N408_2c 67..69

So-called bottom-up fabrication methods aim to assemble and integrate molecular components exhibiting specific functions into electronic devices that are orders of magnitude smaller than can be fabricated by lithographic techniques. Fundamental to the success of the bottom-up approach is the ability to control electron transport across molecular components. Organic molecules containing redox centres—chemical species whose oxidation number, and hence electronic structure, can be changed reversibly—support resonant tunnelling and display promising functional behaviour when sandwiched as molecular layers between electrical contacts, but their integration into more complex assemblies remains challenging. For this reason, functionalized metal nanoparticles have attracted much interest: they exhibit single-electron characteristics (such as quantized capacitance charging) and can be organized through simple self-assembly methods into well ordered structures, with the nanoparticles at controlled locations. Here we report scanning tunnelling microscopy measurements showing that organic molecules containing redox centres can be used to attach metal nanoparticles to electrode surfaces and so control the electron transport between them. Our system consists of gold nanoclusters a few nanometres across and functionalized with polymethylene chains that carry a central, reversibly reducible bipyridinium moiety. We expect that the ability to electronically contact metal nanoparticles via redox-active molecules, and to alter profoundly their tunnelling properties by charge injection into these molecules, can form the basis for a range of nanoscale electronic switches. The approach followed in the present work is summarized schematically in Fig. 1. The redox group (‘redox gate’) forms the backbone of a bifunctional ligand that attaches a gold nanoparticle to a gold substrate (see Methods). Electron (or hole) injection to the redox centre is controlled by the potential applied between the substrate and a counter electrode, and the resulting change in barrier transparency can be used to control electron tunnelling between the nanoparticle and the substrate. The tip of a scanning tunnelling microscope (STM) is positioned above the nanoparticle, and is used to monitor electron tunnelling (see Methods). The redox switchable group used here is a bipyridinium (viologen) moiety located at the centre of a chain of 20 methylene units in an a,v-polymethylenedithiol compound (1 in Fig. 1 inset), previously described in refs 14 and 15. Reduction of the bipyridinium group takes place readily and reversibly in a self-assembled film of the ligand on a gold electrode when Au nanoparticles have been attached to the film; the redox process corresponds to the formation of the bipyridinium radical ion: