| Date | 10th, Jun 2022 |
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As electronic, thermoelectric, and computer technologies have been miniaturized to the nanometer scale, engineers have faced a challenge studying the fundamental properties of the materials involved; in many cases, targets are too small to be observed with optical instruments.
Nanometer-scale quantum dots made of an alloy of silicon and germanium were targeted by researchers at UCI using a technique dubbed “vibrational electron energy loss spectroscopy” in a transmission electron microscope in the Irvine Materials Research Institute. The work resulted in the first atomic-level observation of how phonons behave in nanoengineered quantum dots. Image credit: Chaitanya Gadre, Xingxu Yan, Xiaoqing Pan / UCI
“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.”
Pan added that softened phonons have less energy, which means that each phonon carries 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 key outcomes of the project was the development of a new technique for mapping the direction of the thermal carriers 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,” 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 such a degree that they are now down 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.
“Progress in nanoengineering has outpaced advancements in electron microscopy and spectroscopy, but with this research, we are beginning to catch up,” said co-author Chaitanya Gadre, a graduate student in Pan’s group at UCI.
A likely field to benefit from this research is thermoelectrics – material systems that convert heat to electricity. “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 heating, 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.
Source: UC Irvine
