Scientists record heat moving through materials at speed of sound

Providing unprecedented insight into roles played by individual atomic and nano-scale features, researchers have recorded the first-ever videos showing how heat moves through materials at the nano-scale travelling at the speed of sound.

The groundbreaking videos were made using a state-of-the-art ultrafast electron microscope called FEI Tecnai Femto, which is capable of examining the dynamics of materials at the atomic and molecular scale over time spans measured in femtoseconds (one millionth of a billionth of a second).

According to the study, published recently in Nature Communications, the researchers used a brief laser pulse to excite electrons and very rapidly heat crystalline semiconducting materials of tungsten diselenide and germanium.

They then captured slow-motion videos (slowed by over a billion times the normal speed) of the resulting waves of energy moving through the crystals.

"As soon as we saw the waves, we knew it was an extremely exciting observation," said lead author David Flannigan from the University of Minnesota. "Actually watching this process happen at the nanoscale is a dream come true," he added.

Flannigan said the movement of heat through the material looks like ripples on a pond after a pebble is dropped in the water.

The videos show waves of energy moving at about 6 nanometres (0.000000006 meters) per picosecond (0.000000000001 second).

Mapping the oscillations of energy, called phonons, at the nano-scale is critical to developing a detailed understanding of the fundamentals of thermal-energy motion.

The recordings could help develop better, more efficient materials for electronics and alternative energy.

In many applications, scientists and engineers want to understand thermal-energy motion, control it, collect it, and precisely guide it to do useful work or very quickly move it away from sensitive components, according to Flannigan.

"Because the lengths and times are so small and so fast, it has been very difficult to understand in detail how this occurs in materials that have imperfections, as essentially all materials do. Literally watching this process happen would go a very long way in building our understanding, and now we can do just that," he said.