A First-Principles Study On The Band Structure Tuning Of Graphene/h-BN By Electric Field And The Carrier Mobility Of Zigzag Graphene Nanoribbons

Graphene shows great promise for the next generation of nano devices because of its exceptional and novel electronic, thermal and mechanical properties. It has been found recently that hexagonal 2D BN is an ideal substrate for graphene since graphene is very flat and has a very uniform charge distribution in graphene/h-BN. A hallmark of graphene/h-BN system is that there is a twist angle between them. Therefore, it is necessary to study the electronic properties of this twisted systems and explore the means to open the band gap of graphene on h-BN. Graphene has a remarkably high carrier mobility and hence graphene nanoribbons are supposed to play an important role to transport carriers in graphene-based nano devices. In particular, zigzag nanoribbons have an edge-dependent spin distribution and have significant applications in spintronics. Therefore, it is important to study the carrier mobilities in zigzag graphene nanoribbons. In this thesis, we studied the above two topics by using first-principles calculation:(1) Electric field tuning of the band structure of graphene/h-BNIn the nontwisted AB stacking graphene/h-BN system, there is a 50 meV gap which can be tuned by electric fields. We calculated the electronic structure of the realistic graphene/h-BN moiré structure with different twisting angles(? = 21.78 o, 27.8o, 13.1o). ① In the absence of an external vertical electric field, the twisted graphene/h-BN systems have a band structure with linear dispersion near K, and has a tiny band gap at the K point in the Brillouin zone. ② Unlike the non-twisted AB stacking graphene/h-BN system in which the electric field can effectively tune the band gap, the band gap of the twisted graphene/h-BN is not sensitive to the external electric field; ③ After O adsorption on the twisted graphene/h-BN, a considerable band gap is opened, which can be effectively tuned by external electric field.(2) Carrier mobility in zigzag graphene nanoribbonsThe carrier mobility of zigzag graphene nanoribbons are calculated by using first-principles calculation. ① The carrier mobility in zigzag graphene nanoribbons can reach the order of 103cm2V-1s-1, and increases with the width of the zigzag graphene nanoribbons.Meanwhile, the mobility of the localized state at the edge is about two orders of magnitude smaller than that of the delocalized state, and it’s only about a few tens of cm2V-1s-1. ② O adatom in the middle of the zigzag graphene nanoribbons enhances the electron and hole mobility by 22~198% and 50~201%, respectively. And the effect of the defects on the spin-polarized carriers is dependent on location of the defects.③Stone-Wales defects in the middle of zigzag graphene nanoribbons decreases the carrier(electron and hole) mobility, andelectron and hole mobility decreased by 28~77% and 18~67%.