【作者】 闫循旺；

【导师】 王传奎；

【作者基本信息】 山东师范大学 ， 原子与分子物理， 2007， 硕士

【摘要】 在单分子层次上构建微电子器件,已成为纳米电子学的研究热点,其中单个分子电学特性的实验测量和理论研究是该领域取得成功的关键因素。由于存在着离域电子,π共扼分子被认为是制作单分子电子器件的首选材料。近些年来,人们对π共轭分子,尤其是π共轭小分子的电学性质进行了广泛的理论和实验研究。本文从第一性原理出发,利用密度泛函理论,并结合弹性散射格林函数方法来研究π共扼分子体系电输运性质。单分子器件的模型采用的是由金原子团簇-有机分子-金原子团簇三部分构成的扩展分子体系。首先,本文介绍了单电子近似理论和分子轨道理论。求解多原子体系的薛定谔方程时,用玻恩-奥本海默(Born-Oppenheimer)近似,把原子核运动和电子运动分离为两个方程。电子体系的运动方程可采用哈特利-福克(HF)方法或密度泛函理论(DFT)转变为单电子方程。扩展分子(金原子团簇—有机分子—金原子团簇)轨道表示为各原子轨道的线性组合,即LCAO-MO(Linear Combination of Atomic Orbitals-Molecular Orbital)方法。该方法有利于定量计算影响分子器件电输运性质的重要因素之一——有机分子与金属电极的相互作用能。其次,本文利用扩展分子轨道在各原子上的展开系数和原子轨道之间的相互作用能,在第一性原理的基础上,推导了扩展分子体系中各部分间相互作用能的表示式。对于单分子器件的电子输运机制,仅考虑弹性散射过程。跃迁强度的计算借助格林函数方法,以朗道公式为基础,推导了单分子器件的电流密度表示式,进而得到电流和电导的表示式。最后,把相互作用能和电流电导的表达式应用于具体的π共扼分子体系。理论结果分为三部分。第一,对PDI(1,4-phenylenediisocyanide)分子连接在两金电极之间形成的分子结,理论模型是金原子团簇-PDI分子-金原子团簇形成的扩展分子。在计算PDI分子与金电极的相互作用能时,采用了两种电极原子团簇,一是四个金原子组成正四面体,二是三个金原子组成直线。当电极之间的距离变化时,分子与电极的相互作用能有明显的变化。在四面体型体系和直线型体系中,两电极距离分别为1.338nm和1.378nm时,PDI(1,4-phenylene diisocyanide)分子与两金原子团簇形成稳定的结合。对1,4-苯二甲硫醇(benzene-1,4-dithiol)分子,在其两端加上四面体型金原子团簇组成扩展分子。计算分子与金电极的相互作用能,并与另一种方法计算的数值作比较。二者基本相符,说明了本文相互作用能表示式的正确性。第二,在密度泛函理论(DFT)的基础上,运用弹性散射格林函数方法,研究了PDI(1,4-phenylenediisocyanide)分子在两个金电极之间形成的单分子器件的伏安特性。扩展分子中,PDI(1,4-phenylenediisocyanide)分子两端的金原子团簇分别选用三角形型团簇、四面体型团簇和直线型团簇,比较了不同团簇时扩展分子的几何结构、电子结构、跃迁强度谱、电流电导曲线等。选用不同金原子团簇时,扩展分子体系开启电压有明显的差别;电极之间的距离变化时,体系电流值也发生较大变化。四面体型团簇形成的扩展分子,分子结的电导值在0.2V和0.6V处有明显的增加,其电导曲线与实验曲线相吻合,较好地模拟了分子与金电极的界面接触情况。第三,PDI、BPDI、TPDI分子分别与两个四面体型金原子团簇组成扩展分子。在分子与电极的界面接触情形相同的情况下,比较了扩展分子的能态密度曲线、跃迁强度谱、电流曲线和电导曲线。发现三者能态密度曲线和跃迁强度谱相似,跃迁强度谱的第一个峰的位置都在0.2伏附近,电导曲线中各台阶位置较为接近。本文的第一章阐述了研究背景,第二、三章描述了研究分子器件工作原理的理论方法,第四章研究了电极距离对分子与金电极相互作用的影响。第五章研究分子与电极接触构型和电极距离对单分子器件电子输运性质的影响,第六章比较了PDI、BPDI、TPDI三种分子的电输运性质。更多还原

【Abstract】 In the last few decades, single molecule device construction has become an increasing interest in molecule electronics. At the same time more and more reseachers apply themselves to the research work of single molecule that embrace the electrical characteristics measurement and the understanding of the electronic tranport mechenics.Π-conjugated molecules are considered as appropriate candidates for fabrication of molecular devices because they have delocalized charge inside. Especially, smallπ-conjugated molecules have been studied carefully in theoretical and experimental work in recent years. In this article, we use a quantum chemistry method, which is called elastic scatting Green function method, to describe the electron transport in molecular junction. The theoretical model of single molecule device is the extended molecule composed of aπ-conjugated molecule and two metal atom clusters.Above all, this article introduce single electron approximation and LCAO-MO approximation. While solving Schrodinger’s equation of multiple atoms, with the Born-Oppenheimer approximation, it can be separated into two equations, one is nuclears motion equation, another is electrons motion equation. The latter can be converted to single electron equation by Hatree-Fork method or Density Functional Theory. Extended molecule orbitals are expressed as the linear combination of atomic orbitals, called LCAO-MO approximation, which is suited to quantumly calculate the interaction energy between molecule and electrode.Secondly, basing on ab initio method, we derivate the interaction energy expression. The current density expression also has been obtained by the elastic scattering green function theory combining with Landauer Formular. Moreover, the current formular and conductance formular are deduced on the base of current density expression. Finally, above-mentioned thoretical methods are used to calculate the conductance of some metal-molecule-metal junctions and the interaction energy between molecule and electrode. In these junctions, gold is selected as electrode material and someπ-conjugated molecules, such as BDT (benzene-1,4-dithiol), PDI(1,4-phenylene diisocyanide), BPDI(4,4’-biphenyldiisocyanide) , TPDI (4,4’-p-terphenyldiisocyanide), are bonded to two gold electrodes. Our three main results is listed as follows.1. The interaction energy of metal electrode and molecule has been investigated by taking the 1,4-phenylene diisocyanide (C6H4(CN)2) molecule as an example. Numerical results show that the distance between electrodes is a main factor to the coupling. The coupling energy changes gradually when the distance between two electrodes is broadened. In the tetrahedron-mode system and beeline-mode system, the molecule and electrode couple most tightly at 1.338nm and 1.378nm, respectively. Furthermore, the coupling energy of the junction that consists of BDT and tetrahedron-mode gold atom cluster is calculated and is compared with the numerical value obtained by means of another theory. By and large, two results is consistent with each other, which indicate our interaction energy expression is correct.2. Triangle-mode, tetrahedron-mode and beeline-mode gold atom cluster are combinated with PDI(1,4-phenylene diisocyanide) respectively to form the extended molecules to simulate different interaction between molecule and gold electrode. We caluculate the geometrical structure, electronic structure, transition intensity spectrum, current and conductance graph of three extended molecule systems. The features of conductance curves are shown to be strongly dependent on the detailed contact strucrures. In the tetrahedron-mode extended molecule, the first and second step of the conductance curve is located at 0.2V and 0.6V. The nonlinear electronic transport properties of the molecular junctions have been investigated and the I ? V ,G ? Vcurves have been obtained. For the molecular junction in which 1,4-phenylene diisocyanide link to gold atoms in tetrahedron-typed configuration, good agreement between theory and experiment can be achieved, especially in conductive features.3. State density, transport intensity, current-voltage curve and conductance-voltage curve of isocyanide molecules are studied and compared with each other, including PDI, BPDI and TPDI with the similar contact configuration between electrode and molecule.In this article, the first chapter introduce molecular electronics research background of molecular electronics. The second and third chapter present the theoretical method discribing the electron transport of molecular junction. The effect of distance between two electrodes on interaction energy is shown in the fourth chapter, and the effect of contact configuation on the current-conductance curves of molecular junctions is shown in fifth chapter. In the sixth chapter, electron transport features of three kinds of isocynide molecule are compared.更多还原