  Graphene p-n junctions provide an ideal platform for investigating novel
behavior at the boundary between electronics and optics that arise from
massless Dirac fermions, such as whispering gallery modes and Veselago lensing.
Bilayer graphene also hosts Dirac fermions, but they differ from single-layer
graphene charge carriers because they are massive, can be gapped by an applied
perpendicular electric field, and have very different pseudospin selection
rules across a p-n junction. Novel phenomena predicted for these massive Dirac
fermions at p-n junctions include anti-Klein tunneling, oscillatory Zener
tunneling, and electron cloaked states. Despite these predictions there has
been little experimental focus on the microscopic spatial behavior of massive
Dirac fermions in the presence of p-n junctions. Here we report the
experimental manipulation and characterization of massive Dirac fermions within
bilayer graphene quantum dots defined by circular p-n junctions through the use
of scanning tunneling microscopy-based (STM) methods. Our p-n junctions are
created via a flexible technique that enables realization of exposed quantum
dots in bilayer graphene/hBN heterostructures. These quantum dots exhibit sharp
spectroscopic resonances that disperse in energy as a function of applied gate
voltage. Spatial maps of these features show prominent concentric rings with
diameters that can be tuned by an electrostatic gate. This behavior is
explained by single-electron charging of localized states that arise from the
quantum confinement of massive Dirac fermions within our exposed bilayer
graphene quantum dots.
