Theoretical Study On The Interface Structure And Electrical Transport Properties Of Nitrogen Terminal Molecular Devices

As electronic devices continue to have smaller dimensions,the design and study of molecularscale electronic devices has attracted considerable attention.In general,different molecular junctions exhibit different electron transport properties during their formation,many of those properties can be used to design functional devices in the molecular scale.In molecular electronics studies,experiment techniques,specific design processes and molecular structure can influence the transport properties of molecular devices.Over the past two decades,a variety of different molecular junctions have been investigated,among which the pyridine terminal molecule and the aminobenzene molecule have shown interesting properties.It has been found that the pyridine terminal molecules can produce a unique high/low conductivity transition during binding.In contrast,the different relative positions of the two amino terminals in the aminobenzene molecule lead to a significant difference in the transport properties of the molecule.In order to understand these interesting phenomena,we have performed calculations on molecular junctions based on the first-principle.The structural and electrical transport properties of pyridine molecular devices and aminobenzene molecular devices have been studied from the perspective of the architecture evolution process and quantum interference,respectively.The thesis is divided into the following main sections:1.Structure and evolution of the interface between 4,4’-dipyridine molecules and gold electrodesIn order to determine the possible interface structure between the pyridine molecule and the gold electrode,we first investigated the evolution of the molecule-electrode interface structure as the 4,4′-dipyridine molecule moves away from the gold electrode of different shapes.The four most likely electrode configurations were constructed in the calculations,considering that experimental measurements are generally performed using either the scanning tunneling microscope break junction(STM-BJ)technique or the mechanically controllable break junction(MCBJ)technique.Calculations show that for a triangular prismatic electrode,when the distance between the molecule and the electrode is small,the molecule tilts against the side of the electrode,and when the distance between the molecule and the electrode increases to a certain extent,the molecule adsorbs almost vertically on the second electrode,while the pyridine ring gives a lateral push to the tip gold atom,making it deviate from its original lattice position.With the increase of the distance between the molecule and the electrode,the molecule breaks from the second layer of gold atoms and connects to the tip of the gold atoms,and the second layer of electrodes to break the force required for about 1.3-1.5 nN,but the molecule and the tip of the gold atoms can withstand the maximum force of 0.8-1.0 nN.For the pure planar electrode,the maximum force between it and the molecule is just under 0.5 nN,while the molecule can withstand a force of 1.7n N when attached to an isolated gold atom on the surface of the planar electrode.According to the different electrode configurations and molecular forces,it can be inferred that molecular junctions that produce high/low conductance in the experiment,one electrode(the tip of the probe electrode)is a conical electrode,while the other electrode(the base electrode)should be a flat electrode with isolated gold atoms adsorbed on the surface.2.High/low conductivity conversion of pyridine terminal molecular junctions caused by unique interfacial structural changesAccording to the most likely electrode configurations in the experiment determined by the above studies,the unique high and low conductance conversion phenomena of pyridine molecules substituted by different side groups during the formation of molecular junction were investigated.The conjugated and non-conjugated side groups substituted pyridine terminal molecular junctions were constructed.Adiabatic stretching and compression simulations were carried out based on the first-principle,and the curves of their energy and force with the electrode distance were calculated.The calculations show that for both the 4,4′-dipyridine and 3,3′-dibromo-4,4′-dipyridine molecular junctions,significant high and low conductivity conversion is observed as the pyridine terminal can appear to be vertically adsorbed to the second layer of gold atoms.For the non-conjugated side groups,the oxygen atoms in the larger side branches of the molecule prevent the pyridine ends from being connected vertically to the gold atoms in the second layer of the cone electrode,thus preventing the appearance of a high conductance configuration,resulting in a system with only one conductance plateau.For the conjugated side groups,the aromatic hydrocarbons in the side groups prevent the oxygen atoms from interacting with the electrodes,restoring the high and low conductance transition phenomenon of the system.In particular,for the bending of charge transport paths due to large lateral bases,we have developed the calculation method based on the principle of one-dimensional transmission combined with three-dimension correction approximation(OTCTCA),which enables a better simulation of the electrical transport process.The study reveals a specific contact configuration between the pyridine terminus and the gold electrode,which is very different from that between the other anchoring groups and the gold electrode.This is crucial for the high and low conductance switching behaviour of pyridine terminals and their derivatives.Furthermore,the calculated forces for the high and low conductance states of the pyridine molecule system are consistent with experiment,which further confirms the reliability and scientific validity of the contact configurations and their evolution derived from the adiabatic stretching and compression simulations.Our research shows that adiabatic stretching and compression simulations can be extremely helpful in identifying the contact structure of molecular junctions and revealing their processes.3.Controlling the quantum interference effect of diaminonaphthalene molecules by using molecular terminal sitesThe conductance of NDA molecules with different terminal positions was measured experimentally to give an indication of the effect of different attachment sites on the conductance of the molecules.By Density Functional Theory(DFT)calculations,the effect of quantum interference phenomena on molecular conductance generated at different connection sites is also explained from the distribution of amplitude,phase and transmission eigenstates of the projected eigenvalues.The results show that for ortho-NDA and para-NDA molecules,there is still a large amplitude distribution at the junction with the right electrode.The electron wave incident from the left electrode follows the path of the NDA molecule on two sides through the molecule and arrives at the junction with a phase difference of 0 or 2π.Therefore,there is still a high probability that it will enter the right electrode through the intermediate region.While the amplitude of the metaNDA eigenstate vanishes to zero on the right side,the electron wave incident from the left electrode arrives at the connection with a phase difference of π-phase after passing through the molecule,so it shows a quantum interference phase extinction and the system has no obvious conductance plateau.The thesis is divided into five chapters: The first chapter introduces the development of molecular electronics and points out the progress and problems in the current research field.The second chapter introduces the theoretical methods used for the calculations,including density functional theory,non-equilibrium Green’s function method and one-dimension transmission combined with three-dimension correction approximation methods.The third chapter investigates the interaction between the pyridine terminal molecule and four different configurations of the electrode tip and identifies the most likely electrode configuration for the experiment based on the molecule-electrode interactions.The fourth chapter further investigates the mechanism underlying the high and low conductivity phenomenon of pyridine terminal molecules with different side group substitutions based on the determined electrode configurations.The fifth chapter investigated quantum interference phenomena by changing the junction sites of terminal groups,and studied the effects of this phenomenon on the conductance of molecules.The sixth chapter summarizes the research work and points out the innovative points of the study as well as the remaining problems.