Thesis presented September 08, 2023

Abstract:
Atomic defects play a crucial role in determining the properties of two-dimensional (2D) materials, like graphene. Controlling such defects, including their structure, size, and density can provide an efficient way to regulate the existing properties or integrate new properties in graphene for desired applications. For instance, pores integration in graphene with a narrow size distribution can extend graphene applicability for a wide range of molecular separation membranes, such as gas filtration and water purification. However, integration of atomic defects in graphene with angstrom precision and accessing such precise atomic scale information is extremely challenging yet required for graphene-based advanced technological developments. With the application of low-voltage aberration-corrected transmission electron microscopy (AC-TEM), which is primarily used in this thesis work at the plateform for nano-charecterization (CEA), it is possible to resolve the atomic structure of graphene with single-atom sensitivity. The modern AC-TEM stands out as the most powerful technique for complete study of atomically thin materials, including their atomic structure, chemical composition, and atomic local electric field and related properties around a single atom. Particularly, we aimed to investigate atomic defects in graphene monolayers including nano-meter scale pores by AC-TEM at low acceleration voltage. As a first step, we developed a robust, plasma-based surface cleaning procedure for graphene (in collaboration with LTM, CNRS), which is essential for the formation of size-controlled defects in graphene and to precisely investigate them by high-resolution (HR)TEM. Besides, surface contamination is one of the biggest limitations for graphene to be practically used for many applications. Then, to generate nano-meter scale pores of different sizes, we used another scalable plasma in a remote plasma system, and the interaction between plasma and graphene was investigated by HR-TEM. Further, the structure-wettability relation for the plasma-modified graphene samples was investigated.
The second part of this thesis contributes to the establishment of a new scanning (S)TEM technique, four-dimensional (4D) STEM to extract local atomic electric-field information in 2D materials. This technique has the potential to provide local atomic physical properties fluctuation associated with atomic defects in 2D material that can help to understand and control the final property of materials. By using the atomic electric field information, we demonstrated the applicability of this technique for the detection of a light dopant atom in transition metal dichalcogenide (TMD) material.

Keywords:
2D materials, Atomic defects, Plasma induced defects formation in graphene, Aberration-corrected transmission electron microscopy, Nanopores in graphene for surface-based applications, 4-dimensional STEM