| Summary: | 博士 === 國立臺灣大學 === 化學研究所 === 105 === Three topics are included in this thesis. In the chapter 1, we study the connections between the current distribution within a single molecular junction and molecular orbitals. By using the first order perturbation theory and non-equilibrium Green’s function techniques in the framework of Hückel theory, we derive a simplified local transmittance formula from the local current formula. The simplified local transmittance formula shows that the local transmittances are closely related to the off-diagonal elements of transition density matrices. Moreover, a few simple molecular junctions consisting of single- and multi-ring conjugated systems illustrate the connections between the off-diagonal elements of transition density matrices and the local transmittances. We find that the distribution of local currents inside the molecular junctions can be faithfully described through consideration of only the transition density matrices of degenerate or nearly-degenerate molecular orbitals.
In the second chapter, we study the current hysteresis phenomenon in the current-voltage characteristics. A simple adiabatic theory based on the Longuet-Higgins-Salem (LHS) model for investigating charge transport through a molecular junction consisting of a deformable conjugated two-site molecule bridging in between two electrodes is developed. Influences of the electrode-molecule coupling strengths and bias voltages on the geometry of a deformable two-site molecular bridge are also investigated. In the case of the weak electrode-molecule coupling, we found that the molecule has two steady-state geometries corresponding to the double minima of the potential energy curves at a specific bias regime. In this regime, the current-voltage characteristics can thus exhibit a peculiar hysteresis phenomenon. However, as the electrode-molecule coupling strength increases, the energy barrier of the double minima is gradually reduced and the hysteresis phenomenon is diminished.
In the final chapter, by using the LHS model and the non-equilibrium Green’s function techniques in the framework of lattice relaxation theory, we study the electron transport properties of polymethine molecular junctions and polyacetylene molecular junctions with different chain lengths. In the case without considering the influences of the external electric field, for the molecular junctions with similar chain lengths, the current plateaus in current-voltage characteristics of polymethine molecular junctions show larger current values than that of polyacetylene molecular junctions. The analysis reveals that this is because polymethines have solitons but polyacetylenes have not. In addition, both of the current-voltage characteristics of polymethine and polyacetylene molecular junctions exhibit current hysteresis phenomenon. The widths of the hysteresis regions of both polymethine and polyacetylene molecular junctions increase as the chain length increases. Our analysis shows that the magnitudes of the coefficients on the chain end atoms in the frontier orbitals decrease when the chain length increases. This causes the density of state associated with the frontier orbitals less broadened for the long-chain conjugated molecular junctions. Therefore, the current hysteresis phenomenon in the long-chain conjugated molecular junctions is more apparent. On the other hand, the widths of the hysteresis regions of the polyacetylene molecular junctions with considering the influences of the external electric field are relatively small compared to the case without considering the influences of the external electric field. However, the widths of the hysteresis regions of polymethine molecular junctions in the two cases, with and without considering the influences of the external electric field, are similar. Further investigation reveals that this even-odd effect is due to the solitons inside the polymethine molecular junctions.
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