Studies On Electronic Transport Properties Of Functional Organic Molecular Electronic Devices

The possibility of using single molecules to build electronic devices has attracted much attention in recent decades. Many exciting developments have been made in the field by virtue of technological advances and in-depth understanding of electron transport in molecular junctions. It is known that the detailed geometrical configuration of metal-molecule-metal junctions plays a key role in the charge transport properties. Thus, resolving the configuration of molecular junctions is a key issue for the controlled formation of molecular devices with required functions. However, from an experimental point of view, it is very difficult to ensure that the junction consists of just a single molecule, and,even when such junctions are realized,it is hard to know the microscopic arrangement, e.g., how the molecule is bound to the electrodes or the pathway followed by the electrons in the molecule. Inelastic electron tunneling spectroscopy (IETS) of molecular junctions has been introduced recently to the field of molecular electronics as a way of probing the molecular junctions as well as extracting information about the molecular conformation. Inelastic electron tunneling (IET) is induced by the coupling of electron and nuclear motions in molecules. Moreover, the IET process is also strongly associated with molecular dynamics, charge transfer, and chemical reactions. Therefore, IETS is an promising technique for the development of molecular electronics.Recently, inelastic electron transports in molecular junctions have been studied by several experiment groups, and many valuable achievements have been obtained. Only experimental results are not enough to determine molecular configurations. Theory is thus needed to make accurate assignments and to interpret features of the spectra. Inelastic electron transports in molecular junctions have been studied in theory by using either model calculations or first-principles simulations. The position and intensity of the peaks in the simulated spectra do not always agree with the experimental measurement well. The main reason for the questions mentioned above is that IETS is very sensitive to the molecular geometry, the molecule-metal contact structure, orientation of the molecule adsorbed on the surface and other external factors. In this thesis, a first-principles computational method based on hybrid density functional theory is introduced to simulate the inelastic electron tunneling process of molecular junctions. The influence of the molecule-metal contact structure, orientation of the molecule adsorbed on the surface, the distance between the carbon of the terminal methyl group and the gold surface of the electrode, the intermolecular interaction and the electrodes of different metal element on the IETS of molecular devices are investigated.We study the IETS of decanethiolate related to three molecule-metal contact structures, orientation of the molecule backbone relative to the surface, the distance between the carbon of the terminal methyl group and the gold surface of the electrode. The computational results show that the theoretical simulation has not only reproduced the experimental spectra, but also provided reliable and detailed information about configuration of the molecular junction. The molecular junctions formed by H?llback et al. experimentally are determined. The contact conformation is a triangle gold cluster at one side and a parallelogram gold cluster at the other side with the sulfur atoms are placed above the middle of the triangle, while the carbons of the terminal methyl group are positioned above the middle of the triangle and the center of the parallelogram (i.e. the bridge site). The titled angle is determined to be 20°, while the distance of the terminal carbon from the STM tip is determined to be 3.09 ? .A first-principles computational method based on the hybrid density functional theory is used to calculate the IETS of octanethiolate molecular electronic devices in the nonresonant tunneling regime. The computational results show that the IETS of octanethiolate is very sensitive to the molecule-metal contact structure. The IETS changes a lot with the adjusting of the electrodes'configuration. The IET spectra of a pair of octanethiolates with various molecular backbone to molecular backbone distances are investigated. The results demonstrate that intermolecular interactions affect IET spectra obviously when the distance is about 2.6 ?. It indicates that intermolecular inelastic electron tunneling appears and this process depends on the transverse modes. The IETS of a pair of octanethiolates with molecular backbone partly overlaped with the backbone to backbone distance fixed to 2.6 ? is also investigated. The result shows that the figures of IETS of these systems are quite different with the varying overlap degree.To evaluate the effects of electrodes with different element, the IET spectra of 1,6-hexanedithiol and 1,4-benzenedithiol molecular junctions with different electrodes have been calculated. The calculated results of both 1,6-hexanedithiol and 1,4-benzenedithiol show great discrepancy with different metal electrodes. It indicates that the IETS of both alkanethiols and phenylthoils are influenced by the electrodes with different element. The influence is maybe caused by the different dimension,mass and electron structure with different metal electrodes which induce the variation of the coupling energy between the electrodes and the moleculars.This thesis consists of seven chapters as follows. In the first chapter, the present state of molecular electronic devices, the background of IETS of molecular electronic devices and recent advance of experimental and theoretical work in this field are introduced. The questions needed to be solved in IETS area are also mentioned in this chapter. The density functional theory (DFT) is presented in the second chapter which includes the Hohenberg-Kohn Theorems, the Kohn-Sham equations and the exchange-correlation functionals in DFT. Moreover, the method of exhibiting the vibration of molecule and the vibrational analysis in the Gaussian program are also introduced briefly in this chapter. The computational theory and formulas for the IETS of the molecular junctions are presented in the third chapter. From the fourth chapter to the sixth chapter, the computational work and the main theoretical results are contained. In the fourth chapter, we systematic investigate the influence of the molecule-metal contact structure, orientation of the molecule adsorbed on the surface, the distance between the carbon of the terminal methyl group and the gold surface of the electrode on the IETS. The theoretical work has been compared with the experimental result. The influence of the electrodes contact structures on the inelastic electron tunneling spectroscopy of octanethiolate molecular junction is discussed in the fifth chapter, and the intermolecular interactions is also investigated. We discuss the IETS of 1,6-hexanedithiol and 1,4-benzenedithiol molecular junctions in the sixth chapter, in which the element of electrodes is varied. The seventh chapter draws a conclusion for the whole work of this thesis and gives the prospect on the development of the IETS of molecular electronic devices in the future.