Scientists use imaging techniques to visualize heat

Most people imagine massive vibrations, such as the topic of mobile notifications or the vibrations of electric toothbrushes. But scientists are thinking about vibrations on small scales, and even atoms.

Field’s first researcher, researchers at Grainger Engineering University at the University of Illinois at Urbana-Champaign, directly observed previously hidden branches of vibrational physics in 2D materials using advanced imaging techniques. Their findings published in Science confirm the existence of a class of vibrational modes that we have not seen before, and present the highest resolution images ever taken of a single atom.

Two-dimensional materials are a promising candidate for next-generation electronics as they can reduce their size to just a few atoms thick while maintaining the desired electronic properties. The route to these new electronic devices is at the atomic level by creating so-called Moiré systems. This is a stack of 2D materials whose lattices do not match due to twisting of the atomic layer and other reasons.

Moiré Phonons is a low-frequency vibration mode inherent in twisted 2D double layer materials. Because heat is the result of vibrational patterns, examining different patterns among phonons can help scientists to better understand heat expression. Like phonons, faysons are vibrational modes associated with atomic motion, and are thought to explain some of the unique and desirable properties found in twisted 2D materials. However, up until now, 2D materials have avoided direct observation, rendering predictions about purely hypothetical existence.

“Fayson can’t be easily removed, that’s the blessing and curse,” said Pinshan Fang, a professor of materials science and engineering and a senior author of the paper. “They are constantly hanging out undetected and have changed the properties of 2D moire materials.”

Huang’s interest in electron microscopy prompted questions. Can new advances in imaging technology be used to visualize local vibration modes such as Phasons? To investigate this possibility, Huang joined forces with Yichao Zhang, a post-dokral researcher studying the lead authors of nanoscale heat transport and research.

“Our central goal was to see heat by looking at atoms,” Huang said. “This works by getting a high spatial resolution that can change how the oscillations of atoms are blurred. These movements are small, and you can literally see one atom and see the movements due to heat.”

To acquire these images, the team relied on Electron Ptychography, a recently developed technology that greatly enhances the resolution of existing microscopes. By achieving picometer-scale spatial resolution, researchers directly observed thermal vibrations in twisted double-layered WSE2 atoms.

“The best resolution we thought possible at the start of my career wasn’t under one unstrom,” Huang said. “But when ptychography rolled in a few years ago, we began to see a low number of an anstroms of 0.2. Being able to see heat is an example of how a monumental leap in resolution fundamentally changes what a microscope can do.”

Illinois Grainger engineers are anticipating a future in which pasons can be used to create electronic devices that have different functions than current iterations.

“One potential application of this technique is to create materials that are better thermal conductors,” Zhang said. “You can look at a single atom and identify defects that prevent the material from cooling more efficiently. This could lead to better thermal management techniques at the atomic scale. Looking at atoms one by one, how they respond to thermal vibrations gives you basic knowledge of that type.”