Theoretical Studies On Elastic And Inelastic Electron Transport Of Molecular Junctions

With the minimization of electronic devices, the study of molecular junctions has attracted much attention. All kinds of experimental techniques have been developed to prepare molecular junctions and lots of methods have been used to perform the measurement of the electron transport properties of them. However, the conductance values of the same molecular junctions measured using different techniques by different experimental groups vary significantly. One reason that has been widely accepted is that the difference of the contact configurations of molecular junctions prepared using different methods. Up to now, the uncertainty of the contact configurations of molecular junctions is still one of the important blocks for the development of molecular junctions. Recently, inelastic electron tunneling spectroscopy (IETS) as one of the most important technique to investigate the electron transport properties of molecular junctions has attracted much attention and has developed quickly. The peaks in the IET spectra correspond to the vibration modes of the molecule, so it can provide undirect evidence of the existence of the molecule in the junctions. In addition, the IET spectra are very sensitive to the configurations of molecular junctions; consequently, it is an efficient way to detect the details in the molecular junctions. However, the experiments alone are not capable to provide the structure information in the molecular junctions. It should always combine the theoretical simulation with the experiments to determine the configurations of the molecular junctions.Based on the hydride density function theory, the effect of the aromatic coupling between two molecules in the bi-molecular junctions on the electron transport has been systematically investigated using the elastic scattering Green's function theory. Considering the inelastic scattering process, the inelastic electron tunneling spectra of a serial of molecular junctions have been investigated. The effect of the tilt angle of the molecule, the contact configurations et al. on the IET spectra has been investigated. The main content and results of the research in this thesis are as follows.First, we studied the effect of aromatic coupling between two oligo-phenylene ethynylene (OPE) molecules on the electron transport properties of the bimolecular junction. In molecular junctions, the terminal end group determines the contact character between the molecule and the electrode. When the terminal end group forms the chemical bond with the electrode, the electron can transport freely between the molecule and the electrode. When the molecule contact with the electrode physically, the energy barrier will be very high and it will be difficult for the electron to transport between them. Thiol (-SH) group is one of the most popular end groups, because it can form strong chemical bond with the gold electrode. If the molecule only has one thiol end group, it will be difficult for the side without thiol end group to form strong bond with the electrode in the Mechnical Controlable Break Junction experiments and the molecular junction will be difficult to form. However, it is possible for aromatic molecules to form a bi-molecular junction through theπ?πinteraction, in which each molecule will contact with one of the two electrodes respectively. Based on the elastic scattering Green's function theory, the effect of the relative position of these two molecules on the electron transport properties has been intensively investigated. It is found that the conductance of the bi-molecular junctions decreases with the increasing of the vertical distance between the two molecules. When the vertical distance is fixed, the conductance values of the bimolecular junctions decrease with the widening of the electrode distance. When both the vertical and the horizontal distance are fixed, the conductance values are very sensitive to the tilt of one molecule relative to the other molecule. It is found that the variation of the relative position of the two molecules in the bimolecular junctions influence the aromatic coupling between the two molecules directly, which will induce the change of the electron transport properties. In addition, the electron transport properties of the bimolecular junctions with one carbon atom replaced by one nitrogen atom have also been intensively investigated and the similar electron transport properties have been found.Second, the inelastic electron tunneling spectra of the 1, 4 benzenedithiol molecule sandwiched between two gold electrodes have been investigated based on the inelastic electron tunneling theory. The 1, 4-benzenedithiol molecule is one of the most studied classic molecules, while the conductance values reported by different groups vary by almost four orders of magnitude and are difficult to be reproduced by theoretical work. Although the atom-precision observation is still out of rage, the inelastic electron tunneling spectra technique can provide experimentally indirect structure information through the response of molecular vibrations. In 2009, four different inelastic electron tunneling spectra of the 1, 4-benzenedithiol molecule sandwiched between two gold electrodes have been reported by two different experimental groups using two different experimental techniques at the 4.2 K. There are both similarities and differences for the four spectra. By systematically investigating the inelastic electron tunneling spectra of 1, 4 benzenedithiol molecule at different contact configurations, the configuration-spectra relationship has been found and our calculation successfully reproduced the four experimental IET spectra. It indicated that the differences of these IET spectra come from the variation of the contact configurations in the molecular junctions. Our calculation further indicated that it should always combine the theoretical calculation with the experimental results to determine the configurations of the molecular junctions. Third, the inelastic electron tunneling pectra of the hexadecanethiol molecular junctions and a serial of semifluorinated hexadecanethiol molecular junctions have been investigated theoretically. Alkanethiol molecules have been served as the model systems both for experimental and theoretical study of IETS. However, most theoretical simulations can not reproduce the high intensity of the C-H stretching vibration mode around 0.38V measured in the experiments. In addition, the origin of this vibration mode is still under debate. Based on the inelastic electron tunneling theory, the inelastic electron tunneling spectra of the hexadecanethiol molecule (F0) and a serial of semifluorinated hexadecanethiol molecules (F1, F2, F3, F10) have been studied. It is found that when the molecules are located at the hollow site of the electrode and tilted -40 degree relative the normal of the electrode surface, the IET spectra of these molecular junctions are in good agreement with the experimental results measured by Beebe et.al.. Our calculation further confirmed that the C-H vibration peak around 0.38V comes from the CH2 groups that are close to the sulfur atom.At last, we investigated the inelastic electron tunneling spectroscopy of 1, 3 propanedithiol molecular junction which is the most simple but including all the vibration properties of the alkane molecular junctions. Based on the calculation of the IET spectra of the 1, 3 propanedithiol molecule sandwiched between two gold electrodes with different contact configurations, the theoretical simulation successfully reproduced the two different IET spectra measured by the same experimental group. It indicated again that the uncertainty of the relative direction between the molecule and the electrode is the main factor that influences the spectra characteristics.This thesis consists of nine chapters as follows. In the first chapter, the research background of molecular junctions, the preparation techniques and measurement methods of molecular junctions, the transport mechanisms and the corresponding theoretical methods as well as the basic information of the inelastic electron tunneling spectroscopy have been briefly introduced. The basic quantum chemistry methods to calculate the electronic structures of the molecules including the Hartree-Fock method which is based on the wavefunctions and the Density Function Theory which is based on the electronic density have been presented in Chapter two. Basis sets always influence the precision of the theoretical calculation. Some basis sets usually used in our calculations have been introduced simply. Following, the basic concepts to perform the calculation of the frequencies have been explained and the vibration modes of two kinds of model systems have been presented at the last part of this chapter. In Chapter Three, the elastic scattering Green'function theory is first presented, followed by the inelastic electron tunneling theory. From Chapter Four to Chapter Eight, the applications of both elastic and the inelastic electron tunneling theory are presented. In Chapter Four, the effect of the aromatic coupling on the conductance of the OPE bimolecular junctions has been intensively investigated. In the following chapter, the electron transport properties of the OPE bimolecular junction with one carbon atom replaced by one nitrogen atom have been systematically studied. In Chapter Six, the spectra-configuration relationship has been found by systematically investigating the IET spectra of the 1, 4-benzenedithiol molecular junction at different contact configurations and successfully reproduced the four different IET spectra measured by two different techniques. In Chapter Seven, by investigating the IET spectra of the hexadecanethiol molecule and a serial of semifluorinated hexadecanethiol molecules, we reproduced the experimental results reported by Beebe et al. and confirm their conclusion that the C-H stretching mode comes from the CH2 groups close to the S-Au bond. In Chapter Eight, the IET spectra of 1, 3 propanedithiol molecule have been studied theoretically and reproduced the experimental results measured by the same group. The last 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.