STEM中快电子向球形纳米粒子传递角动量的理论与模拟
Theory and simulations of the angular momentum transfer from swift electrons to spherical nanoparticles in STEM
AI总结：
研究提出基于经典电动力学的理论模型与数值方法，计算STEM快电子向球形纳米粒子传递的角动量，发现传递方向恒定且随粒子半径增大而增加，随电子速度和碰撞参数增大而减少，电场贡献主导磁场贡献。
摘要：
扫描透射电子显微镜（STEM）中的电子束对研究样品施加力和力矩，其量级允许对纳米粒子进行可控操作（一种称为电子镊子的技术）。相关的理论研究主要集中在快电子（如STEM中使用的电子）向纳米粒子传递的力和线性动量上。然而，对相互作用旋转方面的理论研究不仅将促进电子镊子的发展，还将有益于电子显微镜内的其他领域，如电子涡旋。基于经典电动力学描述，我们提出了一个理论模型及高效的数值方法，用于计算STEM快电子向球形纳米粒子传递的角动量。我们展示了对不同尺寸的铝、金和铋纳米粒子传递角动量的模拟。我们发现，传递的角动量始终垂直于系统的对称面，在所有考虑的情况下方向保持恒定。在模拟中，角动量传递随纳米粒子半径增加而增加，但随电子速度或碰撞参数增加而减少。此外，角动量传递的电场贡献主导于磁场贡献，仅在高电子速度（大于光速的90%）时两者才可比拟。另外，对于所研究材料中半径为1 nm的纳米粒子，我们发现了小粒子近似（将纳米粒子建模为电点偶极子）的有效性准则。我们相信这些发现有助于理解STEM实验中的旋转特性，并可能对电子镊子及其他电子显微镜相关技术的进一步发展有所裨益。
英文摘要：
Electron beams in scanning transmission electron microscopy (STEM) exert forces and torques on study samples, with magnitudes that allow the controlled manipulation of nanoparticles (a technique called electron tweezers). Related theoretical research has mostly focused on the study of forces and linear momentum transfers from swift electrons (like those used in STEM) to nanoparticles. However, theoretical research on the rotational aspects of the interaction would benefit not only the development of electron tweezers, but also other fields within electron microscopy such as electron vortices. Starting from a classical-electrodynamics description, we present a theoretical model, alongside an efficient numerical methodology, to calculate the angular momentum transfer from a STEM swift electron to a spherical nanoparticle. We show simulations of angular momentum transfers to aluminum, gold, and bismuth nanoparticles of different sizes. We found that the transferred angular momentum is always perpendicular to the system's plane of symmetry, displaying a constant direction for all the cases considered. In the simulations, the angular momentum transfer increased with the radius of the nanoparticle, but decreased as the speed of the electron or the impact parameter increased. Also, the electric contribution to the angular momentum transfer dominated over the magnetic one, being comparable only for high electron's speeds (greater than 90% of the speed of light). Additionally, for nanoparticles with 1 nm radius of the studied materials, we found validity criteria for the small-particle approximation (in which the nanoparticle is modeled as an electric point dipole). We believe that these findings contribute to the understanding of rotational aspects present in STEM experiments, and might be useful for further developments in electron tweezers and other electron microscopy related techniques.
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