As electronic, thermoelectric and computer technologies have been miniaturized at the nanoscale, engineers have faced a challenge by 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, Irvine, the Massachusetts Institute of Technology and other institutions has found a way to map phonons – vibrations in crystal lattices – with atomic resolution. , enabling deeper understanding of how heat travels through quantum dots, engineered nanostructures in electronic components.
To study how phonons are dispersed by defects and interfaces in crystals, the researchers probed the dynamic behavior of phonons near a single silicon-germanium quantum dot using vibrational electron energy loss spectroscopy in an electron microscope. transmission, equipment housed in the Irvine Materials Research Institute on the UCI campus. The results of the project are the subject of a paper published today in Nature.
“We have developed a new technique to differentially map phonon moments with atomic resolution, which allows us to observe non-equilibrium phonons that exist only near the interface,” said co-author Xiaoqing Pan, professor of materials science. , engineering and physics, Henry Samueli Endowed Chair of engineering and director of IMRI. “This work marks an important advance in the field because it is the first time that we have been able to provide direct evidence that the interaction between diffusive and specular reflection is largely dependent on detailed atomistic structure.”
According to Pan, on the atomic scale, heat is transported into 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 vibration frequency.
Using an alloy of silicon and germanium, the team was able to study how phonons behave in the messy environment of the quantum dot, at the interface between the quantum dot and the surrounding silicon, and around the dome-shaped surface of the nanostructure. quantum dot yes.
“We found that the SiGe alloy exhibited a compositionally disordered structure that prevented the efficient propagation of phonons,” said Pan. “Because the silicon atoms are closer to each other than the germanium atoms in their pure structures, the alloy stretches the silicon atoms a bit. Due to this effort, the UCI team found that the phonons were softened in the quantum dot due to the deformation and engineered binding effect within the nanostructure. “
Pan added that the softened phonons have less energy, meaning that each phonon carries less heat, thereby reducing thermal conductivity. Vibration attenuation is the basis of one of the many mechanisms by which thermoelectric devices prevent the flow of heat.
One of the key results of the project was the development of a new technique for mapping the direction of thermal vectors in the material. “This is analogous to counting how many phonons are going up or down and taking the difference, indicating their dominant direction of propagation,” she said. “This technique allowed us to map the reflection of phonons from the interfaces.”
Electronics engineers have managed to miniaturize the structures and components of electronics to such an extent that they are now on the order of one billionth of a meter, much smaller than the wavelength of visible light, so these structures are invisible to optical techniques.
“Advances in nanoengineering have outstripped advances in electron microscopy and spectroscopy, but with this research we are beginning the recovery process,” said co-author Chaitanya Gadre, a graduate student in Pan’s group at UCI.
One likely field that will benefit from this research is thermoelectrics, the material systems that convert heat into electricity. “Developers of thermoelectric technologies strive to design materials that prevent heat transport or promote charge flow, and atomic-level knowledge of how heat is transmitted through embedded solids as is often the case with defects, flaws and imperfections will help. in this research, “said co-author Ruqian Wu, professor of physics and astronomy at the UCI.
“More than 70 percent of the energy produced by human activities is heat, so it is imperative to find a way to recycle it into a usable form, preferably electricity to power humanity’s growing energy demand,” Pan said.
Gang Chen, a professor of mechanical engineering at MIT, was also involved in this research project, funded by the US Department of Energy, the Office of Basic Energy Sciences and the National Science Foundation; 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.