Microstructures of two-dimensional material based homo- and heterostructures

Abstract

Ever since many different types of two-dimensional (2-D) atomic crystals proved themselves as exciting materials with unusual properties, there have been a lot of efforts to form heterostructures based on these 2-D crystals to engineer unconventional material systems. While tremendous amount of research have been conducted on developing novel 2-D based heterostructures and finding out their intriguing properties, fundamental studies on microstructures of the system have not been done carefully yet. Investigation of microstructural properties is necessary to find out correlation between structural and corresponding physical properties, and thereby making it possible to engineer novel 2-D based heterostructures with desired properties in the future. In this thesis, we study microstructural properties of 2-D based homo- and heterostructures using transmission electron microscopy (TEM) based analysis including electron diffraction, dark field imaging, and spherical aberration (cs) corrected scanning transmission electron microscopy (STEM). First, we investigate the formation of microstructural defects at van der Waals interface between 2-D atomic crystals. With the aid of modern dry transfer technique, artificial bilayer graphene and MoS2 were prepared with controlled twist angle. Depending on the twist angle, the interlayer van der Waals interaction can cause structural phase transition at the interface, and we studied resulting structural properties of the systems such as alternating domain structures and arrays of partial dislocations. By applying classical Frenkel-Kontorova model to our systems, we try to understand the interlayer interaction in more systematical manner, and therefore we could identify microstructural origin that can cause the structural transition in the system. Furthermore, we could also verify that the domain structure and dislocation arrays at the interface and even the type of symmetry elements in the materials can be tuned by controlling the twist angle between the two layers, indicating possibility of defect engineering for advanced properties. Hybrid system composed of compound semiconductor thin films with 2-D substrates is another 2-D based heterostructures of interest in this thesis. While 2-D substrates can offer advanced functionalities to the conventional thin film based devices, effects of 2-D substrates on the formation of defects in thin films have not been understood thoroughly. Using classic TEM based analytical techniques such as electron diffraction and dark field imaging, we investigate microstructural defects in the GaN films grown on graphene layers and figure out their correlation with graphene substrates. Moreover, we study effect of the microstructural defects in the GaN films on luminescence properties by combination of electron backscatter diffraction and cathodoluminescence analysis. In order to further investigate the origin of the luminescence properties of the defects, we examined their atomic and electronic structures with the aid of state-of-the-art cs-corrected STEM and density functional calculation.