Theoretical Studies On Electronic Transport Properties Of Oligoacene Molecules

Molecular electronic devices have attracted more and more attention in the last few years. With continuous development in experimental techniques and treoretics means of micro- electronics ,one developing trend is to construct functional electronic devices by use of single molecule. Many theoretical and experimental groups have devoted to study of electric properties of single molecule and obtained exciting results in recent years. One has noted that small conjugated molecules, single- and multiple-wall carbon nanotubes and macromolecules such as DNA possess many useful device characteristics, for instance, molecular switch, molecular memory, negative differential resistance and single-molecule transistors. Currently, the research area of electric properties of single molecule and atomic cluster has become a independent subject gradually. While experimental techniques and theories in molecular electronics need to be developed, because not only do theoretical results not give a well explanation for experimental measurement, but also the experimental results for the same molecule with different technique show great difference between each other.The main reason for the questions mentioned above is that, compared with the electrode, the molecule is a small system in the size. Therefore the geometric and the electronic structures of the molecule are likely influenced by the change of the external factors. The electronic structure of the molecule dominates the electric properties of the molecule directly. In this thesis, based on the quantum chemistry calculation, applied elastic scattering Green's function method, different gold-molecule-gold systems are investigated. First, the electron transport properties with different electrode distances has been studyed by discussing the relationship between the distance and properties of the molecular devices and the factors that influence properties of the molecular devices. The study shows that the electron transport properties has large difference with different electrode distances. The coupling parameter between molecular junction and the electrode increases with the decreasing distance between two electrodes, as the same of the electron transport properties. Furthermore, the electron transport properties of Oligoacene molecules are compared. The influences of molecular length between two electrodes and different substitutes are discussed. The study shows that the relationship between molecular length and the resistance is consistent with the typical nonresonant equation Rn = R0 exp(β? s). Namely, the resistance show an index increase with the molecular length. Different substitutes have a sharp effect on the effective contact resistance R0 , while have only a little influence on the length-dependent tunneling attenuation factorβ. The charge redistribution of metal- molecule-metal junction and the coupling parameter between molecular junction and the electrode are also analyzed which give a better understanding of the remarkable resistance change. The theoretic calculations show a better simulation of the experiment. In the discussion, the functional molecule devices are proposed.This thesis consists of six chapters. In the first chapter, background and recent development of molecular electronics in the point of experimental and theoretical work are introduced. The questions need to be solved in this area in the future are also mentioned in this chapter. The theory of self-consistent field (SCF) for many-particle system is presented in the second chapter which includes Born-Oppenheimer approximation, Hartree-Fock method and density functional theory. The elastic scattering Green's function method, the computational equation for the coupling parameter and the electronic transition spectra and the function of current-voltage properties for the molecular junctions are introduced in the third chapter. From the fourth chapter to the fifth chapter, the computational work and the main theoretical results are presented. The fourth chapter analyses and compared the electric properties of 1,4-dicyanobenzene conjuctions with different electrode length. The electron transport properties of Oligoacene molecular junctions are discussed in the fifth chapter. The influences of molecular length and different substitutes are discussed. The sixth chapter draws a conclusion for the whole work and views the future development of the molecular electronics.