Theoretical Research On Electronic Transport Of π-conjugated Molecule Junction

In the last few decades, single molecule device construction has become an increasing interest in molecule electronics. At the same time more and more reseachers apply themselves to the research work of single molecule that embrace the electrical characteristics measurement and the understanding of the electronic tranport mechenics.Π-conjugated molecules are considered as appropriate candidates for fabrication of molecular devices because they have delocalized charge inside. Especially, smallπ-conjugated molecules have been studied carefully in theoretical and experimental work in recent years. In this article, we use a quantum chemistry method, which is called elastic scatting Green function method, to describe the electron transport in molecular junction. The theoretical model of single molecule device is the extended molecule composed of aπ-conjugated molecule and two metal atom clusters.Above all, this article introduce single electron approximation and LCAO-MO approximation. While solving Schrodinger's equation of multiple atoms, with the Born-Oppenheimer approximation, it can be separated into two equations, one is nuclears motion equation, another is electrons motion equation. The latter can be converted to single electron equation by Hatree-Fork method or Density Functional Theory. Extended molecule orbitals are expressed as the linear combination of atomic orbitals, called LCAO-MO approximation, which is suited to quantumly calculate the interaction energy between molecule and electrode.Secondly, basing on ab initio method, we derivate the interaction energy expression. The current density expression also has been obtained by the elastic scattering green function theory combining with Landauer Formular. Moreover, the current formular and conductance formular are deduced on the base of current density expression. Finally, above-mentioned thoretical methods are used to calculate the conductance of some metal-molecule-metal junctions and the interaction energy between molecule and electrode. In these junctions, gold is selected as electrode material and someπ-conjugated molecules, such as BDT (benzene-1,4-dithiol), PDI(1,4-phenylene diisocyanide), BPDI(4,4'-biphenyldiisocyanide) , TPDI (4,4'-p-terphenyldiisocyanide), are bonded to two gold electrodes. Our three main results is listed as follows.1. The interaction energy of metal electrode and molecule has been investigated by taking the 1,4-phenylene diisocyanide (C6H4(CN)2) molecule as an example. Numerical results show that the distance between electrodes is a main factor to the coupling. The coupling energy changes gradually when the distance between two electrodes is broadened. In the tetrahedron-mode system and beeline-mode system, the molecule and electrode couple most tightly at 1.338nm and 1.378nm, respectively. Furthermore, the coupling energy of the junction that consists of BDT and tetrahedron-mode gold atom cluster is calculated and is compared with the numerical value obtained by means of another theory. By and large, two results is consistent with each other, which indicate our interaction energy expression is correct.2. Triangle-mode, tetrahedron-mode and beeline-mode gold atom cluster are combinated with PDI(1,4-phenylene diisocyanide) respectively to form the extended molecules to simulate different interaction between molecule and gold electrode. We caluculate the geometrical structure, electronic structure, transition intensity spectrum, current and conductance graph of three extended molecule systems. The features of conductance curves are shown to be strongly dependent on the detailed contact strucrures. In the tetrahedron-mode extended molecule, the first and second step of the conductance curve is located at 0.2V and 0.6V. The nonlinear electronic transport properties of the molecular junctions have been investigated and the I ? V ,G ? Vcurves have been obtained. For the molecular junction in which 1,4-phenylene diisocyanide link to gold atoms in tetrahedron-typed configuration, good agreement between theory and experiment can be achieved, especially in conductive features.3. State density, transport intensity, current-voltage curve and conductance-voltage curve of isocyanide molecules are studied and compared with each other, including PDI, BPDI and TPDI with the similar contact configuration between electrode and molecule.In this article, the first chapter introduce molecular electronics research background of molecular electronics. The second and third chapter present the theoretical method discribing the electron transport of molecular junction. The effect of distance between two electrodes on interaction energy is shown in the fourth chapter, and the effect of contact configuation on the current-conductance curves of molecular junctions is shown in fifth chapter. In the sixth chapter, electron transport features of three kinds of isocynide molecule are compared.