  Massive Dirac fermions are low-energy electronic excitations characterized by
a hyperbolic band dispersion. They play a central role in several emerging
physical phenomena such as topological phase transitions, anomalous Hall
effects and superconductivity. This work demonstrates that massive Dirac
fermions can be controllably induced by lithographically patterning
superstructures of nanoscale holes in a graphene device. Their band dispersion
is systematically visualized using angle-resolved photoemission spectroscopy
with nanoscale spatial resolution. A linear scaling of effective mass with
feature sizes is discovered, underlining the Dirac nature of the
superstructures. In situ electrostatic doping dramatically enhances the
effective hole mass and leads to the direct observation of an electronic band
gap that results in a peak-to-peak band separation of (0.64 $\pm$ 0.03) eV,
which is shown via first-principles calculations to be strongly renormalized by
carrier-induced screening. The presented methodology outlines a new approach
for band structure engineering guided by directly viewing structurally- and
electrically-tunable massive Dirac quasiparticles in lithographic
superstructures at the nanoscale.
