Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China.
School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, China.
Proc Natl Acad Sci U S A. 2023 Jun 20;120(25):e2221956120. doi: 10.1073/pnas.2221956120. Epub 2023 Jun 12.
Investigating coherent acoustic vibrations in nanostructured materials provides fundamental insights into optomechanical responses and microscopic energy flow. Extensive measurements of vibrational dynamics have been performed for a wide variety of nanoparticles and nanoparticle assemblies. However, virtually all of them show that only the dilation modes are launched after laser excitations, and the acoustic bending and torsional motions, which are commonly observed in photoexcited chemical bonds, are absent. Unambiguous identification and refined characterization of these "missing" modes have been a long-standing issue. In this report, we investigated the acoustic vibrational dynamics of individual Au nanoprisms on free-standing graphene substrates using an ultrafast high-sensitivity dark-field imaging approach in four-dimensional transmission electron microscopy. Following optical excitations, we observed low-frequency multiple-mode oscillations and higher superposition amplitudes at nanoprism corners and edges on the subnanoparticle level. In combination with finite-element simulations, we determined that these vibrational modes correspond to out-of-plane bending and torsional motions, superimposed by an overall tilting effect of the nanoprisms. The launch and relaxation processes of these modes are highly pertinent to substrate effects and nanoparticle geometries. These findings contribute to the fundamental understanding about acoustic dynamics of individual nanostructures and their interaction with substrates.
研究纳米结构材料中的相干声振动为光机械响应和微观能量流提供了基本的认识。已经对各种纳米粒子和纳米粒子组件进行了广泛的振动动力学测量。然而，几乎所有的实验都表明，只有膨胀模式在激光激发后被激发，而在光激发化学键中常见的声弯曲和扭转运动则不存在。这些“缺失”模式的明确识别和精细表征一直是一个长期存在的问题。在本报告中，我们使用超快高灵敏度暗场成像方法在四维度电子显微镜中研究了独立石墨烯衬底上的单个 Au 纳米棱镜的声振动动力学。在光激发后，我们观察到在亚纳米颗粒水平上，纳米棱镜角和边缘处存在低频多模振荡和更高的叠加振幅。结合有限元模拟，我们确定这些振动模式对应于平面外弯曲和扭转运动，同时纳米棱镜整体倾斜。这些模式的激发和弛豫过程与衬底效应和纳米粒子几何形状密切相关。这些发现有助于对单个纳米结构的声动力学及其与衬底的相互作用的基本理解。
