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Precursor Geometry Determines the Growth Mechanism in Graphene Nanoribbons
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