Multi-terminal Junctions And Growth Mechanisms Of Carbon Nanotubes

Carbon nanotubes (CNTs) have been widely applied in many fields owing to unique electron transport, mechanics, and gas adsorption properties. The straight carbon nanotubes can be connected into Y-, T-, X-, I- and L-type multi-terminal heterojunctions (MTHJs) by the introduction of defect rings to the perfect hexagonal lattice. Since the potential applications for the miniaturization of electronic devices such as diode and transistor, MTHJs have also attracted the attention of many researchers focusing on the preparations and properties both experimentally and theoretically. Due to the structures of CNTs and MTHJs with nanometer size, which limited in experimental methods and conditions, many unclear or unknown problems still exist in the studies of their syntheses and properties. The structures and properties of CNTs and related MTHJs, however, can be investigated using theoretical methods, which are expected to explain and predict the experimental work.In this dissertation, the structures, properties of MTHJs, and growth mechanism of CNTs are studied using Dmol~3, Gaussian 03 passage, as well as some other program using tight-binding and REBO potential functions. The main contents include:1. The structures, properties and growth mechanisms of the CNTs, as well as the synthetic methods of MTHJs, the relationship between the structure and stability, their electrical, mechanical, thermal and optical properties, and the applications of molecular simulation methods in the formation processes of MTHJs, are reviewed. In addition, their possible applications in nano-electronic devices, hydrogen storage and functional materials are summarized. Furthermore, some questions and the research trends of MTHJs are discussed.2. A variety of multi-terminal junctions, including Y-, T- and X-type carbon nanotubes, were generated by connecting individual single-walled carbon nanotubes with different size and helicity. By optimizing these hetero-structures with different geometric features, the energetically favored structures were proposed. The corresponding energy features of the junctions indicate that the tube diameter, the helicity, and the distribution of defects, play an important role on the stability of junctions. The most stable Y-, T- and X-junctions contain six, six, and twelve heptagons, respectively. These defect rings tend to distributed at the corners of the junctions. The zigzag-type junctions are more stable than the armchair ones. Moreover, the average energy of an individual atom for Y-, T-, and X- junction is found to be a function of the tube diameter.3. The properties of four finite-lengh bent and straight intramolecular junctions (IMJs) connecting two armchair and zigzag single-walled carbon nanotube segments, viz. (3,3)-(6,0) and (4,4)-(8,0), were investigated. Their structures were calculated using the density functional theory (DFT) methods at the B3LYP/6-31G(d) level of theory. The results indicate that the bent junctions are more stable than the straight ones due to the energetically favored defect structures. Remarkable differences of the HOMO and LUMO orbitals appear between the straight and the bent IMJs. The spin-unrestricted calculations at the same level of theory were also performed to obtain the antiferromagnetic-type ground state, suggesting that the spin polarization mainly occur on the zigzag edge and the defect rings of the straight (4,4)-(8,0) IMJ, and induce marked changes of the electronic structures. Additionally, the energy band structures of the four junctions with periodic boundary conditions were calculated based on DFT calculations using generalized gradient approximation (GGA) with the Perdew and Wang function (PW91). The calculated band gaps suggest that the conductance of the straight IMJs is higher than the bent ones.4. The geometry and electronic structure of finite-length (4,4) Y-shaped CNTs were investigated using DFT with GGA/PW91 method. The results indicate that the difference between the Y-shaped CNTs and the pristine one is remarkable due to the influence of the defects in the junctions of the former. Furthermore, the structures and properties of the Y-shaped CNTs are found to be related to the length of the CNT branch. By comparing the properties of the Y-shaped CNTs with different lengths, the length-dependent oscillation behavior including structure, energy gap and electrical property were observed when the length is longer than 10(?). 5. The formation processes of the CNTs with metal catalyst were simulated to decipher the underlying growth mechanism. The metal catalyst is modeled by a cluster with 13 metal atoms. The DFT method is used to investigate the migration of carbon atoms from adsorption sites on the surface to the end of the nanotube, in the process of catalytic growth. All the adsorbed carbon atoms are found to form strong bonds with the nanotube, indicating that adsorptions are an important factor for carbon nanotube growth. By studying the total energy of one carbon atoms adsorbed at all possible sites, it is found that the energy favorable sites are at the interacttion region between carbon nanotube and the discussed catalyst. Calculated migration energies for the adatoms show the migration is highly anisotropic due to the influence of catalyst. Cr, Mn, Fe, Co, Ni, Cu, and Zn are all found to favor the CNT growth, moreover, the Co, Ni and Zn appear to be better than the others. The results may be beneficial supplements for the root-growth mechanisms of carbon nanotubes. The possible growth process rooted on a Co particle was calculated by adding carbon atoms continually. Furthermore, the role of the metal catalyst was discussed, indicating that it will affect the charge distribution of the end part of the nanotube, conducive to the adsorption and migration of the carbon atom to form a perfect tube.