Porphyrin single molecule wires for nanoelectronics

分子纳米电子学的主要目标是制造和互连分子，以取代或至少增强现有的硅基技术，这些分子可以作为互连、开关、晶体管甚至逻辑门。显然，如果这些技术取得成果，将面临巨大的挑战。在这方面，关键的科学挑战之一是合成和可靠地连接单分子导线，这种导线可以长距离传输电荷，也可以执行其他功能，如整流或存储电荷。该提案旨在从一类称为卟啉的分子中合成延伸的分子线，其合成和功能化是牛津小组的重点。卡迪夫大学和利物浦大学将研究这些具有良好特征的分子的电学特性，特别是它们作为分子线的效率、它们与金属电极的接触以及它们在设备中电化学控制的潜力。要合成的卟啉丝是共轭的，坚硬的，并且含有电荷可以定位的位点。其他关键特性包括长度大于10nm，稳定性好，能结合多种金属离子，并能通过电化学或光化学调节。预计它们的特性将使它们能够远距离导电。它们的氧化还原活性和支持悬垂分子基团的能力将反过来为电流整流、开关或电荷存储提供途径。对这些卟啉线的电学性质的研究需要我们把单个分子连接起来。这显然是一个巨大的实验挑战，但利物浦小组最近开发了使用扫描隧道显微镜的新技术，使这个过程更加直接和可靠。这些技术将提供坚固的化学接触的单卟啉分子在两端的金属接触。分子/金属接触的作用仍然是单分子电子学中最不为人所知但又极其重要的方面之一。我们将系统地研究这些接触效应，通过使用几种不同的化学基团结合到金属电极上，并补充确定金属和分子之间的能级阵容。虽然大多数电气特性将在导线两端使用两个金属触点进行，但在项目的后期阶段，将引入一个扫描探针触点，可以沿着导线的长度进行扫描，沿着导线探测电气特性。该项目的最终目标是生产一系列新的卟啉分子线，并定义和理解电子在单分子水平上的传递，以及在不同的环境下，包括特高压和电解质。能够探测一个分子体系的关键变量（温度、环境、氧化态、金属原子、分子刚度、接触化学）将为制定详细的机制提供系统的方法。特别是，接触化学和非理想电荷输运（非弹性散射）过程的限制作用将被定义。理想的卟啉分子线将支持高电流，我们将评估如何接近这个目标（量子电导极限）。我们还将评估这些分子通过沿其长度放置氧化还原可寻址基团作为活性分子线的能力。

The prime aspiration of molecular nanoelectronics is to fabricate and interconnect molecules that can replace, or at least augment, present silicon based technology, with the molecules functioning as interconnects, switches, transistors or even logic gates. Clearly big challenges exist if such technologies are ever to reach fruition. In this respect, one of the key scientific challenges is to synthesise and reliably connect single molecular wires which can transport charge over long distance and also perform other functions such as rectifying current or storing charge. This proposal is aimed at synthesising extended molecular wires from a class of molecules called porphyrins, whose synthesis and functionalisation is the focus of the Oxford group. These electrical properties of these well-characterised molecules will be investigated at Cardiff and Liverpool, particularly their efficiency as molecular wires, their contacts with metal electrodes and their potential for electrochemical control in devices. The porphyrin wires to be synthesised are conjugated, stiff and contain sites where charge can be localised. Other key attributes include lengths greater than 10 nm, remarkable stability, their ability to bind a wide range of metal ions and their capacity to be tuned with electrochemistry or photochemistry. It is expected that their attributes will allow them to conduct electrons over long distances. Their redox activity and ability to support pendent molecular groups will in turn provide avenues for current rectification, switching or charge storage. The investigations of the electrical properties of these porphyrin wires will require us to wire-up single molecules. This is clearly a big experimental challenge but the Liverpool group has recently developed new techniques using the scanning tunnelling microscope which makes this procedure more straightforward and reliable. These techniques will provide robust chemical contact of the single porphyrin molecules at both ends to metallic contacts. The role of the molecule/metal contact remains one of the most poorly understood and yet extremely important aspects of single molecule electronics. We will systematically investigate these contact effects through the use of several differing chemical groups for binding to the metal electrodes and complementary determination of the lineup of energy levels between the metal and the molecule. Although most of the electrical characterisation will be performed with two metal contacts at either end of the wire, in the later stages of the project a scanning probe contact will be introduced which can be scanned along the length of the wire probing electrical properties along the wire. The final goals of the project are to produce a series of novel porphyrin molecular wires and to have defined and understood electron transport across them down to the single molecule level and in different environments including UHV and electrolyte. Being able to probe the key variables (temperature, environment, oxidation state, metal atom, molecule stiffness, contact chemistry) for one molecular system will provide a systematic approach for formulating detailed mechanisms. In particular, the limiting roles of contact chemistries and non-ideal charge transport (inelastic scattering) processes will be defined. The ideal porphyrin molecular wire would support high currents and we will assess how closely this goal (quantum conductance limit) can be approached. We will also have evaluated the ability of these molecules to act as active molecular wires through the placement of redox addressable groups along their length.