It is the nano-equivalent of seeing the first stone disc roll down a prehistoric hill: German and French scientists have, for the first time, observed a nanoscopic wheel rolling over a flat surface.
The researchers created a carbon molecule resembling a pair of wheels just 0.8 nanometres in diameter, joined together by an axle only four carbon atoms long. The achievement was a combined effort by researchers at the Free University of Berlin in Germany and the Center for Material Elaboration & Structural Studies in Toulouse, France.
After being chemically synthesised in the lab, hundreds of these "nanowheels" were sprayed onto a sheet of copper. The researchers then used the tip of a scanning tunnelling microscope (STM) to push individual molecules across a copper surface.
"Using the STM provided a direct readout that shows the wheels [only] roll when pushed in the right direction," perpendicular to the axle, says Leonhard Grill, leader of the German research team, told New Scientist. If nudged the wrong way, the molecules instead skip across the surface.
The tip of the microscope is so sharp, its point is just a single atom wide. This is connected to an electric circuit linked to a substrate that supports a sample. Electrons "tunnel" from the tip to the sample to the substrate beneath and subtle variations in current reveal features of the sample.
The microscope confirmed that the nanowheels will only roll in the correct direction. "When we pushed in a direction that doesn't allow the wheel to rotate we got a different pattern that shows they hop across the surface," says Grill.
Grill acknowledges that other researchers have previously made nanoscale wheels. "They have done good research," he says. "The only thing that was missing was a direct readout of rolling."
One such team is led by James Tour at Rice University in Houston, US (see Nano-car gets an engine). "We say that our wheels are rolling based on the fact that the 'cars' moved forward and back, and never side to side," Tour explains.
"It's fascinating to reproduce movement so important in our world at this scale," adds Grill. Because wheels can only travel in certain directions, they could provide a useful tool for nanoscale manufacturing he suggests: "Any nano-device made from different parts needs to be assembled; rolling could be one way of doing that."
Journal reference: Nature Nanotechnology (DOI: 10.1038/nnano.2006.210)
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