DNA分子器件场效应理论研究
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摘要
DNA分子是一种由核苷酸重复排列组成的长链聚合物,全称脱氧核糖核酸(Deoxyribo Nucleic Acid)。DNA是重要的生物大分子,它是生物的遗传基因的载体。DNA分子的主要组成元素是C、H、O、N、P等。通过DNA生物可以把父代的特征复制传递到子代中,从而完成生物性状的传递,因此DNA又被称为遗传物质。1953年Watson和Crick提出了著名的DNA双螺旋结构,DNA分子包含两条由糖类和磷酸分子组成的核苷链,每个糖分子都与四种碱基里的一种相连接,这两条链反向平行的互相缠绕,其上悬挂的碱基通过互补配对的原则结合。其中,腺嘌呤(Adenine)只与胸腺嘧啶(Thymine)结合,而鸟嘌呤(Guanine)只与胞啼啶(Cytosine)结合。自从Watson和Crick发现了DNA的双螺旋结构以来,关于其生物基因方面的研究开创了生物学的新时代。在另一方面,DNA的导电性引起了其他领域的关注。早在1 962年,Eley和Spivey就提出DNA分子可能具有导电性。在相邻的DNA碱基对间,存在垂直于碱基平面的π电子轨道。这些轨道的交叠就导致了可能的导电性。这就引起了生物、物理和化学多学科领域的关注,一方面,研究DNA分子的电荷输运性质对生物基因的损伤以及修复、DNA的测序有重要意义;另一方面,因为DNA分子的特殊性,它可以在溶液中自动的按照配对原则组装成完整的分子,从而完成自下而上的组装电路。如果DNA分子可以导电,它将是分子电子学良好的备选材料。传统的集成电路的基本单元是硅基晶体管,它们可以实现导通和不导通两种状态。通常的微处理器含有超过百万个晶体管,而面积在几个平方厘米。所以需要将晶体管做的很小,从而实现较高的集成度来提高运算速度。但是普通的硅基半导体在小型化的过程中遇到了瓶颈,继续小型化会造成器件不稳定。分子电子学就是用单个分子来代替硅基的半导体来制作电路,实现器件的高集成度和小型化。随着近年来纳米技术的发展,已经可以将单分子或分子束连接在电极间测量它的导电特性。DNA分子由于特殊的自组装性质,可以进行高精度的制作电路,在工程技术上具有很高的价值。
随着纳米技术的发展,近年来有很多实验直接将单个或一束DNA分子连接到金属电极间,来测量它的电导。这些实验可以更直接的揭示DNA的导电性质,也为DNA作为分子器件的可能性进行了直接的验证。这类实验的结果比较复杂,揭示的DNA的导电性有绝缘性,半导体性,导体甚至超导体。导电性的多样结果一是源于DNA本身的性质,DNA有多种序列,分子形态；二是来源于外界环境的影响,温度,溶液中的离子,电极与DNA分子的接触等。
由于实验结果的多样性,许多理论工作致力于解释DNA中电荷输运的各种现象。现阶段大致有两类理论方法用以研究DNA分子中电荷输运：第一性原理计算和模型化计算。第一性原理计算方法可以将分子的复杂结构和各类相互作用考虑在内,在进行计算的时候仅需要给出原子的种类及其位置,不需要其他的实验的、经验的或者半经验的参量,可以准确的描述DNA分子的电子态和结构特性。模型化计算方法可以抓住研究对象的一些主要特征,通过模型化参数的调控,直观的反映研究对象的物理图像和本质,很多情况下甚至可以得到解析解。同时,模型化方法可以大大降低计算量,适合扩展系统的计算,其计算结果可直接与实验测量进行比较。这两类方法相互联系,又互为补充。
在半导体晶体管中,我们可以通过外加电场和磁场来调节通过晶体管的电流,从而实现电路的开关,放大等功能。在DNA分子器件里,是否可以通过外加电场或磁场来调控体系的输运性质,这将是我们感兴趣的问题。那么,综合前人的相关研究,我们研究了外电场和外磁场对DNA分子器件电荷输运的调控。目的是通过这些研究可以指导实验,拓展DNA分子器件的应用范围。
1.电场对DNA分子器件磁阻的调控研究
磁隧道结在磁性调控器件的应用中占有重要的地位。传统的磁隧道结是把-层薄的非磁材料夹在两层铁磁材料中间,它的电阻对于两边铁磁电极的磁化方向的排列很敏感。可以通过施加一个外磁场来控制铁磁电极磁化方向成平行或者反平行排列,从而控制通过磁隧道结的电流。
因为有机物具有弱的自旋轨道耦合和超精细相互作用,有机自旋电子学成为最近的研究热点,DNA作为有机物也被用来研究自旋相关的输运。Zwolak和Ventra首先研究了DNA作为自旋阀的可能性,他们分别研究了Fe和Ni作为电极的DNA分子器件中的磁电阻,发现Ni做电极时磁电阻可以达到26%,而Fe做电极时磁电阻可以达到16%。最近,Malyshev报道了横向电场对于DNA中电荷输运的影响,他选取人工合成的Ploy(G)-Poly(C)DNA分子为研究对象,发现横向电场压制了通过DNA分子的电流。这是一个非常有意思的结论,由于DNA分子固有的双螺旋结构,横向电场引发了一个螺旋势,这个螺旋势改变了DNA分子的电子结构,从而调整了DNA分子中的导电通道。
在DNA分子器件中,横向电场和铁磁电极的加入扩展了它可能的应用范围。所以我们尝试将横向电场引入“铁磁／DNA／铁磁”分子器件中,来研究横向电场对于DNA分子器件的自旋相关输运的影响。我们的目的是想得到一个增强或者可以调节的磁阻。
1.1基于晶格格林函数和Landauer-Buttiker公式,我们首先计算了“铁磁／DNA／铁磁”分子器件在两边铁磁电极磁化方向平行和反平行排列时的电流电压特性曲线。发现平行排列时的电流要大于反平行时的电流,而且根据此电流计算得到的磁阻与Zwolak和Ventra得到的磁阻相当。
1.2施加横向电场时,我们又模拟了普通金属作为电极的“金属／DNA／金属”分子器件中的电流电压特性曲线。发现在短链DNA分子中,横向电场对于电流的压制作用与横向电场相对于DNA分子的方向有密切关系。通过体系的电流随横向电场方向的改变出现了震荡的行为,随着横向电场的增强,震荡幅度变大。
1.3我们研究了“铁磁／DNA／铁磁”分子器件中的自旋相关输运。我们探讨了利用横向电场来调控该器件磁阻的可能性。发现横向电场的方向和大小都可以有效调控体系的磁阻。在某个特定的方向和大小的横向电场下,体系的磁阻增大。这个磁阻的调制是由于横向电场改变了DNA分子的电子结构,所以这种调制是DNA分子特有的性质。
2.磁场对DNA分子器件电荷输运的调控研究
Aharonov-Bohm效应反映了电子的量子相干性,由于电子的波动性,电子在磁场中获得了一个磁场相关的相位,所以电子不同波束间的相干性就会被改变。在传统的介观器件中,通过施加磁场来改变体系的电导是一种非常有效的手段。当电子通过一个圆环形状的介观导体时,如果在环内施加一磁场,那么电导将随着介观环所包括的磁通量发生周期性的震荡。在实验上,这种类型的震荡已经在微米量级的金属圆柱体和刻蚀成的环中观测到。对于分子级别的纳米器件,最近的研究表明可以通过施加磁场来有效调控体系的电导。
除了双链的DNA分子,还存在多条链组成的DNA分子。G4-DNA分子含有四条链,其中只含有碱基G,相邻的碱基G之间由两个氢键相连接,4个碱基G连接成正方形,然后一层层堆垛成中空的管状结构。由于G4-DNA只含有一种碱基G,是一种非常有序的结构,而且有四条螺旋通道,所以G4-DNA有可能有较好的导电特性。G4-DNA拥有特殊的管状中空结构,管径为2nm,这与Hod等人所研究的原子环的直径相当。所以,G4-DNA的电导有可能具有类似的磁场响应。在本文中我就研究了利用磁场来调控G4-DNA电导的可能性。
2.1选取适当的参数,我们首先计算了“金属／G4-DNA／金属”分子器件中的基本输运性质。发现体系的电导跟费米面处的分子能级和电极与G4-DNA间的连接方式密切相关。当连接方式处于干涉加强时,体系电导处于较大值,而当连接方式处于干涉相消时,体系电导处于较小值。体系的电导还会随着G4-DNA的链长出现周期性的奇偶震荡。
2.2施加上一个平行于G4-DNA轴向的磁场时,体系的电导会受磁场的调制。通过计算电子的透射率曲线,可以看出加磁场后透射峰会发生劈裂。检查G4-DNA的能级,发现原本处于费米面处二重简并的能级在磁场下简并解除,并且关于费米面对称分布。随着磁场的增大,简并解除的能级间距离越来越大。
2.3如果体系处于干涉加强的连接方式,那么施加1T左右的磁场时,体系的电导会下降。相反,如果体系处于干涉相消的连接方式,施加磁场后,体系的电导会增加。在有限偏压和有限温度下,我们仍然可以利用量级为1T的磁场来有效调控“金属／G4-DNA／金属”体系的电导。
2.4界面耦合越弱,则体系对磁阻的响应越敏感。我们还计算了普通双链DNA分子中的磁场响应,发现磁场的作用可以忽略。
DNA (Deoxyribo Nucleic Acid) is a long polymer made from repeating units called nucleotides. It is an important large organic molecule. It contains the genetic instructions used in all living organisms. The main element in DNA molecule is C, H, O, N and P. Organisms can copy the characters of father generation to filial generation from DNA, then organisms complete its inheritance of biological characters. So DNA is also called genetic material. Watson and Crick discovered the famous double-helix structure of DNA in 1953. DNA molecule contains two strands made of sugars and phosphate groups joined by ester bonds. Every sugar is connected with one kind of base. These two strands are intertwined and anti-parallel with each other. The bases are connected with the corresponding bases obeying the complementary base paring law. This is, Adenine pairs with Thymine only and Guanine pairs with Cytosine only.
Since Watson and Crick discovered the double-helix structure of DNA, on the one hand, the research on its function as the biological gene brought about a new era of biology, on the other hand, the potential conductivity of DNA attracts people in other areas. As early as 1962, Eley and Spivey suggested that DNA may conduct current. There areπ-electron orbits perpendicular with the plane of the base, and the interplay of these orbits may induce the potential conductivity. This potential conductivity attracts interests from biology, physics and chemistry etc. On the one hand, the conductivity of DNA has significant importance on its damage and repair. On the other hand, if DNA can conduct current it is the most appropriate candidate for molecule electronics. Because DNA can assemble in solution with the base pairing law, we can build circuits "from bottom to up". The basic unit of traditional integrated circuits is the transistor. The transistor can be switched on or off to complete its function. There are millions of transistors in the micro-processor which is only several square centimeter. So the miniaturization of transistor is needed to enhance the integration level, but the continuing miniaturization in traditional Si based semiconductor is limited by the instability of the transistor. The concept of molecular electronics is to assemble circuits from molecules. In this way the high level of integration and miniaturization can be realized. Recently it is possible to directly detect the conductance of single molecule which is connected between two electrodes. DNA molecule has the special self-assemble property, so it has valuable engineering application in high accuracy circuits
As the rapid development of nano-technology, many direct charge transport experiments on DNA have connected DNA molecules between two metal electrodes to detect the current. These experiments give the direct charge transport information of DNA, and the results can verify the possibility of DNA being the material for molecular electronics. The outcome results of these kinds of experiments are controversial. It is found that DNA can be an insulator, a semiconductor, a conductor or even a superconductor. This diversity of conduction comes from the property of DNA itself and the environments. DNA has various sequences and shapes. The temperature, counter ions in solution, the interface between DNA and electrodes may affect the conductivity of DNA.
Because the experiments on direct charge transport in DNA give many controversial results, many theoretical works focus on the explanation of this charge transport phenomena. There are two categories of theoretical methods:ab initio calculation and model methods. Ab initio calculation takes the complex structure and various interactions into account. The types and positions of the atoms in the system is needed in the calculation. The experimental, empirical or semi-empirical parameters are not needed. So ab initio can give the electronic and lattice structure of DNA exactly. The model method can grasp the main features of DNA molecule. With choosing proper parameters this method can be used to investigate the physical picture and directly clarify the essence. The model method may have analytic solutions sometimes. So this can reduce the amount of calculation. These two kinds of methods are related to each other and work together to arrive the final solution of the problem.
We can use electric and magnetic field to modulate the current in the traditional semiconductor transistors. The purpose is to switch or amplifying the signal. In DNA electronics whether we can use electric or magnetic field to modulate the charge transport properties is what we are interested in. So we investigate the possibility to modulate the charge transport in DNA molecular device based on the earlier researches. These works can be a guide to the experiments on DNA electronics and expand the application scope of DNA molecular devices.
1. Electric field modulated Tunneling Magnetoresistance in a DNA molecular device
Magnetic tunnel junction (MTJ) is extremely important for magnetic device application. Conventional MTJ sandwiches a thin nonmagnetic material between two ferromagnetic metals. The electric resistance of MTJ is sensitive to the relative magnetization of the ferromagnetic electrodes which results magnetoresistance. It is possible to change the resistance by controlling the magnetizations direction of the ferromagnetic electrodes through an external magnetic field.
Organic materials have the advantage of weak spin-orbit and hyperfine interactions, which preserve spin-coherence over times and distances much longer than that in conventional metals or semiconductors. Such features make them the suitable materials for spin injection and transport. DNA is also used to explore the quantum spin transport. Zwolak and Ventra firstly showed that a DNA spin valve can be realized. They studied the DNA molecule contacted with Ni and Fe electrodes. They showed that magnetoresistance values of as much as 26% for Ni and 16% for Fe electrodes can be observed. They suggested the larger magnetoresistance for the Ni electrodes than Fe is due to the larger mismatch between spin-up and spin-down Fermi velocities of Ni with respect to Fe. Wang and Chakraborty further studied the spin flip effect on spin transport in a DNA molecule. They found an enhancement of magnetoresistance for a weak spin flip coupling while an oscillatory behavior for a strong spin flip coupling. Recently, another interesting phenomenon was reported based on a poly(G)-poly(C) DNA device. When a perpendicular electric field is applied on a DNA molecule, Malyshev predicted that the charge current through the molecule will be suppressed. The reason is that transverse electric field induces change of the electronic structure of a DNA molecule, which results in an adjustable conducting tunneling along the double stranded helix.
In a DNA based molecular device, investigation about a transverse electric field or ferromagnetic electrodes broadens the underlying application of the device. In this paper we try to include a transverse electric field into the FM/DNA/FM device. The purpose is to get an enhanced or adjustable MR of in a DNA device. 1.1 We firstly calculate the current voltage characteristics of "FM/DNA/FM" molecular device when the magnetization of the ferromagnetic electrodes are parallel and anti-parallel based on the Lattice Green Function method and Landauer-Buttiker theory. It is found that the current in the parallel case is larger than the anti-parallel case. The corresponding magnetoresistance is equivalent to the results of Zwolak and Ventra. 1.2 The current voltage characteristic of "Metal/DNA/Metal" molecular device is simulated when a transverse electric field is applied. In the case of short chain DNA, the depression of current is related to the relative direction of the transverse electric field. The current through the device begin to oscillate when the direction of the transverse electric field changes. The oscillation becomes more prominent when the strength of the transverse electric field increases.
1.3 We explore the possible usage of using a transverse electric field to enhance the Magnetoresistance of the FM/DNA/FM device. It is found the direction and strength of the transverse electric field will induce a modulation the MR. The MR of the device can be enhanced at some special direction and strength of the transverse electric field. This enhancement of MR is related to electronic structure change induced by the transverse electric field, which is a peculiar characteristic of a DNA device.
2. Magnetic field tuned conductance of DNA molecular device
Aharonov-Bohm effect reflects the quantum interference of electrons. Electrons acquire a magnetic field dependent phase because of its wave nature. Therefore the interference between two electron waves will be changed. Applying magnetic field is a convenient way of affecting the conductance in traditional mesoscopic devices. When electrons pass through a cylindrical mesoscopic conductor aligned in a magnetic field, the conductance will oscillate with the enclosed magnetic flux periodically. These kinds of oscillations have been observed in micrometer-sized thin-walled metallic cylinders and lithographically fabricated rings. As for molecular and nanoscale devices, some recent works show that it is possible to significantly affect its conductance by applying a moderate magnetic field.
Besides the double stranded conformation, other DNA derivatives constructed from guanine(G) nucleotides like the G-quadruplex DNA(G4-DNA), have been widely investigated recently. The tube like structure of G4-DNA is similar with micrometer-sized thin-walled metallic cylinders and carbon nanotubes. The diameter of G4-DNA molecule is about 2nm. This is comparable with the diameter of atom corral in the study of Hod et al. So it may behave the similar magnetic field tuned conductance. So we investigate the possibility of using magnetic field to modulate the conductance of G4-DNA.
2.1 The conductance has higher values at couplingⅠandⅢthan couplingⅡandⅣ. This is related to the interference of electron waves through different paths. They interfere constructively at couplingⅠandⅢ, but destructively at couplingⅡandⅣ. The conductance is also related to the energy level at the Fermi energy. Under constructive coupling the conductance will oscillate with the number of G4
2.2 When the magnetic field is absent, there is a two fold degenerate energy levels at Fermi energy which corresponds to the resonant transmission peak. When the magnetic field switches on, the original resonance peak splits into two peaks. We also check the energy level at each magnetic field. The two fold degenerate energy level at Fermi energy becomes degeneracy eliminated and splits into two separate energy levels. The separation between these two energy levels increases with the magnetic field, which is corresponding to the two separate transmission peaks.
2.3 When the coupling is interference constructive, the magnetic field can reduce the conductance of the device. When the coupling is interference destructive, the magnetic field can enhance the conductance of the device. At higher temperature and bias the tunneling electron is not limited at Fermi level. We can still modulate the conductance of the "Metal/G4-DNA/Metal" device with 1 Tesla magnetic filed.
2.4 the response to the magnetic field becomes more prominent when the interfacial coupling is weak. When the magnetic field is parallel with the axis of double stranded DNA, the electron travels through it clockwise only. So there is no obvious response to magnetic field.
随着纳米技术的发展,近年来有很多实验直接将单个或一束DNA分子连接到金属电极间,来测量它的电导。这些实验可以更直接的揭示DNA的导电性质,也为DNA作为分子器件的可能性进行了直接的验证。这类实验的结果比较复杂,揭示的DNA的导电性有绝缘性,半导体性,导体甚至超导体。导电性的多样结果一是源于DNA本身的性质,DNA有多种序列,分子形态；二是来源于外界环境的影响,温度,溶液中的离子,电极与DNA分子的接触等。
由于实验结果的多样性,许多理论工作致力于解释DNA中电荷输运的各种现象。现阶段大致有两类理论方法用以研究DNA分子中电荷输运：第一性原理计算和模型化计算。第一性原理计算方法可以将分子的复杂结构和各类相互作用考虑在内,在进行计算的时候仅需要给出原子的种类及其位置,不需要其他的实验的、经验的或者半经验的参量,可以准确的描述DNA分子的电子态和结构特性。模型化计算方法可以抓住研究对象的一些主要特征,通过模型化参数的调控,直观的反映研究对象的物理图像和本质,很多情况下甚至可以得到解析解。同时,模型化方法可以大大降低计算量,适合扩展系统的计算,其计算结果可直接与实验测量进行比较。这两类方法相互联系,又互为补充。
在半导体晶体管中,我们可以通过外加电场和磁场来调节通过晶体管的电流,从而实现电路的开关,放大等功能。在DNA分子器件里,是否可以通过外加电场或磁场来调控体系的输运性质,这将是我们感兴趣的问题。那么,综合前人的相关研究,我们研究了外电场和外磁场对DNA分子器件电荷输运的调控。目的是通过这些研究可以指导实验,拓展DNA分子器件的应用范围。
1.电场对DNA分子器件磁阻的调控研究
磁隧道结在磁性调控器件的应用中占有重要的地位。传统的磁隧道结是把-层薄的非磁材料夹在两层铁磁材料中间,它的电阻对于两边铁磁电极的磁化方向的排列很敏感。可以通过施加一个外磁场来控制铁磁电极磁化方向成平行或者反平行排列,从而控制通过磁隧道结的电流。
因为有机物具有弱的自旋轨道耦合和超精细相互作用,有机自旋电子学成为最近的研究热点,DNA作为有机物也被用来研究自旋相关的输运。Zwolak和Ventra首先研究了DNA作为自旋阀的可能性,他们分别研究了Fe和Ni作为电极的DNA分子器件中的磁电阻,发现Ni做电极时磁电阻可以达到26%,而Fe做电极时磁电阻可以达到16%。最近,Malyshev报道了横向电场对于DNA中电荷输运的影响,他选取人工合成的Ploy(G)-Poly(C)DNA分子为研究对象,发现横向电场压制了通过DNA分子的电流。这是一个非常有意思的结论,由于DNA分子固有的双螺旋结构,横向电场引发了一个螺旋势,这个螺旋势改变了DNA分子的电子结构,从而调整了DNA分子中的导电通道。
在DNA分子器件中,横向电场和铁磁电极的加入扩展了它可能的应用范围。所以我们尝试将横向电场引入“铁磁／DNA／铁磁”分子器件中,来研究横向电场对于DNA分子器件的自旋相关输运的影响。我们的目的是想得到一个增强或者可以调节的磁阻。
1.1基于晶格格林函数和Landauer-Buttiker公式,我们首先计算了“铁磁／DNA／铁磁”分子器件在两边铁磁电极磁化方向平行和反平行排列时的电流电压特性曲线。发现平行排列时的电流要大于反平行时的电流,而且根据此电流计算得到的磁阻与Zwolak和Ventra得到的磁阻相当。
1.2施加横向电场时,我们又模拟了普通金属作为电极的“金属／DNA／金属”分子器件中的电流电压特性曲线。发现在短链DNA分子中,横向电场对于电流的压制作用与横向电场相对于DNA分子的方向有密切关系。通过体系的电流随横向电场方向的改变出现了震荡的行为,随着横向电场的增强,震荡幅度变大。
1.3我们研究了“铁磁／DNA／铁磁”分子器件中的自旋相关输运。我们探讨了利用横向电场来调控该器件磁阻的可能性。发现横向电场的方向和大小都可以有效调控体系的磁阻。在某个特定的方向和大小的横向电场下,体系的磁阻增大。这个磁阻的调制是由于横向电场改变了DNA分子的电子结构,所以这种调制是DNA分子特有的性质。
2.磁场对DNA分子器件电荷输运的调控研究
Aharonov-Bohm效应反映了电子的量子相干性,由于电子的波动性,电子在磁场中获得了一个磁场相关的相位,所以电子不同波束间的相干性就会被改变。在传统的介观器件中,通过施加磁场来改变体系的电导是一种非常有效的手段。当电子通过一个圆环形状的介观导体时,如果在环内施加一磁场,那么电导将随着介观环所包括的磁通量发生周期性的震荡。在实验上,这种类型的震荡已经在微米量级的金属圆柱体和刻蚀成的环中观测到。对于分子级别的纳米器件,最近的研究表明可以通过施加磁场来有效调控体系的电导。
除了双链的DNA分子,还存在多条链组成的DNA分子。G4-DNA分子含有四条链,其中只含有碱基G,相邻的碱基G之间由两个氢键相连接,4个碱基G连接成正方形,然后一层层堆垛成中空的管状结构。由于G4-DNA只含有一种碱基G,是一种非常有序的结构,而且有四条螺旋通道,所以G4-DNA有可能有较好的导电特性。G4-DNA拥有特殊的管状中空结构,管径为2nm,这与Hod等人所研究的原子环的直径相当。所以,G4-DNA的电导有可能具有类似的磁场响应。在本文中我就研究了利用磁场来调控G4-DNA电导的可能性。
2.1选取适当的参数,我们首先计算了“金属／G4-DNA／金属”分子器件中的基本输运性质。发现体系的电导跟费米面处的分子能级和电极与G4-DNA间的连接方式密切相关。当连接方式处于干涉加强时,体系电导处于较大值,而当连接方式处于干涉相消时,体系电导处于较小值。体系的电导还会随着G4-DNA的链长出现周期性的奇偶震荡。
2.2施加上一个平行于G4-DNA轴向的磁场时,体系的电导会受磁场的调制。通过计算电子的透射率曲线,可以看出加磁场后透射峰会发生劈裂。检查G4-DNA的能级,发现原本处于费米面处二重简并的能级在磁场下简并解除,并且关于费米面对称分布。随着磁场的增大,简并解除的能级间距离越来越大。
2.3如果体系处于干涉加强的连接方式,那么施加1T左右的磁场时,体系的电导会下降。相反,如果体系处于干涉相消的连接方式,施加磁场后,体系的电导会增加。在有限偏压和有限温度下,我们仍然可以利用量级为1T的磁场来有效调控“金属／G4-DNA／金属”体系的电导。
2.4界面耦合越弱,则体系对磁阻的响应越敏感。我们还计算了普通双链DNA分子中的磁场响应,发现磁场的作用可以忽略。
DNA (Deoxyribo Nucleic Acid) is a long polymer made from repeating units called nucleotides. It is an important large organic molecule. It contains the genetic instructions used in all living organisms. The main element in DNA molecule is C, H, O, N and P. Organisms can copy the characters of father generation to filial generation from DNA, then organisms complete its inheritance of biological characters. So DNA is also called genetic material. Watson and Crick discovered the famous double-helix structure of DNA in 1953. DNA molecule contains two strands made of sugars and phosphate groups joined by ester bonds. Every sugar is connected with one kind of base. These two strands are intertwined and anti-parallel with each other. The bases are connected with the corresponding bases obeying the complementary base paring law. This is, Adenine pairs with Thymine only and Guanine pairs with Cytosine only.
Since Watson and Crick discovered the double-helix structure of DNA, on the one hand, the research on its function as the biological gene brought about a new era of biology, on the other hand, the potential conductivity of DNA attracts people in other areas. As early as 1962, Eley and Spivey suggested that DNA may conduct current. There areπ-electron orbits perpendicular with the plane of the base, and the interplay of these orbits may induce the potential conductivity. This potential conductivity attracts interests from biology, physics and chemistry etc. On the one hand, the conductivity of DNA has significant importance on its damage and repair. On the other hand, if DNA can conduct current it is the most appropriate candidate for molecule electronics. Because DNA can assemble in solution with the base pairing law, we can build circuits "from bottom to up". The basic unit of traditional integrated circuits is the transistor. The transistor can be switched on or off to complete its function. There are millions of transistors in the micro-processor which is only several square centimeter. So the miniaturization of transistor is needed to enhance the integration level, but the continuing miniaturization in traditional Si based semiconductor is limited by the instability of the transistor. The concept of molecular electronics is to assemble circuits from molecules. In this way the high level of integration and miniaturization can be realized. Recently it is possible to directly detect the conductance of single molecule which is connected between two electrodes. DNA molecule has the special self-assemble property, so it has valuable engineering application in high accuracy circuits
As the rapid development of nano-technology, many direct charge transport experiments on DNA have connected DNA molecules between two metal electrodes to detect the current. These experiments give the direct charge transport information of DNA, and the results can verify the possibility of DNA being the material for molecular electronics. The outcome results of these kinds of experiments are controversial. It is found that DNA can be an insulator, a semiconductor, a conductor or even a superconductor. This diversity of conduction comes from the property of DNA itself and the environments. DNA has various sequences and shapes. The temperature, counter ions in solution, the interface between DNA and electrodes may affect the conductivity of DNA.
Because the experiments on direct charge transport in DNA give many controversial results, many theoretical works focus on the explanation of this charge transport phenomena. There are two categories of theoretical methods:ab initio calculation and model methods. Ab initio calculation takes the complex structure and various interactions into account. The types and positions of the atoms in the system is needed in the calculation. The experimental, empirical or semi-empirical parameters are not needed. So ab initio can give the electronic and lattice structure of DNA exactly. The model method can grasp the main features of DNA molecule. With choosing proper parameters this method can be used to investigate the physical picture and directly clarify the essence. The model method may have analytic solutions sometimes. So this can reduce the amount of calculation. These two kinds of methods are related to each other and work together to arrive the final solution of the problem.
We can use electric and magnetic field to modulate the current in the traditional semiconductor transistors. The purpose is to switch or amplifying the signal. In DNA electronics whether we can use electric or magnetic field to modulate the charge transport properties is what we are interested in. So we investigate the possibility to modulate the charge transport in DNA molecular device based on the earlier researches. These works can be a guide to the experiments on DNA electronics and expand the application scope of DNA molecular devices.
1. Electric field modulated Tunneling Magnetoresistance in a DNA molecular device
Magnetic tunnel junction (MTJ) is extremely important for magnetic device application. Conventional MTJ sandwiches a thin nonmagnetic material between two ferromagnetic metals. The electric resistance of MTJ is sensitive to the relative magnetization of the ferromagnetic electrodes which results magnetoresistance. It is possible to change the resistance by controlling the magnetizations direction of the ferromagnetic electrodes through an external magnetic field.
Organic materials have the advantage of weak spin-orbit and hyperfine interactions, which preserve spin-coherence over times and distances much longer than that in conventional metals or semiconductors. Such features make them the suitable materials for spin injection and transport. DNA is also used to explore the quantum spin transport. Zwolak and Ventra firstly showed that a DNA spin valve can be realized. They studied the DNA molecule contacted with Ni and Fe electrodes. They showed that magnetoresistance values of as much as 26% for Ni and 16% for Fe electrodes can be observed. They suggested the larger magnetoresistance for the Ni electrodes than Fe is due to the larger mismatch between spin-up and spin-down Fermi velocities of Ni with respect to Fe. Wang and Chakraborty further studied the spin flip effect on spin transport in a DNA molecule. They found an enhancement of magnetoresistance for a weak spin flip coupling while an oscillatory behavior for a strong spin flip coupling. Recently, another interesting phenomenon was reported based on a poly(G)-poly(C) DNA device. When a perpendicular electric field is applied on a DNA molecule, Malyshev predicted that the charge current through the molecule will be suppressed. The reason is that transverse electric field induces change of the electronic structure of a DNA molecule, which results in an adjustable conducting tunneling along the double stranded helix.
In a DNA based molecular device, investigation about a transverse electric field or ferromagnetic electrodes broadens the underlying application of the device. In this paper we try to include a transverse electric field into the FM/DNA/FM device. The purpose is to get an enhanced or adjustable MR of in a DNA device. 1.1 We firstly calculate the current voltage characteristics of "FM/DNA/FM" molecular device when the magnetization of the ferromagnetic electrodes are parallel and anti-parallel based on the Lattice Green Function method and Landauer-Buttiker theory. It is found that the current in the parallel case is larger than the anti-parallel case. The corresponding magnetoresistance is equivalent to the results of Zwolak and Ventra. 1.2 The current voltage characteristic of "Metal/DNA/Metal" molecular device is simulated when a transverse electric field is applied. In the case of short chain DNA, the depression of current is related to the relative direction of the transverse electric field. The current through the device begin to oscillate when the direction of the transverse electric field changes. The oscillation becomes more prominent when the strength of the transverse electric field increases.
1.3 We explore the possible usage of using a transverse electric field to enhance the Magnetoresistance of the FM/DNA/FM device. It is found the direction and strength of the transverse electric field will induce a modulation the MR. The MR of the device can be enhanced at some special direction and strength of the transverse electric field. This enhancement of MR is related to electronic structure change induced by the transverse electric field, which is a peculiar characteristic of a DNA device.
2. Magnetic field tuned conductance of DNA molecular device
Aharonov-Bohm effect reflects the quantum interference of electrons. Electrons acquire a magnetic field dependent phase because of its wave nature. Therefore the interference between two electron waves will be changed. Applying magnetic field is a convenient way of affecting the conductance in traditional mesoscopic devices. When electrons pass through a cylindrical mesoscopic conductor aligned in a magnetic field, the conductance will oscillate with the enclosed magnetic flux periodically. These kinds of oscillations have been observed in micrometer-sized thin-walled metallic cylinders and lithographically fabricated rings. As for molecular and nanoscale devices, some recent works show that it is possible to significantly affect its conductance by applying a moderate magnetic field.
Besides the double stranded conformation, other DNA derivatives constructed from guanine(G) nucleotides like the G-quadruplex DNA(G4-DNA), have been widely investigated recently. The tube like structure of G4-DNA is similar with micrometer-sized thin-walled metallic cylinders and carbon nanotubes. The diameter of G4-DNA molecule is about 2nm. This is comparable with the diameter of atom corral in the study of Hod et al. So it may behave the similar magnetic field tuned conductance. So we investigate the possibility of using magnetic field to modulate the conductance of G4-DNA.
2.1 The conductance has higher values at couplingⅠandⅢthan couplingⅡandⅣ. This is related to the interference of electron waves through different paths. They interfere constructively at couplingⅠandⅢ, but destructively at couplingⅡandⅣ. The conductance is also related to the energy level at the Fermi energy. Under constructive coupling the conductance will oscillate with the number of G4
2.2 When the magnetic field is absent, there is a two fold degenerate energy levels at Fermi energy which corresponds to the resonant transmission peak. When the magnetic field switches on, the original resonance peak splits into two peaks. We also check the energy level at each magnetic field. The two fold degenerate energy level at Fermi energy becomes degeneracy eliminated and splits into two separate energy levels. The separation between these two energy levels increases with the magnetic field, which is corresponding to the two separate transmission peaks.
2.3 When the coupling is interference constructive, the magnetic field can reduce the conductance of the device. When the coupling is interference destructive, the magnetic field can enhance the conductance of the device. At higher temperature and bias the tunneling electron is not limited at Fermi level. We can still modulate the conductance of the "Metal/G4-DNA/Metal" device with 1 Tesla magnetic filed.
2.4 the response to the magnetic field becomes more prominent when the interfacial coupling is weak. When the magnetic field is parallel with the axis of double stranded DNA, the electron travels through it clockwise only. So there is no obvious response to magnetic field.
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