Molecular Simulations Of Carbon Nanotubes And Related Nanocomposites

As the fourth allotrope of condensed carbon, the carbon nanotubes (CNTs) have been attracted more and more attention and applied widely in many fields owing to their quasi one-dimensional infinite nanostructures and unique mechanics, electron transport and gas adsorption properties. Due to the structures of CNTs with nanometers and limit in experimental methods and conditions, many unclear or unknown problems still exist in the study of their syntheses and properties. The structures and properties of CNTs and related carbon nano-clusters, however, can be simulated and observed using molecular simulation methods, especially the molecular modeling methods with proper potential functions, which can explain and predict the experimental work.In this dissertation, the structures, properties and growth process of CNTs and related carbon clusters or composites are studied using Monte Carlo (MC) and molecular dynamics (MD) simulation methods. The main contents include:1. The structures, properties and synthesis methods of carbon nanotubes, and the features and classification of the molecular simulation methods are reviewed. The structure of carbon nanotubes, the features and contents of Monte Carlo and molecular dynamics methods, the classification and comparison of interatomic potential functions, as well as the study in CNTs by molecular simulation methods are summarized as an emphasis.2. From C₇₂ to C₇₈, the top twenty low-energy isomers screened out from all isomers of each fullerene are optimized and computed by tight-binding Monte Carlo (TBMC), semi-empirical PM3, and ab initio B3LYP/6-31G*//HF/3-21G methods. The comparison results show that the TBMC method can efficiently optimize the structures and correctly predicate the low energy isomers. The relative energies computed by TBMC are in good agreement with the high-lever B3LYP calculation results. Our TBMC and B3LYP results show that the most energetically favorable structure of C72 is not an isomer satisfying the isolated pentagon rule (IPR), which is different with the result by PM3. The symmetry of the most stable IPR isomer tends to low with the fullerene becomes large. Several non-isolated-pentagon structures of low symmetries are found to have low energies close to the most stable isomer.3. Parallel single-walled carbon nanotube (SWCNT) junctions are investigated using tight-binding Monte Carlo simulations. SWCNTs of different size and type, including zigzag (3,0) to (8,0) and armchair (2,2) to (6,6) without defects, are investigated. The calculation results show that the stable parallel junctions can be formed by small size nanotubes, when the two tubes close to each other. The coalescence of tubes to form larger ones and interlinking of two tubes have been observed. The discussion of the effect of size and type of SWCNT, as well as the contact mode, on the formation of junctions shows that thin zigzag tubes with bond crossing hexagon interface are favorable for forming junctions.4. Due to the unique mechanics and transport properties, as well as the potential application, the carbon nanocomposites constructed by fullerenes and CNTs have attracted much attention. Here a new nanocomposites, which are constructed from fullerene C₆₀, C₇₀ and C₈₀ isomers connected by open-end carbon nanotube segments (5,0) and (6,0), are proposed. Using tight-binding Monte Carlo simulations, a series of one-, two- and three-dimensional carbon networks and isolated dumbbell-like structures are obtained. Their structural properties at room temperature are systematically analyzed. The calculation results show that all the composites are stable. And among them, the structure connected by C₆₀ with I_h, symmetry and (6,0) nanotube segments is most stable. The calculated energies show that the one-dimensional linear structures are energetically more favorable than the corresponding dumbbell-like structures. However, the latter shows better mechanical properties and thermal stability. This work implies the potential application of these carbon nanocomposites and may give some inspiration for experimenters in future.5. An improved tight-binding Monte Carlo method is developed, which greatly decreases the CPU time by reducing the number of atoms in calculating tight-binding energy. By simulating classic C₆₀ system, the RMSD of structure at 300K between previous and improved method is 0.0936A, and the consumed time of improved method is only 1/10 of that of previous one. The simulation of the parallel nanotube junction also indicates that the improved method is much more efficient than the previous one. This method is successfully applied in study of the large carbon nanopeapod. The calculation results show that, the fullerene coalescence occurs inside single-walled carbon nanotube at about 2000 K by the formation of vacancies on the close sides of fullerenes, otherwise, the fullerene coalescence is only observed at high temperature of about 4500 K. The orientation of fullerenes does not influence the coalescence.6. The nucleation of nanostructures from multi-layered graphite flakes with different sizes is studied by tight-binding molecular dynamics (TBMD) simulations. Besides classical carbon nanotubes of different diameters with ends open or closed, tetrahedral and spherical cages are also obtained. Most interestingly, in given conditions, the transformations from four or more layers of graphite flakes to nanotube bends and heterojunctions, as well as nanotube T- and Y-junctions, are also observed in the simulations. By analyzing and comparing the nucleation processes of different nanostructures, it is found that structural transformations depend on the shapes of flakes, interlayer distances, temperatures, and so on, in which the interlayer distances are curial to the formation from more than two layers of flakes. Additionally, the double-layered graphite flakes surrounded with randomly distributed carbon atoms are also simulated at different temperatures. The temperatures 1000-1500K together with proper amount of carbon atoms appear to be suitable for nanotube nucleation.