光驱动分子开关电输运性质第一性原理研究

发布时间：2018-06-25 06:55

本文选题：密度泛函理论 + 非平衡格林函数方法　；参考：《山东大学》2017年硕士论文

【摘要】：分子电子学由于其在纳米功能材料领域的潜在应用价值一直备受科研人员关注,主要研究内容为利用单分子及分子团簇,如单壁或多壁的碳纳米管、有机小分子以及高分子等来构造各种功能性的电子器件,并且研究这些分子电子器件的电特性或光特性。近年来,随着实验技术和理论方法的进步和发展,使得人们设计,测量并计算具有不同功能的分子电子的器件成为可能。这些基于单分子的电子器件具有独特的物理性质,包括负微分电阻效应,分子整流现象,开关效应,存储器和二极管等等。其中,由于分子开关具有存储信息和传递信息的特质,对于分子开关的研究成为了前沿课题。根据触发因素的不同,分子开关可以分为两类:电开关和光开关。前者在电场或者电流脉冲等外部触发因素下实现开态和关态的转化,而后者则是在光的触发下实现电流开/关状态的相互转化。由于光的反应时间短而且容易获得,光是非常理想的外部触发因素,因此光开关引起了科学家们的广泛关注。本文,我们利用密度泛函理论与非平衡格林函数相结合的办法,系统地研究了旋转分子马达和二芳基乙烯分子的电子输运性质,探讨其作为分子光开关的可能性。研究内容和主要结论有:1.光驱动旋转分子马达光分子开关Feringa第二代光驱动分子马达在360°旋转过程中,两种异构体分别为anti-folded和syn-folded,计算发现,通过顺势异构体的电流总是大于通过反式异构体的电流,这意味着在一个旋转周期中,分子的导电性改变了四次。这种光引起的周期性电导改变特质使得分子马达具有光分子开关潜质。两种异构体的不同的输运特性主要是由于分子的有效共轭长度。顺势异构体费米能级附近的透射峰来源于HOMO轨道,顺势异构体的HOMO轨道相对于反式异构体来说更加离域。而且反式异构体的透射峰距离费米能级较远,因此相对于顺势异构体,反式异构体的导电性较差。2.二芳基乙烯光分子开关实验上发现了二芳基乙烯分子吸收可见光可以实现双向转变。通过我们的计算模拟发现,通过闭环化合物的电流远远大于通过开环化合物的电流。电流比高达2700,使其呈现出良好的开关特性。另外,在我们的闭环态器件中还出现了负微分电阻现象(NDR)。开环化合物在费米能级附近没有透射峰是其导电能力弱的原因。开环化合物的HOMO和LUMO轨道都是局域的,而闭环化合物的LUMO轨道是离域的,在费米能级附近的透射峰来自LUMO轨道,因此LUMO轨道是主要的电子输运轨道。闭环化合物在偏压0.8V时,偏压窗内的的透射峰积分大于其在偏压为0.9V时的透射峰积分,这是其出现NDR的原因。
[Abstract]:Molecular electronics has attracted the attention of researchers because of its potential application value in the field of nano-functional materials. It is mainly concerned with the use of single-molecule and molecular clusters, such as single-walled or multi-walled carbon nanotubes. Organic small molecules and polymers are used to construct various functional electronic devices, and the electrical or optical properties of these molecular electronic devices are studied. In recent years, with the development of experimental technology and theoretical methods, it is possible to design, measure and calculate molecular electronic devices with different functions. These electronic devices based on single molecule have unique physical properties, including negative differential resistance effect, molecular rectification, switching effect, memory and diode, and so on. Among them, because molecular switches have the characteristics of storing and transferring information, the research of molecular switches has become a frontier topic. According to the trigger factors, molecular switches can be divided into two categories: electrical switches and optical switches. The former realizes the transition of open and off states under external triggering factors such as electric field or current pulse, while the latter realizes the mutual conversion of current on / off states under the trigger of light. Light is an ideal external trigger because the reaction time of light is short and easy to obtain, so optical switch has attracted wide attention of scientists. In this paper, using density functional theory and non-equilibrium Green's function, we systematically study the electron transport properties of rotating molecular motors and diarylethylene molecules, and discuss the possibility of using them as molecular optical switches. The contents and main conclusions of the study are as follows: 1. The two isomers are anti-folded and syn-fold, respectively. It is found that the current of the homeoisomer is always greater than that of the trans-isomer during the rotation of the second generation of the photo-driven molecular motor Feringa. This means that the conductivity of the molecule changes four times in a rotation cycle. The characteristic of periodic conductance change caused by light makes the molecular motor have the potential of photomolecular switch. The different transport properties of the two isomers are mainly due to the effective conjugation length of the molecules. The transmission peak near the Fermi energy level of homeoisomer originates from Homo orbitals, and Homo orbitals of homeoisomers are more delocalized than trans isomers. Moreover, the transmission peak of trans isomer is far away from Fermi energy level, so the conductivity of trans isomer is lower than that of homeopathic isomer. It is found that the visible light absorption of the diaryl ethylene molecule can realize the bidirectional transition in the experiment of the diaryl ethylene photomolecular switch. Our simulations show that the current through the closed loop compound is much larger than that through the open loop compound. The current ratio is as high as 2700, which makes it show good switching characteristics. In addition, the negative differential resistance (NDR) phenomenon appears in our closed-loop devices. The absence of a transmission peak near the Fermi level is the reason for the weak conductivity of the ring opening compounds. The Homo and LUMO orbitals of the open ring compounds are local, while the LUMO orbitals of the closed loop compounds are delocalized. The transmission peaks near the Fermi energy level come from the LUMO orbitals, so the LUMO orbitals are the main electron transport orbits. When the bias voltage is 0.8 V, the transmission peak integral in the bias window is larger than the transmission peak integral at the bias voltage of 0.9 V, which is the reason for the NDR.
【学位授予单位】：山东大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O469

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