【作者】 李延伟；

【导师】 尹鸽平； 赵健伟；

【作者基本信息】 哈尔滨工业大学 ， 化学工程与技术， 2007， 博士

【摘要】 传统硅基电子器件的尺寸逼近纳米数量级时,将面临因量子效应(如隧穿,散射和电子干涉等)而产生的一系列难以逾越的挑战。解决上述问题的出路之一在于发展分子尺度的电子器件(分子电子器件)。扫描探针显微术(SPM)和量子化学计算的发展为研究分子电子器件提供了强有力的工具。尽管目前对分子电子器件的研究已经取得了一些重要进展,但关于功能分子在器件中的行为、分子的电子输运机理等一些最基本的问题还缺乏全面深入的认识。因此研究分子的这些性质对分子电子器件的设计和优化具有非常重要的意义。使用电化学方法研究了Au(111)电极上的烷基硫醇分子自组装膜的电子传递动力学。在KCl溶液中用快速循环伏安法测量了自组装膜的微分电容,以确定自组装膜结构的完好性;以Fe(CN)6-4/-3为探针离子,研究了自组装膜的电子传递特性。结果表明,随着分子链长的增加,分子的隧穿电流减小,测得的电流衰减常数β为0.98/CH2。使用导电原子力显微镜(CAFM)方法研究了在Au(111)表面上的硫醇分子自组装膜的电子输运特性。研究表明,分子的电子输运具有显著的非线性特性,分子的电流电压(I-V)曲线可以用Simmons隧穿模型来很好的拟合。分子电流随着分子链长的增加呈指数衰减,其电流衰减常数β为1.16/CH2,这与使用电化学方法和非平衡态格林函数(NEGF)方法计算得到的β值(分别为0.98/CH2和0.99/CH2)非常接近,说明分子的导电机理主要是隧穿机理。CAFM针尖压力对分子的导电性质有很大影响,随着针尖压力的增加,分子的电流显著增大。NEGF方法可以较好地模拟分子的电子输运特性。为了更加准确地模拟分子电子材料在器件中的行为,提出了一种类似于“现场”的静态理论研究方法。以典型的线型π-共轭分子,聚乙炔(PA)为模型分子,在HF/6-31G*水平上研究了外电场作用下分子性质的变化。结果表明,分子的几何结构和电子结构对外电场有明显的依赖性。随着外电场的增大,分子的碳碳单键变短,碳碳双键变长,分子的共轭程度得到了加强,分子在共轭平面内发生了明显的几何弯曲。外电场使分子的LUMO-HOMO能隙减小,分子的偶极矩增大。外电场作用下,分子的HOMO空间分布移向分子的低电势端,LUMO空间分布移向分子的高电势端。随着分子链长的增加,分子的上述性质随外电场的变化都得到了进一步的加强。对一系列其它类型的线型π-共轭分子(聚对苯撑乙炔(PPV)、聚苯乙炔(PPE)、聚噻吩(PT)和聚苯(PP))进行了类似的计算研究,也得到了相同的结果。使用B3LYP/6-311+G**方法研究了二苯乙炔分子在电场作用下扭转势能面和电子结构。结果表明,随着外电场的增加,分子的扭转势垒增大,并且扭转势垒的高度与外电场的平方具有线性关系;随着扭转角的增加,分子的前线轨道能量和空间分布对外电场的依赖性增强。使用NEGF方法模拟了二苯乙炔分子处于不同构象时的I-V特性。结果表明,分子的I-V特性与分子构象之间的关系可以从分子LUMO-HOMO能隙和前线轨道的空间分布随外电场的变化来解释。使用NEGF方法模拟了一系列具有不同共轭结构的线型π-共轭分子导线(PA、PT、PPV、PPE和PP)的I-V特性,比较了它们的导电性差异,并从分子LUMO-HOMO能隙、分子轨道空间分布和隧穿谱等方面分析了各分子之间导电性差异的原因。在此基础上,还考察了在分子骨架上引入供电子基团(-NH2)和吸电子基团( -NO2)对分子电子输运特性的影响。结果表明,在分子骨架上修饰功能基团可以成功地引入不对称性,进而使分子获得不对称的输运特性。这为分子二极管、分子开关等功能分子电子器件的设计提供了理论参考。使用HF/6-31G*方法探索了一类新型的分子电子材料,苯乙炔大环分子(PAMs)的几何结构、电子结构和环张力特性。基于对大环分子构单元二苯乙炔分子的构象分析,提出了一种新的环张力能分析方法,并且用这种方法分析了分子环张力特性和几何结构特性。与以往的环张力能分析方法相比,这种方法更加的简便、高效并且还能分析出分子环张力能的具体来源。电子结构分析表明,平面构型大环分子的LUMO-HOMO能隙和分子轨道空间分布具有明显的奇偶差异特性。更多还原

【Abstract】 The continuous miniaturization of conventional silicon-based electronics will eventually face the insurmountable challenge of the quantum effect, such as tunneling, diffraction, and interference of electron. One of the most promising solutions is to develop molecular-scale electronics (Molecular electronics). The development of scanning probe microscopes (SPM) techniques and quantum chemistry calculations provides powerful tools for exploring the molecular electronics. Although significant progress has been made during the last two decades, some most basic issues such as the behavior of the functional molecules working in the circuit and the electron transport mechanism through the molecules are still not very clear. So, it is essentially important to investigate these properties of the moelcules for the design and rationalization of molecular electrnics.The electron transfer kinetics of n-alkanethiol monolayers on Au(111) electrode is investigated. The structure of the monolayer is examized by measuring the double layer capacitance in 1 mol/L KCl solution with the rapid cyclic voltammetric method. The electron tunneling parameter for the monolayer is studied by using Fe(CN)6-4/-3 as the redox probe. It demonstrates that with the increase of the chain length of the n-alkanethiols the current decreases dramatically. A decay constantβof 0.98 per methylene group is measured. Electron transport propeties for n-alkanedithiol monolayers on Au(111) is investigated using conducting atomic force microscopy (CAFM). The measured current-voltage (I-V) curves show obvious nonlinear behavior and can be fitted with Simmons tunneling model. The molecular current decays exponentially with the increase of chain lenth. The measured decay constantβis 1.16 per methylene, which is in reasonable agreement with the values of 0.98 per methylene and 0.99 per methylene obtained from the electrochemical method and the nonequilibrium Green’s functions (NEGF) method, respectively, indicating that the main conduction mechanism of the alkane molecules is tunneling. The CAFM tip-loading force is found to dramically influence the molecular electric properties, showing that the molecular current increases dramatically with the the the tip-loading force increasing. The NEGF calculations can simulate the molecular electron transport properties qualitatively. To simulate the properties of the molecular electronic materials working in electronics more precisely, a more likely in-situ theoretical method by considering the interaction from the external electric field (EF) is proposed. Typically, a series of molecular wires, polyacetylens (PA), are systematically studied at the HF/6-31G* level by this method. It proves that both the geometric and the electronic structures of the molecular wires are sensitive to the external EF. In particular, the external EF makes the carbon-carbon single bonds become shorter and the carbon-carbon double bonds become longer, leading to a higher conjugation. The external EF decreases the LUMO-HOMO gap and increases the molecular dipole moment. The spatial distributions of frontier molecular orbitals vary from the fully delocalized form to the partly localized form with the increase of the external EF. All of these features are more pronounced with increasing conjugation chain length. The same calculations are also carried out for several other typical linear conjugated molecules (poly(phenylene vinylene) (PPV), poly(phenylene ethylene) (PPE), polythiophene (PT), and polyphenylene (PP) molecules) and similar results as those of the PA are obtained. Moreover, the torsional potential energy surface and the electronic structure of diphenylacetylene are investigated at the B3LYP/6-311+G** level by considering the influence of the external EF. It domostrates that with the increase of the external EF the molecular torsional barrier increases and the increment is proportional to the square of EF. With the increase of molecular torsional angle, the EF dependence of the LUMO-HOMO gap and the spatial distributions of the HOMO and LUMO are enhanced. The I-V behavior corresponding to different conformers is evaluated by the NEGF method. Further, the evolutions of the LUMO-HOMO gap and the spatial distribution of molecular orbital are used to analyze these structure-property relationships.The I-V property for a series of lineare conjugated molecular wires (PA, PPV, PPE, PT, and PP) with different conjugation strucres is investigated by the NEGF method. The conductivity of these molecules is compared and analyzed from the the LUMO-HOMO gap, the spatial distributions of the HOMO and LUMO, and the transmission spectra of the molecules. Moreover, the effect of introducing functional groups (electron-donating group, -NH2, and electron-withdrawing group, -NO2) into the molecular backbone on the electron transport properties of the molecules is also studied. It demonstrates that the introduction of specific functional groups on the molecular backbone can successfully lead to molecular asymmetry, which may lead the assymmetric I-V behaviors. These results provide a theoretical guidance for the further design of novel molecular electronics, such as molecular diodes, molecular switches, and molecular storage devices.The geometry, electronic structures, and ring strain energy of a kind of new molecular electronic material, phenylene-acetylene macrocycles (PAMs), are explored by HF/6-31G* method. Based on the conformational analysis of diphenylacetylene, which can be viewed as the structural unit of PAMs, a new method for analyzing ring strain energy is proposed and used to analyzing the ring stain energy and the structural characteristics of the PAMs. Compared with the conventional theoretical method for ring strain energy, this new method is more convenient and higher efficient. Moreover, this new method can analyze the origin of the ring strain energy within the macrocycles. In view of their potential applications as electronic materials, the electronic structures of a series of PAMs are also investigated. The LUMO-HOMO gaps and the HOMO spatial distributions of the planar PAMs show obvious odd-even difference behavior.更多还原