A team of researchers has discovered a way to map phonons – vibrations in crystal lattices – in atomic resolution using cutting-edge electron microscopes and novel techniques, allowing for a better understanding of how heat travels through quantum dots, engineered nanostructures in electronic components.
Engineers have faced a challenge studying fundamental properties of the materials involved as electronic, thermoelectric, and computer technologies have been miniaturized to nanometer scale; in many cases, targets are too small to be observed with optical instruments.
A team of researchers from the University of California, Irvine, the Massachusetts Institute of Technology, and other institutions used cutting-edge electron microscopes and novel techniques to map phonons – vibrations in crystal lattices – in atomic resolution, allowing for a better understanding of how heat travels through quantum dots, engineered nanostructures in electronic components.
We developed a novel technique to differentially map phonon momenta with atomic resolution, allowing us to observe nonequilibrium phonons that exist only near the interface. This work represents a significant advance in the field because it is the first time we have been able to provide direct evidence that the interplay between diffusive and specular reflection is heavily influenced by the detailed atomistic structure.
Xiaoqing Pan
To investigate how phonons are scattered by flaws and interfaces in crystals, the researchers used vibrational electron energy loss spectroscopy in a transmission electron microscope on the UCI campus to probe the dynamic behavior of phonons near a single quantum dot of silicon-germanium. The project’s findings are the subject of a paper published today in Nature.
“We developed a novel technique to differentially map phonon momenta with atomic resolution, allowing us to observe nonequilibrium phonons that exist only near the interface,” said co-author Xiaoqing Pan, a UCI professor of materials science and engineering and physics, Henry Samueli Endowed Chair in Engineering, and IMRI director. “This work represents a significant advance in the field because it is the first time we have been able to provide direct evidence that the interplay between diffusive and specular reflection is heavily influenced by the detailed atomistic structure.”
According to Pan, heat is transported in solid materials at the atomic scale as a wave of atoms displaced from their equilibrium position as heat moves away from the thermal source. These waves are known as phonons in crystals, which have an ordered atomic structure: wave packets of atomic displacements that carry thermal energy equal to their frequency of vibration.
The team was able to study how phonons behave in the disordered environment of the quantum dot, at the interface between the quantum dot and the surrounding silicon, and around the dome-shaped surface of the quantum dot nanostructure itself using an alloy of silicon and germanium.
“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 also stated that softened phonons have less energy, which means that each phonon carries less heat, resulting in lower thermal conductivity. One of the many mechanisms by which thermoelectric devices obstruct the flow of heat is vibration softening.
The development of a new technique for mapping the direction of thermal carriers in the material was one of the project’s key outcomes. “This is analogous to counting how many phonons are going up or down and dividing by two to determine their dominant direction of propagation,” he explained. “We were able to map the reflection of phonons from interfaces using this technique.”
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, making these structures invisible to optical techniques.
“Progress in nanoengineering has outpaced advances in electron microscopy and spectroscopy, but with this research, we are beginning the process of catching up,” said co-author Chaitanya Gadre, a graduate student in Pan’s group at UCI.
Thermoelectrics, which are material systems that convert heat to electricity, are likely to benefit from this research. “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.
“More than 70% of the energy produced by human activities is heat, so it is critical that we find a way to recycle this back into a usable form, preferably electricity, to power humanity’s increasing energy demands,” Pan said.
