As electronic, thermoelectric, and computing technologies have been miniaturized to the nanometer scale, engineers have been 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 the latest electron microscopes and new technologies, a team of researchers from the University of California at Irvine, MIT and other institutions has found a way to map phonons — vibrations in crystal lattices — with atomic precision, allowing a better understanding of how heat travels through quantum dots, and engineered nanostructures in the components. e.
To study how phonons scatter due to defects and interfaces in crystals, the researchers investigated the dynamic behavior of phonons near a single quantum point of silicon and germanium using vibrational electron energy loss spectroscopy in a transmission electron microscope, an instrument located at the Institute for Materials Research in Irvine. 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 moment to atomic resolution, allowing us to observe non-equilibrium phonons that exist only near the interface,” said co-author Xiaoqing Pan, a professor of materials science and engineering at UCI. Physics, Henry Samueli Head of Engineering and Director of IMRI. “This work represents a major advance in this field as it is the first time we have been able to provide direct evidence that the interaction between diffusive and specular reflection is largely dependent on detailed atomic structure.”
According to Ban, on the atomic scale, heat is transferred in solids as a wave of atoms displaced from their equilibrium position as heat moves away from the heat source. In crystals, which have an ordered atomic structure, these waves are called phonons: wave packets of atomic displacement that carry heat energy equal to the frequency of their vibration.
Using an alloy of silicon and germanium, the team was able to study the behavior of phonons in the turbulent 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. Himself.
“We found that the SiGe alloy exhibited a compositionally disordered structure that inhibits effective phonon diffusion,” Ban said. “Because silicon atoms are closer together than germanium atoms in their pure structures, the alloy stretches the silicon atoms slightly. Because of this strain, the UCI team discovered that phonons are softened in the quantum dot due to stress and the alloying effect. They are designed within the nanostructure.”
Ban added that attenuated phonons have lower energy, which means that each phonon carries less heat, thus reducing thermal conductivity. One of the many mechanisms by which thermoelectric devices impede heat flow is vibration dampening.
One of the main outcomes of the project was the development of a new technique for mapping the orientation of heat carriers in the material. “It’s like counting the number of phonons going up or down and taking the difference, which indicates their dominant propagation direction,” he said. “This technique allowed us to map the reflection of phonons from the interfaces.”
Electronics engineers have succeeded in miniaturizing structures and components in electronics to the point that they are now on the order of a billionth of a meter, much smaller than the wavelength of visible light, so that these structures are invisible to optical technologies.
“Advances in nanoengineering have outstripped advances in electron microscopy and spectroscopy, but with this research we are beginning to catch up,” said co-author Chaitanya Gadre, a graduate student in UCI’s Pan group.
An area likely to benefit from this research is thermoelectricity – systems of materials that convert heat into electricity. “Thermoelectric technology developers are seeking to design materials that impede heat transfer or enhance the flow of charge, and knowledge at the atomic level of how heat is transmitted through embedded solids often with defects, blemishes, and imperfections will aid in this endeavor,” 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,” Ban said.
Gang Chen, a professor of mechanical engineering at MIT, was also involved in this research project, which was 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, a UCI postdoctoral researcher in materials science and engineering.
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