Theoretical Design Of Functional Molecular Spintronic Devices And Studies On Their Spin Transport Properties

Within this continuously developing information age,miniaturization of electronic components provides an effective way to enhance circuit integration and then to improve performance of electronic equipment,which has been drawing a lot of researching focuses.However,as the size of the components is approaching nano or even sub-nano scale,classical physical theories,current processing technologies and large energy consumption will limit further miniaturization of traditional silicon-based electronic components.As one of solutions to this problem,molecular spintronic devices,which use molecules as functional kernels and aim at realizing certain functions by taking advantage of the spin degree of freedom of electrons,become a research hotspot at present.Compared to traditional silicon-based devices,molecular spintronic devices have smaller size,larger data storage and faster response with lower power consumption,which thus show a promising application prospect.Up to now,researchers have carried out a great many of works to design and fabricate molecular spintronic devices,and also uncovered some novel electron transport phenomena on the molecular-scale platforms made of these devices.In order to advance the development of molecular spintronic devices,devices with higher performance and more abundant functions remain to be further explored,and effects of various factors on spin transport properties in devices also remain to be investigated.Using the fully self-consistent nonequilibrium Green's function method in conjugation with density functional theory,in this thesis,the effect of proton transfer,the kind of transition metal?TM?atoms in transition metal complexes,molecule-electrode bridging manners and gas species in the air environment on spin transport properties of molecular junctions have been investigated,and then some functional molecular spintronic devices with higher performance have been designed.The main contents and results are as follows:For geometrically symmetric molecular junctions along the transport direction,the parity of electronic wave functions plays an important role in determining electron transport properties.We have designed a geometrically symmetric high-performance molecular spin switch consisting of heterocyclic molecules sandwiched between two magnetic zigzag graphene nanoribbon?ZGNR?electrodes.The numerical results show that under parallel or antiparallel spin configurations between two electrodes,the device can be switched between ON and OFF conducting states triggered by intramolecular proton transfer reaction in molecular kernels,and the maximum ON/OFF ratio can reach 341.Furthermore,significant bipolar spin rectifying effect with the giant rectification ratio of ca.10⁵ can be observed under antiparallel spin configuration.Further analysis indicates that both obvious switching effect and rectification are closely related to parity match between wave functions of molecular kernels and electrodes.Proton transfer reaction taking place on molecular kernels alters the parity of wave functions of molecular kernels,and thus changes parity match between them and wave functions of electrodes,resulting in the switch of conducting states.On the other hand,different parity match among wave functions from the left electrode,the molecular kernel and the right electrode under positive and negative bias is the reason for bipolar spin rectify effect in the device.Fabricating molecular spintronic devices with elementary functions to replace traditional silicon-based ones in logical circuits is an effective way to scale down components and then to enhance integration of electric circuits.However,further assembly of logic units from these basic functional molecular spintronic devices still encounters huge difficulties according to current manufacturing techniques.On the contrary,single-molecule logic gates,which are directly built by a single molecule or a set of combined molecules,will have promising prospect in future spin electric circuits owing to simple structures and fast computing speed.Single-molecule junctions consisting of two serially connected transition metal dibenzotetraaza[14]annulenes?TM?DBTAA?,TM=Fe,Co?sandwiched between single-walled carbon nanotube?SWCNT?electrodes via carbon atomic chains?CACs?have been constructed and corresponding spin transport properties have been investigated.The calculated results show that a close dependence of spin-resolved current-voltage characteristics on spin configurations between the left and right molecular kernels as well as the kind of TM atoms.By changing spin configurations,spin-down conducting states in the junctions can be switched between“ON”and“OFF”.It's ascribed to the change of match between spin states from the left and right TM?DBTAA?molecules under different spin configurations.In addition,when TM atoms change from Fe to Co atom,conductance of the junction can be enhanced,which is because the transport mechanism is transformed from non-resonant to resonant tunneling regime.More importantly,if spin polarization of two TM?DBTAA?molecules in the junctions is defined as input signals,and meanwhile spin current or total current is defined as output signals,according to responses of output signals to input signals,the designed junctions can work as“NOR”or“XNOR”logic gate.With the help of spin-dependent Seebeck effect,molecular spin caloritronic devices can convert waste heat?or temperature difference?into electricity with zero emission by taking advantage of electron spin,which has been increasingly arousing researchers'attention especially in this era with energy crisis and environmental problem.Spin transport properties of molecular junctions,which are constructed by transition metal porphyrin?TM?Pr?,TM=V,Mn,Fe,Co?sandwiched between SWCNT electrodes via CACs,have been theoretically investigated.The numerical results show that there is robust Fano resonance in electronic transmission spectra for one of two spin components in each junction.Further analysis indicates that Fano resonance arises from quantum interference between the spin“quasi-discrete state”mainly localized on TM?Pr?molecules and delocalized states inherently contributed by porphyrin-based molecular junctions.Taking full advantage of the Fano resonance,spin-dependent Seebeck coefficients of the junctions for one spin component can be effectively strengthened.Interestingly,it is revealed that eigenvalue,spatial distribution and parity of frontier molecular orbitals of TM?Pr?molecules play a key role in the generation of Fano resonance,which is closely related to the kind of TM atoms.It further provides the possibility to optimize spin-dependent Seebeck coefficient by TM atoms.Based on obtained results,the designed molecular junctions can work as molecular spin caloritronic devices.In the first work of this thesis,we have designed a high-performance single-molecule spin switch by utilizing proton transfer reaction.However,there are few reports on how other factors influence conducting states when proton transfer reaction takes place.As molecule-electrode bridging manner can affect coupling between molecules and electrodes or transmission path of electrons,which play an important role in electron transport properties in devices,exploring the effect of proton transfer reaction on spin transport properties under different bridging manners is one of the valuable topics in molecular spintronics.The molecular spintronic devices have been constructed by a single 5,11-dihydrodibenzo[b,g][1,5]naphthyridine-6,12-dione?Epi?molecule embedded between two magnetic ZGNR electrodes through CACs with different molecule-electrode bridging manners.By means of first-principles molecular dynamics method,intromolecular proton transfer reaction happening in the Epi molecule is confirmed at first.Then,using nonequilibrium Green's function method in combination with density functional theory,spin-dependent transport properties for the designed devices are investigated.The calculated spin-resolved current-voltage curves show that both devices manifest remarkable spin filtering effect before and after proton transfer reaction,which is attributed to strong coupling between spin-up edge states of electrodes and electronic states of Epi molecules.However,under different bridging manners,the control ability of proton transfer on conducting states in the devices is different.When the Epi molecule is connected to ZGNR electrodes with 4,7-sites?named as A bridging manner?via CACs,the electron transport properties of molecular junction are hardly affected by the intramolecular proton transfer.On the contrary,the conductance of the molecular junction is significantly modulated by the intramolecular proton transfer when the Epi molecule is connected to ZGNR electrodes with 4,4'-sites?named as B bridging manner?.This is because compared to A bridging manner,the change of conjugation characteristics of Epi molecule induced by proton transfer reaction is much more easier to modulate transmission pathway of electrons under B bridging manner,which makes the reaction more effective to control conducting states.How to realize the injection of spin current into nonmagnetic molecular devices is one of focuses in molecular spintronics.Exploring the effect of gas molecules in the air on spin transport properties of nonmagnetic molecular junctions provides a solution to this issue.A molecular junction consisting of a nonmagnetic Ni?DBTAA?molecule sandwiched between SWCNT electrodes has been built and the effect of adsorption of a gas molecule?N₂,O₂,H₂O or CO₂?in the air on spin transport properties of the designed junction has been investigated.The calculated results show that the adsorption of N₂,H₂O and CO₂ maintains current spin-degenerate.While the adsorption of O₂ induces the generation of spin polarized current in the junction.The analysis demonstrates that as the ground state for O₂ is triplet state,the transfer of spin-down electrons from the junction to unoccupied orbitals of O₂ breaks spin degeneracy of the junction.The spin-resolved transmission pathway shows that the adsorbed O₂ deeply takes part in spin transport process in the junction,enhancing ability of transmission for spin-up electrons through DBTAA ligand while simultaneously strengthening intensity of reflection for spin-down electrons in molecular kernel.As a result,the junction shows remarkble spin polarized effect.This work offers a method to inject spin currents into nonmagnetic molecular devices.This thesis consists of eight chapters as follows.The first chapter includes the introduction of backgrounds and research methods for molecular spintronics as well as some functional molecular spintronic devices.In chapter two,density functional theory,which is the mainstream tool to study electronic structures,and nonequilibrium Green's function method,which is the state-of-the-art way to calculate electron transport properties in nano devices,have been briefly introduced.Meanwhile,the combination of density functional theory and nonequilibrium Green's function method to self-consistently get electron transport properties has also been presented.Some calculations and the corresponding results obtained by using the aforementioned method are given in chapters three to seven.In chapter three,taking full advantage of the change of parity match between wave functions from electrodes and molecular kernels triggered by proton transfer reaction,the molecular spin switch with high ON/OFF ratio has been designed.In the next chapter,we have designed the molecular spin logic gates by selecting TM?DBTAA?molecules as functional units.By defining different input/output signals,the logic gates can realize“NOR”or“XNOR”Boolean operations.The chapter five investigates the effect of TM atoms in TM?Pr?molecules on spin-dependent Fano resonance in transmission spectra.Utilizing Fano resonance,spin-dependent Seebeck coefficient can be effectively enhanced.In chapter six,we have designed molecular spin filters and revealed that the control ability of proton transfer on spin transport properties is influenced by bridging manner between the molecular kernel and electrodes.In the following chapter,we have explored the change of spin transport properties in the molecular junction when a gas molecule adsorbs on the molecular kernel.Spin-degenerate currents through the junction turn to be significantly spin polarized when the junction adsorbs O₂.The final chapter draws a conclusion for the whole work of the thesis and gives a prospect on the development of molecular spintronic devices in the future.