Medium effects in single molecule electronics

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.

This proposal lies in the area of molecular electronics. It is important to distinguish this from the more mature field of organic electronics. Following the discovery of electrically-conductive polymers in 1977 (Chemistry Nobel Prize 2000), the later (serendipitous) discovery of organic polymer light-emitting diodes in 1989 has since led organic electronics to the cusp of a new industry, in which the U.K. has some leading start-up activity (e.g. CDT, Plastic Logic). In organic electronics, bulk material properties of organic materials (mainly polymers) are used to fabricate electronic devices. In molecular electronics, we are attempting to use single molecules, or a small assembly of molecules, to perform electronic functions. This is still extremely challenging, and is at a much earlier stage of evolution. Nevertheless, there are encouraging signs that real devices can be fabricated in this way, and this could lead to new and presently unforeseen technologies. For example, Philips (Eindhoven) recently published on transistors and simple integrated circuits using self-assembled monolayers of oligothiophenes, closely related to the molecules that form the backbone of this proposal (see ref. 40 in the Case for Support). We currently have an informal collaboration with the latter group, in which we supply molecules for a Ph.D. student who is extending the Nature work, so regular discussion on possible device outcomes for our work are held, and this will continue during the project proposed here. The work in this proposal is fundamental research at a very early stage, and is unlikely to produce commercially-significant results in the short-medium term. It fits within the EPSRC-highlighted Chemistry Grand Challenge of 'Directed Assembly of Extended Structures with Targeted Properties', and as such, it is quite possible that it will have some commercial ramifications over a longer timescale, perhaps in ultra-sensitive (to single molecule level) sensing. The PI is participating in the recently-established Knowledge Centre for Materials Chemistry (http://www.materialschemistry.org/kcmc/news.html). This works in collaboration with industry on translational research projects that will enable companies to develop new commercial opportunities. An independent knowledge transfer team hosted by Chemistry Innovation - the Knowledge Transfer Network for the UK chemistry-using industries - aims to develop and manage collaborative research projects with industry, mainly driven by industry need. Any serendipitous results that do turn out to be of commercial interest will be discussed with appropriate industrial partners, either via the KCMC or via the University of Liverpool Research Business Services department. Additionally, the PI has links with Merck Chemicals Chilworth (fully-funded industrial CASE project on polythiophene synthetic chemistry) and Philips Eindhoven (see above), and has a track record of patenting discoveries with commercial potential (e.g. WO01/94980 'Ionising radiation detector'; P103361GB, 'Polythiophene-based sensors'; EP1 754 736 'Process for the polymerization of ... selenophene derivatives'; WO2007/059849 'Regioregular polyselenophenes').