Step-Assisted On-Surface Synthesis of Graphene Nanoribbons Embedded with Periodic Divacancies

Abstract

The bottom-up approach through on-surface synthesis of porous graphene nanoribbons (GNRs) presents a controllable manner for implanting periodic nanostructures to tune the electronic properties of GNRs in addition to bandgap engineering by width and edge configurations. However, owing to the existing steric hindrance in small pores like divacancies, it is still difficult to embed periodic divacancies with a nonplanar configuration into GNRs. Here, we demonstrate the on-surface synthesis of atomically precise eight-carbon-wide armchair GNRs embedded with periodic divacancies (DV8-aGNRs) by utilizing the monatomic step edges on the Au(111) surface. From a single molecular precursor correspondingly following a trans-and ciscoupling, the DV8-aGNR and another porous nanographene are respectively formed at step edges and on terraces at 720 and 570 K. Combining scanning tunneling microscopy/spectroscopy, atomic force microscopy, and first-principles calculations, we determine the out-of-plane conformation, wide bandgap (∼3.36 eV), and wiggly shaped frontier orbitals of the DV8-aGNR. Nudged elastic band calculations further quantitatively reveal that the additional steric hindrance effect in the cyclodehydrogenative reactions has a higher barrier of 1.3 eV than that in the planar porous nanographene, which also unveils the important role played by the monatomic Au step and adatoms in reducing the energy barriers and enhancing the thermodynamic preference of the oxidative cyclodehydrogenation. Our results provide the first case of GNRs containing periodic pores as small as divacancies with a nonplanar configuration and demonstrate the strategy by utilizing the chemical heterogeneity of a substrate to promote the formation of novel carbon nanomaterials.

Self-Limited Embedding Alternating 585-Ringed Divacancies and Metal Atoms into Graphene Nanoribbons

“…We in-situ-monitor the evolution of the distinct nonplanar configurations and electronic localization of the cascade reaction intermediates using scanning tunneling microscopy/spectroscopy (STM/STS) and noncontact atomic force microscopy (nc-AFM) complemented by first-principles calculations. The presence of trapped Ag atoms at the defective sites provides direct evidence of the metal adatom-mediated cyclodehydrogenation pathways, which is further confirmed by our nudged elastic band (NEB) calculations that unveil the important role played by the metal adatoms in reducing the energy barriers of the intramolecular ring-closure reactions , and the strain effect on the self-limited formation of alternating 585-ringed divacancies and Ag atoms in the GNRs.…”

“…The consecutive H eliminations are different from those with the Ag adatom. For example, the up-tilted H atoms will take a [1,3]-sigmatropic shift to the edge carbons (state 2′ to 3′ and state 6′ to 7′ ) before elimination. , For the elimination of the last down-tilted H atom (state 5′ to 6′ ), C–C coupling undertakes first (state 4′ to 5′ ) to weaken the C–H bond, which gives a much reduced barrier of 0.40 eV. Differently, in the presence of the Ag adatom, its catalytic effect can promote the direct elimination or rotation and elimination processes for the down-tilted and up-tilted H atoms, respectively.…”

“…Periodically embedding structural defects, such as vacancies, , nonhexagonal rings, − and heteroatoms , into the basal hexagonal lattice of graphene, provides a predicable way aiming at electronic property engineering for potential applications . Recent development of on-surface synthesis, − based on rationally designed molecular precursors, has become a remarkable success in atomically precisely fabricating novel graphene superlattices with periodic structural modulations, as exemplified by nanopores in graphene sheets − and graphene nanoribbons (GNRs), − heteroatoms of B , and N − in GNRs, and also a wide range of interesting carbon allotropes − that consist of various nonbenzenoid combinations. One of the interesting nonbenzenoid combinations is the structure of a pair of five-membered rings and an eight-membered ring (585 rings), formed by the loss of two adjacent carbon atoms, also known as divacancy in graphene and carbon nanotubes (CNTs) .…”

“…The difficulty in synthesizing 585-ringed divacancies could come from the existing large H–H steric hindrance in the cove-type inner pores that inhibit transformation. One may expect that this difficulty could be overcome by benefiting the participation of surface metal atoms or adatoms, which have posed important roles in C–H activation. ,, Even with the produced periodic hydrogenated divacancies in GNRs, how to transform them into 585-ringed divacancies remains an open question. Moreover, by considering the highly strained nature of the 585-ringed divacancy, the exploration is necessary for understanding the mechanisms to balance the introduced strain and thus to stabilize the produced GNRs.…”

“…Here, we demonstrate an efficient way in synthesizing GNRs with periodic hydrogenated divacancies followed by formation of alternating 585-ringed divacancies and Ag atoms through intramolecular cyclodehydrogenation reactions in a seven-carbon-wide armchair GNR (7-aGNR) on the Ag(111) surface (Figure ). To overcome the low efficiency of high-barrier cyclodehydrogenation in the formation of the periodic hydrogenated divacancies in GNRs, , we choose a precursor molecule that can facilitate metal adatom-assisted Ullmann-like coupling to directly synthesize nanoporous 7-aGNRs with high efficiency at a temperature of 580 K on Ag(111). At an elevated temperature of 840 K, the final product with alternating 585-ringed divacancies and Ag atoms embedded in the 7-aGNRs can be effectively converted (Figure c).…”