First Principles Studies On Electronic Structures Of Graphene Nanoribbons

Graphene nanoribbons (GNRs) are a relatively new class of carbon-basednano-materials. Due to the quantum confinement and edge effects, GNRs have anopening band gap which can be tuned into a certain extent, depending upon the widthand orientation of the edges. It means graphene nanoribbons have importantimplications for the nano-electronic device development.Using the first-principles density functional theory (DFT) calculations, GNRs’geometry optimization and electronic structure calculations are held. The bandstructure of H-terminated GNRs with different width can be computed numericallybased on the super cell model. The numerical computations reveal that GNRs withunbroken boundary structure are semiconductors with band gaps which dependinversely on the nanoribbons’ width. More specifically, for the armchair graphenenanoribbons (AGNRs), the band gap has an oscillating relationship to the nanoribbons’width index N_awith a period of3, and is close to zero while N_a=3p+2. For the zigzaggraphene nanoribbons (ZGNRs), the band gap decreases with the increasing width.When the width index N_zis small, ZGNRs are semiconducting, when N_zincreases to5,ZGNRs already perform as metal.And then, the electronic properties of AGNRs confined by BN nanoribbons(BNNRs) have been introduced in comparison with that of H-terminated AGNRs. It isfound that AGNRs confined by BNNRs exhibit a considerable band gap with the widthindex of3p+2. And the different behavior from H-terminated AGNRs originates fromthe charge redistribution at the edges of AGNRs confined by BNNRs, which leads tothe change in the potential energy of the C atoms in the AGNR edges that are adjacentto B and N.At last, the effects of edge-vacancy defects on the electronic structure of GNRsare discussed. It is discovered that the edge-vacancy defects decrease the band gap ofAGNRs. And, as these defects get more, the band gap gets smaller, that makes theconversion of semiconducting AGNRs to metallic. While for ZGNRs, the presence ofedge-vacancy defects does not change the physical structure of the zigzag edge. And soZGNRs’ electronic properties don’t be changed almost.In the thesis, GNRs with different width and edge structure have been calculated,which is helpful for us to modify GNRs’ geometry and electronic structure, and designfunctional devices based on GNRs.