Theoretial Studies Of CO Absorption On Carbon Nanotubes And Breaking Mechanism Of Single Molecular Junctions Formed By Octandithiol Molecules And Au Electrodes

Nanotechnology is considered as the greatest invention since the electricity, almost involved in all science fields:nano-materials, nanoelectronics, nano-biology, nanochemistry and so on. With the continuous development in experimental techniques of micro-electronics and improvement of theoretical method, constructing some functional electronic devices based on the single molecules becomes a hot topic in the nanotechnology field. Metal-molecule-metal type molecule junction, with special mechanical and electronical properties, is the most basic and widely studied structure in molecular device. Meanwhile, the carbon nanotubes (CNT) have rapidly attracted a lot of attention in many scientific domains because of their unique physical, chemical properties and structure, as well as their signification potential on a broad range of applications since they have been discovered in 1991 by Doctor Iijima of NEC company. However, the experimentations have certificated that the applications of perfect carbon nanotubes are limited, and realistic carbon nanotubes doubtlessly contained various topological defects which may change the properties and influence the applications of the tubes.Based on the ab initio total energy density function theory (DFT) and the software SIESTA, we present theoretical studies focused on the research about the molecular break junction (MBJ) and another research about the carbon nanotubes with single atomic vacancy. The first part is a theoretical study of the elongation process of molecular junction formed by octanedithiol molecule and Au electrodes. The second part is to analyze the effect of adsorption of small gas molecule CO on the electronic structure modification of the defected carbon nanotubes.The thesis is organized as following four parts:In the first chapter, we introduce the background knowledge of molecular junction, such as junction type, experimental technique, status in quo and problems. Second we review the basic knowledge of carbon nanotube included its allotropes, geometrical structure, sorts, properties of the carbon nanotubes. Then we give a brief description on the carbon nanotube with a single vacancy. Finally we give a brief description of our research contentsAt the beginning of the second chapter, we review the first principles of quantum mechanics which the simulative calculation used, includes Born-Oppenheimer approximation and single electron approximation. Then we emphasize the basic knowledge of density function theory, such as the theory of Hohenberg-Kohn, Local Density Approximation, Generalized Gradient Approximation. Finally self-consistent field calculation and the software SIESTA are briefly introduced.In chapter 3, we present a theoretical study of the MBJ processes of the molecular junction formed by octanedithiol molecules and Au electrodes. In the project, we simulate the contraction and elongation processes of five types of molecular junctions. The variation of the total energy and the average force needed to break the molecular junction are calculated, and each type of molecular junctions is found to have a characteristic breaking force. The results show that the behavior of the end H atom in the -SH group plays a crucial role in the variation of the junction structure and each type of molecular junction has a specific breaking geometry and a characteristic breaking force. We conclude that the breakdown of the H-Au bond may be the most important mechanism in the MBJ experiment if the molecular junction contains SH-Au contacts.Systematical study on surface adsorption of carbon nanotubes with small gas molecules (CO) is presented in chapter 4. Take the (5,5) tubes and the (7,0) tubes for instance, we mainly investigate the adsorption of CO either on a single atomic vacancy or on the defect-free region, and simulate three types of dynamic processes of CO along the defected carbon nanotubes. Then we give an analysis of the change of the adsorption energies, density of states (DOS) and projected DOS (PDOS). It is shown that the adsorbed CO makes similar effect on the (5,5) tubes and the (7,0) tubes. When the CO adsorbed above the defect site, the effect between CO molecule and nanotube is chemical adsorption. However, the effect converts to physical adsorption as the CO molecule far away from nanotube. Additionally, the dynamic processes as ether along the axial or circumferential direction, the systematic energy is the minimum and the system is the most stable when the CO moved to the space site. The oxygen atom of CO molecule may affect the electronic structure of nanotube, which is attributed to the electron on the p orbital of the oxygen atom.The main conclusions are given in the last chapter.