| Several transport problems in molecular scaled sys tems and designing special molecular devices to realize useful transport properties are studied systematically by first principles based on the density-functional theory and the nonequilibrium Green?s functions. We mainly discuss the manetoresistance pheno menon in phthalocyanine molecule and in transition metal phthalocyanine molecules; the width effects on the transport properties of 6, 6, 12-graphyne nanoribbons and in which getting fine manetoresistance phenomenon, negative differential resistance effect and spin-filtering effect; Characteristics of classical Kirchhoff’s superposition law in carbon atomic wires connected in parallel and the stability problem of ?-graphyne nanoribbons.Chapter One introduces the research background of molecular electronics, the experimental methods and main theoretical methods which are used for the study of molecular electronic devices, and what will be studied in this thesis and its importance.Chapter Two focuses on computational methods and theoreical details. Firstly, based on the introduction of what the first principles is, we introduce two important methods used for the first principles: the Hartree-Fork approximation and the density functional theory(DFT). And then we show clearly how to m ake a combination between the density functional theory(DFT)and the nonequilibrium green function(NEGF) to calculate the molecular electric transport properties using the first principle. Finally, we describe the relevant programs we use in the calculation briefly.In Chapter Three, We studies the spin transport properties of singlepht-halocyanine based molecules in the environment of two infinite 8-zigzag-graphene nanoribbon electrodes. The giant magnetoresistance found in the hydrogen phthalocyanine system and in the TM-phthalocyanine systems is much bigger than that found by Schmaus and Iacovita in experiment. The absolute number of the excess spin up electrons of the chromium atom in the chromium-phthalocyanine molecule is bigger than that of other TM-atoms, which demonstrates that chromium-phthalocyanine molecule is the best choice to be used as spin filtering device compared to other TM-phthalocyanine molecules. The bias-dependent spin filter efficiency reaches nearly 100% at a wide bias voltage range(0.1, 1.0) V for anti-parallel chromium phthalocyanine configuration, which cercrifies the above anticipation. These results may be useful in the design of high performance spin filter and spin valve devices.In Chapter Four, unlike ?-graphyne and ?-graphyne that has hexagonal symmetry structure, the 6, 6, 12-graphyne does not have hexagonal symmetry but features two self-doped nonequivalent distorted Dirac cones, suggesting that the electronic and transport properties are even more amazing than that of graphene. So the electronic transport properties of zigzag 6, 6, 12-graphyne nanoribbons(6, 6, 12-ZGYNRs) are investigated by first-principles calculations. The results show that the two bands around the Fermi level of 4-6, 6, 12-ZGYNR are not flat but cambered, which is different from that of zigzag graphene and a-graphyne nanoribbons, and is also different from that of 5-6, 6, 12-ZGYNR. In non-magnetic states, the current across the 5-6, 6, 12-ZGYNR system is almost forbidden even at rather large bias voltages, while in 4-6, 6, 12-ZGYNR system, the current increases linearly with the increase of bias voltage. In spin-polarized state, fine spin filtering effect is gained in anti-parallel configuration of 5-6, 6, 12-ZGYNR system. Moreover, it is found that magnetoresistance exists in the 5-6, 6, 12-ZGYNR system, but not in the 4-6, 6, 12-ZGYNR system.In Chapter Five, to meet the trend of miniaturization in electronic devices, people have imaged electronic devices based on single molecules and carried out lots of interesting work. Special transport behaviors, such as diodes, switchers, spin filters, field effect transistors and magnetoresistance, which exist in the traditional semiconductor devices, are observed in various molecular constructed systems or even in a single molecule device. However, only a few works paid attention to the classical Kirchhoff’s superposition law in molecular scale devices and found only the inability of this law to account for the electron flow in the multilinker systems. The classical Kirchhoff’s superposition law is hard to realize in the molecular scale devices because the coupling between the juxtaposed molecules can lead to constructive or destructive quantum interferences. In view of this, we try to eliminate the quantum interference between the juxtaposed molecules by increasing the distance between them. Simple junctions of carbon atomic wire(s) coupled to zigzag graphene nanoribbon electrodes are chosen as our model. Intere stingly, fine Kirchhoff’s superposition law phenomenon is found when the distance between the two carbon atomic wires reaches 15.5 ?. At the distance 15.5 ?, the conductance for the double carbon atomic wire(DCAW) configuration is 1.96 times of that for single carbon atomic wire(SCAW) configuration and the current across the DCAW con figuration keeps nearly two times of that across the SCAW con figuration at the applied biases. In addition, the conductance superposition effect becomes better when the distan ce between the two wires increases further and the spin filtering effect is enhanced in the DCAW configuration.In Chapter Six, we enhance the stability of α-graphyne nanoribbons by dihydrogenation. Similar to graphene, the stability of Z?GYNRs is also very delicate, decreasing as the ribbon width increases. The energy difference between the ferromagnetic(FM) and antiferromagnetic(AFM) s tates of 12-Za GYNRs is just 2.8 me V. Here we investigate the dihydrogenation effects in Z?GYNRs and armchair ?-graphyne nanoribbons( A?GYNRs) systematically by the first-principles. For dihydrogenated Z?GYNRs, the energy difference between the AFM-FM configurations is greatly increased compared with that for monohydrogenated cases, and thus the AFM state of dihydrogenated Z?GYNRs becomes much more stable. Further, half-metallicity was displayed in the dihydrogenated 8-Z?GYNR at the electrical field strength range(0.04~0.07) V/?. At the electrical field 0.07 V/?, the FM- AFM?E in dihydrogenated 8-Z?GYNR exceeds 35 me V, which means that half metallicity can be stable even at room temperature. Thus the dihydrogenated Z?GYNRs will be more promising than monohydrogenated ones for spintronic devices.In Chapter Seven, the effect of the spin direction of Co atom in Cophthalocyanine molecule in manipunating spin filtering effects are investigated by spin-polarized density-functional theory calculations combined with the Keldysh nonequilibrium Green?s method. When not taking the magnetism of the Co atom into account, the bias depedent spin filtering efficiency is low and thus not good; when taking the magnetism of the Co atom into account, the difference between the spin up and spin down current is increased, especially for parallel case, when the spin direction of the Co atom parallel to the magnetic polarization of the left and right electrodes could improve the spin filtering effect greatly. When the polarization direction of the two electrodes was antiparallel and the polarization of Co atom w as upward, the configuration possesses almost perfectly spin-filter effect. So the spin direction of Co atom in Co- phthalocyanine molecule can manipunate the spin filtering effects greatly. The reason of the perfect spin filtering action was analyzed. |
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