This paper, from a group of scientists at Arizona State University, shows how the problem of connecting a molecular wire to a metal can be overcome. Collaboration between the research groups of chemists Stuart Lindsay, Devens Gust, and Ana Moore has resulted in the measurement of the resistance of a single strand of a molecular wire "soldered" to two gold contacts. The molecule was 1,8-octanedithiol, which consists of a chain of eight carbon atoms with a sulfur atom at each end, and it was these sulfurs that provided the strong connections to the metal through chemical bonding.

The results were not only reproducible, and indeed measurements were repeated more than a thousand times, but they were surprising; the conductivity was much larger than expected. This does not mean that the wiring in molecular computers is ever likely to consist of such hydrocarbon chains, but it suggests that other organic wires might be much more versatile than presently anticipated. However, the real achievement of Lindsay, Gust, and Moore is that they produced the first example of a correctly connected molecular wire.

The collaboration at Arizona State brought together teams researching at the forefront of electron transfer and artificial photosynthesis. The Gust and Moore groups are specialists in the electronic processes that occur in photosynthesis, and they constructed the dithiol-gold circuits. The Lindsay group tested them, using a conducting atomic force microscope (AFM), and proved that they were reliable and gave consistent conductance readings. They also investigated the physical structure of the layer of octanethiol wires by scanning tunnelling microscopy.

Interviewed by Science Watch, Lindsay was enthusiastic about recent developments: "The exciting news is that we are now getting close agreement between experimental data and the theoretical predictions of our colleague, Otto Sankey." Not that these new circuits are entirely free of bugs. As Lindsay says: "There are some issues to do with charge accumulation on the gold spheres, but these are amenable to testing, and we feel that there is now the possibility of both measuring and understanding the electronic properties of a molecule as wired into a metal circuit, something I never dreamed we might even come close to." Their work has attracted outside interest and the researchers have set up a molecule calibration "factory" which involves collaboration with the electronics giant Motorola.

Lindsay, Gust, and Moore have followed up #4 with other research papers. They have published a more detailed account on the making of electrical contacts (see X.D. Cui, et al., Nanotechnology, 13[1]: 5-14, 2002), and this paper has already been downloaded more than 4,000 times. Other papers have dealt with the methodology of measurement (X.D. Cui, et al., Ultramicroscopy, 92[2]: 67-76, 2002), and with single-molecule electronics and tunnelling aspects (S.M. Lindsay, Jap. J. Appl. Phys. Pt. 1, 41[7B]: 4867-70, 2002).

And what of the longer term? Says Lindsay: "It is a priority for us to find ways of building nanostructures that hold molecules between metal gaps in a well defined, well bonded way. Lots of people want to do this, but it’s hard. Even if we are not the ones to do it, we can offer ‘calibrated’ molecules to other people-or we can calibrate their pet molecules for them."