Theoretical Study Of Transport Properties Through Fe-based Molecular Magnets

As an emergent sub-area of nanoscience and nanotechnology,recently,molecu-lar electronics has attracted enormous research attention since it holds promise for the next generation of electronic devices with enhanced functionality and improved per-formance.One of the central issues in this field is to design various functional molec-ular devices based on different single molecules.Here,we investigate the electron-ic structures,spin crossover and transport properties of Fe-based molecular magnets by performing extensive density functional theory(DFT)calculations combining with non-equilibrium Green's function(NEGF)technique.This thesis for Master degree contains the following four chapters.In Chapter 1,we briefly review molecular electronics by introducing the ex-perimental techniques(i.e.scanning tunneling microscopy,conducting atomic force microscopy,and mechanically controllable broken junction)and various functional molecular devices.In Chapter 2,after introducing the basic framework and conceptions of the den-sity functional theory(DFT)and corresponding non-equilibrium Green's function,we review several computational packages,which are adopted to calculate the electronic structures and transport properties in this thesis.In Chapter 3,Fe-based spin crossover(SCO)complexes have been attracted in-creasing attention due to their magnetic bistability between the high-spin(HS)and low-spin(LS)states since they are one of the most possible building blocks in molec-ular spintronics.Here,we investigate the electronic structures and transport properties of the Fe-based SCO magnet(namely,Fe-N4S2 complex)with the LS and HS states sandwiched between Au(111)electrodes by performing extensive DFT calculations combined with non-equilibrium Green's function(NEGF)technique.Theoretical re-sults clearly reveal that the conductance through the Fe-N4S2 complex with the LS state is less than that of the HS state.The obvious spin-filtering effect is observed in the calculated I-V curves of the proposed Fe-based SCO magnet junction with the HS state,and in which the current is mainly contributed by the spin-down electrons under small bias voltage.In addition,the predicted activation barriers reversibly switching between the LS and HS states agree well with the experimental observations.These theoretical findings suggest that this kind of Fe-based SCO magnets holds potential applications in molecular spintronic devices.In Chapter 4,we try to explore the electric field induced switching mechanism in Fe-N6 SCO complexes.In experiments,Fe-N6 SCO complexes with different dipole moments have been successfully prepared,and then the corresponding crystal field around the Fe(II)core can be effectively distorted and tuned by the applied electric field,leading to the SCO phenomenon.Using the DFT+NEGF method,we optimize the geometric structures of the Fe-N6 SCO complexes under different electric fields,build several molecular junctions with different anchoring configurations,and then ex-amine their transport properties.Unfortunately,we can not successfully explain the related experimental observations.