Electronic Transport And Spin Manipulation Of Single Organic Molecule Junctions

With technogical progress and social development,the electronic devices move towards miniaturization and high integration.Conventional microelectronic devices based on silicons and germaniums are facing the unprecedented challenge.The scientists take the goal to the molecules with smaller volume,in which organic molecules have been widely reseached owing to their good performance.By applying the first principles based on density-functional theory and the nonequilibrium Green's functions,we systematically study the electronic structures and electron trasnsport properties of the molecular systems,inculuding graphene nanoribbons,boron nitride nanoribbons,single organic molecule and graphyne nanoribbons.We mainly discuss the effects of doping,connected sites,chemical functionalization,adsorption and gate voltage on the electron trasnsport properties of the molucualar devices.The molecular rectifying behavior and switching behaviors based on spin-filtering effect,magneto resistance effect and field effect are observed.We investigate the spin transport properties of the molecular junctions constructed by a homologous series of 3d transition metal(II)salophens sandwiched between two gold electrodes.It is found that among the four molecular junctions only Co-salophen junction can act as an efficient spin filter distinctively.The conductance through Co-salophen molecular junction is dominated by spin-down electrons.Further studies show that the spin electronic transport properties are also strongly dependent on the contact sites.of the molecule to the electrodes,in which the spin current will srongly response to its tiny change,so as to the controllability on the conductivity of the device.We investigate the effect of boundary types on the rectifying behaviors in heterojunction composed of zigzag graphene and hexagonal boron-nitride(BNC)hybridized nanoribbons.The results demonstrate that the rectifying behavior is strongly dependent on the boundary types,while little affected by the width of BNC hybridized nanoribbons.It is noteworthy that the maximum rectifying ratio of the system at finite bias can be high up to orders of 107 in which atoms carbon in graphene nanoribbon are totally connected with atoms nitrogen in boron-nitride nanoribbon.It is due to the asymmetry distribution of the transmission coefficient near the Ef under positive and negative bias,which finally roots in more localization of LUMOs under positive bias and the absence of PDOSs of BNNR domain at the corresponding energy region under negative bias.The effect of boundary types on the spin transport properties of bare BNC is also studied.The perfect spin filtering and magnetoresistance effect with a ratio high up to 107 is observed.We investigate the spin-dependent transport properties of zigzag—edged?-graphyne nanoribbons(Z ? GYNRs)with symmetric and asymmetric edge hydrogenations,so as their response to external magnetic fields by the non-equilibrium Green's function method combined with the density functional theory..It is found that the symmetric model D-D shows an insulating state till the bias up to some value,while rectifying behavior can be observed in the asymmetric model M-D regardless of external magnetic fields.Moreover,the symmetric model M-M can be switched between a conducting state and a half-metallic state by tuning the magnetic fields,which arises from the matching or mismatching between the ? or?*states of the two ribbons by tuning the magnetic fields.These results indicate that these systems can be designed as multifunctional molecular d spintronic evices,which is important to further improve the level of integration of future atomic-scale circuits.We investigate the transport properties of the single molecule DBTDT sandwiched between two gold electrodes,focusing on the effects of gate voltages.It is found that the gate voltages have much influence on the device,which caused the HOMO molecular orbital moving towards the Fermi level,resulting in the reduction of the HLG.Meanwhile,the positive and negative gate voltages have the distinct effects on the transport properties,in which the former can increase the transport while the latter weaken it.Hence the device can be switched between the high and low conductive states so that it can act as a gate-controlled current switcher.All of these can be helpful to the electronic transport in real molecular devices.