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Imagine you are playing the guitar—each pluck of a string creates a sound wave that vibrates and interacts with other waves.

Now shrink that idea down to a small single molecule, and instead of sound waves, picture vibrations that carry heat.

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Ultra-high vacuum scanning probe setup modified by the Cui Research Group to conduct thermal microscopy experiments.

A team of engineers and materials scientists in the��Paul M. Rady Department of Mechanical Engineering at ������ý Boulder has recently discovered that these tiny thermal vibrations, otherwise known as phonons, can interfere with each other just like musical notes—either amplifying or canceling each other, depending on how a molecule is "strung" together.

Phonon interference is something that’s never been measured or observed at room temperature on a molecular scale. But this group has developed a new technique that has the power to display these tiny, vibrational secrets.

The breakthrough study was led by Assistant Professor��Longji Cui and his team in the��. Their work, funded by the National Science Foundation in collaboration with researchers from Spain (Instituto de Ciencia de Materiales de Madrid, Universidad Autónoma de Madrid), Italy (Istituto di Chimica dei Composti Organometallici) and the ������ý Boulder Department of Chemistry, was recently published in the��.

The group says their findings will help researchers around the world gain a better understanding of the physical behaviors of phonons, the dominant energy carriers in all insulating materials. They believe one day, this discovery can revolutionize how heat dissipation is managed in future electronics and materials.

“Interference is a fundamental phenomenon,” said Cui, who is also affiliated with the��Materials Science and Engineering Program and the��Center for Experiments on Quantum Materials. “If you have the capability to understand interference of heat flow at the smallest level, you can create devices that have never been possible before.”

The world’s strongest set of ears

Cui says molecular phononics, or the study of phonons in a molecule, has been around for quite some time as a primarily theoretical discussion. But you need some pretty strong ears to “listen” to these molecular melodies and vibrations first-hand, and that technology just simply hasn’t existed.

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A sneak peek into the ultra-high vacuum scanning probe microscopy setup used to conduct molecular measurements.

That is, until Cui and his team stepped in.

The group designed a thermal sensor smaller than a grain of sand or even a sawdust particle. This little probe is special: it features a record-breaking resolution that allows them to grab a molecule and measure phonon vibration at the smallest level possible.

Using these specially designed miniature thermal sensors, the team studied heat flow through single molecular junctions and found that certain molecular pathways can cause destructive interference—the clashing of phonon vibrations to reduce heat flow.

Sai Yelishala, a PhD student in Cui’s lab and lead author of the study, said this research using their novel scanning thermal probe represents the first observation of destructive phonon interference at room temperature.��

In other words, the team has unlocked the ability to manage heat flow at the scale where all materials are born: a molecule.

“Let’s say you have two waves of water in the ocean that are moving towards each other. The waves will eventually crash into each other and create a disturbance in between,” Yelishala said. “That is called destructive interference and that is what we observed in this experiment. Understanding this phenomenon can help us suppress the transport of heat and enhance the performance of materials on an extremely small and unprecedented scale.”

Tiny molecules, vast potential

Developing the world’s strongest set of ears to measure and document never-before-seen phonon behavior is one thing. But just what exactly are these tiny vibrations capable of?

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PhD student and lead author of the study Sai Yelishala (right), along with Postdoctoral Associate and second author Yunxuan Zhu (left). Both are members of the Cui Research Group led by Assistant Professor Longji Cui.

“This is only the beginning for molecular phononics,” said Yelishala. “New-age materials and electronics have a long list of concerns when it comes to heat dissipation. Our research will help us study the chemistry, physical behavior and heat management in molecules so that we can address these concerns.”

Take an organic material, like a polymer, as an example. Its low thermal conductivity and susceptibility to temperature changes often poses great risks, such as overheating and degradation.

Maybe one day, with the help of phonon interference research, scientists and engineers can develop a new molecular design. One that turns a polymer into a metal-like material that can harness constructive phonon vibrations to enhance thermal transport.

The technique can even play a large role in areas like thermoelectricity, otherwise known as the use of heat to generate electricity. Reducing heat flow and suppressing thermal transport in this discipline can enhance the efficiency of thermoelectric devices and pave the way for clean energy usage.

The group says this study is just the tip of the iceberg for them, too. Their next projects and collaborations with ������ý Boulder chemists�� will expand on this phenomenon and use this novel technique to explore other phononic characteristics on a molecular scale.

“Phonons travel virtually in all materials,” Yelishala said. “Therefore we can guide advancements in any natural and artificially made materials at the smallest possible level using our ultra-sensitive probes.”