First-principles Study Of Ultra-small Diameter Carbon Nanotubes And Related Structures

The miniaturization of electronic devices is a remarkable feature of the development of science and technology. Carbon nanotubes become one of the hottest topics in the nanotechnology community for their amazing mechanical, electronic properties and wide applications. As known, the excellent properties of carbon nanotubes are mostly attributed to their diameters of nanometer size which therefore brings in a few quantum effects affecting their behaviors. Since their first fabrication in 2000, the 4 A single-wall carbon nanotubes (SWNTs) have exhibited a lot of unique properties for their untra-small diameters, such as superconductivity, photoluminescence, selective adsorptions, high Li storage capacity and so on. Thus further study on these ultra-small diameter SWNTs will not only be necessary due to its scientific importance, but also be opening new windows to the applications in many fields such as electronic techniques, communications and energy storage materials.In recent years, the first-principles calculation based on density functional theory has been widely used in condensed matter physics, quantum chemistry and materials science thanks to the improvement of computational methods and enhancement of computing power. Now it is one of the most useful techniques to study the properties of solids and the calculated electronic, phononic properties are in reasonably agreement with the experimental results. Moreover, the first-principles calculation also provides significant guidance in design of new materials and study of properties of those materials which are not easily accessible to experiments.In this dissertation, we perform first-principles calculations to investigate the structural, energetic, electronic, doping properties of ultra-small diameter carbon nanotubes and related structures, and then explore their possible applications in Li-ion battery anode and hydrogen storage. The dissertation consists of 8 chapters: In chapter 1, we first introduce the discovery and structures of carbon nanotubes and overview their basic electronic, optical, magnetical and vibrational properties as well as their applications. Then we show the fabrication of ultra-small diameter carbon nanotubes and summarize their unique properties and potential applications. At last, we propose our reseach forcus of this dissertation.In chapter 2, we present the theoretical background and framework of first-principles calculation. We introduce the Born-Oppenheimer approximation, Hartree-Fock approximation, Hohenberg-Kohn theorem, Kohn-Sham equation, exchange-correlation energy and so on. At the end of this chapter, we show a flow chart of first-principles calculation.In chapter 3, we investigate the formations of Stone-Wales defects as well as their structural stabilities in 4A SWNTs. By Nudged Elastic Band (NEB) technique, we find formation energy barriers of such defects are very low and they show obvious chirality dependence in 4 A tubes. Among these three tubes, (5,0) is the most probably defective, while (3,3) is the most stable against being defected. Moreover, we study the electronic properties of the defect-containing tubes and find they all exhibit semiconducting behaviors.In chapter 4, we study the interaction between Li and ultra-small diameter SWNTs. First, the binding energies of Li doped in a series of SWNTs with different chiralities and diameters are investigated. The results show zigzag SWNTs are more favorable for Li doping than armchair ones and among zigzag tubes those with small diameters are even more favorable. Then we focus on the Li doping outside ultra-small diameter carbon nanotubes and find Li prefer to reside the top sites of the carbon rings in the wall. Moreover, the Li binding energies show robust chirality dependence as in the case of inside doping. Additionaly, the band structures and charge transfer are discussed and we find the Li storage capacity in these small tubes can reach to LiC2.5, which is much higher than in those graphite intercalated compounds (GICs).In chapter 5, the Li-doping in a double-wall carbon nanotube (DWNT) comprised by (5,0) and (14,0) SWNTs is discussed. Three possible doping sites are considered and the corresponding binding energies, band stuctures and charge transfer are all presented. The interlayer site between the inner and outer tubes is the most favorable for Li and the further Bader charge analysis indicates the inner tube can get electrons more easily from Li than outer one. Together with the other two doping sites, it is found the highest Li storage capacity can reach to LiC4.75.In chapter 6, we investigate the interaction between Li and carbon nanotube-zeolite complex. Four possible doping sites are discussed and their structurs, energies, volume change, band structues and charge transfer have been studied, respectively. At high Li concentration (386 mAh/g), we find this complex show strong structural stability and well reversible cyclicity. Aside from this, our electrochemical calculation suggests the battery voltage can be up to 4 V when the complex is used as Li-ion battery anode.In chapter 7, we study the formations of small bundles from individual (5,0)tubes and the hydrogen storage in the triplet bundle. Due to the strong curvature effect, the (5,0) tubes prefer to form small bundles by covalent bonding rather than van der Waals force. Among all the small bundles, the triplet formed by three (5,0) tubes is the most stable, which exhibits a distinct semiconducting behavior. We find the H2 chemisorptions on the triplet show interesting selective adsorption. Taking the physisorptions into account, the highest H2 storage capacity can reach to 10.4 wt%, which is appealing for its hydrogen storage application.In Chapter 8, we summarize our works briefly and present a few proposals for further research on this topic.