New vsPDF Captures Dynamic Disorder in Germanium Telluride at One Trillionth of a Second

The best digital cameras available can open their shutters for roughly one four-thousandth of a second to take a photograph.

Capturing atomic activity, however, requires a shutter that operates far more quickly.

In 2023, scientists presented a method capable of reaching a shutter speed of just one trillionth of a second – 250 million times faster than those digital cameras. It can capture an important feature of materials science: dynamic disorder.

Dynamic disorder in materials

In simple terms, dynamic disorder occurs when groups of atoms shift and move within a material in particular ways over a period of time, prompted by factors such as vibrations or temperature changes. Scientists do not yet fully understand the phenomenon, although it is essential to the properties and reactions of materials.

This ultra-fast shutter system offers considerably greater insight into the processes behind dynamic disorder. Its developers call the invention variable shutter atomic pair distribution function, shortened to vsPDF.

"It's only with this new vsPDF tool that we can really see this side of materials," said materials scientist Simon Billinge from Columbia University in New York.

"With this technique, we'll be able to watch a material and see which atoms are in the dance and which are sitting it out."

A higher shutter speed produces a more exact instant in time, making it useful for fast-moving subjects such as atoms vibrating rapidly. If a low shutter speed is used to photograph a sporting event, for example, the players will appear blurred in the image.

How the vsPDF neutron camera works

Rather than relying on standard photographic methods, vsPDF achieves its remarkable speed by using neutrons to determine atomic positions. Scientists can monitor how neutrons strike and travel through a material to measure the atoms around them, while shifts in energy levels act as the equivalent of changing the shutter speed.

The ability to vary shutter speed matters as much as the trillionth-of-a-second setting itself. It is crucial for distinguishing dynamic disorder from the related, yet distinct, static disorder: the usual background movement of atoms that merely jiggle in place and do not improve a material's performance.

"It gives us a whole new way to untangle the complexities of what is going on in complex materials, hidden effects that can supercharge their properties," said Billinge.

Germanium telluride under observation

For this study, the team aimed their neutron camera at germanium telluride (GeTe). Because of its particular characteristics, the material is widely used to turn waste heat into electricity or to convert electricity into cooling.

The camera showed that GeTe retained a crystalline structure, on average, at every temperature. At higher temperatures, though, it exhibited greater dynamic disorder: its atoms transferred motion into thermal energy along a gradient aligned with the direction of the material's spontaneous electric polarisation.

A stronger understanding of these physical structures advances knowledge of how thermoelectrics work, helping researchers create improved materials and devices, including the equipment that powers Mars rovers when sunlight is unavailable.

Scientific understanding of these materials and processes can be refined through models built from the observations recorded by the new camera. Nevertheless, substantial work remains before vsPDF is ready to become a widely adopted testing technique.

"We anticipate that the vsPDF technique described here will become a standard tool for reconciling local and average structures in energy materials," the researchers explained in their paper.

The research was published in Nature Materials.

An earlier version of this article was published in March 2023.