The ability of atoms and molecules to self-assemble into spectacular patterns on surfaces is clearly demonstrated in this scanning tunnelling microscope (STM) image taken by Guilluame Schull and Richard Berndt of Christian-Albrechts-Universität in Kiel, Germany, which was presented at the recent Trends in Nanotechnology conference in San Sebastian, Spain (www.tnt2007.org).
Schull and Berndt deposited approximately 0.2 monolayers of C60 molecules on a gold surface under ultrahigh vacuum at room temperature and then imaged the system with a low-temperature STM at 5.7 K. The image on the right, which is 20 nm across, shows that the C60 molecules (yellow and white structures) form a hexagonal lattice with a rhombus-shaped unit cell (yellow lines) that consists of 49 molecules.
The unit cell can be divided into a half that contains a defect and a half that is defect-free. Similar structures have been studied in the 7×7 reconstruction of silicon surfaces for several decades.
Further analysis of the image reveals a layer of complexity that is not found when atoms, rather than molecules, self-assemble on a surface. Although C60 molecules are often compared to footballs, they are not perfectly spherical, which means that they have an orientation or direction (white arrows in inset). On some surfaces the C60 molecules are restricted to 'pointing' in just one or two directions because of the strong interactions between the surface and the molecules, but 11 different orientations can be observed in this system, some of which are shown in the inset.
Moreover, as Schull and Berndt demonstrate, the orientations also display long-range order. The work will be published in Physical Review Letters.
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Rodgers, P. Made to order. Nature Nanotech 2, 672 (2007). https://doi.org/10.1038/nnano.2007.358