  High quality graphene nanoribbons (GNRs) grown by on-surface synthesis
strategies with atomic precision can be controllably doped by inserting
heteroatoms or chemical groups in the molecular precursors. Here, we study the
electronic structure of armchair GNRs substitutionally doped with di-boron
moieties at the center, through a combination of scanning tunneling
spectroscopy, angle-resolved photoemission, and density functional theory
simulations. Boron atoms appear with a small displacement towards the surface
signaling their stronger interaction with the metal. We find two boron-rich
flat bands emerging as impurity states inside the GNR band gap, one of them
particularly broadened after its hybridization with the gold surface states. In
addition, the boron atoms shift the conduction and valence bands of the
pristine GNR away from the gap edge, and leave unaffected the bands above and
below, which become the new frontier bands and have negligible boron character.
This is due to the selective mixing of boron states with GNR bands according to
their symmetry. Our results depict that the GNRs band structure can be tuned by
modifying the separation between di-boron moieties.
