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

Abstract

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.

Comprehensive Electrostatic Modeling of Exposed Quantum Dots in Graphene/Hexagonal Boron Nitride Heterostructures

“…This channel corresponds to the differential tunneling current between the topmost filled states in graphene and states in the STM tip. Consequently, signal traces in this channel will manifest as lines along which graphene retains a constant charge density [ 21 , 34 ]. One such line appears in Figure 4 a enclosed by an orange box.…”

“…The back-gate ( ) connected to the p -doped Si layer is used to remotely tune graphene’s Fermi level ( ). We use these control voltages to create a graphene QD by applying a pulse between graphene and the STM tip while maintaining at a constant value [ 13 , 14 , 15 , 16 , 21 ]. During the application of this pulse, defects in hBN underneath the tip become ionized with opposite polarity to .…”

Gate controlled valley polarizer in bilayer graphene

“…Due to thermal excitation and intrinsic disorder potential, this is expected to reduce the resistance of the insulating state. Further, the lateral confinement can produce additional sub-bands [22][23][24] , which are partially located inside the bandgap, reducing the effective gap even further. Yet, as long as disorder is negligible, sufficiently low temperature will produce insulating behavior.…”

Evolution of quasi-bound states in the circular n–p junction of bilayer graphene under magnetic field

“…It is a gapless semiconductor with a chiral parabolic low-energy band structure 23,24 . Thus, the low-energy electrons in AB-stacked BLG does not satisfy the standard Dirac equation, suggesting that its QBSs have different energy spectra from those in monolayer graphene 22,[25][26][27][28][29][30][31] . Many previous studies have focused on the real bound states in gapped BLG theoretically and experimentally 29,31,32 .…”

“…Thus, the low-energy electrons in AB-stacked BLG does not satisfy the standard Dirac equation, suggesting that its QBSs have different energy spectra from those in monolayer graphene 22 , 25 – 31 . Many previous studies have focused on the real bound states in gapped BLG theoretically and experimentally 29 , 31 , 32 . In contrast, the QBSs in gapless BLG have not yet been fully studied.…”