Bottom-up Assembly of Nanoporous Graphene with Emergent Electronic States

Abstract

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 interfacelocalized 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.

Atomically precise graphene nanoribbons: interplay of structural and electronic properties

“…Although the exterior hydrogen atoms of the ribbons remain unaffected during the cyclodehydrogenation process, by annealing beyond the temperature required for cyclodehydrogenation, a lateral fusion of GNRs can be induced leading to wider ribbons. 80,[111][112][113][114][115][116][117] This method was first used by Huang et al 113 to form 14-and 21-AGNR on Ag(111) from 1. Later, using 4,4 00 -dibromoterphenyl (3), 3n-AGNRs were synthesized on Au(111).…”

“…111,112 Another example is the lateral fusion of chevron-like ribbons which results in the formation of coveted, atomically precise nanoporous graphene. 116,117 While the above described formation methods are useful for scientific characterization of ribbons that are otherwise complicated to synthesize, the synthesis through this pathway suffers from a lack of selectivity and thus, is not suitable for application processes requiring specific GNR widths. However, by using a stepped Au(322) surface Merino-Diez et al 80 were able to preferentially form 6-AGNRs from 3, alleviating this problem.…”

“…7b). 117 In addition, the synthesis of further cGNRs has been successfully achieved. For instance, two heterodoped GNRs were fabricated by Fu et al 203 on Au(111) from the two similar precursors 48 and 50.…”

Fjord-Edge Graphene Nanoribbons with Site-Specific Nitrogen Substitution

“…11,12,13,14,15 Early bottom-up syntheses have focused on GNRs with armchair 16,17,18,19 or zigzag 20 edges. More recently, intricate edge or interior configurations, such as chevron, 11,21,22,23 cove, 24,25,26 fjord 27 or holey, 28,29,30 have been obtained. These novel topologies significantly alter the electronic or magnetic properties of GNRs, as do atomically precise 31 substitutions of carbons with heteroatoms such as boron, 32,33 sulfur, 34,35 or nitrogen.…”

“…23,36,37,41,42,43,44 Gratifyingly, creative ways to synthesize GNRs bearing novel edge or backbone structures are thriving to this day. 29,30,45 Herein, we describe the synthesis of the first eight-atom wide, fjord-edge nitrogen-doped graphene nanoribbons 1a,b (fjordedge N2 [8]GNR; Figure 1). Fjord-edge N2 [8]GNRs 1a,b were obtained in a facile two-step conversion starting from dipyridyl diynes 3a,b.…”