  Precision control of interfacial structures and electronic properties is the
key to the realization of functional heterostructures. Here, utilizing the
scanning tunneling microscope (STM) both as a manipulation and characterization
tool, we demonstrate the fabrication of a heterostructure in a single
atomically precise graphene nanoribbon (GNR) and report its electronic
properties. The heterostructure is made of a seven-carbon-wide armchair GNR and
a lower band gap intermediate ribbon synthesized bottom-up from a molecular
precursor on an Au substrate. The short GNR segments are directly written in
the ribbon with an STM tip to form atomic precision intraribbon
heterostructures. Based on STM studies combined with density functional theory
calculations, we show that the heterostructure has a type-I band alignment,
with manifestations of quantum confinement and orbital hybridization. Our
finding demonstrates a feasible strategy to create a double barrier quantum dot
structure with atomic precision for novel functionalities, such as negative
differential resistance devices in GNR-based nanoelectronics.
