End-Group Dependence of Transport Properties for Biphenyl-Based Molecular Junction System

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The transport properties of junction systems that consist of an $X$–biphenyl–$X$ ($X=\text{O}$, S, Se, and Te) molecule sandwiched between two gold electrodes are studied using the nonequilibrium Green’s function method based on the density functional theory. The end-group atom $X$ has an influence on not only the interaction between the molecule and electrodes but also that between the two phenyl rings. Especially, the junction system with $X=\text{O}$ exhibits much different properties from the other $X$s. The interaction between the molecule and electrodes is weaker and that between $\pi$-type orbitals of the two phenyl rings, which mainly contributes to the transmission around the Fermi energy, is stronger. As a result, this system has a larger transmission around the Fermi energy and unusual behaviors, such as a negative differential conductance and a nonlinear potential drop, are observed. We also studied the dependence on dihedral angle between the two phenyl rings.
2008-06-25