Nanoscale imaging of phonon dynamics by electron microscopy

Chaitanya A. Gadre, Xingxu Yan, Qichen Song, Jie Li, Lei Gu, Huaixun Huyan, Toshihiro Aoki, Sheng-Wei Lee, Gang Chen, Ruqian Wu and Xiaoqing Pan ()
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Chaitanya A. Gadre: University of California Irvine
Xingxu Yan: University of California Irvine
Qichen Song: Massachusetts Institute of Technology
Jie Li: University of California Irvine
Lei Gu: University of California Irvine
Huaixun Huyan: University of California Irvine
Toshihiro Aoki: University of California Irvine
Sheng-Wei Lee: National Central University
Gang Chen: Massachusetts Institute of Technology
Ruqian Wu: University of California Irvine
Xiaoqing Pan: University of California Irvine

Nature, 2022, vol. 606, issue 7913, 292-297

Abstract: Abstract Spatially resolved vibrational mapping of nanostructures is indispensable to the development and understanding of thermal nanodevices1, modulation of thermal transport2 and novel nanostructured thermoelectric materials3–5. Through the engineering of complex structures, such as alloys, nanostructures and superlattice interfaces, one can significantly alter the propagation of phonons and suppress material thermal conductivity while maintaining electrical conductivity2. There have been no correlative experiments that spatially track the modulation of phonon properties in and around nanostructures due to spatial resolution limitations of conventional optical phonon detection techniques. Here we demonstrate two-dimensional spatial mapping of phonons in a single silicon–germanium (SiGe) quantum dot (QD) using monochromated electron energy loss spectroscopy in the transmission electron microscope. Tracking the variation of the Si optical mode in and around the QD, we observe the nanoscale modification of the composition-induced red shift. We observe non-equilibrium phonons that only exist near the interface and, furthermore, develop a novel technique to differentially map phonon momenta, providing direct evidence that the interplay between diffuse and specular reflection largely depends on the detailed atomistic structure: a major advancement in the field. Our work unveils the non-equilibrium phonon dynamics at nanoscale interfaces and can be used to study actual nanodevices and aid in the understanding of heat dissipation near nanoscale hotspots, which is crucial for future high-performance nanoelectronics.

Date: 2022
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DOI: 10.1038/s41586-022-04736-8

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