Tunable Quantum Dots in Bilayer Graphene

Abstract

We demonstrate theoretically that quantum dots in bilayers of graphene can be realized. A position-dependent doping breaks the equivalence between the upper and lower layer and lifts the degeneracy of the positive and negative momentum states of the dot. Numerical results show the simultaneous presence of electron and hole confined states for certain doping profiles and a remarkable angular momentum dependence of the quantum dot spectrum which is in sharp contrast with that for conventional semiconductor quantum dots. We predict that the optical spectrum will consist of a series of non-equidistant peaks.PACS numbers: 71.10. Pm, 81.05.Uw Two-dimensional (2D) carbon crystals, such as singlelayer and bilayer graphene, have been the subject of increasing interest due to the unusual mechanical and electronic properties, which may lead to their use in novel nanoelectronic devices. The relativistic-like properties of carriers in single-layer graphene 1,2,3,4,5 result from the gapless and approximately linear electron spectrum near the Fermi energy at two inequivalent points of the Brillouin zone. The charge carriers in these structures are described as massless relativistic fermions and are governed by the Dirac equation. In contrast, for a symmetric graphene bilayer the spectrum is parabolic at the vicinity of the K points.Among the unusual properties of single-layer graphene is the perfect forward transmission across potential barriers, known as Klein tunneling 6,7 , which is related to the absence of a gap in the carrier spectrum. This effect prevents the electrostatic confinement of charged particles and thus the realization of quantum dots. Recently, alternative strategies have been proposed to confine charged particles by using thin single-layer graphene strips 8,9 or non-uniform magnetic fields 10 . Here we propose a novel approach by considering a bilayer graphene, in which a charge imbalance between the layers gives rise to a gap in the spectrum, that can be used to create potential barriers 11,12 . Such bilayers of graphene can be obtained, e.g., from a graphite crystal by micromechanical cleavage 13 . A recent report described the synthesis of bilayer graphene sheets by graphitization of silicon carbide (SiC) surfaces in which the equivalence of the two graphene layers is broken by their interaction with the SiC substrate as well as by doping one of them with potassium atoms 14 . These recent experimental progresses raise the possibility of introducing a position-dependent modification of the spectrum at the Dirac point by changing the potassium density at different regions of the bilayer graphene sheet or by using microstructured gates. In this letter we propose a position-dependent potassium doping to manipulate the band structure of bilayer graphene in order to create nanometer-scale quantum structures such as quantum dots. Semiconductor quantum dots have been intensively investigated both theoretically and experimentally. Up to now, only quantum dots on graphite 15 have been obtained, with sample thic...

Energy levels of an ideal quantum ring in AA-stacked bilayer graphene

“…This tunable gap can then be exploited for the development of bilayer graphene devices. In particular, the possibility of controlling the energy gap has raised the possibility of the creation of electrostatically defined quantum dots [16,17] and quantum ring [18,19] in bilayer graphene.…”

Giant magnetoresistance in bilayer graphene nanoflakes

“…Energy band gap in BLG nanoribbon and nanoflake could be controlled by substrate properties and the applied vertical electric field, so these structures could be used as a channel material in carbon-based transistors [6][7][8][9][10][11][12] . Moreover, several studies have been performed, both theoretically and experimentally, on transport properties in BLG structure [6][7][8][9][12][13][14][15][16][17] . Interestingly, BLG field-effect-transistors with high on/off current ratios at room temperature have been reported 9 .…”

Electron-electron interactions in bilayer graphene quantum dots

“…In a perpendicular electric field, the spectrum displays a gap which can be tuned by varying the bias 11 . Nanostructuring the gate would allow tuning of the energy gap in BLG which can be used to electrostatically confine QDs 12,13 and quantum rings 14 . Here the electrons are displaced from the edge of the sample and consequently edge disorder and the specific type of edges are no longer a problem.…”