Theoretical Studies Of Atomic Diffusion And Chemical Reactions In Carbon Nanomaterials

In recent years, the discovery of novel carbon allotropes has led to renewed interest of large scientific community in the varied properties of this element. Carbon constitutes a growing family with a variety of fascinating forms and outstanding properties, due to its unique hybridization states (sp, sp2, and sp3). Two natural allotropes of carbon, diamond and graphite, have been investigated for many years. The discovery of C60 in 1985 marked the beginning of an era of synthetic low-dimensional carbon allotropes, including fullerenes (0D), carbon nanotubes (1D), graphene (2D), and graphdiyne (2D). Since the pioneering detection work of Iijima on carbon nanotubes in 1991, there has been rapid progress in understanding their remarkable mechanical, thermal and electronic properties. A large range of other one-dimensional nanomaterials has also been synthesized using carbon nanotubes as the template or the raw material. The rediscovery of graphene, a single graphite sheet, and its fabrication into a field-effect transistor in 2004 by Novoselov and Geim, has opened up a new field of fundamental physics and provided exciting prospects in many areas of nanotechnological applications. More recently, graphdiyne, a new layered carbon allotrope composed of sp-and sp2-hybridized carbon atoms, has been successfully grown on the surface of copper via a cross-coupling reaction using hexaethynylbenzene. This novel carbon allotrope with excellent semiconducting properties, has been recognized as promising candidates of use in next-generation electronic and optoelectronic devices, especially vacuum device applications.The first-principles calculations based on density-functional theory (DFT) have been proved to be a powerful tool to reveal the structures and properties of nanomaterials. The geometric configurations, energetic stability, electronic structures, magnetism, dynamics of chemical reaction etc. can be investigated using this theoretical scheme. The first-principles calculations have also been successfully employed to deal with material design and performance prediction, which can greatly improve efficiency and shorten the development cycle. In this dissertation, we performed first-principles within DFT to reveal the atomic diffusion and chemical reactions in carbon nanomaterials. The carbon materials involved in our work include one-dimensional (1D) single-walled carbon nanotubes, two-dimensional (2D) graphene and graphyne and their corresponding bulk structures. The results in the present work can help us to understand and interpret the experiments at the atomic scale, understand the growth mechanisms of nanomaterials, and explore their potential applications.The thesis is organized as follows:Chapterâ… gives a brief introduction of research background and motivation. Chapterâ…¡introduces the theoretical fundamentals used in our research work. Chaptersâ…¢toâ…¦ describe in detail and summarize the work done during my Ph.D degree studies. The main content and results in this dissertation are listed as follows:1. Dynamics of point defects in graphite is closely correlated to the evolution of defect-induced ferromagnetism in 12C+-irradiated graphite. First-principles calculations were performed to explore the diffusion and coalescence of vacancies and interstitials in graphite. Both non-interacting graphene model and interacting graphite model were considered. The structure distortion and subsequent diffusion behavior of monovacancy revealed in graphite are quite different from those in non-interacting graphene model, indicating that the interlayer interaction should be taken into account in the investigation of point defects in graphite. Our calculations show that the coalescence of monovacancies to form multivacancies unlikely occurs at moderate temperatures due to the high energy barriers for the migration and coalescence of monovacancies, which are 1.26 eV and 2.17 eV, respectively. The interstitial atoms prefer to form a 'bridge' structure at low defect concentration, but a 'spiro' structure at high defect concentration. The 'bridge' interstitials have high mobility with migration energy of only 0.36 eV, and easily recombine with monovacancies or aggregate with other interstitials at moderate temperatures, which diminishes the ferromagnetism of irradiated graphite. Inverse Stone-Wales defects and interstitial clusters can be formed via the aggregation of 'bridge' interstitials. The 'spiro' interstitials prefer to diffuse in the form of 'bridge' structure. The results shed light on the correlation between the thermal behaviors of point defects and the ferromagnetism evolution in 12C+-implanted graphite.2. First-principles calculations were performed to explore the energetics and dynamics of Li in graphyne, a novel carbon allotrope consisting of sp-and sp2-hybridized carbon atoms. In contrast to the commonly-used graphite material where Li diffusion is restricted in the interlayer space (in-plane diffusion), three-dimensional Li diffusion (both in-plane and out-plane) can be achieved by overcoming energy barriers of about 0.53-0.57 eV. The highest Li storage capacity in bulk graphyne can be LiC4, which is higher than that in graphite, LiC6. The Li intercalation has little effect on the interlayer spacing between adjacent graphyne layers and thus advantageous for charging processes. The unique atomic arrangement of graphyne facilitates not only the Li diffusion but also the storage capacity. The high mobility and high Li storage capacity make graphyne a promising candidate for the anode material in battery applications.3. Graphene nanoribbons can be prepared by the oxidative unzipping of carbon nanotubes. We have carried out first-principles calculations to explore the unzipping of single-walled carbon nanotubes with different diameters and chiralities. The nanotube unzipping is triggered by the initial attack of permanganate. Generally, the initial attack takes place on highly-strained C-C bonds ('perpendicular'bonds for armchair tubes and'tilted'ones for zigzag tubes). Once a C-C bond is broken in the initial attack, the following oxidation processes are highly orientation-selective, i.e. breaking the C-C bonds parallel to the ones broken in former processes is barrierless. This gives rise to continuous unzipping of carbon nanotubes, producing graphene nanoribbons with zigzag edges. The initial attack prefers to occur in the middle region for armchair tubes, but at tube ends for zigzag tubes, irrespective of the end terminations or the tube lengths. The energy barriers for the initial attack depend on the tube chirality and increase with increasing of tube diameter. The energetically preferable unzipping paths for armchair nanotubes are straight lines parallel to the tube axis, while those for zigzag and chiral ones are helical curves around the tube axis. Armchair nanotubes are more preferable for producing straight graphene nanoribbons than zigzag ones. The location of the initial attack can be modulated by applying external radial deformation which suggests an effective means to control the shapes of graphene nanoribbons.4. The adsorption, diffusion and reaction of silicon on the exterior surface of carbon nanotubes are crucial for the growth of silicon carbide nanotubes from carbon nanotubes as the template. We have carried out first-principles calculations to explore these processes. Our calculations show that the stable adsorption configurations depend on the chirality. Based on the calculated binding energies, it is found that a single Si atom is most favorably adsorbed at the bridge site above a C-C bond being tilted to the tube axis for both armchair and zigzag nanotubes. The evaluated diffusion barriers suggest that the diffusion is strongly anisotropic. The energetically preferable diffusion pathways are both straight lines in the axial direction for the two types of nanotubes. Silicon substitution reactions in carbon nanotubes follow the kick-out mechanism. The activation barriers involved in these reaction processes are relatively high. The above-mentioned theoretical results provide important information for the controllable growth of silicon carbide nanostructures.