First-principles Investigation For Rectifying Performance Of Functional Molecular Devices

This paper presents briefly the electronic structure calculations based on the nonequilibrium Green's function method and the density-functional theory and the progress on molecular devices at first. The transport properties of some typical organic molecular devices have been demonstrated which include the terphenyl molecule, carbon wire and silane chain based on this method. The effects of the coupling of a molecule and electrodes, a small molecule absorbing on the surface of organic molecular, side groups, substituted atoms, and intermolecular interaction on electronic transport properties of molecular devices are studied systematically. Several proposals for the design of rectifying performance molecular devices are presented after theoretical calculations.The transport properties of a terphenyl molecule connected to the two Y (Y=Li, Al, or Au) metal electrodes through thiol bonds are investigated. The results show that the electrode metals have a distinct influence on rectifying performance of such devices. For the Au electrode system, a best rectifying performance was observed, next for the Al electrode system, and the rectifying effect can be nearly neglected for the Li electrode system. Our findings suggest that the rectifying characteristics are intimately related to electrode materials. There is no obvious change on electronic transport properties of molecular devices when the S end-groups are replaced by Se atoms. The rectifying behavior is reduced significantly when one of two S (Se) atoms located at the right of a molecule is replaced by H atom. The systems with S end-groups have an obvious rectifying performance because the interaction between S and Au electrodes is stronger than that between Se and Au electrodes. Additionally, the transport properties of the Terphenyl molecule adsorbed by different small molecules at the same site then connected to two symmetric Au electrodes by Au-Se bonds are investigated. When a Terphenyl molecule is adsorbed by H2, the rectifying performance is weakened, especially adsorbed by H2O, NH3, or CH4, the rectifying effect can be ignored small. When a Terphenyl molecule is adsorbed by different side groups, the rectifying behavior can be significantly affected by the type and the connected positions of side groups. The-NH2group shows an obvious electron-donating characteristic, whereas-NO2group demonstrates a poor electron-accepting behavior. For other non-symmetric systems with the multi-biphenyl, the electronic transport properties are affected not only by the number of benzene rings but also the relative position of the benzene ring and the end atom which connect the molecule and electrode. For the four (five)-benzene system connected with Au electrodes by S-Au bond, we can observe rectifying performance. While for the six-benzene system, the rectifying behavior is more obvious.The transport properties of a carbon wire connected to two Au electrodes are investigated. The results show that the negative differential resistance and rectifying performance can be observed apparently when a pure carbon chain is connected to two asymmetric Au electrodes. The main origin of the negative differential resistance behavior is a suppression of the highest occupied molecular orbital resonance at certain bias voltage. Also shown is that it is possible to make the negative differential resistance disappear and rectifying performance be weakened only by adding side groups to a wire.The transport properties of functionalized atomic chains of carbon atoms with different lengths are investigated. The results show that the â… -â…¤ evolution and rectifying performance can be affected by the length of a wire when both ends of it is capped with the benzene-thiol attached with an amino group and the pyridine attached with nitro group. But when capped with the benzene-thiol attached with an amino group and the nitro group, we can observe a surprising result that different systems show similar â… -â…¤ characteristics and their transport properties are almost independent of molecular length, which suggests that this is a favorable way to design more ideal molecular wires with a high length-independent rectifying behavior. Finally, the transport properties of the following device have been investigated:a silane chain doped with phosphorus and boron atoms sandwiched between a pair of Au electrodes, and the rectifying effect for this system is considered. A barrier similar to the p-n junction is formed in the molecule for polarization owing to the different electronegativity between phosphorus and boron atoms. Then the electronic transport properties of double-silane chains parallelly positioned between two same electrodes have also been investigated, and the results show that the direct interactions between molecules strengthen the rectifying effect when the distance between the chains is suitable. The splitting of the frontier molecular orbitals due to the intermolecular interaction can contribute to new transport channels and thus leads to rectifying effect disappeared. This shows that a different transport behavior can be observed in the system by changing the intermolecular spacing location.