Nanoscale Imaging of Phonon Dynamics Emergent from Single Defect and Interface by Electron Microscopy

Crystal defects and interfaces affect the thermal and heat-transport properties of materials by scattering phonons and modifying phonon spectra. Spatially resolved vibrational mapping of nanostructures and defects is indispensable to the development and understanding of thermal nanodevices, modulation of thermal transport and novel nanostructured thermoelectric materials. 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 conductivity. There have been no correlative experiments that spatially track the modulation of phonon properties in and around individual defects and nanostructures due to spatial resolution limitations of conventional optical phonon detection techniques. In this talk, I will show that the vibrational spectra of a single defect or interface can be imaged by space- and angle-resolved vibrational electron energy-loss spectroscopy in a transmission electron microscope. It reveals a red shift of several meV in the energy of acoustic vibration modes near a single stacking fault in cubic silicon carbide, together with substantial changes in their intensity. These changes are confined to within a few nanometers of the stacking fault.[1] In a two-dimensional lateral heterostructure, new phonon modes at 27.9 and 41.1 meV are observed at the interface between monolayer thick MoS₂ and WSe₂ [2]. At a Si-Ge interface, localized interfacial phonon modes at ~48 meV.[3] Simulations show that these interfacial phonon modes contribute to the total thermal interface conductance. By tracking the variation of the Si optical mode in a phonon map from a single SiGe quantum dot, the nanoscale modification of the composition-induced red shift is observed.[4] We also developed a technique to differentially map phonon momenta, providing direct evidence that the interplay between diffuse and specular reflection largely depends on the detailed atomistic structure. 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.<br/><br/><b>References:</b><br/>[1] X. X. Yan, C. Y. Liu, C. A. Gadre, <i>et al.</i>, <i>Nature</i> <b>589</b>, 65 (2021).<br/>[2] X. Z. Tian, X. X. Yan, G. Varnavides, et al., <i>Sci. Adv</i>. <b>7</b>, eabi6699 (2021).<br/>[3] Z. Cheng, R. Li, X. X. Yan, <i>et al.</i>, <i>Nat. Commun.</i> <b>12</b>, 6901 (2021).<br/>[4] C. A. Gadre, X. X. Yan, Q. C. Song, <i>et al.</i>, <i>Nature</i> <b>606</b>, 292 (2022).