Heat Travels Through Nanostructures Known as Quantum Dots; New Study Shows How This Happens Through Phonon Dynamics

Using cutting-edge electron microscopes and novel approaches, researchers have found a way to map phonons, and vibrations in crystal lattices, in atomic resolution, allowing deeper insight into how heat travels through engineered nanostructures known as quantum dots in electronic components.

As specified in a Phys.org report, "as electronic, thermoelectric, and computer technologies" have been miniaturizing to the nanometer scale, engineers have encountered a challenge investigating fundamental properties of the materials involved, in many circumstances, are too tiny t be observed with optical instruments.

To examine how flaws and interfaces in crystal scatter phonons, researchers at the University of California, Irvine, the Massachusetts Institute of Technology, and other institutions analyzed the dynamic behavior of phonons close to a single quantum dot of silicon-germanium employing vibration electron energy loss spectroscopy in equipment known as the transmission electron microscope housed in the Irvine Materials Research Institute o the UCI campus.

Relying on Detailed Atomic Construction

According to Xiaoqing Pan, co-author of the study published in Nature, they developed a novel technique to "differentially map phonon momenta" with atomic resolution, which allows them to observe nonequilibrium phonons that only exit near the interface.

Pan, a UCI professor of materials science and engineering and physics, Henry Samuel Endowed Chair in Engineering, and IMRI director, added that their work marks a major advance in the field as it is the first time they have been able to offer direct evidence that interplay between diffusive and specular reflection largely relies on the detailed atomic construction.

The professor also said that heat is transported in solid materials at the atomic scale as a wave of atoms displaced from their equilibrium position as heat repositions away from the thermal source.

Meanwhile, in crystals, which possess an order atomic structure, such waves are identified as phonons, packets of waves of atomic displacements carrying thermal energy equivalent to their frequency of vibration.

How Phonons Behave in Quantum Dots

With the use of an alloy and germanium, the research team was able to study how phonons behave in the quantum dot's disordered environment, in the interface between the quantum dot and the silicon surrounding it, and around the dome-shaped surface of the quantum dot nanostructure, itself.

The team said the SiGe alloy exhibited a compositionally disordered construction that impeded the effective propagation of phonons, explained Pan.

He also said since silicon atoms are close together compared to germanium atoms in their respective pure structures, the alloy is stretching the silicon atoms a bit.

More so, because of this strain, the research team found that phonons were being softened in the quantum dot, which is described in a Nanowerk report, because of the strain and allowing effect engineered within the nanostructure.

Thermoelectric Devices Impeding the Flow of Heat

Pan also explained that softened phonons have less energy which means that every phonon carries less heat, reducing thermal conductivity as an outcome.

Moreover, the softening of vibrations is behind one of the many mechanisms by which thermoelectric devices impede the flow of heat.

One of the project's key results was the new technique's development for mapping the direction of the thermal carriers in the material.

This, the co-author continued explaining, is analogous to counting how many phonons go up or down and taking the difference, specifying their propagation's dominant direction. Such a technique allowed the team to map the phonons' reflection from the interface.

Quantum dot is explained on CNET's YouTube video below:

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