First Principles Investigations Of The Electronic Transport Properties In Benzene-based Molecular Device

In recent years,with the continuous development of micro-electronics and continuous minimization of electronic device,it has been a developing trend to use single molecule,including single-and multiple-wall carbon nanotubes,single organic molecule,macromolecules,to construct functional electronic devices.Furthermore, the research of electric and optical properties for these devices has become an independent subject gradually,i.e.,molecular electronics.Currently,progress in microfabrication and self-assembly techniques,such as scanning tunneling microscope,atomic force microscope and so on,has made it possible to design a single-molecule device.The electronic transport properties of single-molecule junctions have attracted more and more attention because of their novel physical properties,including electrostatic current switching,negative differential resistance (NDR),memory effects,Kondo effects and rectifier,which make it possible to realize the elementary functions in electronic circuits.Following the development in experiments,people use various theoretical methods,such as semi-empirical theories and first-principles methods,to find the mechanism for molecular devices during operation,and explore the correlation between the geometric structures and electronic properties.But in a real experiment,compared with the electrode,the molecule is a small system in size.Therefore the geometric and the electronic structures even the interface structures between molecule and electrode of the molecular device can be affected by external factors during operation,which will strongly influence the performance of the device.Therefore it is also very important to understand the electronic transport of a molecular junction by studying the molecular conformation. In this thesis,based on the DFT+NEGF first-principles method,we construct a lead-molecule-lead sandwich system with single benzene-based molecule to investigate the relationship between the structure and properties of the molecular devices and the factors that influence properties of the molecular devices.Our computational investigations on these molecular devices are primarily concentrated on the effect of contact geometry between molecules and electrodes,the effect of side groups,the effect of different torsion angle between two phenyl rings and the effect of external electric field.This thesis consists of five chapters:In Chapter one,we mainly introduce the research background and development of the molecular electronics,including the experimental fabrication of molecular electronic components and theoretical simulation methods.Afterwards we discuss the questions that need to be solved in this area.Finally,the main contents of this thesis are listed.In Chapter two,we introduce the theoretical methods used in this thesis, including the main first-principles calculation methods,i.e.,the Density-Function (DFT)method and nonequilibrium Green's function.Then Ab initio NEGF formalism is described in detail at the end of this chapter.In Chapter three,we investigate the effect of contact geometry including the relative orientation and the change in the interface structure between electrode and molecule on the electronic transport properties of 1,4′-Dithiolbenzene(DTB) molecule connected to two Au(111)electrodes.Numerical results show that the change in contact geometry strongly affects the conducting behavior of molecular junction.(1)The change of the relative orientation between electrode and molecule gives more various influences on molecular electronic structures and current-voltage characteristics in double DTB system than in single DTB system,which obviously affects the electronic transport properties of metal-molecule-metal systems.We find this change inâ… -â…¤characteristic is mainly induced by the coupling between molecule and electrodes and interaction of two DTB molecules.(2)Then we investigate the effect of the change in the interface structure between electrode and molecule caused by different adsorption sites with the presence of additional Au atom around the metal-molecule contact in the system that DTB sandwiched between two Au(111) leads.The motivation is the variable situations that may arise in break junction experiments.Numerical results indicate that the enhancement of conductance induced by the presence of additional Au is dependent on the adsorption sites of anchoring atom.When molecule is located on top site with the presence of additional Au atom,it can increase molecular conductance remarkably and present negative differential resistance under applied bias which can't be found in bridge and hollow sites.These results indicate that NDR is not only related to the kind of molecule,but also strongly dependent on the number of additional Au atom around the metal-molecule contact and the connection way to electrodes.All these results implicate that the determination of the contact structure is very essential to correctly predict the transport properties of the molecular conductors.In Chapter four,we investigate the effect of different side groups,anchoring groups(dithiocarboxylate and thiol group),torsion angle and external electric field on the electronic transport properties of 4,4′-biphenyldithiol(BDT)molecular junction. Numerical results show that torsion angle plays more important role in conducting behavior of the system with the dithiocarboxylate anchoring group.By changing the torsion angle between two phenyl rings,namely changing the magnitude of the intramolecular interaction,there is an abrupt change in conductance.It indicates that the intramolecule interaction should also be considered in the mechanism of conductance enhancement by dithiocarboxylate anchoring group.Furthermore,the NO₂ functionalized molecule will perform a molecular memory effect with different torsion angles.The shortening of the distance between two electrodes results in a stronger molecule-electrode coupling and lower potential barrier between them, which leads to a larger conductance.The increase in torsion angle between two phenyl rings as external bias increasing results in a weaker intramolecule coupling, which leads to a smaller conductance.Furthermore,this change in molecular conformation from one stable state to another induced by torsion angle can be expected to cause a conductance switching in the system.In Chapter five,we summarize the work of this thesis,and draw a conclusion for the whole work and view the future development of the molecular electronics.