In a recently published study in the journal Nature, researchers developed a technique to map the efficiency of heat transfer by measuring the movement of phonons. This technique will help in the field of thermoelectrics in the future.
A beam of electrons generates vibrational waves in a crystal lattice that are then reflected by quantum dots. Right: Generated vibrations are more easily reflected by abrupt, sharp interfaces of materials than by diffuse ones. Image Credit: Xiaoqing Pan, University of California Irvine
The Science
In crystals, atoms oscillate around their equilibrium positions. These oscillations form collective modes known as "phonons." Measuring phonon properties in nanostructures and at material interfaces offers insights into thermal conductivity between materials.
In this study, the researchers employed energetic electrons to excite and investigate phonon dynamics, developing a novel technique to map changes in phonon momentum. This mapping revealed the direction of phonon propagation and, consequently, the direction of nanoscale heat flow. Overall, the study shows that a sharp boundary between different materials reflects heat more effectively than a gradual, diffuse boundary
The Impact
Materials known as thermoelectrics can be used to turn heat into electricity. Thermoelectrics limit the flow of heat by taking advantage of flaws in their crystal materials. By doing this, the materials' efficiency at converting heat into electricity is increased.
However, scientists' ability to create devices on the nanometer scale has outpaced their understanding of the internal workings of these materials. To advance fields such as energy conversion, quantum computing, and nanotechnology, it is crucial to understand the dynamics of heat transport at the nanoscale.
This study provides insights into the efficiency of heat transfer between different materials. Researchers can leverage this information to develop more advanced nanodevices and tailor thermoelectric properties at the nanoscale.
Summary
As the industry has miniaturized electrical, thermoelectric, and computer technologies to the nanoscale, researchers have faced the challenge of examining the thermal properties of these materials. Electron microscopy, capable of resolving individual atoms, offers significant advantages over conventional techniques. It allows for probing vibrational information at the atomic scale.
This study enhances our understanding of heat transfer mechanisms in nanostructured quantum dot superlattices. Researchers developed and employed advanced monochromated electron energy-loss spectroscopic techniques within a transmission electron microscope to map lattice vibrations (phonons) at atomic resolution.
Focusing on stacked silicon germanium alloy quantum dots separated by silicon, the researchers discovered that strain within the nanostructure caused the quantum dots' vibrations to soften, a process that hinders heat transfer in thermoelectrics.
Crucially, the scientists devised a novel method to map the direction of phonon travel. They found that sharper atomic interfaces between the quantum dots and surrounding silicon more effectively reflected phonons. Conversely, a more gradual change in composition from the quantum dot to silicon resulted in significantly fewer reflected phonons. This work provides a physical basis for targeted engineering and demonstrates the feasibility of using vibrational spectroscopy in electron microscopy for thermoelectric applications.
Funding
The research was primarily supported by the Department of Energy Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, and partially funded by the National Science Foundation.
Journal Reference:
Gadre, C. A., et al. (2022) Nanoscale imaging of phonon dynamics by electron microscopy. Nature. doi.org/10.1038/s41586-022-04736-8