Nishikawa Ayano, Tobita Junichi, Kato Yasuyuki, Fujii Shintaro, Suzuki Masaaki, Fujihira Masamichi
Department of Biomolecular Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8501, Japan.
Nanotechnology. 2007 Oct 24;18(42):424005. doi: 10.1088/0957-4484/18/42/424005. Epub 2007 Sep 13.
The effect of the binding sites of the terminal groups -S on gold on currents through a single molecular junction (MJ) of Au/1,6-hexanedithiol/Au was studied by measuring current-separation (i-s) curves during repeated formation of a break junction in UHV-STM. Three different single molecular conductance (SMC) values (i.e. G(m)(HC), G(m)(MC) and G(m)(LC)) were found by a careful analysis of corrected current histograms for background tunneling currents using a previously developed robust statistical analysis. Here, HC, MC and LC represent a single MJ with high, medium and low conductance, respectively. These three SMC values are attributed to three different contact modes (i.e. strong-strong, strong-weak (or weak-strong) and weak-weak bindings at the two ends). In addition to these three SMC values due to the different contacts, another lower SMC value was newly observed in the corrected histogram. The presence of the fourth SMC is specific to MJs of alkanedithiols and is attributable to LC of a single alkylene chain with gauche rich conformation, which has a lower SMC value than that of LC with all-trans conformation as proposed previously (Fujihira M et al 2006 Phys. Chem. Chem. Phys. 8 3876). Due to the effects of the contact and the conformational change, it was difficult to determine six different SMC values corresponding to two different conformations (i.e. gauche-rich versus all-trans) with three different contacts (i.e. HC, MC and LC). In addition to this complexity, the current steps corresponding to HC, MC and LC almost always appeared in this order in measured i-s curves during separation. The current step observed here could not only be a contribution from a single molecule, but also contributions from a few groups of molecules that happen to link gold atoms of the substrate with those of the tip apex. Therefore, the SMC value for HC obtained as a peak or a set of peaks in the current histogram could be based upon the sum of the current of HC and those of MCs and LCs coexisting in parallel, unless every MJ would change successively from HC to MC and MC to LC. Namely, the currents through coexisting MCs and LCs would raise the intrinsic current observed for HC itself, while those through coexisting LCs would raise the intrinsic current for MC. To avoid such errors in determining the true SMC, we demonstrate here a new method based upon analyses of individual i-s curves referred to as jump height analyses of individual i-s curves. By this method, the true SMC of LC(all-trans) was determined to be 1.6 nS (i.e. G(m) (LC, all-trans) of 2.1 × 10(-5)G(o)) without ambiguity in spite of the possible presence of LCs(gauche rich) in parallel.
通过在超高真空扫描隧道显微镜（UHV-STM）中重复形成断裂结的过程中测量电流-分离（i-s）曲线，研究了金末端基团-S的结合位点对通过Au/1,6-己二硫醇/Au单分子结（MJ）的电流的影响。通过使用先前开发的稳健统计分析仔细分析背景隧穿电流的校正电流直方图，发现了三种不同的单分子电导（SMC）值（即G(m)(HC)、G(m)(MC)和G(m)(LC)）。这里，HC、MC和LC分别代表具有高、中、低电导的单个MJ。这三个SMC值归因于三种不同的接触模式（即两端的强-强、强-弱（或弱-强）和弱-弱结合）。除了由于不同接触产生的这三个SMC值外，在校正后的直方图中还新观察到另一个较低的SMC值。第四个SMC的存在是链烷二硫醇单分子结所特有的，并且可归因于具有丰富gauche构象的单个亚烷基链的LC，其SMC值比先前提出的具有全反式构象的LC的SMC值低（Fujihira M等人，2006年，《物理化学化学物理》8，3876）。由于接触和构象变化的影响，很难确定与三种不同接触（即HC、MC和LC）对应的两种不同构象（即丰富gauche构象与全反式构象）的六个不同SMC值。除了这种复杂性之外，在分离过程中测量的i-s曲线中，对应于HC、MC和LC的电流步骤几乎总是按此顺序出现。这里观察到的电流步骤不仅可能来自单个分子的贡献，还可能来自碰巧将基底的金原子与尖端顶点的金原子连接起来的几组分子的贡献。因此，在电流直方图中作为一个峰或一组峰获得的HC的SMC值可能基于HC的电流与并行共存的MC和LC的电流之和，但前提是每个MJ不会相继从HC变为MC，再从MC变为LC。也就是说，通过并行共存的MC和LC的电流会提高HC本身观察到的固有电流，而通过并行共存的LC的电流会提高MC的固有电流。为了避免在确定真实SMC时出现此类错误，我们在此展示一种基于对单个i-s曲线进行分析的新方法，称为单个i-s曲线的跳跃高度分析。通过这种方法，尽管可能并行存在丰富gauche构象的LC，但仍明确确定了LC（全反式）的真实SMC为1.6 nS（即2.1×10(-5)G(o)的G(m)(LC,全反式)）。
