Theoretical Study On Formation Mechanism And Conductive Properties Of Nano-Junction

With continuous miniaturization of functional semiconductor electronic devices,micro functional electronic devices with stable and controllable performance,which are constructed on the nano-scale level or even on single-molecule level as so-called nano device or molecular device,are expected.The study on designs and properties of nano-and molecular functional devices is a hot research issue in the world.In the past two decades,people have carried out a lot of theoretical and experimental research work in this field.Since the size of the device is only nanometer-scale,it is very sensitive to external influences,among which,the uncontrollability of the electrode-molecule connection interface is an important factor to affect the properties of molecular device.In order to understand the interaction between electrodes and nano functional structure?or molecule?as well as the controlling technique of interface configuration,and guide the experimental fabrication to build more stable molecule devices with strong manipuility,on the bases of ab initio calculations and experimental results,the atomic migration mechanism on the interface of gold nano-junction were studied,and the interactions between electrodes and molecules in nano-junctions were calculated.The main factors affecting the connection of electrode-molecule interface were discussed.Our calculations are not only helpful to systematic understanding of the experiment results,but also give constructive suggestions for experiments successfully constructing functional nanostructures.The thesis mainly includes the following five parts:?1?Atomic"Memory"by Controlling Atoms in Gold Nano-JunctionThe gold nano-junction system was constructed experimentally and theoretically.The process of atomic migration in the gold nano-junctions and the conductance of the junction were calculated by Density Functional Theory?DFT?combined with non-equilibrium Green's function?NEGF?method.By analyzing the minimum energy path of the atomic migration process on the surface of electrode contact,two steady-state with similar energy,but 1 G₀ and 2 G₀ conductance separately,and a sub-stable state with higher energy and 1 G₀ conductance are obtained.It was found that when the electrode bias is greater than 50 mV,the surface atoms with a charge of about 0.8 e can migrate over the energy barrier of the sub-stable state and switch between the two steady-state by electric field with help of vibration.If the electrode bias is less than the bias threshold,the surface atom can not obtain enough energy to pass through the energy barrier,then the conductance state of the system will remain at its original state.Therefore,a"write"operation can be performed by atomic transfer at a higher bias voltage,and the conductance measurement can be seen as a"read"operation at a bias lower than threshold voltage.Thus,an atom scaled memory was obtained.Combined with the experimental measurements provided by Dr.D.Xiang,the bias threshold for the conductance converting between 1 G₀ and 2 G₀ states of gold nano-junction is about 48 mV,which is well consistent with our theoretical calculation.Since vibration plays an important role in the atom migration process,this results in the nano memory needing to operate at an appropriate temperature and pressure circumstances.?2?Fabricating Nano-scale Gap by Field-Aided Atom Migration?FAAM?TechnologyBased on the ab initio calculations,we develop a strategy named as field-aided atom migration?FAAM?method to accurately fabricate atom-sized gap between gold nanoelectrodes.To understand the mechanism of this strategy,the configuration evolutions of gold nano-junction in the stretching/compressing processes are calculated.The numerical results show that,in the repeatedly stretching/compressing process,the gold atoms bridged between two electrodes are likely to form single-atom chains.Moreover,the lattice vacant positions are also easily generated in the stretching/compressing processes,which make it possible for the gap generation by field-aided atom migration.In the field-aided atom migration?FAAM?,the external field can exert driving force,enhance the initial energy of the system and decrease the barrier in the migration path,which makes the atom migration feasible.The conductance and stretching/compressing forces,as measurable variables in the stretching/compressing processes,present very useful signals for determine the time to perform FAAM.This strategy overcomes the difficulty that to fabricate nano-gaps with precise dimension without move electrode,which is very significant in circuit integration by molecular devices in future.According to this desirable strategy,gold nano-gaps with dimension of 0.38±0.05 nm are successfully fabricated in the experiment,which are in a good agreement with our calculations.?3?Tip-Configuration Effect of Gold Electrodes on The Conductance Properties of Aniline Molecular JunctionThe evolutions of aniline molecules in the stretching processes of molecular junctions are calculated based on density functional theory.The numerical results show that the connections between the-NH₂ end of aniline molecule and the gold electrodes are significantly affected by the geometries of the electrode interfaces.When the amino end is connected with the top gold atom of cone-shaped electrode,the electronic structure of the N atom in-NH₂ group is converted from sp²hybridization to sp³ hybridization,which results in stronger couplings between molecule and electrodes.When molecule is connected to plane electrode,the end amino group maintains its sp² hybridization,and the interaction between molecule and electrode is weak.For this case,the molecule and the electrode can only attract each other by van der Waals force.Moreover,the gap between plane electrode and amino end group is wider than that between cone electrode and amino group.Thus,an evident barrier between electrode and amino group is presented,which further hinders electrons transport from electrode to molecule and further to the other electrode,and results in the lower conductance of the molecular junction.According to the OTCTCA method combined with DFT calculation,the conductance of the Cone-Molecule-Cone junction is about 1.5 times of that of the Plane-Molecule-Plane junction.Therefore,by changing the electrode configuration,the connection between the electrode and the amino molecule can be optimized,which further enhance the conductance and the stability of the molecular junction.?4?Differences of Interaction Between Molecule and Electrode Induced by Different Terminals of MoleculeBy using DFT combined with the OTCTCA method,the energies,forces,strain factors and conductances of molecular junctions composed of amino-terminal1,4-diaminobutane molecule or the pyridine-terminated 4,4'-dipyridine with cone or planar electrodes are studied.It was found that the conductance of the system for pyridine-terminated molecule between the cone electrodes is about one order of magnitude smaller than that for the molecule between the planar electrodes.When an isolated gold atom is presented on the surface of the planar electrode,two conductance plateaus with higher and lower conductance values respectively are presented in the conductance trace during stretching process of the molecular junction.Thus,pyridine molecules can be used to identify the configuration of the electrode interfaces.For 1,4-diaminobutane molecule,the N atom in the-NH₂ terminal connected with surface gold atom of electrode by sp³ hybridized bond.Moreover,the terminal N atom can grasp this surface Au atom and make it protrude out of electrode surface and slip on the surface.However,when the molecular junction is broken by stretching force of 1.1 nN,this surface Au atoms can still return to the original lattice position.The conductance of the1,4-diaminobutane molecular junction does not show evident change with the change of the interface structure of the electrode.Therefore,the structural changes of the electrode interface have little effect on the conductivity of 1,4-diaminobutane molecular junction.?5?Hydrogen Bond Rearrangement Induced Conductance Oscillation in Bimolecular JunctionOn the bases of DFT calculations combined with OTCTCA method,the evolutions of bimolecular junction with 1,4-diaminobenzene?M1?and4,4'-diaminobiphenyl?M2?sandwiched between two gold electrodes are studied with the stretching of electrode.The numerical results show that if two electrodes connect with a same molecule in the bimolecular junction,the effect of the H bond between the two terminal amine groups on the conductance of the junction can be neglected,which indicates that the H bond interaction between amine terminals of two molecules has little influence on the coupling between Au and-NH₂.However,when the two electrodes connect with two different molecules,the H bond between the amine terminals of two molecules will become one part of the the electron transport channel of the"electrode-M1-M2-electrode".For this circumstance,the H bond between the amine terminals can alternatively break and reconnect when change the electrode distance,which further affects the conductivity of the system,and results in conductance jumping in the streching process of the molecular junction.At a specific electrode distance,the H bond between the amino groups can repeatedly break and reconnect spontaneously by the vibration inducing of the system,which leads to the spontaneous oscillation of the conductance of the bimolecular system.Although this phenomenon can be observed in experiments,the probability of large-scale conductance oscillations occurring in experiments is generally very small due to the limitation of electrode distance.There are eight chapters in this thesis.The first chapter gives a brief review on the development of molecular electronics and the correlative problems currently faced.The second chapter discusses density functional theory?DFT?and the methods we used to calculate electron transport properties of nano-junction systems.The third chapter focuses on the surface gold atom migration on the surface of pristine gold nanocontact under the action of electric field,and further designs atomic-scale"memory"device applying the phenomenon of field-induced atom migration.In the fourth chapter,the stretching/compressing process of gold nanowires is simulated and the configuration corresponding to the quantum conductance is found.Moreover,the mechanism of field-aided atom migration?FAAM?is systematically discussed.Use gold nanowires and by the strategy of FAAM,nano-gaps with precise dimensions are successful fabricated without moving electrode.The influences of electrode geometries on the conductance properties of amino molecules are discussed in the fifth chapter.In chapter six,the single point energy,force,strain factor and conductance in the stretching processes of molecular junctions with cone-shaped or planar-shaped electrodes for the molecules with different hybrid types of N atoms are discussed.The seventh chapter studies the effect of H-bond interaction between molecular terminals on the conductance property of bimolecular junction,which contain 1,4-diaminobenzene and 4,4'-diaminobiphenyl.The eighth chapter summarizes the above chapters and looks forward to the follow-up work.