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Nature:observation of thermal effect in materials with atomic resolution by transmission electron microscope

With the miniaturization of electronic, thermoelectric and computer technologies to the nanometer level, engineers are facing the challenge of studying the basic properties of related materials. In many cases, the research object is too small to be observed with optical instruments.

A group of researchers from the University of California, Irvine, Massachusetts Institute of technology and other institutions have found a method to draw phonons (vibrations in the lattice) with atomic resolution by using cutting-edge electron microscopy and new technologies, so as to achieve a more in-depth understanding of heat transmission through quantum dots and design nanostructures in electronic components.

In order to study how phonons are scattered by defects and interfaces in crystals, researchers used vibrational electron energy loss spectroscopy in transmission electron microscopy to detect the dynamic behavior of phonons near SiGe single quantum dots. The equipment is located in the UCI campus of Irving Institute of materials. The results of this project were recently published in the journal Nature.

“We have developed a new technology to map phonon momentum with atomic resolution difference, which enables us to observe non-equilibrium phonons that only exist near the interface,” said Xiaoqing pan, co-author, Professor of UCI materials science and engineering and physics, Professor of Henry Samueli School of engineering and director of iMRI.”This work marks a major advance in this field because it is the first time that we can provide direct evidence that the interaction between diffuse and specular reflection depends largely on the specific atomic structure.”

According to Xiaoqing pan, at the atomic scale, heat is transmitted in solid materials, because when the heat is far away from the heat source, the atomic wave will shift from its equilibrium position. In crystals with ordered atomic structures, these waves are called phonons:wave packets of atomic displacement, which carry heat equal to their vibrational frequencies.

Using silicon and germanium alloys, the team was able to study the behavior of phonons in the disordered environment of quantum dots, at the interface between quantum dots and surrounding silicon, and around the dome shaped surface of quantum dot nanostructures.

“We found that SiGe alloy presents a structure with disordered components, which hinders the effective propagation of phonons,” Xiaoqing Pan said.”Since silicon atoms are closer to germanium atoms in their pure structures, the alloys stretch silicon atoms slightly. Due to this strain, the UCI team found that phonons in quantum dots are softening due to the strain and alloying effects designed in nanostructures.”

Xiaoqing pan added that the softened phonon energy is less, which means that each phonon carries less heat, thus reducing the thermal conductivity. Vibration softening is one of the many mechanisms that hinder heat flow in Thermoelectric equipment.

One of the main achievements of the project is the development of a new technology for mapping the direction of heat carriers in materials.”It’s similar to calculating how many phonons rise or fall, and then calculating the difference to prove their main propagation direction,” he said.”This technology enables us to map the reflection of phonons from the interface.”

Electronic engineers have successfully miniaturized the structures and components in electronic equipment to such an extent that they have now dropped to the order of one billionth of a meter, which is far less than the wavelength of visible light. Therefore, these structures are invisible to optical technology.

“The progress of Nano Engineering has exceeded the progress of electron microscopy and spectroscopy, but through this research, we are beginning to catch up,” said Chaitanya gadre, co-author and UCI graduate student of Xiaoqing Pan Group.

One area that may benefit from this research is thermoelectricity, a material system that converts heat energy into electricity.”Developers of thermoelectric technology strive to design materials that hinder heat transfer or promote charge flow, and atomic level knowledge of how to transfer heat through embedded solids, because they usually have faults, defects and defects, which will help this exploration,” said Ruqian Wu, co-author and professor of physics and astronomy at UCI.

“More than 70%of the energy generated by human activities is heat, so we must find a way to recycle it into a usable form, preferably electricity, to meet the growing energy demand of mankind.” Pan said.

Gang Chen, Professor of Mechanical Engineering Department of Massachusetts Institute of technology, participated in the research project funded by the office of basic energy science of the U.S. Department of energy and the National Science Foundation of the United States; Sheng Wei Lee, Professor, Department of materials science and engineering, National Central University of Taiwan, and xingxu Yan, postdoctoral researcher, materials science and engineering, UCI.

About the University of California, Irvine, UCI:founded in 1965, UCI is the youngest member of the prestigious American University Association, and is recognized by U.S. News & World Report ranks among the top 10 public universities in the United States. The campus has trained five Nobel Prize winners and is famous for its academic achievements, the earliest research, innovation and anteater mascot. Under the leadership of President Howard Gillman, UCI has more than 36000 students and provides 224 degree programs. It is located in one of the safest and most economically dynamic communities in the world. It is the second largest employer in Orange County. It contributes $7billion to the local economy and $8billion throughout the state every year.