Theoretical Studies On Inelastic Electron Tunneling Spectroscopy Of Molecular Electronic Devices

One of the exciting recent developments in molecular electronics is the application of inelastic electron tunneling spectroscopy (IETS) for studying the transport properties of molecular electronic devices. The measured IET spectra show well-resolved vibronic features corresponding to certain vibrational normal modes of the molecule. The IETS not only helps us to understand the vibronic coupling between the charge carriers and nuclear motion of a molecule, but also provides us a powerful tool to detect the geometrical structures of molecular electronic devices and bonding situations between the molecule and the electrodes. In fact, the importance of the IETS for the molecular electronic devices can not be overstated, since the lack of suitable tools to identify the molecular and contact structures has hampered the progress of the field for many years.Many experimental and theoretical groups have devoted to the study of IETS of single molecule and obtained exciting results in recent years. While experimental techniques and theories for IETS need to be developed, because not only do theoretical results not give a well explanation for experimental measurements, but also the experimental results for the same molecule with different techniques show great difference among each other. The main reason for the questions mentioned above is that, compared with the electrode, the molecule is a very small system in the size. Therefore the IETS of the molecule is likely influenced by the external factors. In this thesis, the effect of field-induced geometry relaxation on the electron transport properties is studied, and the charge redistribution and the potential variation under the external bias are analyzed. A first-principles computational method based on hybrid density functional theory is introduced to simulate the inelastic electron tunneling process of molecular junctions. The influences of distance between electrodes, the contact structures between the molecule and the metal surface, and the semifluorinated degree in alkanethiol molecules on the IETS of molecular devices are investigated.The non-linear charge transport properties of 4,4'-biphenyldithiol molecular junction have been studied using the generalized Green's function theory. It is shown that the torsion angle between two phenyls is slightly decreased as increase of the external voltage while the whole molecule moves slightly along the reversed direction of the electric field. The coupling constants between the terminal sulfur atom and the gold surface show a non-linear dependence on the electric field strength. The change of the coupling constants is consistent with the change of the bond distance between gold and sulfur atoms. A longer bond distance results in a smaller coupling constant and vice verse. It is found that the energy gap between HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) becomes narrower with the increase of the electric filed. The non-linear behaviors of I-V curves and their asymmetry with respect to the direction of the electric field are clearly demonstrated. There is a conductance turn-on up to 0.7 V, and two conductance peaks appear at higher bias, 1.04 V and 1.28 V, respectively. It is found that with the negative bias, these two conductance peaks are located at -0.88 V and -1.04 V, respectively. Calculation indicates that the inclusion of molecular geometry relaxation can avoid a false prediction of negative differential resistance behavior. The charge redistribution under the external bias results in resistivity dipoles inside the molecule, which leads to the non-linear transport effect. Furthermore, the electrostatic potential analysis indicates that the non-coplanarity of the two phenyl rings has quite negative effect on the electronic transport in this molecular device. The calculated I-V curve of 4,4'-biphenyldithiol molecular junction is consistent with experimental observations in a way.A first-principles computational method based on the hybrid density functional theory is used to calculate the IETS of 4,4'-biphenyldithiol molecular electronic devices in the nonresonant tunneling regime. The influence of the distance of two electrodes and three different contact structures between the molecule and electrodes are investigated. The numerical results show that the change of the distance betwteen two electrodes gives various influence on the geometric structure of the extended molecule, which bring effect on the IETS of the 4,4'-biphenyldithiol molecular junction. The computational results demonstrate that the IETS has certain selection rule for vibrational modes, that's to say the longitudinal modes with the same direction as the tunneling current have greatest contribution to the IETS. The longitudinal modes include the C-S strecthing vibration mode,ν(6a) ring mode ,ν(18a) ring mode,ν(19a) ring mode, and so forth. The values of full width at half maximum (FWHM) for the triangle contact configuration are usually larger than the corresponding ones for the one-gold-atom contact configuration with trans-structure and cis-structure in IETS. It indicates that the molecule-metal bonding in the triangle contact configuration is stronger than that in the one-gold-atom contact configuration for 4,4'-biphenyldithiol molecule. When the temperature is increased from 4.2 K to 50.0 K, some delicate spectral peaks are smeared, and broaden peaks contributed by several vibrational modes are formed.We investigate the IETS of 1-hexadecanethiol molecule and the semifluorinated molecules, including F0, F1, F2, F3, and F10. The careful examination on the atomic vibration and frequency indicates that the C-H stretching peak arises from the stretching of the CH₂ group localized on the region directly adjacent to sulphur atom, not from terminal CH₃ vibration. The calculated result is in agreement with recent experiment result, which can solve this disputed issue about the source of C-H stretching modes in IETS for such alkanethiol molecules. The experimental peak of CH2 wagging vibration might be a sum of several spectral features, including not only the CH2 wagging mode but also the CH₂ twisting and scissoring modes. There are contributions of C-C-C scissoring modes (belong to the fluorinated part of F10) to IET spectrum.The experimental peak ofν(C-C) vibration should comprise the contribution of C-C-C scissoring mode of the F10 junction. It calls for a deeper understanding of the the influence of veracious molecule-metal contact to the IETS for these designed molecular series.This thesis consists of eight chapters as follows. In the first chapter, the background of IETS of molecular electronic devices and recent development in the field of experimental and theoretical work 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. The method of displaying the vibration of molecule and the vibrational analysis in the Gaussian program are introduced briefly in the third chapter. The elastic scattering Green's function method and the computational theory for the IETS of the molecular junctions are introduced in the fourth chapter. From the fifth chapter to the seventh chapter, the computational work and the main theoretical results are presented. In the fifth chapter, we analyse the influence of the the external field on the geometry relaxation, electronic structures, and the current-voltage properties of 4,4'-biphenyldithiol molecular junction. The charge redistribution and the electrostatic potential drop inside 4,4'-biphenyldithiol molecule under the external voltage are also investigated in this chapter. The influence of the electodes distance and the contact structures on the inelastic electron tunneling spectroscopy of 4,4'-biphenyldithiol molecular junction is discussed in the sixth chapter, and the temperature effect is also discussed. We investigate the IETS of the 1-hexadecanethiol molecule and semifluorinated molecules in the seventh chapter, in which the length of the molecular backbone remains constant while the number of fluorine atoms is varied. The theoretical work has been compared with the experimental result. The eighth 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 future.