First-principles Study On Surface Functionalization Of Low-dimensional Carbon Nanostructures

In the past two decades, the low-dimensional carbon nanostructures such as fullerene, carbon nanotubes and graphene have attracted much attention in a number of fields including physics, chemistry and material science. Neverthless, how to control and tune the structure and properties of these concerned carbon nanomaterials with a low dimension remains a challenge. The functionalization of the nano-sized carbon materials is one of the possible approaches to this goal. With this in mind, it is necessary to study the relation between structure and reactivity of the low-dimensional carbon nanostructures, which will help to develop the techniques to functionalize the nano-carbons, and may shed a new light on the design and fabrication of functional materials with the nano-carbons as basic building units. In the thesis, we have examined the relations between the structure and functions of two typical SWNTs with and without defects on the side walls, and the π-π stacking interaction of several aromatic heterocyclic molecules and graphene.The armchair SWNTs with a maximal chiral angle and the zigzag SWNTs with a minimal chiral angle are two representative structures, of which the structure-property relationship needs to be addressed. We studied the side wall reactions on the metallic armchair SWNT (6,6) and the semiconducting zigzag SWNT (10,0) with the same tube length and diameter. A reaction system was constructed, in which a small molecule M2(M=F, OH, NH2, H, CH3, COOH) reacts with the side wall of SWNTs, and the reactions were evaluated in terms of the reaction energy. For the pristine SWNTs, the calculated energies of the side wall reactions are independent on the types of SWNTs, i.e. the side wall reactivity is similar for all types of SWNTs. The functional group adjacent to different sites on the SWNTs, the variation trend of the energies vs. different sites is also the same for the armchair and zigzag SWNTs. For the two groups on the ortho or para sites, their chemical reaction activity is higher than on meta sites.It is known that the defects on the side walls of SWNTs would function as the centers for chemical etching of the tubes. Using the first-principles method, we systematically examined the defect side wall reactivity of the Stone-Wales (SW) defect or vacancy defect on the SWNTs. The results show that the reactivity of the C type SW defect SWNT (6,6) and the A type SW defect SWNT (10,0) is higher than the defect-free pristine SWNTs. The chemical energies of the zigzag tubes due to the SW defect are larger than the armchair one. For the A type SW defect SWNT (6,6) with a short CC bond linking the two pentagon ring that are parallel to the axis of SWNT, no local curvature effect is observed, and the two functional groups have little chance to synchronously react with the two carbon atoms. In other words, if two functional groups react synchronously on the carbon atoms connecting two pentagon rings, the structure of SWNTs will be destroyed. For the fluorination of3DB or5-1DB vacancy defect SWNTs. it is only when a group is on the dangling bond carbon atom that a large increase of the reaction energy will be observed, while in the case of the SW defect SWNTs, it is over60kcal/mol by3DB defect SWNTs, and it is ca.40kcal/mol in the case of5-1DB defect SWNTs.For the six functional reactions of various SWNTs, the trend is also the same:F> OH> H≈NH2> CH3> COOH. The functional reaction of F or OH groups on the side walls of SWNTs is exothermic, while the H, NH2, CH3, COOH functionalization of SWNTs is endothermic.By the first-principles simulation, we performed DFT calculations at the GGA and LDA level on the noncovalent parallel and vertical interaction of graphene by aromatic heterocyclic molecules (thiophene, benzene, benzothiophene, dibenzothiophene, and pyridine). The most stable π-π stacking adsorption configurations prefer the "stack" site. The heterocyclic molecules are charge acceptors for graphene.The polarity and the size of π-conjugation can alter the magnitude of interactions between small aromatic molecules and graphene. According to the Hunter-Sanders model, we have made efforts to evaluate the relative contributions of the π-σ attraction and the π-π repulsion. It is found that the π-σ attraction energy accounts for the most of the energy in terms of the adsorption energy and the differential electron density of the complex systems.