VAN DER WAALS STACKING OF TWO-DIMENSIONAL MATERIALS

Abstract

Two-dimensional (2D) materials, which show a wide range of electrical and optical properties, can serve as building blocks to form stacked 2D systems that are connected by interlayer van der Waals interactions. Any 2D materials can be stacked together, regardless of their crystal structures: this versatility offers an opportunity to realize “materials by design”, where, through stacking, film properties can be directly manipulated and film thicknesses can be precisely controlled. In this dissertation, we focus on studying the fabrication, characterization, and application of the stacked 2D systems on a technologically relevant large scale. We first introduce the growth of wafer-scale homogeneous monolayer transition metal dichalcogenides (TMD) films using metal-organic chemical vapor deposition (MOCVD), a growth technique that enables the fine tuning of precursor kinetics by using gas-phase precursors. After introducing our work on 2D material growth, we switch to 2D stacking and talk about a model 2D stacking system, twisted bilayer graphene (tBLG), where twist angle (θ) serves as a key parameter for manipulating the physical properties. We first discuss the θ-dependent properties in tBLG and introduce a vacuum-assisted direct stacking technique that is able to generate high-quality, θ-controllable tBLG using CVD sample sources. We then focus on small-θ tBLG, where, after presenting a quantitative study of the θ dependence in its Raman R’ process, we introduce a θ measurement method that is developed based on these quantitative results. Finally, we utilize this angle detection method to study the θ-dependent electrical properties in small-θ tBLG. After discussing the 2D stacking model system tBLG, we switch to stacked 2D systems where no apparent changes in properties are introduced by stacking. This kind of stacked system can also have wide applications due to the atomically precise thickness control and well-defined surfaces. As a demonstration, we introduce our preliminary research that builds functional dielectric films from stacked multilayer WS2.