The Theoretical Study Of The Quantum Transport Properties Of Silicon Nanoribbons/nanotubes And The Nonlinear Optical Properties Of Lindqvist/Keggin Derivatives

Molecular electronics as an important branch of electronic nano-science is a very important stage in the development of microelectronics. It is to explore the molecular applications in the field of electronics at the molecular level. With the rising level of experimental techniques, and the development and maturation of quantum transport theory, molecular electronics have been developing rapidly. People have constantly explored new classed of molecules from small organic molecules, inorganic compounds, and even biological protein, the development of research content is also gradually enriched. With the development of related technologies mature, the corresponding theoretical researches in molecular electronics promote practical applications. Quantum transport theory to simulate at the molecular level, can be a better understanding of the basic process of electron transfer. Silicon as the basic element in integrated circuits, has an important role in the microelectronics revolution, which is widely regarded as one of the most important 20 th century material. Silicon nanoelectronics can be perfectly compatible with silicon-based microelectronic, therefore, the silicon nano-materials is particularly important in nanotechnology. Silicon nanoribbons and nanotubes as important members of the silicon material family, have been widespread concern in the experimental and theoretical in recent years. They are developing field of molecular electronics have become an important research direction.Polyoxometalates(POMs), due to its unique and superior physical and chemical properties, has been widely used in many fields. POMs are big electronic reservoir, easy to accept electrons. The POMs connected with organic ligands, which form the donor-acceptor model of organic-inorganic hybrid materials. Such POMs-based hybrid materials have good nonlinear optical properties. And the nonlinear optics of POM has become an important direction.In this thesis, using density functional theory(DFT) and the non-equilibrium Green's function(NEGF) method to explore the effects of deformation of silicon nanoribbons on quantum transport properties, and comparative study of the transport properties of silicon nanotubes and carbon nanotubes. Moreover, using DFT and time-dependent density functional theory(TDDFT), NLO properties of Lindqvis molybdates and Keggin tungstates derived were discussed. Thesis includes six chapters, the first chapter is a review of the molecular electronics, the development of silicon-based materials, and the NLO properties of POMs. The second chapter is calculated theory. The third to the sixth chapters are the main content of the thesis:(1) The effect of torsional deformation in the middle of the scattering region on electron transport through 11-ASi NRs has been investigated using the DFT combined with NEGF method. It is found that the torsional deformation in the scattering region slightly impacts on the transmission properties. From ? = 0° to 60°, the transmission coefficients of these systems are almost the same. It can be seen from calculated I–V curves that the current for ? = 90° and 120° just drops 8% and 22% in comparison with ? = 0°. So the transport properties of 11-ASi NRs are stable. Based on the stability of the electronic and transport properties of Si NTs upon significant structural changes in the scattering region, ASi NRs may be employed in silicon-based electronic nanodevices which possess superior stretchable properties.(2) We have investigated the electronic structure and transport properties of two limiting cases of Si NTs and CNTs by using first-principles calculations. Armchair Si NT possess around double potential charge capacities than the corresponding armchair CNT especially in the high bias voltage range. With regard to the whole transport properties, the metallic nanotubes C(3, 3) and C(6, 0) display more excellent conductive characteristics in low bias voltage range, which reflected in the larger current under the same bias voltage. Si NTs will be one of potential promising materials with contemporary micro- and nanoelectronic silicon-chip devices. The main reason is that the high energy and high-delocalized character of silicon 3p orbitals and the corresponding low energy gap between conduction band and valence band.(3) We systemically investigated the electronic structure, second-order NLO properties of a series of 2D ?-shaped and W-shaped systems based on the chromophore DTE and [Mo6O19]2- derivative with DFT and TDDFT methods. The photoisomerization accompanying conversions from open-ring form to closed-ring form leads to a larger switch of the static first hyperpolarizability. The TDDFT calculations exhibit that the mixing transitions in closed-ring systems, with planar structure provide a good ?-conjugation, significantly improve the NLO properties. These kinds of complexes might be promising candidates for switchable NLO materials.(4) Three pairs of compounds were systematically investigated to explore the one-electron oxidation effect on the static first hyperpolarizibility. The M-salen is the oxidized center by using Fukui function analysis. Three functionals have proved that the one-electron oxidized process obviously enhances the static first hyperpolarizibility. This kind of M-salen-POM compounds might be excellent switchable NLO material. Furthermore, the TDDFT calculations show that the mixing transitions in type II charge transfer might play a key role for effectively reducing the transition energy and improving the static first hyperpolarizability. The analysis of the effect of one-electron oxidation on their optical responses might provide important information of these compounds for switchable NLO applications.