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

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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.