As electronic, thermoelectric, and computing technologies miniaturized to the nanometer scale, engineers were challenged to study the fundamental properties of the materials involved; in many cases the targets are too small to be observed with optical instruments.
Using state-of-the-art electron microscopes and new techniques, a team of researchers from the University of California, Irvine, the Massachusetts Institute of Technology, and other institutions have found a way to map phonons (vibrations in crystal lattices) at atomic resolution. , allowing for a better understanding of how heat moves through quantum dots, the nanostructures engineered into electronic components.
To study how phonons scatter off defects and interfaces in crystals, the researchers tested the dynamic behavior of phonons near a single silicon-germanium quantum dot using vibrational electron energy loss spectroscopy in a electron microscope. transmission, equipment housed 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 to differentially map phonon moments with atomic resolution, which allows us to observe out-of-equilibrium phonons that exist only near the interface,” said co-author Xiaoqing Pan, a professor of materials science and engineering at the ICU. Physics, Henry Samueli Chair of Engineering and Director of IMRI. “This work marks a breakthrough in the field, as it is the first time we have been able to provide direct evidence that the interaction between diffusive and specular reflection is highly dependent on detailed atomistic structure. »
According to Pan, at the atomic scale, heat is transported in solid materials as a wave of atoms displaced from their equilibrium position as the heat moves away from the heat source. In crystals, which 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 phonons in the disordered environment of the quantum dot, at the interface between the quantum dot and the surrounding silicon, and around the domed surface of the quantum dot nanostructure. the same.
“We found that the SiGe alloy exhibited a disordered structure in its composition that prevented effective phonon propagation,” said Pan. “Because the silicon atoms are closer together than the germanium atoms in their respective pure structures, the alloy it stretches the silicon atoms a bit. Due to this strain, the UCI team found that the phonons were smoothed out in the quantum dot due to the strain and the alloying effect. designed within the nanostructure. »
Pan added that the smoothed phonons have less energy, which means that each phonon carries less heat, which reduces thermal conductivity. One of the many mechanisms by which thermoelectric devices impede heat flow is vibration damping.
One of the main results of the project was the development of a new technique to map the direction of heat 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 phonon reflection of the interfaces. »
Electronics engineers have successfully miniaturized structures and components in electronics to such an extent that they are now on the order of a billionth of a meter, much smaller than the wavelength of visible light, making these structures invisible to optical techniques. .
“Advances in nanoengineering have outpaced 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 UCI.
One area likely to benefit from this research is thermoelectricity: material systems that convert heat into electricity. “Thermoelectric technology developers struggle to design materials that prevent heat transport or promote charge flow, and atomic-level knowledge of how heat is transmitted through embedded solids, as they often have flaws, flaws and imperfections, will help in this search,” said co-author Ruqian Wu, professor of physics and astronomy at 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 humanity’s growing energy needs,” said Pan.
Gang Chen, a professor of mechanical engineering at MIT, was also involved in this research project, funded by the US 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 postdoctoral researcher in materials science and engineering.
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