Medium effects in single molecule electronics

在硅芯片技术中，正在开发越来越复杂和昂贵的方法来在硅表面上雕刻越来越小的特征，以产生越来越小的电路元件，如晶体管。随着这些特征的尺寸达到数十纳米的水平，研究人员对使用有机分子组装体作为电路元件的可能性越来越感兴趣，甚至下降到单分子的水平。非常复杂的分子，被设计来模仿简单的电路元件，如二极管，已经被合成。但一个关键问题是必须使这些分子电接触--将它们“连接”到电路中。我们沿着世界各地的许多其他研究人员，一直在研究以硫原子为末端的分子的性质，硫原子很容易附着在金的表面。在我们的工作中，我们已经开发了两个相关的技术，用于测量小整数，n，分子的电导，包括n = 1。分子夹在两个金触点之间。我们通过调整扫描隧道显微镜（STM）的技术来做到这一点，在该技术中，原子级精细金属尖端的位置在金属表面上被非常精确地控制。在适当的条件下，我们可以说服硫封端的分子在金属（金）表面和STM针尖之间桥接。然后我们可以测量流过分子的电流。使用这种方法，我们最近发现，在有水的情况下，被称为寡聚噻吩的单分子的电导率比在实验中小心排除水时高出几百倍。这一点很重要，原因有几个。首先，如果分子的电特性取决于它们的环境，我们将不得不注意在分子电子器件中排除水。但更有趣的是，我们也许可以通过设计“宿主”分子来制造非常敏感的传感器，这些分子在暴露于特定的目标分子时会改变它们的电导率。在这个提议中，我们试图探索这种“介质”效应的起源，并研究如何控制它。我们也想找到更好的方法来计算金属的电导|分子|考虑到水或其它溶剂（“介质”）分子的存在，金属结。

In silicon chip technology, increasingly sophisticated and expensive methods are being developed to carve smaller and smaller features into the silicon surface, to generate ever-smaller circuit components such as transistors. As the size of these features reaches the level of tens of nanometres, researchers are becoming ever more interested in the possibility of using assemblies of organic molecules as circuit components, even down to the level of single molecules. Very sophisticated molecules, designed to imitate simple circuit elements such as diodes, have been synthesised. But a key issue is the necessity to contact such molecules electrically - to 'wire' them into circuits. We, along with many other researchers worldwide, have been studying the properties of molecules terminated with sulfur atoms, which attach readily to gold surfaces. In our work, we have developed two related techniques for measuring the conductance of small integer numbers, n, of molecules, including n = 1. The molecules are sandwiched between two gold contacts. We do this by adapting the technique of scanning tunneling microscopy (STM), in which the position of an atomically-fine metal tip is controlled very precisely over a metal surface. With the right conditions, we can persuade sulfur-terminated molecules to bridge between the metal (gold) surface and the STM tip. We can then measure the current that flows through the molecule(s). Using this method, we recently discovered that the conductance of single molecules called oligothiophenes was several hundred times higher in the presence of water than when water was carefully excluded from the experiment. This is important for several reasons. Firstly, if the electrical properties of molecules depends on their environment, we will have to take care to exclude water in molecular electronics devices. But more intriguing, we could perhaps make very sensitive sensors by designing 'host' molecules that change their conductance on exposure to a particular target molecule.In this proposal, we seek to explore the origins of this 'medium' effect, and to examine how it can be controlled. We also want to find better ways to calculate the conductance of metal|molecule|metal junctions taking into account the presence of water, or other solvent ('medium') molecules.