First Principles Study On Electronic Transport Properties Of CNT\Organic Molecule\CNT Sandwich Structure

With the continuous minimization of electronic device study, it has been a developing trend to use single molecules, including single organic molecule, carbon nanotube, macromolecule, to construct functional electronic devices. Furthermore, the research of electric and optical properties for these devices has become molecular electronics. Progress in microfabrication and self-assembly techniques,such as scanning tunneling microscope, atomic force microscope, 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), rectifier, memory effects. Kondo effects,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. Single wall carbon nanotubes (SWCNTs) have wide variety of superb electronic and mechanical properties.As a quasi-one-dimensional material, SWCNTs can be used for electrodes, and they are more controllable than traditional metal electrodes, opening up different vistas for the design of complex devices. SWCNTs' special geometry structure and semi-conductance has distinct affects to construction, electronic structure, and junction between molecule and electrodes of the molecular device, thus changes the whole electronic properties. It makes sense to study geometry structure of molecular device and SWCNTs' eigen characters.In this thesis, based on the density functional theory plus nonequilibrium Green function (DFT+NEGF) first-principles method, we construct a SWCNT/molecule/SWCNT 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 SWCNT electrodes and torsion angle of the molecule. This thesis consists of four 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 problems 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 DFT method and NEGF. Then Ab initio NEGF formalism is described in detail. We give introduction to eigen characters and applications of carbon nanotubes,recent experiments and theory researches about various types of carbon nanotubes as electrodes at the end of this chapter.In Chapter three, we investigate the effects of different chiral SWCNT electrodes, inner torsion angle of phenyl-ethynyl (PE) molecule, dihedral angle between molecule plane and wall of tube. Carbon nanotubes have special structure, and it is hard to completely control in the self-assemble process,the molecule links to electrodes may have difference in structure. Numerical results show that if molecule is perpendicular to the wall of tube, the planarity ofπ-bonds wil be destroyed mostly, due to low conductance values. It's different between armchair and zigzag nanotube when molecule is linked. Armchair carbon nanotube gives asymmetric effects by the asymmetric edge structure. PE molecule torsion angles larger than zigzag carbon nanotube eletrodes. This kind of deformation will lower the current in transmission. The small energy gap of zigzag carbon nanotube will change the original matching between molecule energy and electrodes chemistry potential.In Chapter four,we discuss about coupling of molecule/electrode junction,the edge state and the spatial voltage drop in the transport of SWCNT/molecule/SWCNT sandwich structure. We find that edge states can afford tunnel near Fermi energy in zigzag SWCNT electrodes structure,but not in armchair system.