Strange 'one-dimensional gas' created by trapping atoms in a tiny tube

Krypton atoms can be forced into a “traffic jam” within a carbon nanotube, unable to pass each other, allowing scientists to more easily observe how they interact. Researchers hope this “one-dimensional gas” could shed light on fundamental physical forces.

Andrei Khlobystov at the University of Nottingham, UK, and his colleagues have spent years investigating chemical reactions inside carbon nanotubes just 1.5 nanometres thick – half a million times smaller than the width of a human hair – as the constricted space limits movement and makes observation easier. They have now developed a method to do the same with atoms of the noble gas krypton, creating what they call a one-dimensional gas.

The team takes buckminsterfullerene molecules – spherical cages formed of 60 carbon atoms – with a single krypton atom trapped inside. These molecules are then sucked into a carbon nanotube by the van der Waals force, the weak attraction caused by fluctuations in the clouds of electrons surrounding the atomic nucleus. Once full, the tube is heated to 1200°C, breaking down the cages. The carbon atoms are absorbed into the nanotube, leaving a line of krypton atoms.

Khlobystov says the result is like a “traffic jam” where atoms can be observed at leisure, rather than zipping around at up to 400 metres per second as they often do in a three-dimensional gas. The group images the atoms with transmission electron microscopy, which allows them to accurately measure the distances between atoms.

“They change their behaviour fundamentally,” says Khlobystov. “This is quite a curious system. We can follow their trajectories – how they move, how they interact. It’s a nice toy to play with noble gases, which will give us some fundamental understanding of the behaviour of atoms under extreme confinement.”

Other researchers have already observed atoms of krypton forming into pairs held together by the van der Waals force – a phenomenon difficult to observe in unconstrained atoms, which could also occur inside nanotubes. Khlobystov says that further experiments will be “full of surprises”.

Future research will explore how temperature affects the one-dimensional gas. While lowering the temperature of a gas in a three-dimensional space causes it to condense into a liquid, then solidify, there is no guarantee that the same rules apply in one dimension.

“Maybe there’s no such thing as a 1D liquid and it will go straight to a 1D solid. It’s a bit of a journey of discovery,” says Khlobystov.