Bottom-up Assembly of Nanoporous Graphene with Emergent Electronic States.

The incorporation of nanoscale pores into a sheet of graphene allows it to switch from an impermeable semimetal to a semiconducting nano-sieve. Nanoporous graphenes are desirable for applications ranging from high-performance semiconductor device channels to atomically-thin molecular sieve membranes, and their performance is highly dependent on the periodicity and reproducibility of pores at the atomic level. Achieving precise nanopore topologies in graphene using top-down lithographic approaches has proven to be challenging due to poor structural control at the atomic level. Alternatively, atomically-precise nanometer-sized pores can be fabricated via lateral fusion of bottom-up synthesized graphene nanoribbons. This technique, however, typically requires an additional high temperature cross-coupling step following the nanoribbon formation that inherently yields poor lateral conjugation, resulting in 2D materials that are weakly connected both mechanically and electronically. Here we demonstrate a novel bottom-up approach for forming fully conjugated nanoporous graphene through a single, mild annealing step following the initial polymer formation. We find emergent interface-localized electronic states within the bulk band gap of the graphene nanoribbon that hybridize to yield a dispersive two-dimensional low-energy band of states. We show that this low-energy band can be rationalized in terms of edge states of the constituent single-strand nanoribbons. The localization of these 2D states around pores makes this material particularly attractive for applications requiring electronically sensitive molecular sieves.