Krypton panopticon: trapped atoms form one-dimensional gas

Scientists looking for a new way to observe atoms have turned to the inert noble gas, krypton.

Chemists were able to imprison krypton atoms inside a carbon nanotube, forcing the atoms to form a one-dimensional gas, so that they can film them with electron microscopes.

Why create this krypton panopticon? Because it’s very difficult to see atoms – and, therefore, to study them.

In gases, atoms zip around at speeds of 400 metres per second, making them tricky to nail down. Plus, at a few tenths of a nanometre in size, atoms are much smaller than wavelengths of light, making traditional microscopy useless for examining them. Researchers need instead to use electron microscopes, which use beams of electrons instead of light.

“As far as we know, this is the first time that chains of noble gas atoms have been imaged directly, leading to the creation of a one-dimensional gas in a solid material,” says krypton captor Professor Paul Brown, director of the Nanoscale and Microscale Research Centre at the University of Nottingham, UK, and co-author on a paper published in ACS Nano.

“Such strongly correlated atomic systems may exhibit highly unusual heat conductance and diffusion properties.”

Krypton (Kr) is an inert noble gas, reacting with almost nothing. Krypton atoms are bigger than those of fellow noble gases like argon or neon, making it a good subject for capturing under an electron microscope.

“Because krypton has a high atomic number, it is easier to observe in a TEM [transmission electron microscope] than lighter elements. This allowed us to track the positions of krypton atoms as moving dots,” says co-author Professor Andrei Khlobystov, a chemist at the University of Nottingham.

Single atoms of Kr entrapped in C60 fullerene cages within nanotube. Credit: University of Nottingham

So how did they make this krypton jail?

With soccer balls – or rather, soccer ball-shaped molecules called buckminsterfullerenes, or buckyballs. These molecules, which are each made from 60 carbon atoms, could act as individual krypton cages.

 “Krypton atoms can be released from the fullerene cavities by fusing the carbon cages. This can be achieved by heating at 1200oC or irradiating with an electron beam,” says Ian Cardillo-Zallo, a PhD student at the University of Nottingham.

Once the buckyballs had been fused together into carbon nanotubes, the krypton atoms were free to move – sort of. They could only move in one direction, along the carbon nanotube, and had to do so slowly.

The nanotubes, which the researchers call “nano test tubes”, are a couple of nanometres in width – about a thousand times smaller than a small bacterial cell.

This allowed the researchers to examine both how the gas atoms behaved, and some rare bonding between krypton atoms to form paired Kr2 molecules.

“These pairs are held together by the van der Waals interaction, which is a mysterious force governing the world of molecules and atoms,” says Professor Ute Kaiser, from the University of Ulm, Germany.

“This is an exciting innovation, as it allows us to see the van der Waals distance between two atoms in real space. It’s a significant development in the field of chemistry and physics that can help us better understand the workings of atoms and molecules.”

The researchers are now hoping to turn their finely-tuned electron microscopes to understand one-dimensional systems and temperature changes.

“Transmission electron microscopy has played a crucial role in understanding the dynamics of atoms in real-time and direct space,” says Brown.

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