Department of Physics and Astronomy, University of California Irvine, Irvine, CA, USA.
Department of Materials Science and Engineering, University of California Irvine, Irvine, CA, USA.
Nature. 2022 Jun;606(7913):292-297. doi: 10.1038/s41586-022-04736-8. Epub 2022 Jun 8.
Spatially resolved vibrational mapping of nanostructures 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 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.
对热纳米器件、热输运的调控以及新型纳米结构热电材料的发展和理解来说，对纳米结构的振动进行空间分辨测绘是不可或缺的。通过对复杂结构（如合金、纳米结构和超晶格界面）的工程设计，可以显著改变声子的传播并抑制材料热导率，同时保持电导率。由于传统光学声子探测技术的空间分辨率限制，目前还没有相关实验能够在纳米结构内部和周围空间上对声子性质的调制进行空间跟踪。在这里，我们通过透射电子显微镜中的单色电子能量损失谱，在单个硅锗（SiGe）量子点（QD）中演示了声子的二维空间测绘。通过跟踪 QD 内部和周围 Si 光学模式的变化，我们观察到了由组成诱导的红移的纳米级修饰。我们观察到仅存在于界面附近的非平衡声子，并且进一步开发了一种新的技术来差分映射声子动量，提供了直接证据表明漫反射和镜面反射之间的相互作用在很大程度上取决于详细的原子结构：这是该领域的一项重大进展。我们的工作揭示了纳米尺度界面处的非平衡声子动力学，可用于研究实际的纳米器件，并有助于理解纳米热点附近的热耗散，这对未来的高性能纳米电子学至关重要。
