Department of Physics, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada.
J Chem Phys. 2012 Jan 7;136(1):014703. doi: 10.1063/1.3671455.
We examine theoretically the effects of the bonding geometries at the gold-thiol interfaces on the inelastic tunneling spectra of propanedithiolate (PDT) molecules bridging gold electrodes and show that inelastic tunneling spectroscopy combined with theory can be used to determine these bonding geometries experimentally. With the help of density functional theory, we calculate the relaxed geometries and vibrational modes of extended molecules each consisting of one or two PDT molecules connecting two gold nanoclusters. We formulate a perturbative theory of inelastic tunneling through molecules bridging metal contacts in terms of elastic transmission amplitudes, and use this theory to calculate the inelastic tunneling spectra of the gold-PDT-gold extended molecules. We consider PDT molecules with both trans and gauche conformations bound to the gold clusters at top, bridge, and hollow bonding sites. Comparing our results with the experimental data of Hihath et al. [Nano Lett. 8, 1673 (2008)], we identify the most frequently realized conformation in the experiment as that of trans molecules top-site bonded to both electrodes. We find the switching from the 42 meV vibrational mode to the 46 meV mode observed in the experiment to be due to the transition of trans molecules from mixed top-bridge to pure top-site bonding geometries. Our results also indicate that gauche molecular conformations and hollow site bonding did not contribute significantly to the experimental inelastic tunneling spectra. For pairs of PDT molecules connecting the gold electrodes in parallel we find total elastic conductances close to twice those of single molecules bridging the contacts with similar bonding conformations and small splittings of the vibrational mode energies for the modes that are the most sensitive to the molecule-electrode bonding geometries.
我们从理论上研究了金-硫醇界面的键合几何形状对丙烷二硫醇（PDT）分子在 bridging 金电极之间的非弹性隧道谱的影响，并表明非弹性隧道谱结合理论可以用于实验确定这些键合几何形状。借助密度泛函理论，我们计算了由一个或两个 PDT 分子连接两个金纳米团簇组成的扩展分子的弛豫几何形状和振动模式。我们根据弹性传输振幅，以术语形式表述了分子在 bridging 金属接触中的非弹性隧道的微扰理论，并使用该理论计算了 gold-PDT-gold 扩展分子的非弹性隧道谱。我们考虑了具有反式和 gauche 构象的 PDT 分子，它们与 gold 团簇以 top、bridge 和 hollow 键合位点结合。将我们的结果与 Hihath 等人的实验数据进行比较[Nano Lett. 8, 1673 (2008)]，我们确定了实验中最常实现的构象是两种电极上都有 top-site 键合的 trans 分子。我们发现实验中观察到的从 42 meV 振动模式到 46 meV 模式的转变是由于 trans 分子从混合 top-bridge 到纯 top-site 键合几何形状的转变。我们的结果还表明，gauche 分子构象和 hollow 位点键合对实验非弹性隧道谱没有显著贡献。对于连接 gold 电极的 PDT 分子对，我们发现总弹性电导接近具有类似键合构象的单个分子 bridging 接触的两倍，并且对分子电极键合几何形状最敏感的振动模式的振动模式能量的分裂很小。
