联苯二硫醇单分子结的倾角形貌和电导的温度依赖性
Tilt-angle landscapes and temperature dependence of the conductance in biphenyl-dithiol single-molecule junctions
摘要：
使用基于密度泛函的输运方法，我们研究了几个联苯衍生的二硫醇（BPDDT）分子连接到金电极的导电性能。BPDDT分子的侧基不同，侧基控制π电子系统的共轭程度。我们分析了低偏压零温电导与两个苯环单元之间的倾斜角φ的关系，发现它严格遵循cos^2（φ）定律，正如有效π轨道耦合模型所预期的那样。我们表明，苯环的倾斜导致的零温电导下降了大约两个数量级，当从一个平面构象的配置，其中的环是垂直的。此外，我们证明，侧基，除了确定φ，电导没有影响。所有这些都与Venkataraman等人最近的实验[Nature 442，904（2006）]一致。最后，我们研究了电导及其波动的温度依赖性，并发现所研究的分子之间的定性差异。在这个分析中，我们考虑两个贡献的温度行为，一个来自费米函数，另一个来自不同的接触配置的热平均。我们说明，由于温度引起的分子的几何结构的变化的电导的波动，可以通过适当的设计减少。
英文摘要：
Using a density-functional-based transport method we study the conduction properties of several biphenyl-derived dithiol (BPDDT) molecules wired to gold electrodes. The BPDDT molecules differ in their side groups, which control the degree of conjugation of the pi-electron system. We have analyzed the dependence of the low-bias zero-temperature conductance on the tilt angle phi between the two phenyl ring units, and find that it follows closely a cos^2(phi) law, as expected from an effective pi-orbital coupling model. We show that the tilting of the phenyl rings results in a decrease of the zero-temperature conductance by roughly two orders of magnitude, when going from a planar conformation to a configuration in which the rings are perpendicular. In addition we demonstrate that the side groups, apart from determining phi, have no influence on the conductance. All this is in agreement with the recent experiment by Venkataraman et al. [Nature 442, 904 (2006)]. Finally, we study the temperature dependence of both the conductance and its fluctuations and find qualitative differences between the examined molecules. In this analysis we consider two contributions to the temperature behavior, one coming from the Fermi functions and the other one from a thermal average over different contact configurations. We illustrate that the fluctuations of the conductance due to temperature-induced changes in the geometric structure of the molecule can be reduced by an appropriate design.
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