Qi Ruishi, Li Ning, Du Jinlong, Shi Ruochen, Huang Yang, Yang Xiaoxia, Liu Lei, Xu Zhi, Dai Qing, Yu Dapeng, Gao Peng
Electron Microscopy Laboratory, School of Physics, Peking University, Beijing, China.
International Center for Quantum Materials, Peking University, Beijing, China.
Nat Commun. 2021 Feb 19;12(1):1179. doi: 10.1038/s41467-021-21452-5.
Directly mapping local phonon dispersion in individual nanostructures can advance our understanding of their thermal, optical, and mechanical properties. However, this requires high detection sensitivity and combined spatial, energy and momentum resolutions, thus has been elusive. Here, we demonstrate a four-dimensional electron energy loss spectroscopy technique, and present position-dependent phonon dispersion measurements in individual boron nitride nanotubes. By scanning the electron beam in real space while monitoring both the energy loss and the momentum transfer, we are able to reveal position- and momentum-dependent lattice vibrations at nanometer scale. Our measurements show that the phonon dispersion of multi-walled nanotubes is locally close to hexagonal-boron nitride crystals. Interestingly, acoustic phonons are sensitive to defect scattering, while optical modes are insensitive to small voids. This work not only provides insights into vibrational properties of boron nitride nanotubes, but also demonstrates potential of the developed technique in nanoscale phonon dispersion measurements.
直接绘制单个纳米结构中的局域声子色散，有助于我们深入了解其热学、光学和力学性质。然而，这需要高检测灵敏度以及空间、能量和动量分辨率的结合，因此一直难以实现。在此，我们展示了一种四维电子能量损失谱技术，并给出了单个氮化硼纳米管中位置依赖的声子色散测量结果。通过在实空间中扫描电子束，同时监测能量损失和动量转移，我们能够揭示纳米尺度下位置和动量依赖的晶格振动。我们的测量表明，多壁纳米管的声子色散在局部上接近六方氮化硼晶体。有趣的是，声学声子对缺陷散射敏感，而光学模式对小空隙不敏感。这项工作不仅为氮化硼纳米管的振动性质提供了见解，还展示了所开发技术在纳米尺度声子色散测量中的潜力。
