First Principles Investigation On Molecular Electronic Devices With Graphene Nano-electrodes

For organic molecular devices with controllable structure,assembly cutting and longer spin life,electrode material and the interactive properties of molecular interface and electrodes play a crucial role on electron injection and transport.It is difficult to experimentally control the contacting interface between conventional metal electrodes and molecule,which results in poor conclusion reproducibility and a great difficulty of comparison with theoretical calculation.Recently,graphene,silicene,phosphorene and other two-dimensional structures have been successfully prepared,and they have definite conformation with molecular interface.Moreover,there exit many excellent features,such as high Dirac electron mo-bility,large coherent length,easily controlled electronic structure and so on.We concentrate attention on spin-dependent charge transport properties for several typical organic molecules between two graphene nanoribbon(GNR)leads by us-ing a combination of density functional theory(DFT)and non-equilibrium Green's function(NEGF)formalism based on ab initio calculation method of quantum chemistry in this dissertation.Some interesting results on the corresponding molec-ular junctions have been obtained,which is different from the one constructed by metal electrodes.Besides,the physical and chemical analysis on the transmission coefficients,local density of states(LDOS),band structure of electrode,molecular projected self-consistent Hamiltonian(MPSH)eigenstates and frontier orbitals are used to expound the relevant microscopic mechanisms of spin-dependent charge transport.The purpose of our thsis is to provide more selective electrode materi-als for the exploration of new molecular spintronic devices and the realization of experimental preparation.We divide full thesis into seven chapters,including the third to six chapter for our own work.We primarily introduce the research progress of spin molecular electronics and devices.The microstructure and electrical properties of graphene,graphyne and its nanoribbons,as well as the research progress and present situation of the corresponding molecular electrodes are described in detail.Secondly,this paper briefly discusses the basis and background of the project,as well as main research content and scientific significance.In addition,the quantum transport theory and numerical calculation methods of nano/molecular systems are introduced,includ-ing the first principles calculation method,DFT,Green function method and the related ab initio software package.In chapter 3,we study spin transport property for a molecular junctions consisting of an oligo(p-phenylenevinylene)(OPV)molecule sandwiched between two zigzag-edged GNR(ZGNR)leads.Obviously,a number of electrical func-tions,involving switching,spin-filtering,spin-diode and negative differential re-sistance(NDR),are numerically found in our proposed molecular junctions by conformational regulation between OPV molecule and ZGNR leads.By analyzing the spatial distribution of LDOS and spin transmission coefficients,the formation mechanism of molecular switch effect is elaborated.Further,the performance of spin-filtering and-rectifying is resulted from the asymmetry distribution of the molecular orbitals in central region as well as the coupling between the OPV molecule and ZGNR leads.In chapter 4,we investigate the spin electron property for a molecular junc-tion of an OPV molecule without or with different side groups between two ZGNR leads.Moreover,the influences of NDR and spin-rectifying in 1-V characteristics are revealed and explained for our proposed molecular junctions.The analysis imply that obvious NDR behavior comes from the conduction orbital being sup-pressed under certain bias voltage,while the rectifying effect is resulted from the asymmetry distribution of the highest occupied molecular orbital(HOMO)or the lowest unoccupied molecular one(LUMO)as well as the degree of coupling between the molecule and leads.Interestingly,the transport property of molecular junction can be improved by adding amino-nitro side groups to OPV molecule.The NDR behavior can be much enhanced for OPV molecule with NH2 side groups,and the rectifying can be improved for OPV molecule with NH2 and NO2 side groups.To our surprise,the peak to valley ratio(PVR)of NDR behavior can be much en-hanced and a molecular rectification effect which offers rectifying ratio more than three orders of magnitude up to 2863 by introducing amino and nitro side groups to OPV molecule,respectively.In chapter 5,we investigate the spin-resolved charge transport on a molecu-lar junction consisting of a single pyridine-linked(PDL)molecule embedded be-tween two ZGNR leads,where a 4,4'-bipyridine,4,4'-vinylenedipyridine and 4,4'-ethylenedipyridine molecules are considered,respectively.Our result demonstrates that the spin-charge transport can be modulated by performing different mag-netic configuration in the ZGNR leads.Specifically,we find that the proposed PDL molecular junctions exhibit several interesting behaviors,involving(dual)spin-filtering,rectifying,NDR and magnetoresistance behavior.For the junction consisting of a 4,4'-bipyridine molecule with proper magnetic configuration in two ZGNRs,a perfect spin polarization with filtering efficiency up to 100%can be ob-served,the maximum value of rectification ratio can reach up to 104,and the PVR of NDR is up to 328,respectively.Furthermore,it is of interest to note that the magnetoresistance ratio for this junction can go up to 106%.Besides,the physical mechanisms for those phenomenons are revealed and analyzed by the evolution of the frontier molecular orbital,the spin-resolved transmission spectrum associated with the LDOS around the EF at zero bias and the MPSH level.In chapter 6,so as to study the impact of symmetric and asymmetric edge hy-drogenation or fluorination acted on a-graphyne nanoribbons,we design Z-shaped armchair-edged a-graphyne nanoribbon is sandwiched between two zigzag-edged a-graphyne nanoribbon(Z?GYNR)electrodes with parallel and antiparallel mag-netic configuration.Our result illustrates that the edge passivation have a sig-nificant impact on the Z-shaped ?GYNRs junctions and the electron transport of nanoribbons depends on edge state.Furthermore,the edge fluorination can enhance charge transport but the edge double fluorination can suppress the cor-responding edge transport channel.In particular,there exists obvious NDR effect in our designed molecular junction with PVR reaching up to 4696.Finally,the corresponding physic mechanisms are presented for these phenomena.