Theoretical Studies On Electronic Transport Properties Of Organic Molecular Junctions

Construction of molecular junctions with various special functions using singlemolecules is one of branches of Nanoelectronics. Within the last decade, study of themolecular junction became an important part of the molecule electronics. The basicstudy correlated with molecule electronics is being spread all over the world, whichplays a very important part in science and extended application. We have appliedDensity Functional Theory and elastic-scattering Green' function theory to study theelectron transportation processes in organic molecular junctions.In our calculation, some organic molecules are chosen to structure molecularjunctions, in which the metal electrodes are made up of gold atoms. In order to studythe interaction between molecule and metal electrodes, finite gold atoms are chosen tocompose gold clusters to simulate connection between electrodes and molecule. Theorganic molecule is sandwiched between two gold clusters to form the extendedmolecule. The gold clusters composed of three gold atoms may simulate the interactionbetween bare molecule and the gold (111) surface finely. The interaction between baremolecule and the surfaces of the gold clusters has taken place by hybrid of the orbits ofthem, and the result of the hybrid is to make the coupling of the original orbits ofsubsystems take place to form a new set of orbits. Some of these new orbits extendthroughout the extended molecule, and provide the channel for electronic transport.Other orbits only localize on some atoms of the extended molecule, which have littlecontribution to the electronic transport.Recently,many studies of molecule electronics have been carried out with thearomatic molecule based on benzene, for it has delocalized electrons——π electrons.Heterocyclic molecules containing nitrogen have similar π electrons,so they also aregood choice for molecule junction. So far, the research based on heterocyclic moleculesis seldom, and in terms of the terminal atoms, sulfur atom is chosen in mostexperimental systems, whereas other atoms almost haven't been used. In this paper, theelectronic transport properties of molecular junctions constructed by the six-memberedheterocyclic molecules-pyridazine-2,5-dithiol, pyrazine-2,5-dithiol,pyrimidine-2,5-dithiol-in contact with gold electrodes have been calculated andcompared . For the heterocyclic molecule pyridine, the influence on the current-voltagecharacteristics of the choice of terminal atoms has also been obtained. It shows thatpyridazine-2,5-dithiol has the higher current and conductance, and the conductance ofpyrimidine-2,5-dithiol is very small when the bias voltage is low. When selenium atomis chosen as the terminal atom, the conductivity of the molecule pyridine is better thanthat when oxygen atom or sulfur atom is, respectively. When the bias voltage is 2.0V,the conductance of pyridine come into being a ratio of 1:4.3:5, with oxygen atom, sulfuratom or selenium atom as the terminal atom, respectively.When molecule is placed between two metal electrodes, the shape of the I-Vcharacteristic is determined by many factors, such as the geometry structure of themolecule, the property of the bonding between the molecule and electrode, the externalelectric field, temperature and so on. The different orientation of the molecule can alsoaffect the electronic character effectively. In this work, we have investigated electronictransport properties of metal-molecule-metal junctions by taking the 1, 8-octanedithiolmolecule as an example. The results show that the different orientation of the moleculeresults in the changes of the molecular geometric and electronic structures, and furtherinfluences the current-voltage characteristics of the junction. When the angle betweenthe S-S axis of 1, 8-octanedithiol molecule and the normal direction of the gold (111)surface is 25°, the calculated current-voltage curve consists with the experimentalresults preferably.The thesis consists of six chapters which are as follows. In the first chapter, thecurrent development of the molecular electronics is introduced from the point oftheoretical work and experimental work. The basic concept and progress of densityfunction theory is introduced in the second chapter. When we use density functionaltheory to do calculation, the selection of basis sets is needed, which is also showed inlast part of this chapter. In the third chapter, we detail the elastic scattering Greenfunction method that is used in investigating the current-voltage properties. And ourresearch process and numerical results are discussed in the fourth and fifth chapter. Themain conclusions are given in the last chapter.