As electronic, thermoelectric and computing technologies have been miniaturized to the nanometer scale, engineers have faced a challenge in studying 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 at Irvine, the Massachusetts Institute of Technology and other institutions have found a way to map phonons – vibrations in crystal lattices – in atomic resolution, allowing a better understanding of how heat moves through quantum dots, the engineered nanostructures in electronic components.
To study how phonons are scattered by defects and interfaces in crystals, researchers probed the dynamic behavior of phonons near a single silicon-germanium quantum dot using vibrational electron energy loss spectroscopy in a transmission electron microscope, 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 momenta with atomic resolution, which allows us to observe out-of-equilibrium phonons that exist only near the interface,” said co-author Xiaoqing Pan, UCI Professor of Materials Science and Engineering and Physics, Henry Samueli Endowed Chair of Engineering and Director of IMRI. “This work marks a major advance in the field as it is the first time that we have been able to provide direct evidence that the interplay between diffusive and specular reflection is largely dependent on detailed atomistic structure. »
According to Pan, on 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 thermal 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 frequency of vibration.
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 quantum dot nanostructure. himself.
“We found that the SiGe alloy exhibited a compositionally disordered 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 strain, the UCI team discovered that the phonons were softened in the quantum dot due to strain 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 thermal conductivity. Damping of vibration is one of the many mechanisms by which thermoelectric devices impede heat flow.
One of the main results of the project was the development of a new technique to map 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 a degree that they are now on the order of a billionth of a meter, far smaller than the wavelength of visible light, so that these structures are 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 the UCI.
One area likely to benefit from this research is thermoelectricity – systems of materials that convert heat into electricity. “Thermoelectric technology developers strive to design materials that impede heat transport or promote charge flow, and atomic-level knowledge of how heat is transmitted through embedded solids as they are often with flaws, defects and imperfections, will help in this quest. “, 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 the growing energy needs of the world. humanity,” Pan said.
Gang Chen, a professor of mechanical engineering at MIT, also participated 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.
