Precursor Geometry Determines the Growth Mechanism in Graphene Nanoribbons

Abstract

On-surface synthesis with molecular precursors has emerged as the de facto route to atomically well-defined graphene nanoribbons (GNRs) with controlled zigzag and armchair edges. On Au(111) and Ag(111) surfaces, the prototypical precursor 10,10′-dibromo-9,9′-bianthryl (DBBA) polymerizes through an Ullmann reaction to form straight GNRs with armchair edges. However, on Cu(111), irrespective of the bianthryl precursor (dibromo-, dichloro-, or halogen-free bianthryl), the Ullmann route is inactive, and instead, identical chiral GNRs are formed. Using atomically resolved noncontact atomic force microscopy (nc-AFM), we studied the growth mechanism in detail. In contrast to the nonplanar BA-derived precursors, planar dibromoperylene (DBP) molecules do form armchair GNRs by Ullmann coupling on Cu(111), as they do on Au(111). These results highlight the role of the substrate, precursor shape, and molecule–molecule interactions as decisive factors in determining the reaction pathway. Our findings establish a new design paradigm for molecular precursors and opens a route to the realization of previously unattainable covalently bonded nanostructures.

Imaging on-surface hierarchical assembly of chiral supramolecular networks

“…Fig. 2c shows the resulting structural model of the organometallic chain, in line with recent results presented by Schulz et al 38 In Fig. 2b, arrows indicate the protruding C-H bonds within the molecule.…”

“…This observation indicates that the molecular coordination involves aryl group interaction, in agreement with previously proposed models based on STM images. 36 However, the distance between the molecules (1.2 nm) is larger than the one found in anthracene chains on Au (0.85 nm), 44 suggesting the presence of bridging Cu adatoms, 36,38 usually not visible in STM images. [54][55][56] Since the adatoms are expected to sit closer to the substrate than the molecular protruding lobes, they cannot be detected by constant-height AFM.…”

“…In this work, we report the study of chirality expression during 10,10 0 -dibromo-9,9 0 -bianthryl (DBBA) self-assembly on Cu(111), combining state of the art scanning probe microscopy (SPM) techniques. In fact, despite DBBA has been extensively studied on the Cu(111) surface as precursor for the on-surface synthesis of chiral graphene nanoribbons, [36][37][38] little attention has been devoted to the molecular structure of Ullmann intermediates.…”

Tracking On-Surface Chemistry with Atomic Precision

“…[68][69][70][71] In order to solve the controversy, we started by growing GNRs from DBBA on Cu(111) in the same way as Han and Simonov, and performed nc-AFM experiments to indisputably reveal the product to be the 3,1-chiral GNR. 72 A simultaneous effort by Sánchez-Sánchez et al corroborated these results. 73 The inevitable conclusion is that the Ullmann coupling, which should interconnect the C(10) positions of bianthryls, is compromised; in contrast, the chiral nanoribbons feature bonds between the CH(2) positions of adjacent monomers.…”

“…72 This reaction only involves planar products, and indeed allowed us to identify both organometallic and covalent stages in the reaction. By combining all the information, we could establish the mechanism by which the nanoribbons form on Cu(111).…”