First-principles Study Of Graphene Quantum Dots And Graphene/Hexagonal Boron Nitride

Graphene has attracted great attattention because of its unique structure and exceptional properties. It is found that graphene is very suitable for the fabrication of quantum dots and nano devices. By using the first-principles study, we explored how the performance of SET would be affected by the intrinsic properties of Coulomb island made of graphene quantum dots. Recent experiments produced high quality graphene/hexagonal boron nitride (graphene/hBN). Morie patterns were observed due to the rotation between graphene and hBN layers. In comparison to the calculated50meV energy gap in the graphene(1×1)/hBN(1×1), the energy gap was found to be negligible in experiments. Furthermore, we can change the rotation angle to manipulate the periodic potential, which will surely lead to many interesting novel properties.(1) By performing density functional theory calculations, we studied the quantum confinement in charged graphene quantum dots (GQDs), which is found to be clearly edge and shape dependent. It is found that the excess charges have a large distribution at the edges of the GQD. The resulting energy spectrum shift is very nonuniform and hence the Coulomb diamonds in the charge stability diagram varies irregularly, in good agreement with the observed nonperiodic Coulomb blockade oscillation. We also illustrate that the level statistics of the GQDs can be described by a Gaussian distribution, as predicted for chaotic Dirac billiards.(2) We studied the graphene/hBN systems by first-principles calculations to address the important effects of the periodic potential induced by the moire patterns. The energy gap at Dirac point of moiré patterns is negligibly small, in good agreement with the experiments. We studied the evolution of the electronic structure with respect to the rotation angle. It is found that the Fermi velocity at the Dirac point is reduced with the decrease of the rotation angle. At a critical rotational angle, an asymmetry-reversion of the electron and hole velocities is observed. We show that apart from the change of the periodic moire potential, the buckling of the graphene layer plays an important role in the Fermi velocity modulation by rotation angles. In the local density of states of the C atoms above the N atoms or B atoms, there is one dip below and one dip above the Fermi level, respectively, due to the charge transfer between C atoms and N or B atoms. The dip approaches the Fermi level as the rotation angle decreases, agreeing well with the experiments.