Theoretical Design And Mechanism Study Of Single-molecule Rectifiers And Spin Filters

In recent decades,with the development of the minimization of electronic devices,traditional classical theories have been no longer applicable.It's necessary to introduce quantum theories to study electronic properties of devices.Currently,molecular devices in nano scale have been drawing people's extensive concern.And a variety of functional molecular devices have been designed and synthesized theoretically and experimentally such as molecular rectifiers,molecular switches,molecular spin filters and molecular memory etc.Among these devices,as important basic components in nanoscale circuit,molecular rectifiers and molecular spin filters,have attracted a great deal of attention in recent years.However,core molecules,as important parts of molecular devices,have a great effect on rectification and spin filtering properties of molecular devices due to their different intrinsic properties.Recently,arylethynylene molecules and aromatic chain molecules with different conjugation have been synthesized experimentally,and their applications in design of molecular devices have been systematically studied experimentally or theoretically.Even so,there are still many issues,such as how to improve performance of rectifiers and spin filters as well as how to explain the working mechanism and so on.In this thesis,based on density functional theory(DFT)with non-equilibrium Green's function(NEGF)method,influences of conjugation in central bridge in donor-bridge-accepter(D-B-A)molecular devices consisting of arylethynylene thiolate molecules on rectification properties and effects of conjugation in aromatic chain molecules on spin filtering properties have been investigated,respectively.Main contents and corresponding results of this thesis are as follows.1.The theoretical study on the effect of conjugation on performance of rectification in D-B-A molecular devices comprising arylethynylene thiolate molecules.Asymmetric geometry of molecules can affect spatial distributions of frontier molecular orbitals as well as alignments of corresponding molecule energy levels with energy bands of electrodes,and then further affect performance of rectification of molecular rectifiers.Thus,how to improve rectifying performance of molecular rectifiers by designing asymmetrical molecules is a crucial issue.For arylethynylene molecules,we employ asymmetrical terminal groups(thiol group and cyano group)to introduce asymmetry in the molecules and to design the central bridging fragment with different conjugation,obtaining three kinds of D-B-A molecules which are cross-conjugated 9,10-anthraquinone(AQ),linearly-conjugated anthracene(AC),and broken-conjugated 9,10-dihydroanthracene(AH),respectively.Effects of conjugation in center bridging fragment on rectification of D-B-A arylethynylene thiolate molecules are explored.Calculated results show that both the rectifying performance and rectification direction are highly dependent on conjugation properties of the central bridging segment.When the central bridge changes from crossed conjugation to linear conjugation,rectification performance is improved and the rectifying direction is inverted.More interestingly,when the bridge fragment changes from linear conjugation to broken conjugation,the rectification performance of molecular junction is enhanced remarkably and the inverted rectification ratio rapidly increases from 3.7 to 58.2.Further analysis reveals that the broken conjugation of the central bridging fragment induces strongly localized frontier molecular orbitals,resulting in obvious asymmetric evolutions under positive and negative bias voltages,which is responsible for the great enhancement in rectification ratio for AH.2.Effects of conjugation strength of aromatic chain molecules on the spin filtering properties of molecular devices.The angle between adjacent aromatic rings in aromatic molecule has an obvious effect on molecular charge transport,and the relationship between conductance and angle is G?cos~2?.So how does the angle affect spin filtering properties once the spin degree of freedom of electrons is taken into consideration?In order to answer this question,the molecular devices comprised of aromatic chain molecules and graphene nanoribbons are considered investigated here.In aromatic chain molecules,the magnitude of the angle between adjacent benzene rings directly impacts the strength of molecular conjugation.Zigzag graphene nanoribbon(zGNR)with finite width is magnetic,which is ultilized widely in the field of designing spin molecular devices.In this work,spin molecular devices are constructed using aromatic chain molecules sandwiched between two zGNR electrodes with the width of 8 atoms to study effects of conjugation strength in aromatic chain molecules on spin filtering properties.Calculated results show that conjugation strength of aromatic chain molecule can greatly influence the spin filtering effect.The weakly conjugated aromatic chain molecular junction shows spin-up dominated filtering effect when two z GNR are antiparallely spin-polarized.While the strongly conjugated aromatic chain molecular junction shows spin-down dominated filtering effect when two zGNR are parallely spin-polarized configurations.Further analysis reveals that the change of torsion angle between two benzene rings alters intensity of conjugation as well as spatial distribution of spin-resolved electronic states under different bias voltages,resulting in distinctly different spin filtering effect in molecular junctions.This thesis consists of five chapters as follow.The first chapter is preface,including a simple introduction of molecular electronics and molecular spintronics as well as the recent development of molecular rectifiers.In the second chapter,we briefly introduce theoretical methods(DFT and NEGF)that are widely used for calculating the electronic structure of multi-electron system and electronic transport properties of molecular devices and we also describe the self-consistent process when they are combined to calculate electronic structures of molecular devices.In the third and fourth chapters,we focus on presenting the works conducted by using the theoretical method introduced above.In chapter three,we have studied influences of conjugation of the central bridge in the donor-bridge-accepter(D-B-A)arylethynylene thiolate molecule on rectification of molecular devices,finding that when the conjugation of the bridge fragment is broken,the rectification performance is remarkably enhanced and direction of rectification is inverted.In chapter four,we have explored effects of conjugation strength of aromatic chain molecules on spin filtering properties of molecular junctions,finding that the molecular junctions with different conjugation strength of aromatic chain exhibit completely different spin filtering properties when electrodes are parallely/antiparrallely spin-polarized.The final chapter includes a conclusion about the content above and a prospect for functional molecular devices.