Theoretical Studies On Electronic Transport Properties Of Molecular Junctions

Within the last decade, there is no denying the fact that an increasing interest in molecular electronics has developed and the study of molecular electronics is one of the branches of the Nanoelectronics. With molecules as a drive of information processing, lots of electric characters in the molecular space can be researched in the molecular electronics. Nowadays, the basic science on which a molecular electronics technology would be built is now unfolding in the world, and the science and applications that are emerging are tremendously exciting. For example, current research is using molecules in such electronics applications as interconnects, switches, rectifiers, transistors, nonlinear components, dielectrics, photovoltaics, memories and so forth. 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. Semiempiric methods have been used, as well as first principles techniques. It is realized gradually in the study that, when the molecule is placed between two 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. In our calculations, an organic molecule 4,4'-biphenyldithiol is chosen to structure molecular junctions, in which the metal electrodes are made up of gold atoms. In order to study the interaction between molecule and metal electrodes, finite gold atoms are chosen to compose gold clusters to simulate connection between electrodes and molecule. The organic molecule is sandwiched between two gold clusters to form the extended molecule. The purpose of using gold clusters is to simulate the interaction between molecule and gold surface. The gold clusters composed of three gold atoms can simulate the interaction between bare molecule and the gold (111) surface finely. The results we get by investigating the extended molecule exhibit that the net charge of bare molecule is negative, and the net charge of gold electrodes is positive, which indicates that the bond between sulfur atoms and gold clusters not only has covalent property, but also has some portion of electrovalent property. The interaction between bare molecule and the surfaces of the gold clusters has taken place by hybrid of the orbits of them, and the result of the hybrid is to make the coupling of the original orbits of subsystems take place to form a new set of orbits. Some of these new orbits extend throughout the extended molecule, and provide the channel for electronic transport. Other orbits only localize on some atoms of the extended molecule, which have little contribution to the electronic transport. The equilibrium Fermi energy level lies between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Basing on ab initio methods and the elastic scattering Green function theory, we have investigated electronic transport properties of metal-molecule-metal junctions. The molecular geometric structures, electronic structures, and current-voltage characteristics have been studied for varying the distance between the two electrodes. The numerical results show that the changes of the electrodes'distance give a various influence on the extended molecule's geometric and electronic structures, which bring effects on the electronic transport properties of the molecular systems. The equilibrium state of the extended molecule is not the best situation for electronic transportation. The characteristics of electronic transportation can thus be improved by adjusting the distance between the two electrodes. The external electric field has great effect on the geometric structures of the extended molecular systems. In addition, the changing of the molecular geometric structures and the electric field cause the changing of the electronic structures, which is 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, the declining of Fermi energy level, and so forth. The nonlinear electronic transport properties of the molecular junctions are investigated by the elastic scattering Green function method we have developed by us recently, and compared them with the experimental results. The results are well agreed with the experimental results, which implies that the effects of the electric field on the molecular structure should be investigated carefully.