Research will be useful in the design of future electronic and thermoelectric technologies – ScienceDaily

As electronic, thermoelectric and computer technologies are miniaturized to nanometer scale, engineers are challenged to study the fundamental properties of the materials used; in many cases the 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, the Massachusetts Institute of Technology and other institutions has found a way to map phonons – vibrations in crystal lattices – to atomic resolution, allowing a deeper understanding of how the heat passes through the quantum dots designed nanostructures into electronic components.

To study how phonons are scattered by defects and interfaces in crystals, the researchers studied the dynamic behavior of phonons near a single silicon-germanium quantum dot using vibrational spectroscopy of electron energy loss in a transmission electron microscope, equipment at the Institute for materials research in Irvine. on campus at UCI. The results of the project are the subject of a publication published today in nature.

“We have developed a new technique for differential mapping of phonon pulses with atomic resolution, which allows us to observe nonequilibrium phonons that exist only near the interface,” said co-author Xiaoqing Pan, UCI’s End Samli Professor of Materials Science and Engineering and Physics, Henry Samue Department of Engineering and Director of IMRI. “This work has made great progress in this area, because for the first time we have been able to provide direct evidence that the interaction between diffuse and mirror reflection depends to a large extent on the detailed atomistic structure.

According to Pan, on an atomic scale, heat is transferred to solid materials such as a wave of atoms displaced from its equilibrium position as heat moves away from the heat source. In crystals that have an ordered atomic structure, these waves are called phonons: wave packets of atomic displacements that carry heat 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 quantum dot medium, in the interface between the quantum dot and the surrounding silicon, and around the domed surface of the quantum dot nanostructure itself.

“We have found that the SiGe alloy is a compositionally disordered structure that prevents the efficient propagation of phonons,” Pan said. “Because silicon atoms are closer to each other than germanium atoms in their respective pure structures, the alloy stretches the silicon atoms slightly. Due to this stress, the UCI team found that phonons soften at the quantum dot due to the stress and alloying effect within the nanostructure. “

Pan added that softened phonons have less energy, which means that each phonon carries less heat, resulting in reduced thermal conductivity. Vibration attenuation is behind one of the many mechanisms for how thermoelectric devices impede the flow of heat.

One of the key results of the project was the development of a new technique for mapping the direction of heat carriers in the material. “It’s analogous to counting the number of phonons up or down and taking the difference, which shows their dominant direction of distribution,” he said. “This technique has allowed us to map the reflection of phonons from interfaces.”

Electronics engineers have managed to miniaturize electronics structures and components 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 that these structures are invisible to optical techniques. .

“Advances in nanoengineering have outpaced advances in electron microscopy and spectroscopy, but with this study we are beginning the catch-up process,” said co-author Caitanya Gadre, a student in the UCI Pan Group.

Probably the field that will benefit from this study is thermoelectricity – material systems that convert heat into electricity. “Developers of thermoelectric technology seek to design materials that either hinder heat transport or promote the flow of charges, and knowledge at the atomic level of how heat is transferred through embedded solids, such as those with defects, defects and imperfections, will help in this search, “said co-author Rukian Wu, a professor of physics and astronomy at UCI.

“More than 70 percent of the energy produced by human activities is heat, so it is imperative that we find a way to recycle it back in usable form, preferably electricity, to feed humanity’s growing energy needs,” Pan said.

This research project, funded by the U.S. Department of Energy’s Office of Basic Energy Sciences and the National Science Foundation, also involved Gang Chen, a professor of mechanical engineering at the Massachusetts Institute of Technology; Sheng-Wei Lee, Professor of Materials Science and Engineering at the National Central University, Taiwan; and Xingxu Yan, UCI Postdoctoral Fellow in Materials Science and Engineering.

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