Theoretical Study On Spin Transport Properties Of Iron-based Molecular Devices

With the rapid development of information technology,the size of electronic devices approaches the nanometer and even sub nanometer scale.The miniaturization of traditional silicon-based semiconductor electronic devices is about to reach the limit.People begin to explore and find the processing methods and candidate materials of a new generation of information technology.Molecular electronics,which constructs devices at the atomic and molecular scale,has become a frontier research field at home and abroad.As one of the most promising components in molecular spintronics,spin crossover(SCO)complexes have attracted more and more attention because of their magnetic bistability.In this paper,the first principle calculation method combining density functional theory and nonequilibrium Green's function will be used to construct molecular devices based on Fe(?)spin crossover system.The internal transport mechanism of molecular devices will be explored from the aspects of changing the types of substituents,molecular electrode bridging atoms and electrodes in transition metal complexes,predict and explain the changes of molecular configuration and the influence of external environmental factors on the transport behavior of molecular devices,and put forward feasible design schemes of functional molecular devices,so as to construct and optimize functional molecular spintronic devices,such as molecular spin valve,molecular spin switch,molecular spin filter,single molecule spin logic gate,etc.The main research contents of this paper are:We explore the electronic structure and transport properties of a four-coordinate bis(formazanate)Fe(?)SCO complex with two different spin configurations(S=0 and S=2),by performing extensive density functional theory calculations combined with non-equilibrium Green's function technique.The calculated results reveal that the spin transition between the HS and LS states can be achieved by changing the Fe–N bond lengths and the dihedral angle between the ligand coordination planes under external stimuli,which is in good agreement with previous experimental measurements.To explore the transport properties,the model molecular junction was constructed by sandwiching the Fe(?)-N₄ SCO complexes with different spin configurations between two periodic Au(111)electrodes.According to the calculated transport results,the currents through the molecular junctions with different spin configurations are both low when the bias is less than 0.1 V.Whereas,as the bias above 0.1 V,the current through the HS-configured molecular junction increases rapidly and is significantly higher than that of the LS-configured molecular junction.The bias-induced shift of the transmission peak contributed by the perturbed HOMO towards the Fermi level is responsible for the rapid increase in the current through the HS-configured molecular junction.Moreover,the transport properties of the SCO complex with the HS state are dominated by the spin-up electrons,indicating a strong spin-filtering effect.Based on this Fe(?)SCO complex,a bifunctional molecular device integrating switching and high spin-filtering efficiency can be constructed,marking its promising application in molecular electronics and spintronics.(1)The electronic structure and transport properties of tetra coordinated bis(methoxylate)Fe(?)SCO complexes with two different spin configurations(S=0 and S=2)were investigated by density functional theory calculation combined with nonequilibrium Green's function technique.The calculated results show that under external stimulation,the spin transition between HS and LS states can be realized by changing the dihedral angle between Fe-N bond length and ligand coordination plane,which is very consistent with the previous experimental results.In order to explore its transport properties,Fe(?)-N₄ SCO complexes with different spin configurations were sandwiched between two periodic Au(111)block electrodes to construct a model molecular junction.According to the calculated transport results,when the bias voltage is less than 0.1 V,the current through molecular junctions with different spin configurations is very low.When the bias voltage is greater than 0.1 V,the current of molecular junction in HS configuration increases rapidly,which is significantly higher than that in LS configuration.The shift of the transmission peak to the Fermi level caused by the disturbed Homo is the reason for the rapid increase of the current through the HS configured molecular junction.In addition,the transport properties of SCO complexes with HS state are mainly controlled by spin up electrons,indicating that they have a strong spin filtering effect.Based on this Fe(?)SCO complex,a bifunctional molecular device with switching and high spin filtering efficiency can be constructed,which indicates that it has broad application prospects in molecular electronics and spintronics.(2)The molecular junctions of Fe(?)-N₄ SCO complexes with different spin configurations and periodic Au(100)nanowire electrodes were studied by density functional theory and nonequilibrium Green's function.The results show that the current of Fe(?)-N₄ SCO device with Au nanowire electrode HS configuration has good spin filtering effect and negative differential resistance(NDR)behavior.Under small bias voltage,the current of spin down electrons passing through HS complex Fe(?)-N₄ is very weak,no more than 20 n A.The current is mainly contributed by spin up electrons.With the increase of voltage,the current trend first increases and then decreases.The current reaches the highest value at 0.2 V(1577.3 n A),and then begins to decline,and the peak valley ratio is 606.The electron donor and electron withdrawing substituents(B-OCH₃,C-OH,D-NH₂,E-CH₃,F-F and G-NO₂)were selected to replace the h in the complex Fe(?)-N₄ molecule.The results show that the spin filtering effect and NDR of devices B and D are better,and the current change trend of spin up electrons in HS configuration first increases and then decreases,It reaches the maximum at about0.25V(3036 n A,2514 n A).The types of bridging atoms,i.e.anchored atoms,have a great impact on the transport capacity of the electronic structure devices of HS.After replacing the anchored atom s with Se and Cl atoms,it is found that the conductivity of HS configuration complexes decreases sharply,and the highest current values are only71 n A and 26 n A respectively.Spin filtering effect and NDR behavior can hardly be observed through the current of HS complexes,It shows that the connection effect of Se and Cl atoms is far less than that of S atoms.After replacing Au nanowires with Ag nanowires,it was found that the conductivity of HS complex also decreased sharply,with the highest value of only 15.46 n A,resulting in the current through LS complex being greater than that through HS complex.These theoretical findings show that the complexes Fe(?)-N₄ in HS configuration of s-connected Au nanowire electrode and the devices modified by electron donor groups B-OCH₃ and D-NH₂ have potential application value in the design of molecular spintronic devices with spin filtering effect and negative differential resistance(NDR) effect.