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We show that the precursor geometry controls whether we form regular armchair or chiral graphene nanoribbons (GNRs). On Cu(111), bianthryl precursors (dibromo-, dichloro-, or halogen-free bianthryl) unexpectedly yield chiral GNRs. 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.

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