Surface Physics Laboratory, Fudan University, Shanghai 200433, China.
J Chem Phys. 2009 Dec 28;131(24):244712. doi: 10.1063/1.3276280.
Effects of binding modes and anchoring groups on nonequilibrium electronic transport properties of alkane molecular wires are investigated from atomic first-principles based on density functional theory and nonequilibrium Green's function formalism. Four typical binding modes, top, bridge, hcp-hollow, and fcc-hollow, are considered at one of the two contacts. For wires with three different anchoring groups, dithiol, diamine, or dicarboxylic acid, the low bias conductances resulting from the four binding modes are all found to have either a high or a low value, well consistent with recent experimental observations. The trend can be rationalized by the behavior of electrode-induced gap states at small bias. When bias increases to higher values, states from the anchoring groups enter into the bias window and contribute significantly to the tunneling process so that transport properties become more complicated for the four binding modes. Other low bias behaviors including the values of the inverse length scale for tunneling characteristic, contact resistance, and the ratios of the high/low conductance values are also calculated and compared to experimental results. The conducting capabilities of the three anchoring groups are found to decrease from dithiol, diamine to dicarboxylic-acid, largely owing to a decrease in binding strength to the electrodes. Our results give a clear microscopic picture to the transport physics and provide reasonable qualitative explanations for the corresponding experimental data.
基于密度泛函理论和非平衡格林函数理论，从原子的第一性原理出发，研究了烷烃分子线的结合模式和锚定基团对非平衡电子输运性质的影响。在两个接触点之一处考虑了四种典型的结合模式：顶、桥、hcp 空穴和 fcc 空穴。对于具有三种不同锚固基团（二硫醇、二胺或二羧酸）的分子线，四种结合模式产生的低偏压电导都被发现具有高或低值，这与最近的实验观察结果非常一致。这种趋势可以通过小偏压下电极诱导的能隙态的行为来解释。当偏压增加到更高的值时，来自锚固基团的状态进入偏压窗口，并对隧穿过程做出显著贡献，从而使四种结合模式的输运性质变得更加复杂。还计算并比较了其他低偏压行为，包括隧穿特征的倒数长度标度值、接触电阻和高低电导值的比值。发现三种锚固基团的导电能力从二硫醇、二胺到二羧酸逐渐降低，这主要归因于与电极的结合强度降低。我们的结果为输运物理提供了一个清晰的微观图像，并为相应的实验数据提供了合理的定性解释。
