磁性单分子器件电子输运性质及其自旋调控

发布时间：2018-06-01 15:59

本文选题：分子电子器件 + 非平衡格林函数　；参考：《湖南大学》2016年博士论文

【摘要】：本文采用第一性原理计算方法,系统地研究了分子尺度体系输运性质中存在的几个问题以及通过设计特殊分子器件实现有意义的输运特性。重点探讨了酞菁分子和过渡金属酞菁分子的磁致电阻效应、宽度对6,6,12石墨炔输运性质的影响,得到了好的磁致电阻效应,负微分电阻和自旋过滤效应,并行碳链中的基尔霍夫定律以及石墨炔纳米带的稳定性问题。主要内容如下:第一章介绍了分子电子器件研究的发展概况、人们探索分子尺度器件的实验方法以及当前研究分子器件的主要理论方法,最后阐明了本论文研究的主要内容和意义。第二章是理论基础和计算方法部分。在介绍什么是第一性原理计算的基础上,详细介绍了两种主要的第一性原理方法Hartree-Fork近似方法和密度泛函理论方法的理论基础和实际应用。最后详细地介绍了如何把密度泛函理论和非平衡态格林函数方法结合起来,进行分子导体输运特性的第一性原理计算及所采用的计算程序。第三章是基于实验上测得酞菁分子和钴酞菁分子具有巨磁电阻效应,我们研究了酞菁分子和过渡金属酞菁分子的磁致电阻效应,得到了比实验上大得多的磁致电阻效应。通过分析多种过渡金属酞菁分子的自旋磁矩,铬酞菁分子中的过渡金属抺原子拥有最大的自旋磁矩,因此最适合做自旋过滤器件,我们的计算验证了这一点。第四章6,6,12-graphyne,虽然不像石墨烯,alpha石墨炔那样具有六角对称结构,但却具有不对称的狄拉克锥。这预示着6,6,12-graphyne具有奇特的电子特性和输运性质。因此我们对锯齿型的6,6,12-graphyne纳米带的输运性质进行了研究。研究发现,4个宽度此纳米带费米能级附近两条能带不像石墨烯那样是平的,而是弧形的,也不同于5个宽度此纳米带的,这就导致了非常不一样的输运性质。如在不考虑磁性的情况下,5个宽度6,6,12-graphyne的体系中,电流是被抑制的。在反平行的自旋极化下,5个宽度6,6,12-graphyne的体系中出现了磁致电阻效应,负微分电阻和自旋过滤效应。第五章是为了实现电子器件的不断小型化,人们基于单分子设计出了具有各种功能的电子器件。传统器件中的二极管效应,开关效应,场效应管,自旋过滤和磁致电阻效应等,都已经在分子体系,甚至单分子体系中实现了。然而,在分子器件中实现传统的基尔霍夫定律的研究工作却很少。仅有的几篇相关文献均没能在分子尺度器件中实现传统的基尔霍夫定律,原因是并行分子链间存在耦合,这种耦合会产生量子相干或退相干作用。鉴于此,为了在分子器件中实现基尔霍夫效应,我们试图通过增加分子间的距离来减少或消灭量子相干效应;同时我们选择了分子本身宽度很窄的碳原子链作为材料。结果和我们的预想一致,在我们设计的电路中显示出了完美的基尔霍夫叠加定律:当双碳链之间的距离调整到15.5?时,双链体系的电导是单链体系的1.96倍,同时在一个大的电压范围内,通过双链的电流几乎是通过单链的2倍。此外,增加双链之间的距离,电导叠加效果更好;相比于单链,双链体系中的自旋过滤效果得到了加强。第六章跟石墨烯纳米带一样,石墨炔纳米带的磁性稳定性也不理想。如纳米带12个宽度时,铁磁态和反铁磁态的总能相差只有2.8me V。我们对锯齿型石墨炔纳米带三种磁态的稳定性进行了初步计算,计算结果显示,边沿双氢化的石墨炔纳米带反铁磁态的稳定性较单氢化的明显增强。边沿双氢化的石墨炔在没有加外电场时,铁磁态与反铁磁态的总能差值为20.3me V(而单氢化时仅为8.2me V)。随着外电场的介入,这种总能差值随着电场的增加而增加,在0.07 V/?时达到了最大值35 me V。这个总能差值是令人兴奋的,因为它相当于408K的温度差,这表明石墨炔在电场0.07 V/?时可以在常温下保持稳定。我们的计算还发现在电场范围(0.04~0.07)V/?内,自旋向上的能带通过了费米能级,而自旋向下的能带是直接带隙的。也就是说通过调节外电场的值,我们得到了半金属性,而且这种半金属性是稳定的,这预示着石墨炔可以制作成稳定的自旋过滤器件。第七章采用密度泛函理论+非平衡格林函数方法的算法,通过调控酞菁分子中钴原子的自旋方向来调控其自旋过滤效果。当不考虑钴原子磁性方向时,钴酞菁分子体系中的自旋过滤效果不佳。考虑钴原子的磁性方向后,自旋向上和自旋向下电流的差值变大了。对于电极磁性平行的情况,当把钴原子的磁性方向调制与电极自旋方向一致时,自旋过滤效果从差变到很好。同样对于电极磁性方向反平行时,把钴原子的磁性方向调制成与左电极方向一致,自旋向上的电流在整个考虑的电压范围内明显增大,而自旋向下的电流保持遏制,自旋过滤效果变得更加完美。因此,钴酞菁分子中钴原子的自旋方向可以极大地提高自旋过滤效应。
[Abstract]:In this paper, several problems in the transport properties of the molecular scale system are systematically studied by the first principle calculation method and the significant transport characteristics are realized by the design of special molecular devices. The magnetoresistance response of phthalocyanine and transition metal phthalocyanine molecules and the effect of the width of the 6,6,12 graphite acetylene transport properties are discussed. Good magnetoresistance effect, negative differential resistance and spin filtering effect, Kirchhoff's law in parallel carbon chain and stability of graphite acetylene nanoribbons are obtained. The main contents are as follows: the first chapter introduces the development of molecular electronic devices, and people explore the experimental methods of molecular scale devices and the current research points. The main content and significance of the research in this paper are explained at the end of the paper. The second chapter is the theoretical basis and the part of the calculation method. On the basis of the introduction of what is the calculation of the first principle, the theoretical basis of the two main first principles, Hartree-Fork near similar method and density functional theory method, is introduced in detail. At last, we introduce how to combine the density functional theory with the nonequilibrium Green function method to calculate the first principles of the transport properties of molecular conductors and the computational procedure. The third chapter is based on the experiment that the phthalocyanine molecules and the cobalt phthalocyanine molecules have the giant magnetoresistance effect. The magnetoresistance effect of the phthalocyanine molecules and the transition metal phthalocyanine molecules has been obtained. The magnetoresistance effect is much larger than that in the experiment. By analyzing the spin magnetic moments of various transition metal phthalocyanines, the transition metal atoms in the chromphthalocyanine molecule have the maximum spin moment, because this is the most suitable spin filter device, our calculations are tested. Fourth chapter 6,6,12-graphyne, although unlike graphene, alpha graphite alkyne has a symmetrical structure of six angles, but it has an asymmetric Dirac cone. This indicates that 6,6,12-graphyne has peculiar electronic and transport properties. Therefore, we have studied the transport properties of the serrated 6,6,12-graphyne nanoribbons. It has been found that two bands near the 4 width of this nanband Fermi energy level are not flat unlike graphene, but arc and are different from the 5 width of the nanoscale. This leads to very different transport properties. In the case of no magnetism, the current is suppressed in the 5 width 6,6,12-graphyne system. Under the spin polarization, the magnetoresistance effect, the negative differential resistance and the spin filter effect appear in the 5 width 6,6,12-graphyne systems. The fifth chapter is to realize the continuous miniaturization of the electronic devices. The electronic devices with various functions are designed based on the single molecule. The diode effect, the switching effect, the field effect in the transmission device are used. Tube, spin filter and magnetoresistance effect have been implemented in molecular and even single molecular systems. However, there are few studies on the implementation of the traditional Kirchhoff's law in molecular devices. Only a few related literatures have not been able to realize the traditional Kirchhoff's law in molecular scale devices. The reason is the parallel division. In order to achieve the Kirchhoff effect in molecular devices, we try to reduce or eliminate the quantum coherence effect by increasing the distance between molecules. At the same time, we choose a carbon atom chain with a very narrow molecule itself as a material. As we expected, the perfect Kirchhoff superposition law was shown in the circuit we designed: when the distance between the two chains is adjusted to 15.5? The conductance of the double chain system is 1.96 times that of a single chain system, and in a large voltage range, the current through the double chain is almost 2 times that of the single chain. In addition, the double stranded chain is increased. The effect of distance and conductance superposition is better; the spin filtration effect in the double chain system is strengthened compared to the single strand. The sixth chapter, like the graphene nanoribbons, is not ideal for the magnetic stability of the graphite alkyne nanoribbons. For example, the total energy difference between the ferromagnetic and antiferromagnetic States is only 2.8me V. when the nanoribbons are wide, and we have the zigzag graphite acetylene nanoscale The stability of three kinds of magnetic states is preliminarily calculated. The results show that the stability of the antiferromagnetic state of the border double hydrogenated graphite alkynes is obviously stronger than that of the mono hydrogenated. The total energy difference between the ferromagnetic state and the antiferromagnetic state is 20.3me V when the edge double hydrogenated graphite alkyne is without external electric field (and only 8.2me V is only hydrogenated). This total energy difference increases with the increase of the electric field. The total difference of the maximum value of 35 me V. is exciting at 0.07 V/? As it is equivalent to the temperature difference of 408K, which indicates that the graphite acetylene can be stable at 0.07 V/ at the electric field. Our calculation also found that the electric field range (0.04~0.07) V/? Inside, the spin upward energy band passes the Fermi level, and the spin down energy band is direct band gap. In other words, by adjusting the external electric field value, we get the semi gold attribute, and this semi gold attribute is stable, which indicates that the graphite acetylene can be made into a stable self rotating filter device. The seventh chapter uses the density functional theory + non flat. The algorithm of the Green function method regulates the spin filtering effect by regulating the spin direction of the cobalt atom in the phthalocyanine molecule. When the magnetic direction of the cobalt atom is not considered, the spin filtration effect in the cobalt phthalocyanine molecular system is not good. The difference between the spin upward and the spin down currents becomes larger when the magnetic direction of the cobalt atom is considered. When the magnetic direction is parallel to the magnetic direction of the cobalt, the effect of the spin filtration is very good when the magnetic direction of the cobalt atom is conformed to the direction of the spin of the electrode. The magnetic direction of the cobalt atom is conformed to the direction of the left electrode when the magnetic direction of the electrode is anti parallel. The spin upward current is obviously increased in the whole range of voltage considered. The spin filtering effect becomes more perfect when the spin downward current is kept in control. Therefore, the spin direction of the cobalt atom in the cobalt phthalocyanine molecule can greatly improve the spin filtering effect.
【学位授予单位】：湖南大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O469

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