Theoretical Studies On Electronic Transport Properties Of Molecular Junctions

Great development of single-molecule science has taken place in theoretical regions as well as in experimental regions in recent years. It is known that if a molecule has the similar characteristics as electronic devices such as diode or triode, the molecule must possess delocalized electrons. Therefore, the aromatic molecule is usually chosen to be studiedbecause it has delocalized electrons- electrons. In addition, the molecule may serviceas electronic device greatly when it is chemically adsorbed on the surface of the metal. The experimental results show that when a molecule having thiol end groups connected with a gold cluster, the hydrogen is desorbed easily and the sulfur atom is directly bound to the gold surface. The bonding between sulfur and gold cluster is covalent interaction, which results in the hybrid of the molecular orbitals of the bare molecule and the metal, and forms the channels for the electrons to transport.In our calculations, an organic molecule benzene-1,4- dithiol is chosen to structure molecular junctions, and the metal electrodes are presumed to be made up of gold. 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 molecule is a system liable to being affected by the electric field, therefore when the electronic transport properties of molecule are studied, the effect of electric field should be considered, which induce the nonlinear electronic transport through the molecule. Based on the hybrid density functional theory, the effect of the electric field on the extended molecule and the potential distribution properties of several extended molecular systems are investigated at ab initio level. Then the nonlinear electronic transport properties of the molecular junctions are studied.At first, we investigate one extended molecule without adding electric field. The results exhibit that the net charge of bare molecule is negative, and the net charge of gold electrodesis 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 orbitals of them, and the result of the hybrid is to make the coupling of the original orbitals of subsystems take place to form a new set of orbitals. Some of these new orbitals extend throughout the extended molecule, and provide the channel for electronic transport. Other orbitals 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), and is nearer to the LUMO.The external electric field has great effect on the bonds of the S-C and the distance of between sulfur atoms and the surfaces of the gold clusters, which consequently change 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, the less expansibility of the orbitals and the changing of the peak values of the energy density with the increasing of electric field.The numerical results of potential distribution of the molecular system indicate that the potential distributions along the axis of the systems are different for the extended molecules connected with different gold cluster models, while the variation trend of the potential distributions are similar in the direction of external field. In the weak field, the potential distributions a...