Imaging and atomic structure engineering of quasi-two-dimensional materials encapsulated between graphene sheets

石墨烯片是由单原子层组成的机械坚固和化学惰性膜。由于这些特性，石墨烯已被证明是使用像差校正透射电子显微镜（AC-TEM）成像分子和纳米结构的理想衬底。实验表明，如果将物体封装在两片石墨烯之间，石墨烯可以减少撞击和电子束诱导的电离损伤。这个过程使原子结构的可视化，否则是不可能的。由于石墨烯和其他二维（2D）材料，如六边形BN和过渡金属二硫族化物，对水和水溶液是不渗透的，因此它们可用于限制液体材料。由于这种能力，入射电子可以在封装材料内部诱导形成新的二维相，否则这些相可能不稳定。在这个项目中，我们的目标是将交流高分辨率（HR） TEM实验与原子模拟相结合。这种方法将使我们能够揭示石墨烯片和其他二维材料之间的新二维材料的形成过程、结构和性质。由于这些封装结构不稳定，我们称之为准二维材料。具体来说，水、盐的水溶液和熔融温度较低的金属（汞、镓）将在20-80千伏的温度和低电子电压范围内使用HRTEM进行封装和研究。我们将首次使用我们新开发的SALVE机器，它提供了卓越的分辨率，因为它配备了球面和色差校正器。由于电子辐照通过多种机制和化学反应诱导封装材料中缺陷的形成，我们将利用电子束来设计新的受限纳米结构和准二维晶体。为了全面了解光束诱导转换和辐射诱导缺陷的作用，将进行多尺度原子模拟。具体而言，我们将开发基于非绝热Ehrenfest动力学与时变密度泛函理论相结合的新计算技术，在专用计算机软件中实现它们（也适用于块状材料和生物系统），并将它们与动力学蒙特卡罗方案连接起来，以描述系统在宏观时间尺度上的演化。我们还将使用包括DFT和分析势方法在内的标准技术对准二维材料的性质进行广泛的计算。我们的研究结果不仅为在原子尺度上限制低维系统的物理学提供了基本的见解，而且还使我们能够探索新的封装纳米结构的工程结构和性能的有前途的途径。

Graphene sheets are mechanically robust and chemically inert membranes consisting of a single atomic layer. Owing to these properties, graphene has been proven to be an ideal substrate for imaging molecules and nanostructures using aberration-corrected transmission electron microscopy (AC-TEM). Experiments have shown that graphene reduces knock-on and electron-beam-induced ionization damage if the object is encapsulated between two sheets of graphene. This procedure enables the visualization of the atomic structure which otherwise is not possible. Because graphene and other two-dimensional (2D) materials, such as hexagonal BN and transition metal dichalcogenides, are impermeable for water and aqueous solutions, they can be used for confining liquid materials. Owing to this ability, the incident electrons can induce within the encapsulated materials the formation of new 2D phases which may not be stable otherwise.In this project, we aim to combine AC-high-resolution (HR) TEM experiments with atomistic simulations. This approach will enable us to unravel the formation process, the structure, and the properties of the new 2D materials between the sheets of graphene and other 2D materials. Since these encapsulated structures would otherwise be unstable, we call them quasi 2D materials. Specifically, water, aqueous solutions of salts, and metals with low melting temperature (mercury, gallium) will be encapsulated and studied using HRTEM in a wide range of temperatures and low electron voltages in the range of 20-80 kV. For the first time, we will use our newly developed SALVE machine, which provides exceptional resolution, because it is equipped with a spherical and chromatic aberration corrector. Since electron irradiation induces the formation of defects in the encapsulated materials through several mechanisms and chemical reactions, we will use the electron beam for engineering new confined nanostructures and quasi-2D crystals. To obtain complete understanding of the beam-induced transformations and the role of radiation-induced defects, multiscale atomistic simulations will be carried out. Specifically, we will develop new computational techniques based on the non-adiabatic Ehrenfest dynamics combined with time-dependent density-functional theory, implement them in the dedicated computer software (applicable also to bulk materials and bio systems), and connect them to the kinetic Monte-Carlo schemes to describe the evolution of the system on a macroscopic time scale. We will also carry out extensive calculations of the properties of the quasi-2D materials using standard techniques including DFT and analytical potential approaches. Our results should not only provide fundamental insights into the physics of confined low-dimensional systems on an atomic scale, but also enable us to explore promising avenues for engineering the structure and properties of novel encapsulated nanostructures.