Studies On Mechanism Of High/low Conductance Switching In Stretching/compressing Processes Of Molecular Junctions

With the continuous development and improvement of single-molecule experimental technology and theoretical methods,the design and research of molecule-scale electronic devices have received great attention and have become a hot research topic.It has been found that the conductance of some molecular devices can be freely switched between high and low steady states by properly controlling of the electrodes.According to the different physical mechanism of this phenomenon,more and more important molecular devices such as molecular switches,molecular memory or molecular sensors can be designed by using the corresponding molecules.In recent decades,scientific researchers have carried out many explorations and studies on bistable molecular devices,not only looking for molecular materials with bistable properties,but also trying to realize the controllable and reusable bistable transitions.The most important thing to achieve this goal is to first clarify the inherent physical mechanism of the high/low bistable conductance conversion phenomenon of the molecular junctions.In order to explore the feasible way of using single molecules to achieve high/low conductance switching and understand its inherent physical mechanism,we developed the molecule-junction adiabatic simulation method based on first-principles calculations,and presented the configuration-evolution processes of molecular junction by computational simulations.The changes in the configurations of the molecule-electrode interfaces provided an in-depth understanding of the mechanism that leads to changes in the electrical transport properties of the molecular junctions during the stretching/compression processes.Particularly worthy of attention is that through this technology,the unique physical process corresponding to the high/low conductance switching phenomenon generated during the stretching/compression of the bipyridine molecular junction has been successfully solved.The investigations are mainly divided into the following two parts:(1)High/low conductance switching of 4,4'-bipyridine molecular junctionsThe electron-transport properties of 4,4'-dipyridine molecular junction in the stretching/compressing processes are studied based on ab initio calculations.In the study,the electrode interface configurations that may be generated during the stretching and compressing with the scanning tunneling microscopy-break junctions(STM-BJ)technique are considered.The numerical results show that the arising of the high/low conductance switching phenomenon of the molecular junction is due to the abrupt changing of the molecule-electrode interface configuration in the stretching/compressing processes of the molecular junction.When the electrode gap is relatively large,the molecules tend to adsorb on the tip atom of the electrode which results in a low conductance state.On the one hand,when the molecular junction is compressed,the coupling between the ? electron of the pyridyl and the electrode is increased.At the same time the pyridyl group produces a special lateral driving force,which pushes the gold atom at the tip of the electrode aside,and causes the molecule adsorbing on the second layer of gold atom vertically.Therefore,the molecular junction shows high conductivity.Due to the elastic relaxation of the molecular junction during the stretching/compression process,the system configuration will remain relatively stable during the period of elastic configuration changes.When the force between the molecule and the electrode reaches the threshold to break the N-Au bond,a nonelastic configuration change occurs,which leads to the observation of high/low conductance switching in the experiment.Our results not only reveal the intrinsic physical mechanism of high/low conductance switching phenomenon of 4,4'-bipyridine molecular junction,but are also helpful in identifying the electrode interface configuration at the atomic scale level.(2)The influence of side group substitution on the switching properties of molecular junctionsWe further carried out numerical simulations to study the effects of side substituent group on the high/low conductivity switching properties of 4,4'-bipyridine-based molecular junctions.The results show that when the side group is small,its influence on the configuration evolution of the molecular junction system is small.When the side group is large,it will produce strong steric hindrance,which hinders the pyridine ring moves closely to edge of the electrode.When the side group shows strong electronegativity,it tends to couple with the electrode and hinders the conductance switching.When the side groups contain benzene rings,the conductance switching phenomenon can be restored,which due to that the ? electron orbital can couple with the tip atom of the electrode and restore the unique lateral driving force of the pyridyl end on the tip gold atom.This discovery not only solves the incomprehensible phenomenon in the experiment once again,but also further clarifies how to control the conductance switching of the molecular junction by chemical design and substitution,which is helpful to guide the design of functional molecular devices with conductance switching performance.This article is divided into five chapters.The first chapter briefly introduces the development of molecular electronics,and discusses the problems existing in the current research and the content of this article.Chapter two briefly introduces the theoretical methods used in the studies,including the density functional theory,non-equilibrium Green's function method and onedimension transmission combined with three-dimension correction approximation.In Chapter three,we discuss the phenomenon of conductance switching of 4,4'-bipyridine molecular junction under mechanical control.In Chapter four,the effect of side substituent group on the high/low conductance switching phenomenon of molecular devices is studied.The fifth chapter summarizes the work and prospects of the whole paper,and analyzes the innovation and shortcomings of this paper.