| With the development of miniaturization of electronic device, the size of the traditional silica-based material integrated circuits is constantly shrinking. According to the prediction of famous Moore’s law, the size of silica-based material will shrink to 1～2nm, which is in the size of region of atom or molecule. Whether or not it is possible to use single molecules as active elements in nanocircuits becomes an issue that people are very concerned about. To be pleased, single molecule can indeed mimic traditional silica-based electronic device, and it can display the function whose traditional silica-based device has. Molecular electronics, is defined as method which using single atom or single molecule to build functional parts in electronic circuits. All the time, People are looking for and design electronic devices in molecular size.A famous American physicist Feynman made a famous speech, "Plenty of Room at the Bottom" at the annual meeting of the American Physical Society in 1959, which proposed an idea of building electronic devices from atoms or molecules, then assembling them to make circuits. This idea had far-reaching influence at the time. The traditional idea is to contribute the macroscopic system and element, and then mimic their size. However, the Feynman’s idea is to build electronic device in atomic scale, and then assembling them to make circuits. Compared to traditional silica-based electronic device, molecular device has many advantages, for example, (i) size:the reduce size of small molecules and high packing density of devices with the subsequent advantages in efficiency and power dissipation, (ii)speed:good molecular wires could reduce the transit time of transistors, reducing the time needed for an operation. (iii)assembly and recognition:One can exploit specific intermolecular interactions to form structures by nanoscale self-assembly. Molecular recognition can be used to modify electronic behavior, providing both switching and sensing capabilities on the single-molecule scale, (iv)synthetic tailorability:by choice of composition and geometry, one can extensively vary a molecule’s transport. (v)new functionalities: Special properties of molecules, like the existence of distinct stable geometric structures or isomers, could lead to new electronic functions that are not possible to implement in conventional solid state devices.With the development of experimental technology, more and more molecular electronic device can be prepared, for example:(i)scanning tunneling microscope, which is the most versatile tools for the fabrication of atomic-size contacts. In the standard application of an STM is putting solution of target molecule on the metallic substrate, and using STM probe indent into the metallic substrate and carefully withdrawn, finally, formed two-probe system of probe target molecule and metallic substrate, (ii)atomic force microscopes, (iii)transmission electron microscope, (iv) mechanically controllable break-junctions. With the development of experimental technology, theoretical research is continuous innovation to describe the issue of electronic transport more precise. Presently, the method combining density functional theory with non-equilibrium Green’s function is the preferred method for electronic transport, and many types of molecular structures can be treated in this theoretical framework like single molecular cluster, molecular wire, graphene, and nanotube.Since the experimental discovery of graphene by Novoselov and Geim in 2004, this material has attracted considerable attention. And in this period, two-dimensional plane materials become the research hot spot. By cutting the graphene sheet along two different directions, two kinds of graphene nanoribbon can be realized, according to the edge of the style, the graphene nanoribbon can be named as armchair graphene nanoribbons (AGNRs) and zigzag graphene nanoribbons (ZGNRs). Among them, ZGNRs have received special attention, and most studies have focused on the electronic transport properties of ZGNRs connected to ZGNRs electrodes. In this situation, the contact position effects of ZGNRs molecules and ZGNRs electrode have been ignored. Moreover, ZGNRs can form edge state which give rise to flat bands in the electronic band structure and thus to generate a pronounced peak in the electronic density of state (DOS) at Fermi level. This may lead to distinct transport properties.The exploration of new hybridization of carbon allotrope is ongoing when people are focus on the sp2 hybridization of graphene. Graphyne has attracted considerable interest in recent years due to its unique sp and sp2 hybridization, and Hirsch believe that the era of graphyne has come. Although most studies have focus on the synthesis of graphyne and properties, the transport properties are rarely reported. Furthermore, the π-orbital in graphyne are form by carbon with different hybridization, thus, the transport properties of graphyne will be different with graphene. So, it is necessary to study the electronic transport mechanism of sp and sp2 hybridization. And the works in this dissertation are based on the two systems.1. The transport properties of ZGNRs electrode and graphyne substructureTaking into account the graphyne contains sp2 and sp hybridization, it has fewer research on the physical mechanism of transport properties, so it is very important and practical value to explore theoretical transport mechanism of sp2 and sp hybridization in the electron transport system. We choose ZGNRs as an electrode due to all of them are two-dimensional material, and ZGNRs can form edge state which give rise to flat bands in the electronic band structure and thus to generate a pronounced peak in the electronic density of state (DOS) at Fermi level. This may lead to distinct transport properties based on ZGNRs electronic device. By applying nonequilibrium Green’s function formalism in combination with density functional theory, we have investigated the electronic transport properties of graphyne substructure attached to ZGNRs electrode, and study the effect of the different contact position between graphyne and ZGNRs electrode and the different width of ZGNRs electrode. (ⅰ) All the two-probe systems perform semi-conduction behavior regardless of the linking position of graphyne and ZGNRs electrode, which is determined by the HOMO-LUMO gap of graphyne. (ⅱ) the current of graphyne connected to the edge of ZGNRs electrode is bigger than that connected to the center of ZGNRs electrode, and the current will become small and small when the link position is more and more close to the center of ZGNRs. (ⅲ) when the system is mirror symmetry under the xz midplane, parity limitation tunneling effect can completely destroy electron tunneling process. The mechanisms of the three phenomena are proposed.2. Rectification properties of ZGYRs and ZGNRs heterojunction From a traditional view, any asymmetric junctions may result in rectification. By applying nonequilibrium Green’s function formalism in combination with density functional theory, we have investigated the rectification characteristic of ZGNRs and ZGYRs heterojunction, and found that asymmetrical structure does not always lead to rectification behavior. In order to further investigate the issue, we use oxygen to substitute ZGYRs and find that the oxygen-unsubstituted ZGYRs/ZGNRs heterostructure has no rectification, which is due to the energy band structure of the heterostructure is on the symmetry of Fermi level. Under the positive or negative bias, the current distribution is symmetry. However, when the ZGYRs are substituted by oxygen, the rectification direction has changed, which is sensitive to the position of oxygen atoms in the ZGYRs. Moreover, if the ZGYRs is replaced by oxygen atom symmetry, then the system will show a bias-induced rectification inversion phenomenon.3. Transport properties of ZGNRs under vertical-strain Former workers found that ZGNRs with even width has a mirror symmetry, and then its current shows current suppression effect due to the parity limitation tunneling effect. We can’t help thinking that when the symmetry of even N-ZGNRs systems is partially broken, whether the bias induced conductance suppression around the Fermi level is disappeared. By applying nonequilibrium Green’s function formalism in combination with density functional theory, we have investigated the transport properties of ZGNRs under vertical-strain, we focus on the detailed procedure of the change in electronic transport properties of ZGNRs under different magnitude of localized vertical strain and different center of the circular deformation zone. The results indicate that all the two-probe systems with vertical strain exhibit almost the same transport characteristics as pristine 6-ZGNR, and present current suppression phenomenon under small strain. Continue to increase strain, some of the systems show localized states induced by strain, which obstruct the electronic transport under zero bias. However, localized state induced by localized vertical strain can enhance the transport properties of symmetric system under bias voltage. It is because the localized state has broken the symmetric of electron density under bias voltage. Further studies show that different scope of strain also influences the electronic transport properties of the systems, the strain acting on the edge of the atom changes less than that in the central region, and the responsiveness of the electron density distribution according to the strain will also affect the transport properties. |
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