First-principles Investigation On Transport Properties Of Molecular Junctions With Graphene/Graphyne Electrodes

With the continuous development of science and technology,the emergence of micro-assembly technology and manipulation technology makes it possible to manipulate a single molecule on the nanoscale.Therefore,the manufacture of molecular devices with specific functions becomes a reality.Due to the limitation-s of size,the traditional silicon-based electronic devices can not be continuously miniaturized,and molecular electronic devices may be the best alternative to con-ventional silicon-based devices.The successful preparation of two-dimensional materials,such as graphene and graphyne,has attracted more and more people to pay attention to the molecular electronic devices with graphene electrodes because of the high electron mobility.In addition,the electronic structure of graphene and graphyne is very easy to control,and their molecular interface configuration is easy to determine.Therefore,they may be the most ideal molecular junction electrode material.In this dissertation,we concentrate on spin-dependent charge transport prop-erties for several typical organic molecular junctions by the combination of density functional theory(DFT)and non-equilibrium Green's function(NEGF).And we also discuss the effects of side groups,the molecular oxygen and connected sites on the transportation of electrons.Some interesting results of the corresponding molecular junctions have been obtained.The physical and chemical analysis of the transmission coefficients,local density of states(LDOS),the electron transmission pathways,molecular projected self-consistent Hamiltonian(MPSH)eigenstates and frontier orbitals are used to expound the relevant microscopic mechanisms of spin-dependent charge transport.These results are of great importance for un-derstanding and experimentally constructing molecular devices.The full text is divided into six chapters,the main contents are as follows:The first two chapters of this thesis mainly introduce molecular electronics and molecular devices,as well as the research progress and unique properties of molecular devices.The microstructures and electrical properties of graphene,graphyne and their nanoribbons are introduced in detail.The content,purpose and significance of this thesis are also illustrated.In addition,we present the theoretical basis and method used in this paper,including density functional theory and non-equilibrium Green's function.In chapter 3,we study the properties of electron transport of molecular de-vices composed of the fused oligothiophenes molecule or aromtic molecule.We studied the effects of amino groups,the oxygen absorption on molecule and con-nected sites on the transportation of electrons.The results show that molecular devices composed of oligothiophene molecules have obvious negative differential resistance effect,while the addition of amino makes the current increase.In par-ticular,the junction with side group shows more obvious NDR.We study the prop-erties of electron spin-charge transportation of an aromatic molecule sandwiched between two pyridines and graphene electrodes.The computational result shows that the multifunctions of perfect spin-filtering,spin-rectifying with efficiency ap-proaching nearly 100%,giant magnetoresistance with ratio up to 105 and negative differential resistance effects are exhibited in this all-carbon system.Importantly,these functionalities can be qualitatively adjusted by the absorption of the oxygen on molecule,the variation of the electrode initial magnetic orientations and the molecule pyridine connection,respectively.The physical and chemical mechanism-s are revealed and discussed in terms of the spin-resolved transmission spectrum,the evolution of the frontier molecular orbitals,the local density of states ai ound the Fermi level at zero bias,and the molecular projected self-consistent Hamilto-nian.Our conclusion may indicate a direction for designing all-carbon spintronic nanodevices based on graphene.In chapter 4,we explores the electronic structure and magnetic properties of zigzag-?-graphyne nanoribbons,and designs a new type of molecular device.The results show that the zigzag-?-graphyne nanoribbons have very similar properties compared with graphene.And the device we design has a variety of interesting effects,such as negative differential resistance effect,spin-filters effect,rectification.The mechanisms are explained by analyzing the spin electron reansmission spectra near the Fermi level,local density of states(LDOS),spin transport pathway.Graphene and graphyne have great potential for the applications of spintronic devices due to their exceptionally good electrical and mechanical properties in the pristine form.In chapter 5,we study the properties of spin-charge transportation for monolayer graphene/graphyne zigzag-edged nanoribbon heterojunctions by em-ploying the ab initio calculations,where two types of ?-and(6,6,12)-graphynes are considered,respectively.Our important result is that these heterojunctions can exhibit an interesting variation of magnetoresistane by applying ferromag-netic stripes or external magnetic fields onto the ribbons to initially orient their spin configuration.Specifically,the magnetoresistance ratio can be up to 104 for graphene/?-graphyne heterojunction,but only 300 for the graphene/(6,6,12)-graphyne one.In addition,the effects of spin-filtering and negative differential resistance are also observed in those heterojunctions.the maximum spin filtering efficiency can be up to 99%.The mechanisms are revealed and analyzed by the evolution of the spin-resolved electron transmission spectra and pathways around the Fermi level at zero bias.In chapter 6,a summary and outlook are presented.