Theoretical Studies Of Single Molecular Electronic Devices

It is well known that, one of the electronic research subjects in the 21th century is to make the traditional device based on silicon minimize to a nanometer size. With the size of the semiconductor transistors close to the nanometer level, quantum effects will have an important impact on device performance. In recent years, with the development of scanning probe microscopy techniques and the research on nanometer size devices, one has a certain degree of knowledge on the nano or even molecular scale of microscopic world. This builds a solid foundation for a new generation of molecular electronic devices. At the same time, because of the freedom of molecular design, flexible synthetic methods, widely available self-assembly means, super small size of molecules and their rich properties, scientists have capacious space for development and preparation of molecular devices.In the late 90s, several research groups carried out experimental research work on a number of molecular devices, using STM technology, light camera technology, LB film technology, self-assembly technology, organic molecular beam epitaxy technology. They found that some molecular devices have many useful functions, such as molecular switches, molecular memory, negative differential conductance, molecular FET characteristics.Although molecular electronics has made considerable progress, these molecular devices are still in principle. Their reliability, repeatability, and cost need a lot of work to do. Furthermore, there are still some controversies in the experiments of some molecular devices and different groups even within the same group gave different conclusions for the same molecule. Regarding the differences in these reports, there are two reasons. On the one hand, the technique is not mature enough. The contact structures between molecules and the electrode, the distance between the electrodes, and the number of molecules are difficult to control. On the other hand, due to lack of awareness of the molecular characteristics and electronic transport mechanism, one cannot guide experiments well. Therefore, it is important for theorists to find molecular materials with different functions, and explore the working principle of molecular devices. Because the design of molecular function devices directly in experiments requires a lot of attempts, and is limited by the technology, equipment, cost and other factors, therefore, the design and simulation of molecular devices in theory can provide theoretical guidance and reduce the twists for experiments.On the basis of quantum chemical calculations, we develop the elastic scattering Green function method, and form a molecular devices QCME package for calculation of electronic transport properties. The geometry optimization and electronic structure calculations are performed on the package of GAUSSIAN03, the hybrid density functional theory (B3LYP) is used and LanL2DZ is selected as the basis set. Two systems are often used to study the molecular devices, the aromatic hydrocarbon system and thiol member chain. We study the electronic transport properties of such molecules, discuss and analyze the effects of functional groups, solvents, intermolecular interactions, pressure, end group and molecular chain length on the electrical properties of molecules. We also investigate the switching characteristics of single-molecule junctions formed by the two isomers of 4,4- dicarboxylate 1,2- stilbene and two gold electrodes, and compare the theoretical results with the experimental results. Our work can provide theoretical basis for assemble and regulation of molecular devices, and useful suggestions for the design of molecular function device. It is also conducive to search for the relationships between the molecular devices performance and molecular structure comprehensively and accurately.The aromatic hydrocarbon system formed by benzene ring is often used to make a molecular wire. We take the anthracene in the terphenyl and the fused ring aromatic hydrocarbon as the study objects, and carry out the calculation of electronic structure on the member composed by molecular and the golden electrode and the voltage-current characteristic research. Couplet aromatic hydrocarbon molecules are taken as the example to discuss the effect of adding different functional groups in the same location of the molecule on the molecular transport properties. We also carry out the molecular dynamics simulation in solvent on the couplet aromatic hydrocarbon molecules replaced by amino and hydroxyl and obtain the results of the effect of hydrogen bond on electronic transport properties. The calculation results show that the band-gap between HOMO and LUMO, the number ofπelectrons, interacting between molecules and gold surface, the molecular orbital expansion and other factors affect the electron transport properties in the molecular wire. Adding a functional group changes the molecular geometry and electronic structure, thus affects the electrical transport properties of the molecules. The strong polar functional group is more remarkable on the electrical transport properties of molecular. As a result of the hydrogen bond influence, the geometry structure and the electronic structure of molecular are changed, especially the expansion of the occupied orbital is much bigger, thereby the molecular conductivity is modified. The study shows that we must consider the impact of the solvent in the design of molecular devices.Thiol molecules can form a stable and dense ordered self-assembed monolayers (SAMs) membrane in the metal surface. This preparation method of this film is simple and controllable, and it is widely used in research field of molecular devices. We carry out the theoretical simulation and initial calculation on the electrical properties of thiol membrane, and calculate the effect of molecular interactions on the electrical properties of molecules membrane. In order to investigate the changes of the electronic transport properties of molecular films with the different applied pressure, we use a different angle to simulate the pressure change. The impact of different terminal groups on molecular junction conductivity and the alkane thiol of different lengths on electrical transport properties are also investigated. The calculated results indicate that, the conductivity of molecular films increases 2-3 magnitudes due to the interaction of molecules. With the increase of applied pressure, conductance of each molecular junction is significantly increased. That is, the conductivity of the molecular junction is increased with the pressure increasing. For the molecule junction composed by single molecule, the increase of current is mainly due to changes of the coupling of single molecule and electrode caused by change of electrodes'distance. But for a molecular film composed by a number of molecules, there are three reasons. The first reason is the enhanced coupling of molecule and electrode caused by change of electrodes'distance. The second reason is that the distance of molecular chains decreases as cosθchanges because of the pressure, and the interaction between molecular chains is increased. These two points mentioned above increase the electron tunneling probability of the molecular chain, which leads to an enhanced conductivity. The third reason is that, because of the smaller distance of the molecular chains, the tunneling current of molecular film chains become larger, thus causing the increase of currents of entire self-assembly in molecular membrane. By the simulation of different end group, we find that the end groups have a significant effect on the molecular conductivity. This can be attributed to the exectronegative oxygen atom. We calculate the electronic structures of molecular junction formed by alkane thiols of different lengths and gold electrode. The result shows that, at low bias voltage, the current value of alkane thiols decreases exponentially with increasing the length of molecular chain. These results are compared with experimental results and have good agreements on both qualitative and quantitative.Molecular switch is a bistable quantum system. When external conditions change, the molecules can switch between the two states, and these two states including high and low conductance correspond to the states on or off in circuit. The single-molecule junction formed by two isomers of 4,4 - dicarboxylate 1,2 -stilbene and two golden electrodes is systematically studied. The results show that, the end carboxyl group of such molecules by chemical adsorption on the gold surface can be used to build molecular switches. This phenomenon is attributed to the different contact configuration between molecules and electrodes, and theoretical results are in good agreement with measurements.There are seven chapters in this paper. The first chapter is the review section. It briefly introduces the challenges of traditional electronic devices and the birth of molecular electronics, the current development of molecular devices in experimental and theoretical research, and analyzes the existing problems and causes. The second chapter describes the theoretical basis of quantum chemical calculations, including the Born-Oppenheimer approximation, Hartree-Fock approximation, density functional theory, molecular orbital theory, the choice of basic functions and so on. The third chapter introduces the elastic scattering Green function theory in the calculation of electronic transport properties of molecular devices in the applications. Chapter four to Chapter six introduce the computational work and research done in this paper. The fourth chapter takes the aromatic system as the research object, discusses electronic transport properties and analyzes the functional groups and solvent on the molecular electronic transport properties of the device. Chapter five takes the thiol monolayer as the research object, studies the electronic transport properties of such molecules, and discusses the effect of molecular interaction, extra pressure, end group, molecular chain length and other factors on the electrical properties of molecules. Chapter six studies the switching characteristics of single-molecule junction formed by the two stilbene isomers of 4,4-1,2-dicarboxylate two gold electrodes systematically. The seventh chapter summarizes the full work of this paper prospects the future development of molecular electronics field.