卟啉基分子器件的电磁输运性质研究

发布时间：2018-01-22 17:58

本文关键词： 第一性原理密度泛函理论电磁输运特性分子-电极接触构型负微分电阻效应 ATK软件　出处：《苏州大学》2016年博士论文　论文类型：学位论文

【摘要】：分子电子学起源于诺贝尔物理学奖获得者费恩曼(R.P.Feynman)提出的“从单个分子甚至原子进行组装”的伟大设想。1974年,Avriam和Ratner设计了具有非对称结构的分子,研究了给体-受体-分子桥的整流特性,成功预示了该分子呈现出分子整流器的功能。随着实验技术的完善,人们利用扫描隧道显微镜技术,力学可控劈裂法,纳米电极技术等方法制备出各种具有特殊功能的分子器件,包括分子导线,分子开关,分子整流器,分子存储器等。如何构筑、表征稳定的分子结是分子器件实验研究中面临的最困难的问题,尤其是分子器件的特性对于分子与电极之间的界面非常灵敏,导致了不同研究小组的一些测量结果相差较多,甚至出现互相矛盾。而在实验上要实现分子与电极之间的精确控制是非常困难的。因此,计算模拟方法在分子器件研究中起着越来越重要的作用。随着计算机功能的急速增强,分子电子学的理论模拟计算对分子材料的开发和分子器件的设计逐渐成为研究热点。近年来,由于卟啉分子高共轭的平面结构,优异的化学稳定性以及独特的光电特性,可与多种金属离子形成稳定的金属配合物等性质受到国内外各研究小组的普遍关注。由于分子的电磁输运特性与金属电极和有机分子接触面的特性密切相关,本文利用非平衡格林函数与密度泛函理论相结合的第一性原理方法,应用Atomistix Tool Kit(ATK)计算软件,详细研究了苯环-卟啉-苯环分子(BPB)与电极间的耦合方式,中心配位金属,功能性基团替代等因素对卟啉类分子器件输运性质的影响,得到了一些有意义的结论。主要研究内容如下:(1)在无限大平面金电极基础上构造尖端金字塔型电极结构吸附BPB分子,并逐渐减少电极前端的Au原子个数,破坏金字塔尖端电极接触方式,系统研究了分子与电极的接触构型对BPB分子结电输运特性的影响。分子电极接触构型的不同导致了电流-电压(I-V)特性曲线呈现出明显不同的结果,特别是两端金字塔电极的硫-金(S-Au)顶位接触的BPB分子结,在外加电压-0.3V时出现了明显的负微分电阻(NDR)现象。随着尖端接触对称性被逐渐打破,在两端都是大平面电极接触构型的BPB分子结中没有观察到低压负微分电阻现象。一侧是金字塔电极接触,另一侧是平面电极接触的不对称构型的BPB分子结,不仅具有低压负微分电阻效应,还同时具有较强的整流特性。进一步通过透射谱和分子投影自洽哈密顿(MPSH)进行分析,表明这些传输特性与金电极的尖端接触构型密切相关。分子与金字塔构型的电极接触时,尖端金原子与BPB分子耦合作用引起了主导输运的分子前沿轨道HOMO能级,使得费米面附近的透射峰随电压发生变化,导致了低压负微分电阻效应。(2)系统研究了BPB分子与不同金属配位的复合物(M-BPB,M=Zn,Ni,Cu,Mg,Co,Ca)的电输运性质,我们选择直径约4.5埃的金(111)纳米线作为电极材料,构造尖端金字塔型电极结构,研究了在不同对称性、三种键合方式(顶式、桥式、空式)的M-BPB分子结的电输运特性。首先将苯环-钴卟啉-苯环分子(Co-BPB)与金纳米线电极构成不同S-Au键合方式的分子结,研究键合方式对Co-BPB在宽电压范围内电输运性质的影响,计算结果发现该分子结在不同电压范围出现了多重NDR现象。低压区域(0-1.0V),Co-BPB分子与金电极顶式键合构型具有NDR效应,中间电压区域(1.0-2.0V),出现了两种不同特性的NDR现象,高压区域(2.0-3.0V),所有S-Au键合构型在外加偏压为2.8V左右都具有负微分电阻效应。这些结果表明宽电压范围内,低压区域与高压区域的负微分电阻效应机制并不相同。我们的分析表明分子与电极通过S-Au顶式键合时,尖端金原子与硫原子的耦合能级中,近费米面的能级对低压区电输运起主要作用,随着电压逐渐增加,离费米面较远的Co-BPB分子的本征能级逐渐对输运起主导作用,这两种机制的结合和竞争导致了中间电压区域的NDR效应。中心配位金属对M-BPB电输运特性的影响结果表明,Ca-BPB与Co-BPB由于相似的扩展的分子轨道空间分布,具有良好的电输运特性。而Cu-BPB和Ni-BPB电子态的空间分布呈现出很强的局域性,导致费米面附近的透射率很小,具有较弱的电输运特性。(3)进一步研究了S-Au三种键合构型Co-BPB分子结自旋极化输运性质,结果发现Co-BPB分子与金电极顶式键合的I-V曲线特性显示出不同程度的自旋分立的现象,尤其是在低压区自旋分立更为明显。在Co-BPB分子中增加功能性基团氨基(NH2)和硝基(NO2),研究基团替代对Co-BPB分子结的自旋极化输运性质的影响,结果表明基团的种类以及替代位置都可以有效调控Co-BPB分子结的自旋输运性质。中间卟啉环的H原子被NH2替代可以获得更大的自旋向下电流,侧苯环的H原子被NH2替代可以获得反向整流效应。相同位置采用NH2替代,比NO2具有更强的自旋分立和低压负微分电阻效应。计算结果表明自旋极化的转移电荷导致了基团替代的Co-BPB分子自旋向下轨道的HOMO能级向费米面移动,增强了分子与电极间的耦合作用,获得了更好的自旋输运特性。本文的研究结果为新型多功能分子器件的设计提供了理论参考。
[Abstract]:Molecular electronics origin Feinman who won the Nobel prize in Physics (R.P.Feynman) to put forward "from a single molecular or even atomic assembly" is a great idea in.1974, Avriam and Ratner designed molecules with the asymmetric structure, the donor acceptor molecular bridge rectifier characteristics, indicates the molecule showing a successful molecular rectifier function. With the improvement of experimental technique, people use the technology of scanning tunneling microscope, splitting method of controllable mechanical, nano electrode technology prepared a variety of molecular devices with special functions, including molecular wire, molecular switch, molecular rectifiers, molecular memory. How to build, is the most stable characterization of molecular junctions the difficult problems of experimental research in molecular devices, especially the properties of molecular devices between the molecule and the electrode interface is very sensitive, leading to different research group Some measurement results differ more, even contradictory. But in the experiment to realize accurate control between molecule and electrode is very difficult. Therefore, numerical simulation method plays a more and more important role in the study of molecular devices. With the function of computer rapid enhancement, molecular electronics simulation development theory and molecular devices of molecular materials has become a research hotspot. In recent years, due to the planar structure of porphyrin molecules highly conjugated, excellent chemical stability and unique optoelectronic properties, with a variety of metal ions to form stable physical properties such as metal complexes has received widespread attention at home and abroad of the research team. Because the molecules of electromagnetic transmission transport properties and the metal electrode and the organic molecules contact surface is closely related to the characteristics, this paper using the nonequilibrium Green function and density functional theory combined with the first The principle of method, application of Atomistix Tool Kit (ATK) software, a detailed study of the phenyl porphyrin - benzene molecule (BPB) and coupling between the electrodes, central metal, alternative functional groups and other factors investigated transport properties of porphyrin molecular devices, obtained some valuable conclusions. The research contents are as follows: (1) to construct Pyramid tip electrode structure with the adsorption of BPB on gold electrode based on the infinite plane, and gradually reduce the number of Au atoms at the front end of the electrode contact, failure mode of Pyramid tip electrode, and the electrode contact configuration of molecular transport properties of BPB molecule junction was studied. The contact electrode configuration of molecules has different current voltage (I-V) curves show different results, especially the ends of Pyramid sulfur gold electrode (S-Au) molecular BPB top contact node, when the applied voltage in -0.3V The apparent negative differential resistance (NDR) phenomenon. With the tip contact symmetry is broken gradually, in both observation low negative differential resistance phenomenon to the BPB junction plane electrode in the contact structure. One side is Pyramid electrode contact, the other side is a BPB molecule asymmetric configuration of planar contact electrode. Not only has the advantages of low pressure, negative differential resistance, but also has the characteristics of strong rectification. Further through the transmission spectrum and molecular projected self consistent Hamiltonian (MPSH) analysis showed that the tip contact configurations of these transmission characteristics with the gold electrode is closely related to the molecular configuration of the electrode contact with Pyramid, the tip of gold atoms and BPB molecules coupling by the frontier molecular orbital energy level of HOMO leading transport, the transmission peak near the Fermi level changes with voltage, low voltage leads to negative differential resistance. (2) of B PB molecules with different metal coordination complexes (M-BPB, M=Zn, Ni, Cu, Mg, Co, Ca) of the electrical transport properties, we select the diameter of about 4.5 Au (111) nanowires as electrode materials, structure of Pyramid type tip electrode structure is studied under different symmetry, three bond (top, bridge, empty type) of the electronic transport properties of molecular junctions. Firstly, M-BPB benzene COBALTPORPHYRIN - benzene molecular (Co-BPB) of different molecular junction S-Au bond with the Gold Nanowire Electrode on bonding effect of Co-BPB in the wide range of voltage electrical transport properties the results show that the molecular structure in different voltage range appeared multiple NDR phenomenon. The low pressure region (0-1.0V), Co-BPB and gold electrode top bonding configuration with NDR effect, the middle voltage region (1.0-2.0V), there are two different characteristics of the NDR phenomenon, the high-pressure region (2.0-3.0V), all the S-Au key in the configuration The bias is about 2.8V with negative differential resistance. These results indicate that the wide range of voltage, the negative differential resistance mechanism of low pressure region and high pressure regions are not the same. Our analysis shows that the molecule and the electrode through the S-Au top type of bonding, coupling level of gold atoms and sulfur atoms tip in near Fermi level in the area of low pressure electric transport plays a major role, as the voltage increases, the energy levels of Co-BPB molecule is far away from the Fermi gradually on transport plays a dominant role in binding and competition of these two mechanisms lead to the effect of NDR middle voltage region. Central metal of M-BPB electrical transport properties the results show that Ca-BPB and Co-BPB due to the distribution of molecular orbital space similar to the expansion, has good electrical transport properties. Cu-BPB and Ni-BPB electronic states localized spatial distribution showed a very strong, resulting in the Fermi level Near the transmittance is very small, the electron transport properties is weak. (3) further study of S-Au three kinds of bonding configuration of Co-BPB molecule junction spin polarized transport properties, the results showed that Co-BPB molecules with the gold electrode top I-V curve bonding showed different degrees of separation of spin phenomenon, especially in low spin separation is more obvious. The increase of amino functional groups in Co-BPB molecule (NH2) and (NO2), nitro substituted on transport properties of spin polarized Co-BPB molecular junctions, results show that the group type and position can replace transport properties to effectively control the spin Co-BPB molecular junctions. H atom intermediate porphyrin ring substituted by NH2 can obtain more spin down current, H atom side benzene ring substituted by NH2 can reverse rectifier effect. NH2 is adopted to replace the same position, spin is stronger than the NO2 and low voltage Negative differential resistance effect. The calculation results show that the charge transfer of spin polarization leads to the HOMO level of Co-BPB molecular spin substituted downward track moves to the Fermi level, enhance the coupling between the molecule and the electrode, obtained better spin transport properties. The research results of this paper provide a theoretical reference for the design of new multi function molecular devices.

【学位授予单位】：苏州大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O561

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