  Bottom-up graphene nanoribbon (GNR) heterojunctions are nanoscale strips of
graphene whose electronic structure abruptly changes across a covalently bonded
interface. Their rational design offers opportunities for profound
technological advancements enabled by their extraordinary structural and
electronic properties. Thus far the most critical aspect of their synthesis,
the control over sequence and position of heterojunctions along the length of a
ribbon, has been plagued by randomness in monomer sequences emerging from
step-growth copolymerization of distinct monomers. All bottom-up GNR
heterojunction structures created so far have exhibited random sequences of
heterojunctions and, while useful for fundamental scientific studies, are
difficult to incorporate into functional nanodevices as a result. Here we
describe a new hierarchical fabrication strategy that allows deterministic
growth of bottom-up GNRs that preferentially exhibit a single heterojunction
interface rather than a random statistical sequence of junctions along the
ribbon. Such heterojunctions provide a viable new platform that could be
directly used in functional GNR-based device applications at the molecular
scale. Our hierarchical GNR fabrication strategy is based on differences in the
dissociation energies of C-Br and C-I bonds that allow control over the growth
sequence of the block-copolymers from which GNRs are formed, and consequently
yields a significantly higher proportion of single-junction GNR
heterostructures. Scanning tunnelling spectroscopy and density functional
theory calculations confirm that hierarchically-grown heterojunctions between
chevron GNR (cGNR) and binaphthyl-cGNR segments exhibit straddling Type I band
alignment in structures that are only one atomic layer thick and 3 nm in width.
