【作者】 盖家祥；

【导师】 王传奎；

【作者基本信息】 山东师范大学 ， 凝聚态物理， 2010， 硕士

【摘要】 利用功能单分子构建光电子器件是纳米学和分子电子学研究的目的之一。分子器件是指构建在单个分子或有限个分子上的具有特定功能的器件,其尺寸在纳米量级,使用的材料有纳米线、纳米管、纳米颗粒、有机小分子、生物分子、DNA等.近年来,在实验和理论上对单分子的电学特性的测量和研究已经成为分子电子学研究的重要内容。其研究成果一旦推向应用领域,将极大地推动社会生产力的发展。芳香类分子由于含有能够在分子中自由移动的π电子,具有较好的电学性质,所以很多实验和理论工作者都选取这一类的分子作为对象进行研究。本文的工作是利用密度泛函理论,结合弹性格林函数方法来对芳香类分子的电输运性质进行理论研究。我们选取四个金原子组成正四面体的金原子团簇来模拟电极,与自由分子相连,形成电极—分子—电极的扩展分子模型。首先研究了芳香类分子的平面性和长度对其电输运性质的影响。选取了4,4’-diaminobipheny(C12H12N2)和4,4’’-diamino-p-terphenyl(C18H16N2)进行研究。通过分析各扩展分子的几何结构、伏安特性曲线、电子输运谱和分子轨道的电子分布,发现分子中的芳香环发生扭转后,其电子输运能力相应减小。芳香分子越长,其电流值在相同偏压下越小,从而说明分子内的扭转角度的出现以及分子长度的增加都不利于电子的输运。其次,研究了芳香环个数对芳香类分子电输运性质的影响。选取了芳香类分子1,4-diaminobenzene(C6H8N2) , 1,4-diaminonaphthalene(C10H10N2) ,9,10-diaminoanthracene(C14H12N2) , 2,6-diaminonaphthalene(C10H10N2) ,2,6-diaminoanthracene(C14H12N2)作为对象,通过分析各扩展分子的几何结构、电子输运谱、能级和分子轨道的电子分布和分子的电导值,发现随着芳香环个数的增加,分子HOMO被抬高,HOMO和LUMO能量间隔减小,从而有利于电子的隧穿,其开启电压也越来越低。当分子长度相同时,随着芳香环个数增加,其电导值随着变大;然而当分子长度也在增长时,其电导值却反而下降,说明在一定程度上,分子长度对电导的影响比芳香环的个数的影响要大。再次,研究了芳香类分子的同分异构对其电输运性质的影响。选取了芳香分子C10H10N2的四种同分异构体1,5-diaminonaphthalene , 2,6-diaminonaphthalene , 1,4-diaminonaphthalene , 2,7-diaminonaphthalene作为对象,讨论了分子构造对该系列分子的电输运性质的影响,研究了该系列分子结的电子输运谱,比较了局域分子轨道和扩展分子轨道(LUMO+2),并说明了各分子的第一个输运峰来自于扩展分子轨道LUMO+2。本论文共由七章组成:第一章为综述部分,简要介绍了分子电子学的研究意义,产生背景和目前存在的主要问题;第二章介绍了多粒子体系单粒子近似,包括波恩—奥本海默近似、哈特利—福克近似、密度泛函理论;第三章,主要介绍计算分子器件伏安特性的理论方法即弹性散射格林函数方法;第四章、第五章、第六章介绍了本文工作的计算过程和研究结果,并对研究结果进行了分析;第七章对本文工作进行了总结,并对分子电子学领域未来的发展进行了展望。更多还原

【Abstract】 One of research topics in nanotechnology and molecular electronics is constructing photo-electronic devices with functional single molecule. Molecular devices refer to the devices sized in nanoscale with a specific functionality that are built up by a single or a few molecules. The materials of the devices are different, such as nanowires, nanotubes, nanoparticles, small organic molecules, biological molecules, DNA and so on. In recent years, measurements and studies on electrical properties of single molecule from experiments and theories have played an important role in molecular electronics. The development of social productive forces will be greatly promoted, once the research results were put into application. Aromatic molecules have good electrical properties because of containingπ-electrons which can move freely in the molecular, which are chosen as the prototype candidates. The present work uses density functional theory combined with elastic scattering Green’s function theory to study the electronic transport properties of aromatic molecules theoretically.We simulate the electrodes by tetrahedral gold atoms clusters, and molecules are sandwiched between two gold clusters to form an extended molecular system. First, we study the planarity and length impact on the electronic transport properties. We choose 4,4’-diaminobipheny(C12H12N2) and 4,4’’-diamino-p-terphenyl(C18H16N2) to study their electronic transport properties. Through the analysis of their geometrical structures, extended molecular I-V curves, electronic transport spectra and molecular orbital electronic distributions, we find that the conductance decreases when the aromatic ring is twisted. The longer the aromatic molecule, the lower the current at the same bias. It means that the appearance of the twist and the increase of molecular length are disadvantaged factors to electronic transport.Secondly, we study the impact from different number of aromatic rings and length of molecules on the electronic transport properties. We choose the aromatic molecules 1,4-diaminobenzene(C6H8N2) , 1,4-diaminonaphthalene(C10H10N2) ,9,10-diaminoanthracene(C14H12N2) , 2,6-diaminonaphthalene(C10H10N2) ,2,6-diaminoanthracene(C14H12N2 ) as our models. Through the analysis of the extended molecular geometrical structures, electronic transport spectra, energy levels, molecular orbital electronic distributions and molecular conductance, we find that as the number of aromatic molecules increases, the energy of highest occupied molecular orbital (HOMO) rises, HOMO-LUMO(lowest unoccupied molecular orbitals) gap decreases, and electron tunneling becomes easy and the set-on voltage is lower. The conductance increases with the increase of the number of the aromatic ring when the molecular length keeps the same. When the molecular length is also increased, however, its conductance decreases. It means that in a certain extent, the influence of molecular length is greater than that of number of aromatic rings on the conductance.Thirdly, we study the isomeric compound of aromatic molecules influence on the electronic transport properties. Four isomers of C10H10N2 are chosen, which are 1,5-diaminonaphthalene , 2,6-diaminonaphthalene , 1,4-diaminonaphthalene , 2,7-diaminonaphthalene.We discuss the molecular structure impact on electronic transport properties, and study the molecular electronic transport spectrum, and compare the local molecular orbital of extended molecular orbital (LUMO + 2). The first electronic tunneling peak is attributed to the molecular orbital LUMO + 2.This paper consists of seven chapters: the first chapter gives a brief introduction of molecular electronics, including research significance and the existing problems. The second chapter introduces the single-particle approximation of multi-particle systems, including Born-Oppenheimer approximation, Hartee-Fock approximation, and density functional theory. The third chapter mainly introduces the elastic scattering green’s function method for calculating current-voltage characteristic of molecular junctions. Chapter 4, 5 and 6 contain the simulation results and discussions. Chapter 7 summarizes the research and prospects the future development of molecular electronics.更多还原