【作者】 冷建材；

【导师】 王传奎；

【作者基本信息】 山东师范大学 ， 原子与分子物理， 2010， 博士

【摘要】 近年来,利用单分子组装电子器件引起了人们极大的兴趣。随着实验手段的不断进步和人们对分子器件电输运理论的深入研究,在单分子器件研究领域产生了许多令人鼓舞的成果。单分子器件研究领域的最大的挑战之一就是如何表征电子是通过连接在两个金属电极之间的有机分子进行传输的。众所周知,分子器件内部的微观结构在金属层-有机分子-金属层隧道结的电子输运过程中起着决定性的作用。因此,确定分子器件的几何接触构型在制作分子功能器件过程中变得非常重要。单纯从实验的角度来讲,很难确定分子结是否由一个分子构成。即使能够制备出单分子结,分子结内部结构的确定也相当困难。非弹性电子隧穿谱技术作为探测分子结内部接触构型的有效手段,已经被广泛的应用于分子电子学领域。有机分子的电子非弹性隧穿过程的产生源于电子与有机分子振动的耦合,它与分子动力学以及电荷的转移、发生化学反应的过程等诸多因素密切相关。因此,电子非弹性隧穿的研究工作对分子电子学的研究和实验技术的发展都有十分重要的意义。近年来,国际上已经有多个实验组对多种分子功能器件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。尽管非弹性电子隧穿谱技术是非常灵敏有效的探测手段,然而由于缺乏可以参考的标准,因此仅仅依靠实验很难提供足够的信息来确定分子结的内部结构。这就需要发展理论来指认和解释分子器件非弹性电子隧穿谱的谱峰。在理论方面,目前已有多个研究小组利用数学建模的方法或者采用第一性原理计算来模拟分子器件的非弹性电子隧穿谱,也取得了与实验符合较好的结果。然而理论模拟的结果在谱峰的位置和相对强度上很难与实验符合得较好,不同研究小组对同一分子的研究结果也有较大差异。存在以上问题的主要原因有:影响分子的非弹性电子隧穿谱的因素很多,不但受分子结构的影响,而且依赖于电极与分子的接触方式和接触形状,此外还受分子相对于衬底的取向以及外界其他因素的影响。本论文在杂化密度泛函理论的基础上,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,详细讨论了电极构型,分子的倾斜取向以及末端甲基的碳原子相对金探针的距离对分子非弹性电子隧穿的影响,并研究了分子间的相互作用以及不同电极对分子非弹性电子隧穿的影响。癸烷硫醇分子的非弹性电子隧穿谱的理论计算表明分子结的电极构型、分子的倾斜取向以及末端甲基的碳原子相对金探针的距离对癸烷硫醇扩展分子非弹性电子隧穿谱有很大的影响。理论模拟结果较好地符合了实验结果,且给出了该分子结的内部结构信息。理论计算结果表明Hallb?ck小组实验中测量的分子结接触构型是末端甲基上的碳原子位于金电极的桥位,另一端的硫原子处于金衬底的空位,分子相对金衬底的倾角θ=20°,而末端甲基上的碳原子与STM探针的距离是0.309nm。从第一性原理出发,我们系统研究了电极与辛烷硫醇分子间距和两分子交叠程度对双辛烷硫醇扩展分子体系的非弹性电子隧穿谱的影响。理论计算结果表明,辛烷硫醇扩展分子体系的非弹性电子隧穿谱与电极接触构型有着密切的联系,当调整扩展分子的电极接触构型时,体系的非弹性电子隧穿谱随之出现较大的变化。通过计算不同链间距下的双辛烷硫醇扩展分子体系的非弹性电子隧穿谱,发现当两分子间距为2.6?时,体系的非弹性电子隧穿谱有了明显的变化。我们推断此时在分子间作用力的影响下,可能出现了电荷的链间非弹性隧穿。同时,还研究了在分子间距为2.6?不变的情况下部分交叠的双辛烷硫醇扩展分子体系的非弹性电子隧穿谱,计算结果表明分子的交叠程度对有机分子的非弹性电子隧穿谱有很大的影响。选取1,6-己二硫醇分子和1,4-二巯基苯分子为研究对象,初步研究了不同元素的金属电极对链烃硫醇分子和共轭分子两类分子的非弹性电子隧穿谱的影响。计算结果表明,采用不同元素的金属电极时1,6-己二硫醇分子和1,4-二巯基苯分子的非弹性电子隧穿谱有较大变化,由此表明金属电极的材料对链烃硫醇分子和由π键构成其主体结构的共轭分子的非弹性电子隧穿谱均有较大影响。我们认为这主要是由于不同的金属原子的线度、质量和电子结构不同,从而导致电极与分子间的相互作用能发生变化,由此影响了有机分子的非弹性电子隧穿谱。此结果有助于帮助确定相关实验中各振动模式的贡献。论文共由七章组成:第一章为综述部分,简要介绍了分子器件的研究现状,非弹性电子隧穿谱技术的产生、发展和在分子电子学领域的应用及目前存在的不足;第二章简单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,而且总结了分子的振动模式以及Gaussian程序中的振动分析方法;在第三章中详细地推导了分子器件非弹性电子隧穿谱的理论公式;第四章到第六章介绍了本人所做的工作和计算结果,第四章系统地讨论了电极接触构型、末端甲基碳原子相对金探针的距离以及分子主链相对金属衬底的倾斜角度对癸烷硫醇分子非弹性电子隧穿谱的影响,并且与实验结果进行了比较。第五章讨论了不同的电极接触构型对辛烷硫醇分子非弹性电子隧穿谱的影响,同时讨论了分子间相互作用对非弹性电子隧穿谱的影响。第六章研究了不同电极材料对1,6-己二硫醇分子和1,4-二巯基苯分子的非弹性电子隧穿谱的影响;第七章是对论文工作的总结和对下一步研究工作的展望。更多还原

【Abstract】 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.更多还原