【作者】 范志强；

【导师】 陈克求；

【作者基本信息】 湖南大学 ， 凝聚态物理， 2010， 博士

【摘要】 近年来,随着微观操控技术和微观组装技术的快速发展,人们可以在纳米尺度操控单个分子并将其制造成具有特定功能的分子器件。这些分子器件被认为是慢慢接近尺度极限的传统电子器件的最合适的替代者。因此在实验和理论两方都吸引了越来越多的关注。本论文利用基于密度泛函理论的第一性原理方法研究了分子器件输运性质中的几个问题,重点探讨了分子受外压形变、分子与电极连接位置的变化、外加门压、侧基团取代以及非对称电极连接对分子器件输运性质中负微分电阻(NDR)和整流的影响。其主要内容如下：研究了单个C60分子与金属铝电极组成的分子器件的电子输运性质,同时还研究分子受外压发生形变和外加门压对器件电子输运性质的影响。结果表明,当单个C60分子通过范德华力吸附在金属铝电极上,其电流并不是按照传统的欧姆定律随着电压的增大而增大,而是在特定的电压区间随着电压的增大而减小表现出奇特的负微分电阻效应。与此同时,我们还发现当分子受到外加压力时,这种负微分电阻效应可以伴随着分子的径向形变先增大后减小直至消失。此外,我们还发现门压对分子器件电子输运性质的影响也很强烈。通过调控门压可以使分子器件在高导态与低导态之间切换从而实现分子电流开关功能。此外,我们还发现门压也可以调控分子器件的负微分电阻效应使其减弱或消失。研究了侧基团取代对有机共轭分子OPV电子输运性质的影响。计算结果表明,当OPV分子被氨基取代后,分子的占据态轨道被局域。然而,当OPV分子被硝基取代后,分子的非占据态轨道被局域。当氨基或者硝基单独取代分子时,可以增大器件的电子传输能力。但是,当这两个基团同时取代分子时却减弱器件的电子传输能力。更有趣的是,当且仅当OPV分子被两个氨基共同取代后,器件会出现负微分电阻效应。研究了双分子器件的电子输运性质。我们将两个OPV分子平行放置于金电极之间,重点研究了侧基团的相对取代位置对双分子器件电子输运性质的影响。由于分子层间强烈的相互作用,双OPV分子器件的轨道和输运系数比单OPV分子器件要复杂的多。同时,侧基团的取代作用也大不相同。结果表明,在双分子器件中,侧基团对器件输运性质的影响强烈地依赖于取代位置。氨基在同侧取代双OPV分子后,器件的电流要大于氨基在异侧取代分子后的电流。对于硝基来说,情况恰恰相反。更重要的是,我们发现当且仅当氨基在同侧取代分子后,器件的电流曲线会出现负微分电阻效应。研究了单phenalenyl分子与金电极构成的分子器件的电子输运性质。Phenalenyl分子是一个高度对称(D3h)的有机自由基,其分子自身有两个不同的连接位置可以与外界电极相连接。计算结果表明,分子自身的连接位置是影响器件输运性质的重要因素。当连接位置为相对于中心原子的两个二近邻原子或者一个二近邻和一个三近邻原子,器件的电流曲线会呈现出强烈的负微分电阻效应。同时,我们还发现当连接位置为相对于中心原子的一个二近邻和一个三近邻原子,器件的电流会呈现出整流效应。研究了非对称电极对单个C60分子器件电子输运性质的影响。由于金电极与碳纳米管电极在费米能级附近差异很大,使Au-C60-CNT器件的电流在小偏压范围内与同性质电极所构成的器件相比缩小了三个数量级。同时,我们还发现器件的电流曲线呈现出整流效应,并可以通过外加门压对整流比进行调控。更多还原

【Abstract】 In recent years, the rapid progresses in micro-fabrication and self-assembly tech-niques have made it possible to control the molecules in nanoscale and to assemble them as the molecular device. These molecular devices are considered the suitable candidate of electronic devices which trend toward the ongoing miniaturization. For this reason, the experimental and theoretical studies on the molecular device have attracted more and more attentions. In this dissertation, we use the first-principles in combination with the density-functional theory to study the transport properties of some molecular devices and mainly discuss the effect of deformation, connected sites, gate voltage, side groups and asymmetric electrodes on the negative differential resistance (NDR) and the rectifying behavior.We investigate the transport properties of a single molecular device constructed by one C60 molecule sandwiched between two Al electrodes. At the same time, we also in-vestigate the effect of deformation and gate voltage on the device’s current-voltage prop-erties. The calculated results show that the currents of the device which the C60 connects with the Al electrodes by van der waals force not accord with the traditional Ohm theo-rem. In the special voltage region, the currents decrease with the increase of bias voltages and show the NDR behavior. In addition, we find that the NDR behavior can be enlarged or reduced and shut off by squashing the molecule on the vertical direction. Further study indicates that the gate voltage can also affects the device’s transport properties intensively. As a result, we can modulate the device between the high and low conductive states and make it as a gate-controlled current switch. In the same way, the NDR behavior also can be reduced and shut off by the gate voltage.We study the effect of side groups on the transport properties of a linearπconju-gated molecule OPV sandwiched between two Au electrodes. Our calculation explicitly demonstrates that when the molecule modulated by amino, the highest occupied molecu-lar orbitals are localized, while the molecule modulated by nitro, the lowest unoccupied molecular orbitals are localized. The electron transport of device will be enhanced when it modulated by amino or nitro, but will be weakened when it modulated by both of them. More interesting, negative differential resistance is only observed when the molecule modulated by two amino at the same time.Then, we investigate the transport properties of a bimolecular device. The two par- allel OPV molecules are sandwiched between two Au electrodes and the effect of side groups is studied again. Due to the intensive interaction between the two molecules, the molecular orbitals and transport properties of the bimolecular device are more complex than the single molecular device, so does the side groups. The results show that the side groups can modulate the bimolecular device’s transport properties by the substituted posi-tion. The current of the device substituted by two amino groups on the same side is bigger than that on the different side. Contrarily, the current of the device substituted by two nitro groups on the same side is smaller than that on the different side. More importantly, the NDR behavior can be observed only when the system is substituted by two amino groups on the same side.We study the transport properties of a single phenalenyl molecular device. Phenalenyl is a well known stable organic radical with high symmetry (D3h) and has two differ-ent sites to connect with the electrodes. The results show that the electronic transport properties are strongly dependent on these contact sites. The negative differential resis-tance behavior with large peak to valley ratio is observed when the molecule contacts the Au electrodes through two second-nearest sites or one second-nearest site and one third-nearest site, while the rectifying behavior is observed only when the molecule contacts the Au electrodes through one second-nearest site and one third-nearest site.We perform a theoretical study of a single C60 sandwiched between Au electrode and nanotube electrode. Due to the huge difference, the matching of orbitals around the Fermi energy among the two electrodes and the molecule is not very well. So the current value of the Au-C60-CNT is much smaller than the C60 device combining the same electrode up to three orders of magnitude. Moreover, the rectifying behavior is observed in this device and the rectification ratio can be modulated by the gate voltage.更多还原