Design Of Molecular Spintronics Devices Based On Several Transition Metal Complexes

With the development of electronic devices towards“smaller,faster and colder”,when the size of the device is getting smaller and smaller,its size will soon reach the molecular or atomic scale.At this scale,quantum effects cannot be ignored,and this will lead to serious obstacles to the further miniaturization of traditional silicon-based electronic components.In the background,putting some molecules or even a single molecule between two electrodes to achieve some of the most basic digital circuit functions（such as molecular conductance switching,rectification and information storage）has become a hotspot of research and application in the field of microelectronic devices.At the microscopic and even atomic and molecular scales,material design methods based on first-principles,especially ab initio methods based on density functional theory（DFT）,have begun to play an increasingly important role in the design of emerging materials.In this paper,the spin-resolved electron configuration and quantum transport properties of several transition metal complexes were studied by DFT combined with non-equilibrium Green’s function（NEGF）.By clarifying the quantum behavior and mechanism in the process of electron transport,their applications in the field of molecular spintronics can be predicted.The main body of the paper contains the following four parts:First,we have researched the electronic configuration and transport properties of the complex Cp Fe·corannulene under the different coordination modes.After structural relaxation,it was found that there were three energy-stable isomers,of which the two isomers coordinated in theη⁶mode show obvious spin polarizated characteristics,and their transmission channels of spin-down are almost closed.However,the isomer Cp Fe·η⁵（exo）-corannulene coordinated in theη⁵mode shows unpolarized characteristics,but shows the largest current value.Through the investigation of the transition states,it was found that the temperature and thermodynamic control can be realized by the switching between three isomers,thereby it can realize the function of molecular spin conductance switch.Specifically,under low temperature,isomers with high spin states contributed the main conductance characteristics of these devices,and the electrons transport depends on the transmission channel of spin-up.As the temperature increases,the complex can change from a high spin state to a low spin state.The current intensity ratio between high and low spin states is about 3:1.Under higher temperatures,as the concentration of diamagnetic isomers increases,the tunneling channels of two spin states are switched on,and the current intensity of spin up and down is close to each other,then the device is stable at the“off”state of the conductance.Second,the effect of molecular structural stretching on the spin electron configuration and transport properties of the double nickel lantern complex[Ni₂（SOCH）₄（NCS）₂]was studied.The results showed quintet as the complex’s ground state and the electronic structure exhibited high tolerance to structural stretching.By the investigation of transport properties,we found that negative differential resistance,rectification effect and perfect spin filtering effect can be detected in molecules with different sizes.Combining with the crystal field theory,the key factor of that the complex shows a quintet ground state deponds on theπbackbonding between axial NCS ligands and the central Ni（Ⅱ）ions.The d_xzand d_yzorbitals which is the source of the magnetic moments are occupied by unpaired electrons,and there is no overlap between the orbitals occupied by the unpaired electrons in the two central metal ions,so that the switching between high and low spin states（HS-LS）cannot be triggered by stretching of the complex.Therefore,in this bimetallic system,the high-spin state is protected by coherent spin coupling and is more energetically stable with different molecular sizes.Third,the transport properties of the binickel lantern complex[Ni₂（OOCH）₄（NCS）₂]with structural symmetry were studied.Compared with the asymmetric complex[Ni₂（SOCH）₄（NCS）₂],the symmetric complex exhibited the similar spin-polarized transport characteristics,but due to the symmetry of the structure,the conductance does not show rectification effect.At the same time,it was found that the change of bone ligands only had a weak effect on the spin electron configuration of the complex.The choice and properties of the axial ligand are the key factors that determine the electronic structure of the molecular ground state.The configuration is mainly controlled by the axial ligand NCS.Compared with the asymmetric complex,it can be seen that theπbackbondings between the axial ligands and the metal ions make metal ions exhibit high spin states,and the weak intermetallic coupling prevents the spin electrons from delocalizing between the metals and resists spin decoherence.The change of the framework coordination environment has no obvious effect on the electronic configuration,which also shows that the axis ligand is the key factor to determine the electronic configuration of the complex.The transmission eigenstates of molecular junctions indicate that the transmission of spin-down is mainly determined by the conductive LUMO orbital near the Fermi level at low bias,which is characterized by the overlapping of d_z2orbitals.As the bias increases,the alternation of transmission eigenstates within the bias window is the cause of NDR effect.Fourth,we have studied the electronic configuration and transport behaviors of a series of planar and twist metal porphyrin derivatives.Based on this,the influence of the presence of transition metal ions and the distortion of the geometric configuration on the electronic configuration and transport properties of the porphyrin derivatives are clarified.Compared with the monomer,the energy gap of the dimer is reduced due to the quantum confinement effect and the extension of the conjugated system.Zn porphyrin derivatives have similar electronic configurations as porphyrin derivatives;in Fe porphyrin molecules and derivatives,the d orbitals of the Fe ion give an important contribution to the composition of frontier orbitals.The porphyrin ring is injected into the spin polarization feature by hybridization with the d orbitals of Fe ion.In addition,the distortion of the structure delays the onset of the molecular junction NDR effect.The electron transmission pathways show that in the molecular junctions of porphyrins and derivatives,the electron transmission is mainly completed by the edge conjugated system.Even if Fe ions have obvious participation in frontier orbitals,the local current from the porphyrin ring to the metal ion is still very small,and the main function of Fe（Ⅱ）ions is to provide a paramagnetic environment for electron transmission.