Theoretical Studies On Designs And Rectification Tuning Of Single-Molecule Heterojunction

With continuous development of single-molecule-control technique and electronic transport theory of micro-system,the design and investigation of molecule-scaled devices have become an essential topic for many countries in pursuit of the commanding heights of science and technology.While the theoretically design idea of single-molecule P-N junction was initially put forward by Aviram and Ratner in 1974,due to the limitation of experimental technology,this topic did not attract researchers'attention for nearly two decades.In order to make the molecular rectifier device be applied,it is of great significance to fully understand the working mechanism of the molecular rectifier device from the most basic physical perspective.Especially to analyze the key factors that have effects on its rectification performance,such as the interface configuration between the molecule and the electrodes,the coupling between the molecule and the electrodes,the relative position between the frontier molecular orbital and the Fermi level of the electrodes and so on,will provide a new idea for design of molecular rectifier device with ideal rectification properties.Because there are many different factors that affect the rectification properties of molecular devices,in order to have a better understanding of the rectification mechanism of molecular rectification devices and to design molecular rectification devices with ideal rectification properties,in this paper,the rectifying properties of PyridinoParacyClophane molecules?PPC?containing branch-gated groups are systematically studied by changing the functional groups in the side branches and the interface between the electrode and the molecule.The effects of branch-gated groups with different electronegativity and electrode interfaces with different configurations on the rectification properties of PPC molecules are inverstigated,and the intrinsic mechanisms affecting the rectification are analyzed.The studies not only find that the changes of electrode interface cause remarkable rectiflication inversion,but also provide an effective stretage to dramatically enhance the rectiflication ratio of the system by introducing electrode bias in side-gated groups.It is generally believed that the energy level position and spatial distribution of the frontier molecular orbitals are important factors which will affect the electronic transport properties of molecular devices,and as a result of that,the gating groups can effectively tune frontier molecular orbitals and affect the electronic transport properties of molecular devices.However,it was found in the experiment that the rectifying properties significantly of PPC molecules are not changed significantly by changing branch-gated group with different electronegativity.In order to understand the intrinsic mechanism and its working principle of the effects of side groups on the rectifying properties of PPC molecules,the calculations based on density functional theory?DFT?and non-equilibrium Green's function?NEGF?are performed,which show that the asymmetric evolution of the lowest unoccupied molecular orbital?LUMO?with bias voltage is the reason for the rectification of the system.However,the change of the side group has small effect on the width and height of the transmission peak of the frontier molecular orbitals.The changes of side groups changes the spatial distribution of the frontier molecular orbitals,but in the experiment,the influences of the side groups on the main chain of the PPC molecule are indirect for the isolation of side linkers,so the effects on the spatial distribution of frontier molecular orbitals of molecular main chain is weak,which further results in small changes of the charge transport and rectification properties of the system.Although the changes of side groups have little influence on the spatial distribution of frontier molecular orbitals and the rectification properties of the system,the existence of side groups can provide facilities to introduce electrode influence on the electronic transport of the system.Thus,the effects of the connection between electrode protrusions and side groups on the electronic transport properties and rectification behaviors of PPC molecules are studied.Based on the DFT and NEGF methods,we have calculated the effects of eight different electrode contact configurations on the rectifying properties of PPCNMe₂ molecules.The results show that when the end of PPCNMe₂ near the pyrimidine ring is connected with the tip of electrode pyramid configuration,some of the frontier molecular orbitals of the system enter the bias window at lower negative bias.Although their wave functions mainly localize on the pyrimidine rings,the contributions to the electronic transport still increase the strength of the current under negative bias,which further causes the remarkable rectification inversion of the system.If the-NMe₂ side group directly interacts with the electrode protrusion,the LUMO and the LUMO+1 under the positive bias are lowered and enter the bias window,which significantly improves the electronic transport capacity under the positive bias.However,the electrode under negative bias has little effect on the frontier molecular orbitals,thus the system rectification ratio has been greatly improved by about 6 times.This phenomenon indicates that the introduction of electrode bias through side groups is a novel effective method for controlling the rectification ratio of molecular devices.The content of this thesis is divided into five chapters.The first chapter summarizes the origin and development of molecular electronics,and introduces the development process of single-molecule rectifier devices systematically.The second chapter briefly describes the theoretical methods required for the calculation of charge transport in molecular devices.Chapter3 studies the rectification tuning effects of Pyridinoparacyclophane molecular junctions by side branch functional groups.Chapter 4 discusses the influence of the contact configuration of different electrode interfaces and electrode protrusion on the single-molecule junction rectification characteristics.And chapter 5 summarizes all the work we have done,the innovation as well as shortcomings of it,and makes an outlook for future work.