【作者】 李明君；

【导师】 徐慧； 龙孟秋；

【作者基本信息】 中南大学 ， 物理学， 2013， 博士

【摘要】 近年来,分子电子学的发展取得了巨大的进步,尤其是微观表征和操控技术的快速发展,极大地提高了分子器件的研究能力,可在纳米尺度观察研究并操控单个分子来构筑具有特定功能的分子器件。有机共轭分子具有结构可设计、易于合成、结构丰富及良好的半导体性能等特点,成为分子器件中研究的热点。本文采用基于密度泛函理论和非平衡格林函数方法相结合的第一性原理方法,对苯基D/A分子器件的电荷输运性质进行了计算模拟,并对其分子器件的设计提出了构想。详细考察了影响分子器件设计的一些关键物理和化学因素,如分子与电极间的键合方式、分子的化学修饰、电子给体-受体功能分子中共轭桥的选择以及分子间相互作用等对分子器件导电性的影响,得到了一些有意义的结论。主要研究内容如下：首先,本文研究了Donor-Bridge-Acceptor (D-B-A)结构分子器件中桥键极性与长度对电子输运性质的影响。选用四种不同桥键连接电子给体(苯胺)-受体(硝基苯)构建异质结分子器件,探讨不同极性和不同长度的桥键对分子器件输运性能的影响。研究发现,在D-A-B结构的单分子器件中,桥键的极性对分子器件的导电性起着重要作用。极性桥键连接的分子器件比非极性桥键连接的分子器件能提供更多更好的电子输运通道,并且还出现了负微分电阻现象；对于相同非极性桥键组成的分子器件,随着桥键长度的增加,电流减小,分子器件的传输性能减弱。其次,本文选取四组不同锚定基团将NBPDA分子为中心的分子异质结连接到两个半无限长金电极上,计算模拟分子与电极之间键合方式不同对分子器件电荷输运性质的影响。研究发现,分子导电性对分子和电极间的键合方式比较敏感。电极-分子间的耦合的不同导致分子轨道的能量和空间分布发生改变,产生了负微分电阻现象。接着,本文采用化学修饰的手段,研究了氟化反应对DT-TTF分子器件电荷输运性能的影响。研究发现氟原子基团的引入调整了分子器件的结构和电子特性。氟原子基团由于其强吸收电子能力,降低电荷注入势垒,利于电子的注入与传输,并改变分子与电极间的耦合,从而影响分子与电极间的电荷转移。高度氟化后分子器件的透射峰向低能区移动,并调整了分子轨道的局域化。氟化减弱了分子与电极间的耦合,弱化了电流,出现库仑阻塞现象,导致负微分电阻现象的产生。最后,本文研究了两个平行分子结不同相互作用对分子器件输运性质的影响。研究发现分子间耦合作用的强弱显著影响分子器件的输运性能,分子间距离较近,相互作用较强时,更多的输运通道被打开,引起分子轨道分裂,导致出现负微分电阻现象。希望以上计算结果能对有机分子器件的设计提供有益的参考。更多还原

【Abstract】 TIn recent years, molecular electronics has become a significant and rapidly growing field, which made tremendous progress. With to the fast development of microscopic characterization and manipulation technology, the research of molecular device is greatly improved. People can observe and manipulate single molecules at the nanometer scale, which can be built into the specific functional molecular device. Due to their designable and rich structures, easy synthesis and good semiconductor properties, organic conjugated molecules has drawn great attention in research of the molecular device research.Using the approach of density functional theory (DFT) combined with nonequilibrium Green’s functions (NGF), the electronic transport properties of benzene-based D/A molecule have been investigated and the design of molecular devices are proposed in this thesis. Considering some key physical and chemical factors that impact the charge transport properties of the molecular device, such as anchoring groups, molecular chemical modification, donor-acceptor molecular conjugated bridge selection and intermolecular interaction, some useful conclusions has been obtained. The main contents are listed as follows.First, the electronic transport properties of different bridges connected to benzene-based heterostructure molecular devices have been studied. Focusing on the effects of the polarity and length of the bridge bond on the molecular device of Donor-Bridge-Acceptor (D-B-A), it is found that the polarity of the bridging bond in the molecular device plays a significant role in charge transport in D-B-A molecular junction. The molecular device with connection of the polar bond has higher conductivity than that with connection of the non-polar bond. The NDR behavior and current platform are observed within a certain bias voltage range for the polar bonding systems.Second, the electronic transport characteristics of NBPDA molecular heterojunction, wihich connected by four different anchoring groups, are discussed. The molecular heterojunction is positioned between two semi infinite gold electrodes. Results show that the anchoring group plays a crucial role in determining the overall conductivity of the molecular junctions. The NDR behavior originates from the changes of transferred charges and redistribution of the frontier molecular orbitals at different bias.Third, fluorination effects on the electronic transport in DT-TTF are explored based on the first-principle calculation and theoretical analysis. It is found that the structure and electronic properties of the molecular device have been modified. The fluorine atoms group can decrease charge injection barriers and then do good to the electron injection and transmission due to their strong electronic absorbing ability. Meanwhile, the addition of fluorine atoms can modify the couple of DT-TTF molecular junction and the electrodes, thereby affecting the charges transfer between them. The transmission peaks of fully fluorated molecular shift to the low energy region, and adjust the localization of molecular orbit. At the same time, the Coulomb Blockade effect and NDR behavior can be observed within a certain bias voltage range since the coupling between molecular junction and the electrodes has decreased and the current becomes weak.Finally, the intermolecular coupling effects on the electronic transport have been investigated towards the dimolecule device. It is found that the intermolecular coupling effect plays an important role in the conducting behavior of the system. The nearer distance among the molecular, the more transport channels will be opened so leading to the split-up of molecular orbits and thus appears negative differential resistance within a certain bias voltage range.These results will shed light on establishment of the structural properties of organic molecular devices.更多还原