Mechanism Research On Switch And Rectifier In Molecular Scale

By applying the density-functional theory and the nonequilibrium Green’sfunctions, we systematically study the electronic structures and electron trasnsportproperties of the molecular systems, inculuding graphene nanoribbons, boron nitridenanoribbons, organic single molecule and carbon nanotube. We mainly discuss theeffects of doping, chemical functionalization, adsorption, mechanical torsion andmagnetic ordering on the electron trasnsport properties of the molucualar devices. Themolecular rectifying behavior and switching behaviors based on spin-filtering effect,magnetoresistance effect and negative differential resistance effect are observed.We investigate the electronic transport properties of nitrogen-doped armchairgraphene nanoribbons devices. For the electronic devices, an N dopant is consideredto substitute the center or edge carbon atom of one electrode. The results show thatthe electronic transport properties are strongly dependent on the width of the ribbonand the position of the N dopant. Obvious rectifying effect can be observed and can bemodulated by changing the width of the ribbon or the position of the N dopant.Further studies show that the corresponding narrow matching region between theenergy bands of both electrodes and very weak coupling between the correspondingmolecular orbital and the doping state of the electrode lead to obvious rectifyingeffect of the device.We investigate the spin transport properties of edge hydrogenated zigzag edgedgraphene nanoribbon heterojunctions. Results show that a perfect spin-filtering effectand a rectifying behavior with a ratio larger than105can be realized bydihydrogenation, which can also be modulated by changing the widths of the twocomponent ribbons. Further analysis shows that these interesting effects are attributedto the intrinsic transmission selection rule of π and π subbands in zigzag edgedgraphene nanoribbon.We investigate the effect of the weak intermolecular interaction on electronictransport properties in a bilayer graphene nanoribbon device. The results show thatswitching and rectifying phenomena can be observed by adjusting the π πinteraction between two graphene nanoribbon molecules. It is suggested that thechange of the coupling strength between molecular orbitals and electrodes isresponsible for these transport phenomena.We investigate the electronic structures and transport properties of fluorinated zigzag-edged boron nitride nanoribbons. The results show that the transition betweenhalf-metal and semiconductor in zigzag-edged boron nitride nanoribbons can berealized by fluorination at different sites or by the change of the fluorination level.Moreover, the negative differential resistance and varistortype behaviors can also beobserved in such fluorinated zigzag-edged boron nitride nanoribbon devices. Theseresults indicate that these two systems can be designed as multifunctional molecular dspintronic evices, which is important to further improve the level of integration offuture atomic-scale circuits.We investigate the transport properties of manganese porphyrin-based spintronicdevices constructed by two manganese porphyrin molecules connected withp-phenylene-ethynylene group. The interesting spin filtering and magnetoresistanceeffects can be observed in the device. Especially, after the overlap of π channelsbetween manganese porphyrin and phenyl ring parts are broken, the spin filteringeffciency and magnetoresistance ratio of the device can be effectively increased.Moreover, electrically induced switching behavior based on negative differentialresistance is also observed in our model. The distribution difference of electronicstates for spin-up and spin-down states leads to the spin-filtering effect in the device,while electronic states mismatching in the various parts of the device is responsiblefor magnetoresistance and negative differential resistance effects.Moreover, We investigate the spin-dependent transport properties of aFe-porphyrin-like carbon nanotube spintronic device. The results show thatmagnetoresistance ratio is strongly dependent on the magnetic ordering of theFe-porphyrin-like carbon nanotube. Under the application of the external magneticfield, the magnetoresistance ratio of the device can be increased from about19%toabout1020%by tuning magnetic ordering of the Fe-porphyrin-like carbon. Ourresults confirm that magnetic ordering is a key factor for obtaining ahigh-performance spintronic device.