  The influence of substrate steps on the bottom-up synthesis of atomically
precise graphene nanoribbons (GNRs) on an Au(111) surface is investigated. A
straight surface step is found to promote the assembly of long and compact
arrays of polymers with enhanced interchain pi-pi stacking interaction, which
creates a steric hindrance effect on cyclodehydrogenation to suppress the H
passivation of polymer ends. The modified two-stage growth process results in
periodic arrays of GNRs with doubled average length near step edges.
