First Principles Investigations Of The Electronic Transport Properties In Biphenyl-dithiol Molecular Devices

With the development of micro-electronics and minimization of electronic device, it has been a trend to use single molecules, including single-wall carbon nanotubes, multiple-wall carbon nanotubes, single organic molecules, biomolecules, to construct functional electronic devices. Furthermore, the research of electrical and optical properties for these devices has become an independent subject, that is, 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 switch effect, negative differential resistance (NDR) effect, memory effect, Kondo effect and rectifying effect, 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. As the electronic transport plays a key role in the operation of molecular devices, it is of fundamental importance to obtain a comprehensive understanding of the electronic transport properties of molecular junctions.In this thesis, by using first principles density functional theory combined with non-equilibrium Green’s function, we focus on the investigations of electronic transport characteristics in molecular devices. The detailed research and main results are given below:Biphenyl-dithiol (BPD) molecule has attracted much attention because of its simplicity and chemisorption property, thus the BPD molecular junction is an ideal subject to experiment and theoretical researches. Up to now, many experimental studies has demonstrated that side groups have great importance on electronic transport properties of molecular junctions. However, a lot of issues of side groups such as the effects of their substitution positions or the kinds of side groups on the electrical transport were paid less attentions. So we construct a Au electrodes/BPD molecule/Au electrode sandwich system in order to study the above problems. Our calculations show that the positions of amino groups can modulate the transport properties. When two substitutions are on one phenyl ring of the biphenyldithiol molecule, the current of the system with two amino groups on the same side is smaller than that on the different side. On the contrary, the current of the system with same side substitutions is bigger when two side groups are on different phenyl rings, furthermore platform like curves can be observed. By analyzing the transmission spectrum and MPSH states at zero bias, we find that both the energy levels’ sites of the extended molecule and the localization properties of the MPSH orbitals of the corresponding energy levels in an electric field lead to these phenomena. The Ⅰ-Ⅴ characteristics for BPD molecule with different side groups show that the current of the system substituted by two amino groups is bigger than that with two amino groups. However, the systems substituted by one nitro groups and one amino group have poor transport properties.