Irvine scientists observe effects of heat in materials with atomic resolution

Using cutting-edge electron microscopes and novel techniques, a team of researchers at the University of California, Irvine, the 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, engineered nanostructures in electronic components.

To investigate how phonons are scattered by flaws and interfaces in crystals, the researchers probed the dynamic behavior of phonons near a single quantum dot of silicon-germanium using vibrational electron energy loss spectroscopy in a transmission electron microscope, equipment housed in the Irvine Materials Research Institute on the UCI campus.

"We developed a novel technique to differentially map phonon momenta with atomic resolution, which enables us to observe nonequilibrium 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 in Engineering, and IMRI director.

According to Pan, at the atomic scale, heat is transported in solid materials as a wave of atoms displaced from their equilibrium position as heat moves away from the thermal source.

In crystals, which possess an ordered atomic structure, these waves are called phonons: wave packets of atomic displacements that carry thermal energy equal to their frequency of vibration.

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 dome-shaped surface of the quantum dot nanostructure itself.

"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."

"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 and astronomy.

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 scholar in materials science and engineering.

Research Report:Nanoscale imaging of phonon dynamics by electron microscopy.