April 15, 2016
- Using a state-of-the-art ultrafast electron microscope, assistant professor David Flannigan, materials science graduate student Daniel R. Cremons and chemical engineering graduate student Dayne A. Plemmons have recorded the first-ever videos showing how heat moves through materials at the nanoscale traveling at the speed of sound. Their research, "Femtosecond Electron Imaging of Defect-Modulated Phonon Dynamics," is published in Nature Communications and provides unprecedented insight into roles played by individual atomic and nanoscale features that could aid in the design of better, more efficient materials with a wide array of uses, from personal electronics to alternative-energy technologies.
Materials scientists and engineers have spent decades researching how to control thermal energy at the atomic level in order to recycle and use it to dramatically increase efficiencies and ultimately drive down the use of fossil fuels. Such work would be greatly aided by actually watching heat move through materials, but capturing images of the basic physical processes at the heart of thermal-energy motion has presented enormous challenges. This is because the fundamental length scales are nanometers (a billionth of a meter) and the speeds can be many miles per second. Such extreme conditions have made imaging this ubiquitous process extraordinarily challenging.
To overcome these challenges and image the movement of heat energy, the researchers used a cutting-edge FEI Tecnai™ Femto ultrafast electron microscope (UEM) 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). In this work, the CEMS team 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.
“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,” Flannigan said. “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.” The research was funded primarily by the National Science Foundation through the University of Minnesota Materials Research Science and Engineering Center.
The image (above) is a false-colored ultrafast electron microscope (UEM) snapshot of a thin semiconducting crystal. The image was captured with an extremely fast shutter lasting only a few hundred femtoseconds (a millionth of a billionth of a second).
Related Link: http://www.nature.com/ncomms/2016/160415/ncomms11230/full/ncomms11230.html