  The success of all-graphene electronics is severely hindered by the
challenging realization and subsequent integration of semiconducting channels
and metallic contacts. Here, we comprehensively investigate the electronic
transport across width-modulated heterojunctions consisting of a graphene
quantum dot of varying lengths and widths embedded in a pair of armchair-edged
metallic nanoribbons, of the kind recently fabricated via on-surface synthesis.
We show that the presence of the quantum dot enables the opening of a
width-dependent transport gap, thereby yielding built-in one-dimensional
metal-semiconductor-metal junctions. Furthermore, we find that, in the vicinity
of the band edges, the conductance is subject to a smooth transition from an
antiresonant to a resonant transport regime upon increasing the channel length.
These results are rationalized in terms of a competition between
quantum-confinement effects and quantum dot-to-lead coupling. Overall, our work
establishes graphene quantum dot nanoarchitectures as appealing platforms to
seamlessly integrate gap-tunable semiconducting channels and metallic contacts
into an individual nanoribbon, hence realizing self-contained carbon-based
electronic devices.
