Research will benefit the design of future electronic and thermoelectric technologies

As electronic, thermoelectric, and computer technologies have been miniaturized to the nanometer scale, engineers have faced a challenge in studying the basic properties of the materials involved; in many cases the targets are too small to be observed with optical instruments.

Using advanced electron microscopes and new techniques, a team of researchers from the University of California at Irvine, the Massachusetts Institute of Technology and other institutions have found a way to map phonons – vibrations in crystal lattices – in atomic solution, providing a better understanding of how heat moving through quantum dots, the engineered nanostructures of electronic components.

To study how phonons are scattered by defects and interfaces in crystals, researchers examined the dynamic behavior of phonons near a single silicon-germanium quantum dot using vibrating electron energy loss spectroscopy in a transmission electron microscope, equipment placed at the Irvine Materials Research Institute. on the UCI campus. The results of the project are the subject of an article published today in Nature.

“We have developed a new technique for differentially mapping phonon moments with atomic resolution that allows us to observe out-of-equilibrium phonons that only exist near the interface,” said co-author Xiaoqing Pan, UCI Professor of Materials Science and Engineering and Physics, Henry Samueli gifted as chairman of engineering and director of IMRI. “This work marks a major step forward in the field, as it is the first time we have been able to provide direct evidence that the interplay between diffuse and speculative reflection depends to a large extent on detailed atomistic structure.”

According to Pan, on an atomic scale, heat in solid materials is transported as a wave of atoms that are displaced from their equilibrium position as the heat moves away from the thermal source. In crystals that have an ordered atomic structure, these waves are called phonons: wave packets of atomic displacements that carry thermal energy equal to their vibrational frequency.

Using an alloy of silicon and germanium, the team was able to study the behavior of the phonons in the disordered environment of the quantum dot, at the interface between the quantum dot and the surrounding silicon, and around the vaulted surface of the nanostructure of the quantum dot. himself.

“We found that the SiGe alloy exhibited a composition-disrupted structure that prevented effective phonon propagation,” Pan said. “Because the silicon atoms are closer together than the germanium atoms in their respective pure structures, the alloy stretches the silicon atoms a bit. Due to this load, the UCI team discovered that the phonons were softened in the quantum dot due to the load and the alloying effect. designed within the nanostructure. »

Pan added that softened phonons have less energy, which means that each phonon carries less heat, thus reducing the thermal conductivity. One of the many mechanisms by which thermoelectric devices inhibit heat flow is vibration damping.

One of the main results of the project was the development of a new technique for mapping the direction of thermal carriers in the material. “It’s like counting the number of phonons going up or down and taking the difference, indicating their dominant direction of propagation,” he said. “This technique allowed us to map the reflection of phonons from interfaces.”

Electronics engineers have successfully miniaturized structures and components in electronics to such an extent that they are now on the order of one billionth of a meter, far less than the wavelength of visible light, so that these structures are invisible to optical techniques.

“Advances in nanotechnology have surpassed advances in electron microscopy and spectroscopy, but with this research we are beginning the process of catching up,” said co-author Chaitanya Gadre, a graduate student in Pan’s group at the UCI.

One area that is likely to benefit from this research is thermoelectricity – systems of materials that convert heat into electricity. “Developers of thermoelectric technology striving to design materials that inhibit heat transport or promote charge flow, and knowledge at the atomic level of how heat is transmitted through embedded solids, as they often are with defects, imperfections and imperfections, will help in this quest.” , said co-author Ruqian Wu, professor of physics and astronomy at the UCI.

“More than 70% of the energy produced by human activities is heat, so it is imperative that we find a way to recycle it into a usable form, preferably electricity, to meet the world’s growing energy needs. Humanity,” he said. Pan.

Gang Chen, a professor of mechanical engineering at MIT, also participated in this research project, funded by the U.S. Department of Energy’s Office of Basic Energy Sciences and the National Science Foundation. Sheng-Wei Lee, Professor of Materials Science and Engineering at National Central Taiwan University; and Xingxu Yan, UCI postdoc researcher in materials science and engineering.

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