U researchers record video of heat waves

Using an ultrafast microscope, the team made groundbreaking nano-scale observations.

Kevin Beckman

Two graduate students at the University of Minnesota have recorded the first-ever videos of nanoscale heat waves.
Using a brief laser pulse, researchers excited electrons to rapidly heat materials of tungsten diselenide and germanium, two semiconductors that can be used to make electronic devices.
The team, under the direction of chemical engineering and material science professor David Flannigan, was able to observe and record the heat movement on video — marking a new scientific achievement. They published their research on Friday in Nature Communications.
“What we were asking was, ‘What does this actually look like? How does it work, and can we actually do experiments to directly watch this happen?’” Flannigan said. 
Flannigan said the way energy waves move a through material affects how electrons in the material itself interact. Being able to actually observe the movement of energy could help engineers create more efficient devices, such as transistors.
The observations could also improve and reduce research’s application of heat energy, which could help scientists recommend how to decrease use of fossil fuels, Flannigan said.
For example, he said, about 70 percent of the energy in gasoline is wasted as heat in automobile engines. But the speed and scales of heat energy has rendered observing the process difficult. 
Scientists measure waves in nanometers — a billionth of a meter — and at speeds between 4 and 6 miles per second. 
“You combine that high velocity with these really small scales, and you’re talking about very brief, very fast, very short-lived phenomena,” Flannigan said. 
In order to record the process, researchers used an ultrafast electron microscope capable of examining the material dynamics at an atomic and molecular scale and in femtoseconds — one millionth of a billionth of a second. 
“With this crazy gizmo, we can actually image this energy moving through materials at the speed of sound,” Flannigan said. 
In order to see the waves move, researchers slowed their videos by over a billion times the normal speed. 
“We’ve never been able to do that before,” Flannigan said. “We’ve always just kind of speculated and built our theories on what we think it might look like and how we think it might move, but now we can actually watch this happen.” 
Like ripples on a pond after a pebble is dropped into it, heat energy undulates, Flannigan said. 
Flannigan said he and doctoral students Dayne Plemmons and Dan Cremons started trying to observe the waves in June 2015 and drafted their initial observations in September. 
“It wasn’t too long, and that really was a testament to [Dayne and Dan],” Flannigan said. “Really it was their hard work that made this possible.” 
Dayne Plemmons was the first researcher to observe the energy waves while going through image scans of tungsten diselenide.
“I would see this jittering that occurred, and I was thinking to myself, ‘That actually looks like it’s moving,’ ” Plemmons said. 
He took his findings to Flannigan shortly after. 
“He showed me one of the videos, and I immediately knew what he had,” Flannigan said. “I just knew this was a breakthrough.”
Cremons said he had observed the same phenomenon in germanium roughly 24 hours after Plemmons saw it in tungsten diselenide. 
“It was just really funny that they’re working on two different projects in parallel, and they discover the same thing at almost the same time,” Flannigan said.
Plemmons said observing the same event in both germanium and tungsten diselenide, which have very different structural compositions, meant their observations could be generalized to several other different types of materials. 
“Nobody else in the world had seen anything like it,” Cremons said. “So it was kind of hard to wrap our minds around what we had seen.”