Scientists use imaging technology to visualize heat

Most people envision vibration on a large scale, like the buzz of a cell phone notification or the oscillation of an electric toothbrush. But scientists think about vibration on a smaller scale—atomic, even.

In a first for the field, researchers from The Grainger College of Engineering at the University of Illinois at Urbana-Champaign have used advanced imaging technology to directly observe a previously hidden branch of vibrational physics in 2D materials. Their findings, published in Science, confirm the existence of a previously unseen class of vibrational modes and present the highest resolution images ever taken of a single atom.

Two-dimensional materials are a promising candidate for next-generation electronics because they can be scaled down in size to thicknesses of just a few atoms while maintaining desirable electronic properties. A route to these new electronic devices lies at the atomic level, by creating so-called Moiré systems—stacks of 2D materials whose lattices do not match, for reasons such as the twisting of atomic layers.

Moiré phonons are low-frequency vibrational modes unique to twisted 2D bilayer materials. Because heat is a consequence of vibrational patterns, examining different patterns among phonons can help scientists better understand heat expression. Like phonons, phasons are vibrational modes associated with atomic movement, and they are thought to explain some of the unique and desirable properties seen in twisted 2D materials. But until now, phasons in 2D materials had eluded direct observation, rendering predictions about their existence purely hypothetical.

“You can’t easily get rid of phasons; that’s the blessing and the curse,” said Pinshane Huang, a professor of materials science & engineering and the senior author of the paper. “They’ve always been hanging around undetected, changing the properties of 2D moiré materials.”

Huang’s interest in electron microscopy prompted the question: Can new advancements in imaging technology be used to visualize local vibrational modes such as phasons? To investigate this possibility, Huang joined forces with Yichao Zhang, then a postdoctoral researcher studying nanoscale heat transport and the study’s lead author.

“Our central goal was to see heat by looking at an atom,” Huang said. “This works by getting such high spatial resolution that the vibrations of atoms change how blurry the atoms appear. These motions are tiny, and we are literally able to look at one atom at a time and see how they are moving due to heat.”

To obtain these images, the team relied on electron ptychography, a recently developed technique that massively enhances the resolution of existing microscopes. By achieving picometer-scale spatial resolution, the researchers directly observed thermal vibrations in twisted bilayer WSe2 atoms.

“At the start of my career, the highest resolution we thought was possible was just under one angstrom,” Huang said. “But when ptychography rolled around a few years ago, we started seeing numbers as low as 0.2 angstroms. That got us thinking, ‘hey, heat vibrates atoms by roughly 0.05 angstroms.’ Being able to see heat is one example of how a monumental leap in resolution fundamentally changes what microscopes can do.”

The Illinois Grainger engineers anticipate a future in which phasons may be used to create electronics that function differently than current iterations.

“One potential application of this technique is making materials that are better heat conductors,” Zhang said. “We could look at a single atom and identify a defect that’s preventing the material from cooling down more efficiently. This could lead to better thermal management techniques at the atomic scale. Looking at atoms one by one and how they respond to thermal vibrations will give us that type of fundamental knowledge.”