| Recently, due to the development of various experimental technologies such as STM (Scan Tunneling Microscope) and SAM (Self Assembled Monolayer), molecular electronics has been an interesting and popular field, which is to construct smaller and faster electronic devices with the utilization of organic materials or even single molecules. Organic electronics has many advantages than traditional materials, for example, low cost and large-area fabrication, can be tailored with chemical or physical methods, easy to form stable contact with electrode. Due to the diversity organic material can be used as conductor or insulators. It has been proved that some conjugated polymers such as benzene and polythiophene may conduct current as molecular wires, while alkyl has large resistance as insulator because it only consists ofσbonds. Some more complex biomolecules, such as DNA, even showed different conducting behaviors including conductor, semiconductor and insulator in different experiments. It was also reported that molecular device may serve as conduct switch, diode or even memorizer, which is dependent on the experimental details.Although a lot of experimental researches about molecular devices have been extensively carried out, a complete theoretical understanding on electron transport in molecular device is necessary. Most of current theoretical calculations are based on the combination of ab initio calculation and non-equilibrium Green's function transport theory, such as the quantum chemical software TranSIESTA. Such calculations are able to give an exact description of electronic structure of small molecular device, and then calculate the electron transport property through an open quantum system. Based on the related studies in recent years, electron transport in molecular devices has been well understood including the transport mechanism and the possible mechanisms for some interesting phenomena. However, many issues in this field are still obscure. It has been indicated that during the electron transport, the molecular may be excited with contemporary lattice distortion due to the strong electron-lattice coupling. The effect of lattice distortion on molecular conductance is not well understood. The experiment has found that the thiophene-thiazole co-oligomer can serve as intrinsic molecular diode, but a clear physical explanation is absent. Furthermore, the complex interface between molecule and electrode is needed to be studied in detail. To get a comprehensive understanding for above issues, it is necessary to capture the intrinsic softness of organic materials as well as the nonequilibrium electron transport process.On the other hand, the further consideration in electron spin degree of freedom opens the field of organic spintronics. It is well known that the spin-orbit coupling and hyperfine interaction in organic materials are very weak. So the spin relaxation time in organic materials is relative long. The researches in organic spintronics are usually divided into two areas: One is the spin injection and transport from magnetic electrode to normal organic materials, such as spin injection from CMR to sexithienyl or Co/Alq3/LSMO spin valve structure. The spin polarized electron transport or large magnetic resistance has been obtained in above structure. Xie et al. have theoretically studied spin polarized injection and transport from magnetic electrode to organic materials with both classical and quantum method, where the softness of organic materials is emphasized.Another area is to utilize magnetic molecule to realized spin functionality. Organic magnet is the combination of organic material and magnet, which has been investigated in the past decade. Up to now, several organic magnets have been synthesized, such as organic ferromagnet poly-BIPO, which substitutes part of H atoms in polyethylene with magnetic side radicals to achieve magnetism. Previous studies focused on the exploitation of magnetic mechanism as well as effects of electron-electron correlation and boundary conditions on spin density wave in such material. However, electron transport through organic ferromagnet is seldom involved. There existsπorbital in organic ferromagnet, which can be used as conduct channels. In the presence of intrinsic spin correlation, electron transport in such materials should be spin dependent. It is meaningful to study to electron transport in organic ferromagnet, which is necessary for the further application of magnetic molecule in organic spintronics. To study electron or spin transport in organic device, the softness of organic material should be emphasized. In this doctoral thesis, by combining SSH (Su-Schrieffer-Heeger) model and Green's function method, we investigated the effect of various lattice distortions on molecular conductance. In addition, we calculated the nonequilibrium electron transport through thiophene-thiazole co-oligomer molecular diode and gave an explanation of rectification. With SSH+Heisenberg model, the spin polarized transport through organic ferromagnet was studied. Then the related phenomena of spin filtering and spin rectification was discussed and studied based on our proposed model. The detailed research and main results are given below:1. Effect of lattice distortion on the conductance of molecular wireIt has been indicated that the lattice distortion is possible during the electron transport process, and then influence molecular conductance. Some experimental phenomena, such as conducting switch, are related to the lattice distortion. Distinct from inorganic materials, in the presence of strong electron-lattice interaction, the lattice atoms in molecular will be dimerized. When there exists charge accumulation, polaron or bipolaron lattice distortion is also possible. In the case of odd-number chain, the dimerization leads to appearance of a kink in molecular wire. The effects of beforementioned lattice distortion on molecular conductance are studied respectively.1.1 It is found that in the presence of dimerization, the electron transmission spectrum is divided into two subbands by large Peierls gap. The transmission coefficient near the Fermi energy is reduced. It is because that the electron-lattice interaction in the chain localizes the electronic state near the Fermi energy, and then decreases the molecular conductance.1.2 The polaron-like or bipolaron-like lattice distortion will form when the molecule wire carries one or two extra electrons. In nanometer scale, the polaron (bipolaron) level is assisted for electron transport. In contrast with dimerization, the charged state has lower resistance and threshold voltage. The transition between ground and charged states may result in the switch behavior.1.3 For an odd-number molecular wire, the presence of kink localizes the electronic state at the Fermi energy. The wave function of electron state is localized in the middle of the chain, whose width is related to the strength of electron-lattice coupling. When the molecular length is in this range, the electronic state can work as valid conducting channel and the molecular conductance decreases quickly with molecular length. If the length is beyond this scope, the molecular conductance at Fermi energy is zero.2. Rectification mechanism in co-oligomer molecular diodesIntrinsic molecular diodes are the aim of both experimentalists and theorists, which constructs diodes with the asymmetry of molecular structure. An apparent rectification has been observed in thiophene-thiazole co-oligomers in recent experiments. However, a clear physical explanation for the intrinsic mechanism is still absent. Using nonequilibrium Green's function method and extended SSH model, we calculated the charge transport through such molecules and gave our explanation.2.1 With proper parameters the electronic structure of organic molecule at zero bias is calculated. The front molecular orbitals HOMO (highest occupied molecular orbital) and HOMO-1 are close to the Fermi energy of Au electrode. But they are localized in the thiophene part due to the lower ionization potential. Such orbitals have no contribution to the molecular conductance.2.2 Applying a positive bias, the induced electric potential distribution along the co-oligomer tends to match the molecular levels of the two segments. The hybridized HOMO-1 and HOMO become delocalized with the increasing bias and leads to the increase of current; a negative bias tends to enlarge the mismatch of their energy levels and form localized molecular orbital. The front orbitals have no contribution to current.2.3 The single-double alternate molecular structure and large Peierls gap induced by electron-lattice coupling is an essential requirement for co-oligomer to constitute diode and orbital hybridization. The lattice distortion under a bias is not apparent and not crucial to the rectification. 3. Spin polarized charge transport through organic ferromagnetDue to the combination of organic softness and magnetic property, the study of electron through organic ferromagnet is very meaningful. Here, we constituted an metal/organic-ferromagnet/metal sandwich device structure. Based on SSH+Heisenberg model, the spin-dependent transport through the model device is calculated with Landauer-Buttiker formula.3.1 At ground state, the radical spins form a ferromagnetic order. The current through the device is spin polarized. The spin polarization oscillates with the bias. The reason is that the spin-split electronic states begin to conduct in turn with the increasing bias.3.2 At a low bias, the spin polarization is near 100% and the device serves as a spin filter. The reason is the coexistence of large Peierls gap and spin split.3.3 The current spin polarization is sensitively dependent on the spin dependent interfacial coupling between magnetic molecule and electrode. By adjusting the interfacial coupling, both the magnitude and spin direction of the spin polarization may be changed.3.4 Effect of thermal fluctuation of radical spins on spin polarization is investigated. The deviation angles from ground state of each radical spins were simulated with a square-random model. It is found that the current polarization is reduced as the fluctuation becomes stronger and stronger.3.5 The organic ferromagnet will be excited when the spin direction of part of radical spins is reversed. With the increase number of flipped radical spins, the current and its spin polarization through the device may be adjusted.4. Spin-current rectification in organic mangetic/nonmagnetic co-oligomerDesigning molecular device with electron spin is an interesting topic now. Recently, the concept of spin-current rectification has been proposed. It has been theoretically predicted that spin-current rectification may be realized in a magnetic metal/molecule/nonmagnetic metal asymmetric structure. We proposed a new intrinsic spin diode model based on organic magnetic/nonmagnetic co-oligomer. Both the charge current and spin current through metal/co-oligomer/metal device were calculated. Two kinds of functionality may be realized as flowing:4.1 Parallel spin-current rectification without charge-current rectification. If the Fermi energy of electrode lies in the middle of the gap of co-oligomer, the charge channels are symmetric about the Fermi energy while the spin channels are asymmetric. Reversing the applied bias, a parallel spin-current rectification is realized with the polarized direction of spin current reversed, but the charge current is symmetric. The spin-current rectification comes from the asymmetric energy shift for different spin channels under different polarity of applied bias.4.2 Antiparallel spin-current rectification together with charge-current rectification. Adjusting the Fermi energy of electrode to break the asymmetry of charge channels, both spin-current rectification and charge-current rectification can be realized simultaneously. Reversing the bias, the spin direction of spin current is not changed but its amplitude is asymmetric. It is because that the electronic localization of conducting channel is asymmetric under different bias.
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