Theoretical Study Of Field Effect On DNA Molecular Devices

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 circuitsAs 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 deviceMagnetic 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 deviceAharonov-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 G42.2 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.