【作者】 邹斌；

【导师】 王传奎；

【作者基本信息】 山东师范大学 ， 原子与分子物理， 2008， 博士

【摘要】 近年来,非弹性电子隧穿谱测量技术作为研究有机分子器件电输运性质的重要手段引起人们广泛注意并迅速发展起来。由于隧穿谱的峰值对应于有机分子的振动模式,因此,测量分子器件非弹性电子隧穿谱不仅可以用来理解隧穿电子与分子振动模式的耦合作用,而且能够提供分子器件几何和接触构型等各类信息。由于非弹性电子隧穿谱测量技术是目前确定分子和金属电极接触形状最为有用的手段之一,因此该项技术在分子电子学的发展过程中具有十分重要的作用。许多实验和理论研究组对单分子的非弹性电子隧穿谱进行了相关研究,并且取得了很多有意义的成果。然而目前在该研究领域的实验技术和理论水平都还不够成熟,不但从理论上很难与实验结果完全符合,就是不同的实验组对同一类分子进行研究的结果之间也会存在很大的差别。存在以上问题的主要原因是:与电极相比,有机分子是体积很小的体系,因此外界因素的变化对分子非弹性电子隧穿谱的影响会很明显。本论文在杂化密度泛函理论的基础上,详细讨论了电场对分子器件电输运性质的影响,并分析了有外加电场情况下分子器件的电子重新分布和空间电势变化情况。发展了第一性原理的理论方法来模拟分子器件的非弹性电子隧穿过程,研究了电极距离、分子与金属间的接触构型、分子的氟化程度等因素对分子非弹性电子隧穿的影响。在弹性散射格林函数方法基础上对4,4’-联苯二硫酚分子器件的非线性电输运特性进行研究,结果显示分子体系的扭转角随电场的增大而单调递减,4,4’-联苯二硫酚分子沿电场的反方向有微小的移动。终端S原子与Au原子团簇之间耦合系数随着电场强度的变化呈现非线性变化趋势,这种变化趋势与S原子到Au平面垂直距离的变化一致:距离越大耦合系数越小,距离越小耦合系数越大。随着电场的增加,最高占据分子轨道和最低未占据分子轨道轨道之间的能隙变窄。电场方向的改变导致非线性I-V曲线是不对称的:4,4’-联苯二硫酚分子的电导值在0.7 V开始开启,并且在1.04 V和1.28 V分别出现两个电导峰值;在负向电压情况下两个峰值位置分别出现在-0.88 V和-1.04 V。对于该分子器件而言,不同电场情况下对分子的优化过程可以有效避免因不优化分子而得到的负微分电阻。计算结果表明有外加电压情况下电荷的重新分布在分子与电极的接触点附近产生了附加电偶极子,并进而引起非线性输运效应。通过对分子电势分布情况的分析发现,4,4’-联苯二硫酚分子两个苯环不共面,会对该分子器件电输运产生不利影响。我们的计算工作较好地符合了实验结果。电极距离以及分子与金属的接触构型是影响分子器件非弹性电子隧穿谱的两个重要因素。通过对4,4’-联苯二硫酚分子器件非弹性电子隧穿谱的计算表明,电极距离的不同会改变分子几何结构,从而影响分子体系的非弹性电子隧穿谱。通过分析4,4’-联苯二硫酚分子的非弹性电子隧穿谱,发现垂直于表面的振动模式对非弹性电子隧穿谱具有较大地贡献,表明了非弹性电子隧穿谱存在着取向择优性。较大相对谱强度主要是来自于ν（C-S）,ν（6a）,ν（18a）和ν（19a）等简正振动模式的贡献。对于每种振动模式所对应的非弹性电子隧穿谱半高全宽,基本上都是正三角形的比单个Au原子电极构型的要大一些,这表明正三角形构型情况下4,4’-联苯二硫酚分子和金属电极的相互作用要比单个Au原子情况下的强一些。随着温度由4.2 K逐步升高到50.0 K,非弹性电子隧穿谱中原先比较尖锐、易辨别的峰逐渐变得模糊不易分辨,而且谱峰宽度逐渐变宽。通过对十六烷硫醇分子及其部分氟化分子（F0,F1,F2,F3和F10）等五种烷烃分子的非弹性电子隧穿谱的理论计算发现,隧穿谱中C-H伸缩振动模式的贡献应该是来源于链烃分子中的与S原子相邻的亚甲基（-CH₂-）基团伸缩振动模式,而不是来源于分子终端的甲基（-CH₃）基团。该项结果与实验结论相一致,我们的理论工作有助于澄清类似的链烃硫醇分子非弹性电子隧穿谱中关于C-H伸缩振动模式来源的疑问。我们认为标记为”CH₂ wag”的实验峰可能包含CH2面外摇摆振动、CH₂扭绞振动、变形振动模式等一系列振动模式的贡献。计算发现F10分子非弹性电子隧穿谱114 mV附近存在被氟化区域的C-C-C变形振动模式对隧穿谱的贡献,实验中F10分子隧穿谱中标记为”CH₂ wag”的实验峰应该含有C-C-C变形振动模式的贡献。此外需要更深入的理论工作来研究十六烷硫醇系列分子与金属电极接触方式对分子器件非弹性电子隧穿谱的影响。论文共由以下八章内容组成:第一章为综述部分,简要介绍了分子器件非弹性电子隧穿谱的产生背景、该领域实验和理论发展`现状和目前存在的主要问题;第二章介绍了密度泛函理论（DFT）的基本理论,包括Hohenberg-Kohn定理、Kohn-Sham方程和交换关联泛函等;分子振动模式以及Caussian程序中的振动分析方法在第三章作了总结;弹性散射格林函数理论以及分子器件非弹性电子隧穿谱计算方法在第四章中作了详细地推导;第五章到第七章介绍了本文所做的计算工作和研究结果,第五章分析了外加电场对4,4’-联苯二硫酚分子的几何结构、电子结构和伏安特性的影响,并描述了有电场情况下的电荷重新分布以及电势的变化情况。第六章讨论了不同的电极距离和接触构型对4,4’-联苯二硫酚分子非弹性电子隧穿谱的影响,同时讨论了温度的影响。第七章对十六烷硫醇分子及其部分氟化分子等系列烷烃分子的非弹性电子隧穿谱进行了讨论,考察了氟化程度对分子器件非弹性电子隧穿谱的影响,并且与实验结果进行了比较;在第八章中对本论文工作进行了全面总结,并对分子器件非弹性电子隧穿谱研究领域未来的发展进行了展望。更多还原

【Abstract】 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.更多还原