1204 Microsc. Microanal. 27 (Suppl 1), 2021 doi:10.1017/S1431927621004530 © Microscopy Society of America 2021 Phonon Reflections from Nanostructured Interfaces Imaged by Momentum- Averaged and Resolved Vibrational EELS Chaitanya Gadre 1 , Xingxu Yan 2 , Qichen Song 3 , Gang Chen 3 and Xiaoqing Pan 1 1 Department of Physics and Astronomy, University of California, Irvine, CA 92697, Irvine, California, United States, 2 Department of Materials Science and Engineering, University of California - Irvine, Irvine, California, United States, 3 Michigan Institute of Technology, United States Phonon transport is the dominant factor in determining thermal conductivity in semiconductors and is greatly modulated by nanostructures, interfaces, and defects but the physics of the nanoscale process has hardly been explored by experimental means. The phonon resistance provided by these structures are often a result of a mismatch between the local phonon density of states (LDOS) [1], studies of which have eluded optical measurement techniques due to their insufficient spatial resolution. Additionally, optical, and other conventional phonon spectroscopies have no way of investigating the phonon dynamics of individual interfaces and as a result, cannot aid in the physical understanding of nanoscale thermal transport. However, recent developments in electron microscopy have enabled the acquisition of vibrational spectra at sub-nanometer resolutions via monochromated electron energy loss spectroscopy (EELS) [2]. Armed with this technical capability, we demonstrate a mapping of phonons revealing interface dynamics at the nanometer scale. Utilizing momentum- averaged and resolved EELS, we image phonon reflections from nanostructure interfaces revealing the dynamics of phonon propagation and reflection. We employ a momentum averaged beam scattering geometry depicted in Fig. 1A to study the intensity of the Si optical mode (OM). Fig. 1B shows a high angle annular dark field (HAADF) image of the left side of a QD consisting of a gradual Si-SiGe at the top and a more abrupt SiGe-Si interface at the bottom elucidated by the composition gradient map in Fig. 1C. Fig. 1D is a mapping of the Si OM revealing an abnormal intensity enhancement in the presence of the two interfaces. It can be seen from the profile in Fig. 1E, the region towards the center of the QD, that the Si OM mode intensity is significantly greater than that at the top of the QD while at the edge of the QD (Fig. 1F) that the intensity in the vertical direction is far more uniform. The compositional gradient plots in Figs. 1E-1F suggest that the sharpness of the interfaces is modulating this intensity. To see propagating modes, a 3 mrad convergence semi-angle probe was used to enable the investigation of phonon modes at specific points in k-space within the first Brillouin zone (FBZ) (Fig. 2A). By using post-specimen lens deflectors, the scattered off-axis beam was collected at points Δ+ and Δ- denoted in Fig. 2B. By conservation of energy and electron-phonon momentum, these points in momentum space within FBZ correspond to downward and upward propagating phonon modes, respectively. The difference then provides net momentum information i.e. direction of propagation. Utilizing this novel technique, the differential momentum mapping in fig. 2C reveals that there is indeed a strong reflection from the abrupt interface while the gradual interface provides a weaker directionality. The abrupt change in LDOS creates an environment where propagating Si optical modes experience a shortage of corresponding modes on the other side of the interface, leading to a reflection. Our work showcases the versatility of vibrational EELS for studying nanoscale physics previously invisible to experimental techniques. By utilizing averaged and resolved momentum conditions, phonon https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1431927621004530 Downloaded from https://www.cambridge.org/core. IP address: 3.80.168.101, on 05 Nov 2021 at 00:33:06, subject to the Cambridge Core terms of use, available at