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Graphene p-n junctions provide an ideal platform for investigating novelbehavior at the boundary between electronics and optics that arise frommassless Dirac fermions, such as whispering gallery modes and Veselago lensing.Bilayer graphene also hosts Dirac fermions, but they differ from single-layergraphene charge carriers because they are massive, can be gapped by an appliedperpendicular electric field, and have very different pseudospin selectionrules across a p-n junction. Novel phenomena predicted for these massive Diracfermions at p-n junctions include anti-Klein tunneling, oscillatory Zenertunneling, and electron cloaked states. Despite these predictions there hasbeen little experimental focus on the microscopic spatial behavior of massiveDirac fermions in the presence of p-n junctions. Here we report theexperimental manipulation and characterization of massive Dirac fermions withinbilayer graphene quantum dots defined by circular p-n junctions through the useof scanning tunneling microscopy-based (STM) methods. Our p-n junctions arecreated via a flexible technique that enables realization of exposed quantumdots in bilayer graphene/hBN heterostructures. These quantum dots exhibit sharpspectroscopic resonances that disperse in energy as a function of applied gatevoltage. Spatial maps of these features show prominent concentric rings withdiameters that can be tuned by an electrostatic gate. This behavior isexplained by single-electron charging of localized states that arise from thequantum confinement of massive Dirac fermions within our exposed bilayergraphene quantum dots.

Visualization and Control of Single Electron Charging in Bilayer Graphene Quantum Dots