Engineers have had difficulty analyzing the fundamental properties of the materials used as electronic, thermoelectric, and computer technologies have been scaled down to the nanoscale level; in many cases, targets are too small to be viewed using optical tools.
Researchers from the Massachusetts Institute of Technology, the University of California, Irvine, and other institutions have developed new methods and cutting-edge electron microscopes to map phonon vibrations in crystal lattices with atomic resolution. This has allowed them to gain a better understanding of how heat moves through engineered nanostructures like quantum dots and electronic components.
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, apparatus housed in the Irvine Materials Research Institute on the UCI campus, to learn more about how phonons are scattered by flaws and interfaces in crystals. A study about the project’s outcomes was just published in Nature.
“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,” said co-author Xiaoqing Pan, UCI professor of materials science and engineering and physics, Henry Samueli Endowed Chair in Engineering, and IMRI director.
“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.”
According to Pan, as heat moves away from the thermal source, a wave of atoms that have been knocked out of equilibrium travels through solid materials. These waves, which transport thermal energy proportional to their frequency of vibration, are known as phonons in crystals, which have an organized atomic structure.
The team was able to investigate the behavior of phonons in the disordered environment of the quantum dot, at the interface between the quantum dot and the surrounding silicon, and all around the dome-shaped surface of the quantum dot nanostructure itself using an alloy of silicon and germanium.
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. 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.
Xiaoqing Pan
“We found that the SiGe alloy presented a compositionally disordered structure that impeded the efficient propagation of phonons,” said Pan. “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.”
In addition, according to Pan, softened phonons carry less heat per unit of energy, which lowers thermal conductivity. One of the various processes by which thermoelectric devices block the transfer of heat is the softening of vibrations.
The creation of a novel method for tracking the path of the heat carriers within the material was one of the project’s major outcomes.
“This is analogous to counting how many phonons are 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.”
Because the structures and components in electronics have been successfully reduced to sizes on the order of a billionth of a meter, which is far smaller than the wavelength of visible light, they are now invisible to optical techniques.
“Progress in nanoengineering has outpaced advancements 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.
Thermoelectrics material systems that convert heat to electricity are likely to gain from this research.
“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,” said co-author Ruqian Wu, UCI professor of physics & astronomy.
“More than 70 percent 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,” Pan said.
Also involved in this research project, which was funded by the U.S. 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.
