New ultra-fast shutter speed captures the movement of atoms

While digital cameras on the market offer shutter speeds ranging from 30 seconds to 1/8000 of a second, researchers at Columbia Engineering (New York) and the University of Burgundy have developed a new device that demonstrates incredibly fast shutter speeds of approximately 1 picosecond (10-12 seconds), in a million million times faster than conventional camera shutters! Their system, in particular, will allow a better understanding of how materials conduct heat.

Shutter speed refers to the speed at which the camera’s shutter closes: a fast shutter speed results in a shorter exposure time (exposure time), which means more light enters the device. In the case of this new device, the shutter speed is so fast that it allows you to see the “dynamic disorder” of atoms – the collective fluctuations of clusters of atoms inside materials that are caused by external phenomena such as vibration or temperature changes. .

However, a better understanding of dynamic disorder in materials could lead to more energy efficient thermoelectric devices. Unfortunately, this disorder is especially difficult to study because clusters of atoms are not only very small and disordered, but also fluctuate with time. In addition, there is also “static disorder” in materials, which does not improve the properties of the material (therefore lacks interest), but interferes with observations of dynamic disorder (traditional crystallography fails to distinguish between the two). A new system of ultra-fast shutter speeds developed by the researchers helps to circumvent this problem.

The dynamics of the crystal structure has finally been revealed!

The shutter speed of the device is variable; while slowing down, the team observes an atomic structure that appears ordered but blurry as the atomic clusters are in motion. At higher speeds, an even more accurate picture can be taken, showing a clear, complex pattern of dynamic movements. This system is called “Variable Gate PDF” or vsPDF (for Variable Gate Atom Pair Distribution Function).

At low exposures, the atomic structure of germanium telluride (GeTe) looks ordered, but diffuse. Faster exposures show a complex pattern of dynamic shifts. © Jill Hemman/ORNL, US Department of State. energy

” [Le vsPDF] offers us a whole new way to unravel the intricacies of what goes on in complex materials, the hidden effects that can enhance their properties. Using this technique, we will be able to observe the material and see which atoms are dancing and which are not,” says Simon Billinge, professor of materials science, applied physics and applied mathematics at Columbia University and co-author of the study. article representing the device.

This system clearly has nothing to do with a conventional camera: it uses neutrons (from a source at the US Department of Energy’s Oak Ridge National Laboratory) to measure the position of atoms. In practice, this method consists of studying how neutrons travel through a material to measure the displacement of surrounding atoms – neutron scattering pictures are taken by changing the shutter speed from slow to fast.

The team pointed this “neutron chamber” at a material called germanium telluride (GeTe), which has special electronic properties. It exhibits semi-metallic conductivity and ferroelectric behavior. Solid GeTe can transition from an amorphous state to a crystalline state: the crystalline state has a low resistivity, while the amorphous state (obtained at very high temperature) has a high resistivity. It is an important material for thermoelectricity and converts waste heat into electricity (or electricity into cooling).

Towards better conversion of waste heat into electricity

The vsPDF tool showed that, on average, GeTe retains its crystal structure at all temperatures. But at higher temperatures, it exhibits “anisotropic anharmonic dynamics” in which atoms convert their motion into thermal energy, following a gradient corresponding to the direction of the material’s spontaneous electrical polarization.

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Using insights provided by vsPDF, the team developed a new theory that shows how such local fluctuations can form in GeTe and related materials. This will help scientists identify new materials that exhibit this particular effect, as well as determine the external forces required to influence this effect, which could lead to more energy efficient thermoelectric devices such as refrigerators and heat pumps.

This understanding of dynamic disorder could also lead to a better recovery of usable energy from waste heat (such as car and power plant exhaust) by converting it directly into electricity. The researchers also raise the possibility of developing new instruments capable of powering rovers when sunlight is not available.

At the moment, the equipment is not ready for use, they note. But taking it further, it could become a standard tool used in many systems where atomic dynamics are important: for example, one could observe the movement of lithium in battery electrodes or study the dynamic processes that occur when water separates. under the influence of sunlight.

S. Kimber et al., Nature Materials.

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