Exciton Effects In Strained Quasi-one-dimensional Carbon Nano-materials

Nano-materials are new materials in the 21st century, among which the quasi-one-dimensional (Q1D) carbon nano-materials, such as carbon nanotubes (CNTs) and graphene nanoribbons (GNRs), have attracted much attention because of their amazing properties and potential for broad application.The optical properties of these Q1D nano-materials have long been one of the hot research topics; nevertheless, previous studies of them have not gone beyongd the independent-particle approximation. However, recent theoretical and experimental studies have shown that the exciton effects dramatically alter the optical response of these Q1D carbon nano-materials. Theoretical work has been carried out to study the exciton effect, but the effect of deformation on the exciton effect is relatively small. Therefore, in the study of the optical properties of quasi-one-dimensional (Q1D) carbon nano-materials, the influence of deformation is considered in the calculation.In this dissertation, we have studied the exciton effects in the zigzag carbon nanotubes (SWNTs) and the armchair graphene nanoribbons (AGNRs), under the uniaxial strain were studied within the nonorthogonal tight-binding (TB) model, supplemented by the long-range Coulomb interactions. The dissertation is organized as follows:In chapter 1, we firstly introduce the history and background of the researches on the CNTs and GNRs, and then, we have reviewed their geometrical and electronic structures.In chapter 2, we firstly introduce the nonorthogonal tight-binding (NTB) model, which are widely used to calculate the electronic structure and optical properties of the carbon materials. Then, the exciton effects in the Q1D carbon nano-materials are introduced. Finally, we introduce the tight-binding (TB) approximation method to calculate the exciton effects.In chapter 3, we have studied the exciton effects in 8,9 and 10-AGNRs under the uniaxial strain within the NTB model. The variation of the excitation energy and exciton binding energy under the uniaxial strain depends on the ribbon family. For 8-AGNR, both the excitation energy and exciton binding energy decrease with increasing strain, while the situation for 9-AGNR is opposite. For 10-AGNR, the excitation energy increases first and then decreases under the uniaxial strain. These results can be understood by the change of electron band gap for AGNRs under the uniaxial strain.In chapter 4, we have studied the exciton effects in the CNTs with zigzag shaped edges under the uniaxial strain within the NTB model. It is found that the excitation energy of the excition in the carbon nano-tube is strongly dependent on the type of the carbon tube, and the excitation energy is linearly related to the deformation. The binding energy of the exciton can only be related to the type of carbon nanotubes with the change of the deformation.In chapter 5, we summarize the work of this paper.