First-principles Study Of Graphene/hBN Moiré Superlattice

Graphene is an excellent two-dimensional material with properties such as ultra-high thermal and carrier mobility,ultra-high mechanical strength,and dissipationless ballistic charge transport,which provides broad prospects for carbon-based nanodevice applications.In practical applications,graphene must be grown on some kind of substrate,which usually leads to the degradation of graphene performance.h-BN is a two-dimensional material with a honeycomb structure,wide band gap and only single atom layer thickness,which is an ideal substrate to support graphene.It not only preserves the quality of graphene,but also brings about new properties by the periodic modulation of BN.Although the expected change in the morphology of graphene is small,the experiment found that BN significantly modulates the Fermi velocity and density of states of graphene.In this thesis,first-principles calculations have been performed on graphene/BN to explore the effect of hBN on the graphene it supports,and to reveal the mechanisms behind the experimentally observed phenomena.The main results are as follows:Moirésuperlattices are formed in graphene/BN due to interlayer rotation and lattice mismatch,in which local quasi-AA and quasi-AB stacking regions exist.Although the graphene(1×1)/BN(1×1)structures of the AA and AB stackings have appreciable band gaps,respectively,the band gap of the graphene/BN moiréstructure is found to be almost zero,which is consistent with the experiments.This is due to the staggered arrangement of the band gaps of the AA and AB stacking regions,making the overall band gap of the moiréstructure very small.It is found that with the increase of the Moirésuperlattice period,the Fermi velocity decreases,and dip appears in the density of states and the dip position is closer to the Fermi level,which is consistent with the experiment.By calculating a series of graphene/BN moiréstructures with different rotation angles and spatial periods,a unified explanation for these phenomena is proposed:the BN substrate causes graphene P_z orbitals to lose electrons,weakens the interlayer interaction of C atoms and generates the in-plane electric field,resulting in the reduction of the Fermi velocity.The periodic potential imposed by BN leads to an uneven charge distribution,which increases the electron-electron repulsive interaction,and moves the highest occupied state S at theΓpoint upward,causing a dip in the density of states at the energy of the S state.With the increase of the spatial period of the graphene/BN moiréstructure,the P_z orbital interaction will be further weakened and the S state will be further moved up,so that the Fermi velocity is further reduced,and the dip in the density of states is closer to the Fermi level.