Nanotech rolls on

There's a nice little paper about to appear online in Nature Nanotechnology (advance online publication, doi:10.1038/nnano.2006.210). Christian Joachim and his colleagues have made a molecule consisting of a two wheels 8 Å wide on an axle. The wheels are in fact three-bladed paddlewheels, which may rotate by a third of a turn when pushed with the tip of a scanning tunnelling microscope. This sounds rather like Jim Tour's nanocar, in which the wheels are fullerenes. That's altogether a more sophisticated contraption: a four-wheeler with a full chassis, shown in the picture above. But Jim could only infer rolling from the nature of the path that the cars took when pulled with the STM. It looked pretty persuasive to me, especially as tripod cars could only go in circles. But Christian's work has more direct proof of rolling. By looking closely at the way the two-wheeled molecule moves, he and his colleagues can distinguish between simple hopping between the grooves of the atomic corrugations on the metal surface, smooth pushing when the corrugation is very shallow, and jumps due to rotation of one wheel by 120 degrees. Only one wheel turns at a time, so in that case the molecule pivots as it is pushed.