Electron Microscopy for Nanophononics: A Review

Phonons, the quantized vibrations of crystal lattices, are essential to determining the material properties. At (sub)nanoscale, surfaces, heterointerfaces, crystal defects, and other imperfections generate localized phonon modes that can significantly alter materials' thermal, electrical, optical, and mechanical properties. However, conventional techniques often struggle to probe these localized phonon modes due to limitations in spatial resolution, momentum transfer, and penetration depth. Recent advancements in electron energy loss spectroscopy in scanning transmission electron microscopy (STEM-EELS) have overcome these challenges, achieving (sub)nanometer spatial resolution, several Brillouin Zones of momentum transfer, and millielectronvolt energy resolution, providing opportunities to probe the local phonons and their properties. This Review examines the theoretical framework of resolution control and local phonon detection in STEM-EELS, while comprehensively discussing experimental advancements and their applications in various materials systems. Given its flexibility to finely tune both spatial and momentum resolutions, this approach enables precise detection of local phonon density of states in defect structures, heterointerfaces, and nanoscale systems, as well as achieving nanometer-scale spatial resolution for phonon dispersion measurements. Additionally, the application and potential of such a method for studying thermal, electrical, and optical properties as well as detecting vibrations in molecules are also discussed. With rapid development, vibrational STEM-EELS is expected to play an increasingly important role in materials science, condensed matter physics, and chemistry in the future.