One atom, one heartbeat: Scientists map heat inside 2D electronics

Heat is usually treated like an invisible fog that slips through solids as they warm or cool. Until now, nobody had watched its tiniest messengers move that heat, one by one. New research now shows that it is phasons that dominate thermal motion.

Using cameras sharp enough to see a hydrogen atom jitter, researchers from the University of Illinois at Urbana-Champaign have recorded heat in action inside a single sheet of tungsten diselenide.

The work is directed by Pinshane Huang of Grainger College of Engineering, with imaging specialist Yichao Zhang supplying the ptychography expertise.

Unseen vibrations at the scale of atoms

Every crystal vibrates, but moiré phonons arise only when two lattices are stacked with a slight twist, creating a larger superlattice pattern.

Among these modes sit the ultrasoft phasons, with frequencies so low that standard spectroscopy misses them.

The Illinois team used a powerful imaging technique called electron ptychography, which can capture details as small as one-fifth the width of a carbon atom.

That sharpness lets scientists measure how much the image of a single atom blurs as it vibrates, thus turning a microscope into a motion detector.

Phasons emerged as localized patches where the rigid crystal briefly shears, a bit like tectonic plates sliding at faults, yet on a scale of picometers and picoseconds.

These motions carry heat in directions that ordinary phonons cannot, rewriting expectations for thermal transport in layered devices.

Imaging heat, one atom at a time

Traditional electron microscopes treat atoms as static dots, and average their motions into still pictures. Ptychography, instead, records thousands of diffraction patterns while the beam scans, then computationally stitches them into a phase map that preserves minute displacements.

“This works by getting such high spatial resolution that the vibrations of atoms change how blurry the atoms appear,” explained Zhang, who is now at the University of Maryland.

The team reached a time-averaged positional accuracy of below fifteen picometers, which is enough to separate thermal blur from instrumental noise.

The researchers studied a special type of ultra-thin material made by stacking two layers of tungsten diselenide slightly out of alignment, by less than two degrees.

Their images showed stronger atomic vibrations in specific areas of the material, matching what scientists expected to see if phasons were present.

Simulations that were run alongside the images matched amplitude maps across the sample, giving confidence that the camera truly watched heat moving rather than electron-beam artifacts.

Meeting the moiré family of phonons

Phonons normally ferry heat as collective waves of stretching and squeezing. Moiré superlattices add a second scale, so their vibrational menu broadens to include zone-folded acoustic branches and ultrasoft shear oscillations.

Phasons sit at the bottom of that menu, with calculated frequencies below one wavenumber – far beneath the reach of Raman or infrared probes.

Their long periods let them couple efficiently to defects, impurities, and interfaces, turning them into gatekeepers of thermal flow.

That sensitivity opens a path to engineer heat management from the bottom up: tweak the twist angle, layer count, or composition, and phasons should tune themselves accordingly.

Engineers already adjust electronic band structures with moiré patterns – manipulating vibrational spectra may be just as powerful.

At room temperature, a single layer of this material doesn’t move heat very well. It’s almost ten times worse at it than graphene, mostly because the atoms are heavier and vibrate more slowly.

Phasons could make it even harder for heat to spread or change the direction it flows, giving engineers new ways to keep tiny devices cool. 

Why phasons could remake electronics

Modern chips throttle performance when hotspots rise by a few degrees, and 2D materials promise compact transistors but worsen cooling by stacking device layers. If phasons dominate thermal motion in these stacks, understanding them becomes urgent.

“We could look at a single atom and identify a defect that’s preventing the material from cooling down more efficiently,” explained Zhang.

Such pinpoint inspection could drive a new generation of thermal interface materials and on-chip heat spreaders.

Phasons also interact with electrons and excitons, influencing optical response and superconductivity as reported in twisted graphene.

Imaging them may therefore unlock links between heat, charge, and light in van der Waals heterostructures.

Future devices might exploit controlled phason populations as thermal switches, shunting heat away when open and trapping it when closed – all without moving parts.

Ptychography captures details of delicate materials

In just five years, imaging tools improved enough to see five times more detail than before. Because ptychography uses a gentler beam, it can capture images of delicate materials without damaging them, making it useful for studying things like proteins and other soft substances.

Automated parameter tuning now uses Bayesian optimization to pick scan steps, defocus, and aperture size, all of which trim human bias while squeezing extra clarity from noisy data. Those tools turn once-esoteric setups into push-button accessories for modern transmission microscopes.

Metrology agencies already cite ptychography as the highest-resolution imaging method on record, and laboratories worldwide race to pair it with cryogenic stages, ultrafast beams, and holographic probes. Each add-on promises sharper movies showing matter in motion.

As microscopes evolve, theoretical models must keep pace, integrating quantum molecular dynamics with machine-learned potentials to simulate the sprawling moiré supercells that are seen experimentally.

What comes next for cooler chips

Phasons have moved from mathematical footnotes to observable actors, yet many questions linger. Do they scatter with electrons strongly enough to limit mobility, or might they carry spin information in magnetic layers?

Temperature-dependent studies will test whether phason amplitudes freeze out at cryogenic temperatures or persist to influence low-temperature quantum phases.

Pump-probe experiments, meanwhile, could capture their real-time evolution after a laser pulse, measuring lifetimes directly.

Device engineers will watch closely, because controlling phasons might spell the difference between a 2D logic chip that throttles under load and one that cruises at full speed.

Tailored twist angles, patterning, or substrate choice could shepherd heat exactly where designers want it.

For now, the Illinois team shows that heat need not stay hidden. Atoms vibrate, the camera rolls, and the secret life of crystals steps into view.

The study is published in Science.

Image credit: (left) atoms present in the 2D material. (right) photos of single atoms. The Grainger College of Engineering at the University of Illinois Urbana-Champaign

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