performed in conjunction with first-principles calculations confirms that these GNR superlattices possess a non-zero density of states (DOS) through E_F, a hallmark of metallicity. Strategies for increasing the bandwidth of metallic GNRs will be discussed.