Theoretical Studies On Electronic Transport Properties Of Molecular Junctions

Great development of the study on molecular electronics and molecular devices has taken place in theoretical regions as well as in experimental regions in recent years, and which has been one of the branches of Nanoelectronics, and will have important value to scientific research and application. Molecular devices have began to play a important role in the area about interconnects, switches, rectifiers, transistors, nonlinear components, dielectrics, photovoltaic, memories and so forth.Many studies of molecular electronics have been carried out with the aromatic molecule based on benzene , for it has delocalized electrons -Ï€electrons, it can take on special properties of electronic transportation. In order to study the interaction between molecule and metal electrodes, finite gold atoms are chosen to compose gold clusters to simulate the connection between electrodes and molecular. This system forms the extended molecular. Since the net charge of bare molecule is negative, and the net charge of gold electrodes is positive, which indicates that the bond between molecule and gold clusters not only has covalent property, but also has some portion of electrovalent property. The interaction between bare molecule and the gold clusters has taken place by hybrid of the orbits of molecule and gold clusters. The result of the hybrid is to make the coupling of original orbits of subsystem take place to form a new set of orbits. some of these new orbits extend throughout the bare molecular and gold clusters, and provide channels for electronic transport, the others only locate on some atoms of extended molecular, which have little contribution to electronic transport, because its extend properties are poor. Fermi energy level lies between the highest occupied molecular orbit (HOMO) and the lowest unoccupied molecular orbit (LUMO).These studies differ in the way they take the electronic levels of the molecules, their modification by the coupling to the leads, and the change of electrostatic potential due to bias into account. To this end a number of theoretical studies have been performed with the aim of reproducing measured I-V characteristics. It is realized gradually in the study that, when the molecule is placed between electrodes, the shape of the I-V characteristic is determined by the electronic structure of the molecule in contact with the electrodes and in the presence of the external electric field.Basing on ab initio and elastic scattering Green function theory, by taking bis-(4-mercaptophenyl)-ether molecule as an example, the geometrical structure and electronic structure have been calculated with different models of electrodes and different distance between two electrodes. It is shown that the electronic transport properties have a great difference with the different models of electrodes, only the model which consist of three gold atoms is well agreed with the experimental results. The changes of the distance between two electrodes bring effects on the electronic transport properties of the molecular system. The conductance , current and coupling coefficient will drop with the increasing of the distance between two electrodes. The equilibrium state of the extended molecule has no special meaning to the properties of electronic transport.The external electric field has great effect on the properties of electronic transport also. The changing of the electronic structure which exhibited by the changing of the net charge of subsystems, the moves of the energy levels, the decrease of the gap between HOMO and LUMO leads to the nonlinear character of the electronic transport properties, which is well agreed with the experiment results. It is shown that it is necessary to take the influence of electronic field into account.The thesis consists of six chapters which is as follows: In the first chapter, the current development of molecular electronics is introduced from the point of experimental work and theoretical work. In the second chapter, the theory of independent particle model for many-particle system i.e. Hartree-Fork and density function theory is presented. In the third chapter, the theory of molecular orbit i.e. the selection of basis sets and the expending of molecular orbits is introduced. In the fourth chapter, the elastic scattering Green function method that is used in investigating the current-voltage properties is detailed. In the fifth chapter, the calculating process and its results are discussed. The main conclusion are given in the last chapter.