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Engineers have faced a challenge when studying the basic properties of the materials involved in thermoelectric electronics, and computer technologies as they have been miniaturized to nanometer scale. And in many cases, targets are too small to be seen with optical instruments.
With the help of cutting-edge electron microscopes and novel methods, a research team at the University of California, Irvine, the Massachusetts Institute of Technology, and other institutions has discovered a method to map phonons — vibrations in crystal lattices — in atomic resolution. This allows deeper knowledge of the way heat tends to travel via quantum dots and engineered nanostructures in electronic components.
To examine how phonons are distributed by flaws and interfaces in crystals, the scientists probed the dynamic behavior of phonons next to a single quantum dot of silicon-germanium utilizing vibrational electron energy loss spectroscopy in a transmission electron microscope, equipment that is kept in the Irvine Materials Research Institute on the UCI campus. The outcomes were published in the Nature journal.
“We developed a novel technique to differentially map phonon momenta with atomic resolution, which enables us to observe nonequilibrium phonons that only exist near the interface,” stated co-author Xiaoqing Pan, UCI professor of materials science and engineering and physics, Henry Samueli Endowed Chair in Engineering, and IMRI director.
Pan adds, “This work marks a major advance in the field because it’s the first time we have been able to provide direct evidence that the interplay between diffusive and specular reflection largely depends on the detailed atomistic structure.”
Pan feels that, at the atomic scale, heat is transported in solid materials as a wave of atoms that have been moved from their equilibrium position as heat shifts away from the thermal source.
In crystals, which hold an ordered atomic structure, these waves are known as phonons: wave packets of atomic displacements tend to carry thermal energy equal to their frequency of vibration.
Utilizing an alloy of germanium and silicon, the team was capable of studying how phonons tend to behave in the disordered surrounding of the quantum dot, in the interface between the quantum dot and the encircling silicon and next to the dome-shaped surface of the quantum dot nanostructure itself.
We found that the SiGe alloy presented a compositionally disordered structure that impeded the efficient propagation of phonons. Because silicon atoms are closer together than germanium atoms in their respective pure structures, the alloy stretches the silicon atoms a bit. Due to this strain, the UCI team discovered that phonons were being softened in the quantum dot due to the strain and alloying effect engineered within the nanostructure.
Xiaoqing Pan, Study Co-Author and Professor, Materials Science and Engineering and Physics, University of California–Irvine
Pan added that softened phonons consist of less energy, implying that every phonon carries less heat, thereby decreasing thermal conductivity as a consequence. The softening of vibrations is behind one of the several mechanisms by which thermoelectric devices tend to hinder the flow of heat.
One of the main results of the project was the development of a new method for mapping the direction of the thermal carriers in the material.
This is analogous to counting how many phonons are going up or down and taking the difference, indicating their dominant direction of propagation. This technique allowed us to map the reflection of phonons from interfaces.
Xiaoqing Pan, Study Co-Author and Professor, Materials Science and Engineering and Physics, University of California–Irvine
Electronics engineers have been successful in miniaturizing components and structures in electronics to such a degree that they are currently down to the order of a billionth of a meter. This is much smaller compared to the wavelength of visible light, thus these structures are invisible to optical methods.
“Progress in nanoengineering has outpaced advancements in electron microscopy and spectroscopy, but with this research, we are beginning the process of catching up,” stated co-author Chaitanya Gadre, a graduate student in Pan’s group at UCI.
A field that is likely to benefit from this study is thermoelectrics (material systems that transform heat into electricity).
“Developers of thermoelectrics technologies endeavor to design materials that either impede thermal transport or promote the flow of charges, and atom-level knowledge of how heat is transmitted through solids embedded as they often are with faults, defects, and imperfections, will aid in this quest,” stated co-author Ruqian Wu, UCI professor of physics and astronomy.
More than 70% of the energy produced by human activities is heat, so it is imperative that we find a way to recycle this back into a useable form, preferably electricity to power humanity’s increasing energy demands.
Xiaoqing Pan, Study Co-Author and Professor, Materials Science and Engineering and Physics, University of California–Irvine
Also included in this research project, which was financially supported by the US Department of Energy Office of Basic Energy Sciences and the National Science Foundation, were Gang Chen, MIT professor of mechanical engineering; Sheng-Wei Lee, professor of materials science and engineering at National Central University, Taiwan; and Xingxu Yan, a UCI postdoctoral scholar in materials science and engineering.
Gadre, C. A., et al. (2022) Nanoscale imaging of phonon dynamics by electron microscopy. Nature. doi.org/10.1038/s41586-022-04736-8.
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