As electronic, thermoelectric, and computing technologies have been miniaturized to the nanometer scale, engineers have been faced with the challenge of learning the fundamental properties of the materials used; in many cases the targets are too small to be observed with optical instruments.
Using modern 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 – with atomic resolution. , allowing a better understanding of how heat travels through quantum dots, engineered nanostructures in electronic components.
To study how phonons are scattered by defects and interfaces in crystals, the researchers examined the dynamic behavior of phonons near a single silicon germanium quantum dot using vibrational electron energy loss spectroscopy in a transmission electron microscope equipped at the Irvine Materials Research Institute. on the UCI campus. The results of the project were the subject of an article published today in Nature.
“We have developed a new atomic-resolution differential phonon momentum mapping method that allows us to observe non-equilibrium phonons that exist only near the interface,” said co-author Xiaoqing Pan, professor of materials science and engineering at UCI. Physics, Henry Samueli, Chair of Engineering and Director of IMRI. “This work marks a significant advance in this field, as we have been able to provide direct evidence for the first time that the interaction between diffuse and specular reflection is highly dependent on the detailed atomistic structure. »
According to Pan, on an 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 with an ordered atomic structure, these waves are called phonons: wave packets of atomic displacements that carry thermal energy equal to the frequency of their vibrations.
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 surrounding silicon, and around the domed surface of the quantum dot nanostructure. myself.
“We found that the SiGe alloy has a compositionally disordered structure that prevents efficient 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 deformation, the UCI team found that the phonons in the quantum dot soften due to the deformation and the fusion effect. developed in nanostructure. »
Pan added that softened phonons have less energy, meaning that each phonon carries less heat, reducing thermal conductivity. Vibration damping is one of the many mechanisms by which thermoelectric devices inhibit heat flow.
One of the main results of the project was the development of a new technique for mapping the direction of heat carriers in the material. “It’s like counting the number of phonons going up or down and taking the difference to indicate their dominant propagation direction,” he said. “This technique allowed us to map the reflection of phonons from interfaces. »
Electronics engineers have successfully miniaturized the structures and components in electronics to the extent that they are now on the order of one billionth of a meter, much smaller than the wavelength of visible light, such that these structures are invisible to optical methods.
“Advances in nanoengineering have outpaced advances in electron microscopy and spectroscopy, but with this research we are starting a catching-up process,” said co-author Chaitanya Gadre, a PhD student in Pan’s group at UCI.
One area likely to benefit from this research is thermoelectricity – systems of materials that convert heat into electricity. “The developers of thermoelectric technologies seek to develop materials that impede heat transfer or facilitate the flow of charge, and knowledge at the atomic level about how heat is transferred through embedded solids, since they are often flawed, defective and imperfect, 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 critical that we find a way to convert it into a usable form, preferably electricity, to meet the growing energy needs of the world,” Pan said.
Gang Chen, professor of mechanical engineering at the Massachusetts Institute of Technology, 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, National Central Taiwan University; and Xingxu Yan, UCI Postdoctoral Researcher in Materials Science and Engineering.