Theoretical Study On The Structural Defects And Surface Decoration Of Carbon Nanotubes

Carbon nanotubes(CNTs) are promising materials for nanodevices,due to their novel physical and chemical properties.However,a realistic carbon nanotube doubtlessly contains various structural defects which may influence the properties of the tube significantly.Therefore,the defects in the tubes may hinder potential application of CNTs.Thus improving the quality of a CNT is of importance.In this dissertation,based on the tight-binding(TB) potential and density functional theory (DFT) calculations,we systematically study surface decoration of CNTs with the hydrocarbon radicals,and find that the radicals can not only heal the topologic structure of a single vacancy in a CNT,but also modulate the electronic structures of the tube.Meanwhile,we also study the vibrational properties of a carbon nanotube either with a single vacancy or with adsorption of a hydrocarbon radical.In addition, the structures and some typical properties of a novel hybrid carbon material,nanobud, are investigated in this dissertation.This dissertation consists of five chapters.In the first chapter,we review the progress of low-dimentional carbon based materials.Then we introduce the structures of the CNTs and give a brief description on their main properties and their potential applications,such as hydrogen storage, carbon compound materials,fields emission,and fibre materials.At the beginning of the second chapter,we compare classical potential computational method,tight banding potential method and the first principle DFT computational method with each other,which are commonly used to calculate the systems of condensed matter.Then DFT theory and TB potential method,as well as the parameterized forms for the latter are also illustrated in details.At the end of this chapter,we introduce the algorithms for structural optimization in brief.Systematical study on surface decoration of CNTs with the hydrocarbon radicals (CH,CH₂ and CH₃) is presented in chapter 3.Based on the TB potential calcultation, we investigate the adsorption of CH,CH₂,and CH₃ either on a single vacancy or on the defect-free region,and find that the adsorbed CH and CH₂ on the single vacancy can heal or improve the local defective structure at low temperatures.On the other hand,the adsorbed radicals on the defect-free region can diffuse to a single vacancy in the tube at room temperature,and either CH or CH₂ can improve the local structure of this defect,while CH₃ can not anyway.Our results explain well the observation of the related experiment.Moreover,based on the DFT calculation,we also investigate the influence of the adsorbed radicals on the electronic structure of the system.Vibration is the fundmental property of a system,which is closely coupled with the atomic structure.Essentially,the changes of the structure must be reflected on the vibrational spectrum of the sample.So vibrational spectrum can be regarded as the fingerprint of a sample.In chapter 4,we mainly study the vibrational properties of the carbon nanotubes with the single vacancies and that with adsorption of a hydrocarbon radical.Our calculations reveal that two kinds of characteristic vibrational modes for a single vacancy can act as the indicators of this defect.In addition,we find the main vibrational features for the adsorbed radicals on a CNT.According to the different number and frequencies of stretching modes for C-H,the adsorbed radicals can be identified from each other.A novel hybrid carbon material,nanobud,was discovered in experiment,in which fullerenes are covalently bonded to the outer surface of the single walled carbon nanotubes.In chapter 5,we firstly study the structures for nanobuds,in which C60 covalently connects to a armchair carbon nanotube.Moreover,the electronic, optical and vibrational properties of these nanobuds are discussed in this chapter.