As electronic, thermoelectric and computing technologies have been reduced to the nanometer scale, engineers faced a challenge in studying fundamental properties of the materials involved; in many cases, targets are too small to be observed with optical instruments.
Using advanced electron microscopes and new techniques, a team of researchers from the University of California, Irvine, Massachusetts Institute of Technology and other institutions has found a way to map phonons — vibrations in crystal lattices — in atomic resolution, enabling deeper understanding. of the way heat travels through quantum dots, constructed nanostructures in electronic components.
To investigate how phonons are scattered by flaws 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, equipment housed at 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-momenta with atomic resolution, allowing us to observe unbalanced 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 of Engineering and IMRI Director. “This work marks a major advance in the field, as it is the first time we have been able to provide direct evidence that the interplay between diffuse and specular reflection largely depends on the detailed atomistic structure.”
According to Pan, atomic-scale heat is transported in solid materials as a wave of atoms that is displaced from their equilibrium position as heat moves away from the thermal source. In crystals, which have an ordered atomic structure, these waves are called phonons: wave packages of atomic displacements that carry thermal energy equal to their vibrational frequency.
Using an alloy of silicon and germanium, the team was able to study how phonons behave in the disordered environment of the quantum dot, in the interface between the quantum dot and the surrounding silicon, and around the domed surface of the quantum dot nanostructure itself.
“We found that the SiGe alloy exhibited a compositionally disordered structure that hindered the efficient propagation of phonons,” Pan said. “Because silicon atoms are closer together than germanium atoms in their respective pure structures, the alloy stretches the silicon atoms a bit. Through this tension, the UCI team found that phonons in the quantum dot were softened due to the tension and the alloy effect developed within the nanostructure.”
Pan added that softened phonons have less energy, meaning each phonon transports less heat, reducing thermal conductivity. The softening of vibrations is behind one of the many mechanisms of how 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 the thermal carriers in the material. “This is analogous to counting how many phonons go up or down and take the difference, which indicates their dominant direction of propagation,” he said. “This technique allowed us to map the reflection of phonons from interfaces.”
Electronics engineers have succeeded in miniaturizing structures and components in electronics to the order of a billionth of a meter, much smaller than the wavelength of visible light, so these structures are invisible to optical techniques.
“Advances in nanoengineering have surpassed advances in electron microscopy and spectroscopy, but with this research we are starting to catch up,” said study co-author Chaitanya Gadre, a graduate student in Pan’s group at UCI.
A likely area to benefit from this research is thermoelectricity – material systems that convert heat into electricity. “Developers of thermoelectric technologies strive to design materials that impede thermal transport or promote the flow of charges, and knowledge at the atomic level of how heat is transferred through solids embedded, as often with flaws, defects and imperfections, will aid in this quest,” said co-author Ruqian Wu, UCI professor of physics and 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 usable form, preferably electricity to meet humanity’s increasing energy demand. to feed,” said Pan.
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 scientist in materials science and engineering.
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