分子结中金-硫醇和金-硫醇盐界面的非弹性调谐光谱：氢的作用
Inelastic Tunneling Spectroscopy of Gold-Thiol and Gold-Thiolate Interfaces in Molecular Junctions: The Role of Hydrogen
摘要：
人们普遍认为，当具有硫醇（S-H）端基的分子桥接一对金电极时，S原子键合到金上，硫醇H原子从分子上脱离。然而，很少有人知道这个过程的细节，它的时间尺度，以及是否有和没有硫醇氢原子的分子可以共存于分子结。在这里，我们从理论上探讨如何非弹性隧穿光谱（IETS）可以揭示这些问题。我们目前的几何形状，低偏压电导和IETS的丙二硫醇和丙二硫醇分子结与金电极的计算。我们表明，IETS可以区分没有，一个或两个硫醇氢原子的分子之间的结。我们发现，在大多数情况下，在Hihath等人的IETS实验中的单分子结[Nano Lett. 8，1673（2008）]没有硫醇H原子，但是具有单个硫醇H原子的分子可能偶尔桥接它们的连接点。我们还考虑了IETS光谱的演变，作为一个黄金STM尖端接近完整的S-H基团在其另一端绑定到第二个电极的分子的末端。我们预测的硫醇H原子的振动模式的频率增加的一个因素\sim 2的尖端和分子之间的间隙变窄。因此，IETS应该能够跟踪尖端朝向分子的硫醇基团的接近，并且在发生硫醇H原子从分子脱离时检测硫醇H原子。
英文摘要：
It is widely believed that when a molecule with thiol (S-H) end groups bridges a pair of gold electrodes, the S atoms bond to the gold and the thiol H atoms detach from the molecule. However, little is known regarding the details of this process, its time scale, and whether molecules with and without thiol hydrogen atoms can coexist in molecular junctions. Here we explore theoretically how inelastic tunneling spectroscopy (IETS) can shed light on these issues. We present calculations of the geometries, low bias conductances and IETS of propanedithiol and propanedithiolate molecular junctions with gold electrodes. We show that IETS can distinguish between junctions with molecules having no, one or two thiol hydrogen atoms. We find that in most cases the single-molecule junctions in the IETS experiment of Hihath et al. [Nano Lett. 8, 1673 (2008)] had no thiol H atoms, but that a molecule with a single thiol H atom may have bridged their junction occasionally. We also consider the evolution of the IETS spectrum as a gold STM tip approaches the intact S-H group at the end of a molecule bound at its other end to a second electrode. We predict the frequency of a vibrational mode of the thiol H atom to increase by a factor \sim 2 as the gap between the tip and molecule narrows. Therefore, IETS should be able to track the approach of the tip towards the thiol group of the molecule and detect the detachment of the thiol H atom from the molecule when it occurs.
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