Nanoscale imaging of phonon dynamics by electron microscopy

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.

Atomic-scale origin of the low grain-boundary resistance in perovskite solid electrolyte Li0.375Sr0.4375Ta0.75Zr0.25O3

“…To resolve this issue, we map the Li distributions by probing Li-O vibrations. We employ a state-of-the-art vibrational EELS technique enabled by high energy resolution monochromation that has facilitated the spectroscopy of vibrational excitations at the nanometer 53,54 and even atomic scales 52,55 . We employ a dark field vibrational EELS (DF VibEELS) beam-detector geometry (Supplementary Fig.…”

Imaging Vibrational Excitations in the Electron Microscope

“…In parallel, other researchers measured the phonon modes in strontium titanate−calcium titanate superlattices 50 and silicon−germanium interfaces 51 to map the spatial extent of interfacial phonon modes. Recent work by Gadre et al 52 observed that abrupt Si−Ge interfaces impede the phonon propagation to a higher degree than gradual interfaces, thus explaining the decrease in the thermal conductivity at abrupt interfaces. These studies illustrate how q-EELS can experimentally probe localized interfacial phonon modes and can provide unique insights into the engineering of material interfaces for desirable electrical and thermal transport properties.…”

Exploiting the Momentum Distribution in Atomically Confined Plasmonic Fields by Inelastic Scatterings

“…In this work, we propose a novel exploitation of the momentum distribution via inelastic scatterings of the atomically confined plasmonic fields, where the energy and momentum conservation laws together govern the corresponding optical processes (Figure a). A perfect subject is to measure phonon dispersions that play an essential role in many properties, such as mechanical stabilities, electrical/thermal conductivities, , and optical transitions, in solid states. To this end, the Raman processes of a single trans -polyacetylene (t-PA) chain are taken as a proof of concept, which leads to the precise determination of all phonon branches of t-PA in the whole Brillouin zone from the Raman images for the first time.…”