Theoretical Studies Of Electronic Transport In Bimolecular Junctions

In recent years, nanotechnology and supramolecular chemistry have experienced rapiddevelopment and much attention has been attracted from numerous scientists in a diverse rangeof research fields. Despite there ware many unsolved issues in molecular electronics, importantand solid advances have been made over the past decade in areas of the molecular electronicdevices, especially for single molecular devices. There have been a lot of researches concerningthe effect factors of electronic transport in molecular junctions such as molecule-electrodecontacts, molecular structures, the distances between electrodes, and the applied electric field.Most of researchers try to study on the electron transport properties of single molecular bridge,while seem to ignore the influence of the molecular interactions. According to the traditionalviewpoint, if the open ends of molecules form strong chemical bonds with gold electrodes viathe thiol groups, which have been widely used as terminal anchor groups of molecules toconstruct stable molecular junctions, electronic transport ability will be promoted efficiently, yetcontrary to the physical contact. Recently, Wu et al. have prepared a series of molecularjunctions—bimolecular junctions throughπ-stacking of adjacent molecules by the approach ofmechanically controllable break junction (MCBJ), constructed by two oligomeric phenyleneethynylene molecules with a single thiol anchor group (HS-OPE) attached to the adjacent goldelectrodes. Analysis of their experimental results showed that the electronic properties of thisseries of molecular junctions was wonderful and desirable, which stimulated an intensiveresearch interest into this field.Using elastic scattering Green’s function theory, we studied the conductance-voltagecharacteristics and discussed the underlying mechanism for electron transport in thosearomatically coupled bimolecular junction. Besides, we also studied the effect of the side groupson intermolecularπ-πinteractions and electronic transport properties. Other influential factorsare also studied，such as the distances between adjacent OPE planes, the distances betweenelectrodes and the slide distance. The present article addresses some controversial questions.Theoretical results of bimolecular junctions are in good agreement with experimentalfindings.The main content and results of the research in this thesis are as follows: First, we have studied OPE bimolecular junctions with t-butyl and o-methyl substituents onthe phenyl rings to study the effect of side groups on intermolecularπ-πinteractions andelectronic transport properties. Numerical results show that these substituents on phenyl ringshave significant influences on intermolecularπ-πinteractions. When the substituent is o-methyl,it will promote theπ-πinteraction and electronic transport ability, while the t-butyl substituentleads to a decrease inπ-πinteraction to reduce electronic transport. As overlap distance of OPEmolecules was increased and distances between adjacent OPE planes is decrease, thesupramolecularπ-πinteractions is decrease, which promote electronic transport properties ofbimolecular junctions. In order to further indicateπ-πcoupling interaction is the main influencefactor of electronic transport properties for the bimolecular junctions, the influence of slidedistance on electron transfer ability of molecular junction is discussed too.Second, we also present a systematic study of the current-voltage characteristics of thebimolecular junctions to expand our understanding of the effect of side groups on electronictransport properties of bimolecular junctions. Furthermore, the influence of junction’s structuresis discussed from the standpoint of arene dimers. The numerical results show that, for abi-OPE-molecule junction, the offset face-to-face (OSFF) configuration induces moredelocalized molecular orbitals, and results in more conductive for it than parallel face-to-face(PFF) configuration. The side substituent groups which contain with rich delocalized electronscan strengthen the coupling of the two single OPE molecule and raise the resonant electronictransport of bimolecular junctions. The resonant conductances become smaller with the weakerof the substituent electronegativity. The numerical results show that the OSFF configurationinduces more delocalized molecular orbitals in the bias windows, which results in the OSFFconfiguration more conductive than the PFF configuration in the resonant transport regime. Theconfigurations can be regulated through the temperature, to achieve a desired result of molecularswitches.This thesis consists of six chapters as follows. The first chapter is the review section. Itbriefly introduces the preparation techniques and measurement methods of molecular junctions,the corresponding theoretical methods and the research background of bimolecular junctions isdescribed emphatically. The second chapter describes the theoretical basis of quantum chemicalcalculations, including the Born-Oppenheimer approximation, Hartree-Fock approximation,density functional theory and so on. The third chapter introduces the elastic scattering Green function theory in the calculation of electronic transport properties of molecular devices in theapplications. Chapter four and Chapter five introduce the computational work and research donein this paper to study the effect factors on electronic transport properties of bi-OPE-monothiolmolecular junctions, suah as the side groups, the distances between adjacent OPE planes, thedistances between electrodes and the slide distance. The switch function of bimolecular junctionsis researched too. The last chapter draws a conclusion for the whole work of this thesis and givesthe prospect on the development of the bimolecular junctions in future.