Molecules with a need for speed - The smallest car race in the world takes place in a vacuum

Molecules with a need for speed - The smallest car race in the world takes place in a vacuum

Nanocars are tiny – consisting of just a single molecule. When this type of “race car” took part in a race a few weeks ago in Toulouse, France, it was conducted under extreme conditions: in an ultra-high vacuum and at temperatures close to absolute zero.

This race doesn't feature any droning engines and risky overtaking manoeuvres; and the speed isn't exactly frantic, measured as it is in nanometres rather than kilometres per hour. Nonetheless, the competition between these tiny participants is uniquely exciting, as it involves controlling individual molecules, atoms and subatomic particles.

A free run thanks to vacuum

The racecourse in question is hundreds of nanometres long and made of pure gold. What's more, the nano-race takes place at extremely frosty temperatures. To make it possible to direct the competitors along the racecourse, the molecules' own movement has to be minimized, so the holding location is therefore cooled to minus 269°C using liquid nitrogen and helium. Almost nothing moves of its own accord so close to absolute zero.

In order to give the vehicles a free run, the racecourse needs to be completely clear of foreign atoms. Even the smallest contamination, such as a molecule of atmospheric oxygen, could throw a nano-car from the track. To maintain these conditions, the contest takes place in an ultra-high vacuum. With the aid of turbo and ion getter vacuum pumps the pressure is reduced to 10-11 mbar.

Atoms as route-marking

The golden racecourse is also thoroughly cleaned before the race: its surface is bombarded with ions and then heated, in order to ensure a flat track. Nano-scientists then add individual gold atoms to mark the course of the vehicles along this track.

The next challenge is to place the molecular cars onto the course, without destroying them in the process. To achieve this, they are heated in a preparatory chamber until they evaporate. The right candidate for the race is then selected from the molecules that subsequently settle on the surface of the gold racecourse.

Detected using tunnel flow

The tiny competitors, each approximately the size of just one hundred-thousandth of the diameter of a human hair, cannot be seen using optical microscopes as the light wavelength is too large. For this reason, the teams of researchers track what happens in the race using a scanning tunnelling microscope. In this tool, an electrically conductive probe, the tip of which consists of just a single atom, is moved over the surface of the race track in a grid shape, at intervals of just a few nanometres.

Although the probe tip and the object of investigation never come into contact, in accordance with the rules of quantum mechanics, a tunnel flow is created when low voltage is applied. The volume of this flow is very much dependent on the distance between the object and the probe. When scanning the gold foil, the tip is precisely controlled in such a way that the distance and flow volume always remain constant. This allows scientists to create a height profile for the racecourse, with a topographic representation of the nanocars.

Eight hours to the finish line

In fact, the researchers do not just use the probe to make their race cars visible, but also to control their movement. In addition, they also use the tunnel flow. In Toulouse, this method was used to raise a few molecular vehicles to a higher energy level through the addition of an electron. The vehicles immediately re-released this energy and converted it into movement. Other race teams used the electrostatic repulsion of the electrodes to move forward.

It would also be possible to steer the vehicles to their destination mechanically using the tip of the probe, but during the race this was only permitted in the event of a dire emergency. The victorious Swiss team didn't need to resort to this type of manipulation. Their Swiss Nano Dragster reached the – possibly record-breaking – speed of around twelve nanometres per hour, and was the fastest to cross the golden finish line after approximately eight hours. Just as at large-scale races, the success was celebrated with a shower of champagne.

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The scientists were only allowed to use around one hundred atoms for the construction of their miniature cars. However, there were no strict specifications for how they should be composed or their external appearance.

The principle of an air cushion vehicle wins

The vehicle bodies provided by the six different teams of researchers were certainly diverse: nanocars from France, the USA, and an American-Austrian collaboration were in fact similar to familiar four-wheeled competitors, while Japan's vehicle was reminiscent of a bone, and Germany brought a windmill-like structure to the starting grid. The successful design from the University of Basel followed the principle of an air cushion vehicle. The flat molecule consisted of four carbon rings arranged in a Y shape. The engine of the structure was formed by three carbon atoms, while a methyl group is used as a front spoiler. However, the nano-scientists from Basel did not develop the molecule specifically for taking part in the miniature car race: in everyday research conducted by the Swiss team, the Swiss Nano Dragster functions as a component of organic solar cells.

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