Design And Study Of Properties Of Functional Molecular Devices

With continuous development of micro-electronics and continuous minimization of electronic device, one developing trend is to construct functional electronic devices by use of single molecule. Many theoretical and experimental groups have devoted to study of electric properties of single molecule and obtained exciting results in recent years. One has noted that small conjugated molecules, single- and multiple-wall carbon nanotubes and macromolecules such as DNA possess many useful device characteristics, for instance, molecular switch, molecular memory, negative differential resistance and single-molecule transistors. Currently, the research area of electric properties of single molecule and atomic cluster has become a independent subject gradually. While experimental techniques and theories in molecular electronics need to be developed, because not only do theoretical results not give a well explanation for experimental measurement, but also the experimental results for the same molecule with different technique show great difference between each other.The main reason for the questions mentioned above is that, compared with the electrode, the molecule is a small system in the size. Therefore the geometric and the electronic structures of the molecule are likely influenced by the change of the external factors. The electronic structure of the molecule dominates the electric properties of the molecule directly. In this thesis, based on the quantum chemistry calculation, applied elastic scattering Green's function method, different gold-molecule-gold systems are investigated by studying the relationship between the structure and properties of the molecular devices and the factors that influence properties of the molecular devices. Furthermore, the transport properties of different isomers are compared. The influences of contact structures between the molecule and the metal electrodes, the electrodes distance, the external field, the molecule length and the gate voltage on the electric properties of molecular devices are discussed. The charge redistribution and the potential variation upon the formation of metal-molecule-metal junction are also analyzed. In the discussion, the functional molecule devices are proposed. Basing on the investigation of the electric properties of different molecular systems, one can find that the metallic atom that nearest to the molecule plays key role for the structure and the properties of the molecule. Extended molecular orbital and coupling parameter between molecule and electrode are the two important factors that influence electronic transport property of the molecule. The extended molecular orbital is the main factor that results in the occurrence of the plateau feature for the conductance curve of the molecular system. The extended molecular orbital and the coupling parameter between the molecule and the electrode are both influence the height of the plateau.The atoms that form 4,4'-bipyridine and its isomers are completely the same, but for the difference of the molecular structures, there exist great difference between the electronic transport properties of the isomers. The nitrogen atoms and the carbon atoms in the pyridine rings contribute electrons to form extended molecular orbitals together. While for the strong oxidbillity of the nitrogen atom,compared with coupling parameter of the carbon atom at the counterpart position, the coupling parameter between the nitrogen atom and electrode is a little weaker. The conductance curves of the isomers have good plateau features. While in the bias regime under investigation, the number of the plateaus is different. Because the contributions of different terminational atoms to the extended molecular orbitals is notably differentThe geometric optimization of the 4,4'-bipyridinejunction shows that it is energetically favorable for the teminational nitrogen to sit on the top of the gold atom, which is different from the molecule with thiols as the terminational atom clusters. It seems that, for 4,4'-bipyridine, the dihedral between the pyridine ring and the coupling parameters between the molecule and the electrodes have the minimum absolute values simultaneously. Because the molecule contribute charges to the electrodes and there exist potential barriers at the interfaces between molecule and electrodes, upon the formation of the gold-4,4'-bipyridine- gold junction, charge density and potential have an obvious variation near the interface. Moreover charges are accumulated inside of the interface and depleted outside of the interface. Thus two additional electric dipole moments with reversed direction are produced near the two metal-molecule interfaces. The changes of charge density and potential in interior of 4,4'-bipyridine molecule are slight. The changes of charge density and potential upon the formation of the molecular junction are different for different molecular system. For small molecular system, it is also obvious in the interior of the molecule, such as benzene-1,4-dithiolate.The 4,4'-bipyridine molecular junction is studied for contacts with planner, chain and pyramid configurations. Similar electrodes distance dependence has been observed for the current flow and conductance of the molecular system indicating that the major contribution comes from the gold atom that is bonded to the molecule. The conductance is exponentially dependent on the electrodes distance, such distance dependence is a result of the strong distance dependence of the couplings between molecule and electrodes. The shortening of the distance between two metallic electrodes results in a stronger coupling and lower potential barrier between them, leading to larger conductance. Good agreement between theory and experiment can be achieved when the contacts are of chain configuration and separated by a distance of 1.08nm. When the electric field is considered, there exist two kinds of charge transfer with the molecule contributes electrons to the electrodes and the charge redistributes inside the molecule, which leads to the presence of resistivity dipoles at the molecule-metal interfaces. When the bias voltage is in the lower region, the electronic transport properties of 4,4'-bipyridine molecular junctions are a little influenced.Compared with the 3T1DT (oligothiophene with three thiophene rings) molecule, the longer 4T1DT (oligothiophene with four thiophene rings) is found to be more conductive, which is agreed with the experiment measurement very well. This unusual length dependence is due to the short distances and larger couplings between the terminational atoms of 4T1DT and the electrodes. The coupling parameters between 4T1DT and gold electrodes are increased with the increase of the gate voltage, while the absolute value of Fermi energy have decrease trend with it. These two factors lead the current of the 4T1DT increase with the increase of the gate voltage, which is consistent with the experiment result. Under negative gate voltage, the 4T1DT molecule is oxidated since the 4T1DT contributes electrons to the electrodes, while under positive gate voltage the 4T1DT obtains electrons and is reduced.For the molecule with thiols as the terminational atom clusters, when the sulfur atom sits on the hollow position of Au(111) surface, the onset bias voltage of the first transport channel is about 0.8V. While when the sulfur atom on the top of a gold atom, the onset bias of the first transport channel is generally less than 0.5V.This thesis consists of eight chapters. In the first chapter, background and recent development of molecular electronics in the point of experimental and theoretical work are introduced. The questions need to be solved in this area in the future are also mentioned in this chapter. The theory of self consistent field (SCF) for many-particle system is presented in the second chapter which includes Born-Oppenheimer approximation, Hartree-Fock method and density functional theory. The elastic scattering Green's function method, the computational equation for the coupling parameter and the electronic transition spectra and the function of current-voltage properties for the molecular junctions are introduced in the third chapter. From the fourth chapter to the seventh chapter, the computational work and the main theoretical results are presented. The fourth chapter analyses and compared the electric properties of 4,4'-bipyridine and its isomers. The influences of the contact structure and the electodes distance on the coupling parameters and the transport properties of 4,4'-bipyridine molecular junction are discussed in the fifth chapter. In this chapter the influence of the external electric field on the electronic structures and current-voltage properties of 4,4'-bipyridine molecular junction is also studied. The charge redistribution upon the formation of the molecular junction is investigated in the sixth chapter. The seventh chapter compares the conductivity of oligothiophenes, and discusses the factors that influence the conductibility of oligothiophenes. In this chapter, the influence of gate voltage on the coupling parameters and on the transport properties of 4T1DT are also discussesed. The eighth chapter draws a conclusion for the whole work and views the future development of the molecular electronics.