Synthesis and Optical Spectroscopy Study of Two-Dimensional Crystals: Graphene and Beyond

The discovery of graphene, a single atomic layer of hexagonally packed carbons, can be considered as the opening of an epic of two-dimensional (2D) materials. With the restriction in dimensions and the weak interlayer reactions, these isolated layer crystals have exhibited distinctive properties from their bulk forms. Moreover, integrating heterogeneous layered crystals is expected as a promising approach to produce materials with complex structures and well-engineered properties. Therefore, this dissertation emphasizes on synthesizing ideal 2D building blocks, graphene and transition metal dichalcogenides (TMDCs) crystals via chemical vapor deposition (CVD), exploring their optical spectroscopic signatures, and verifying the practicability of artificial stacking with the simplest case, twisted bilayer graphene (tBLG). After evaluating several key factors, millimeter size of monolayer hexagon graphene crystals have been harvested with designed recipe. Synthesis of bilayer hexagon graphene domains, monolayer and few-layer MoS2 and WS2 crystals in micrometer size have also been demonstrated. In addition, hundreds-of-micrometer sized twisted bilayer graphene domains have been achieved by artificial staking of two monolayer graphene hexagons, of which crystal orientation mismatch can be directly estimated by the misalignment of edges. Raman and photoluminescence spectroscopy are utilized not only to determine the number of layers and the quality of as-grown samples, but also to characterize the underlying crystalline and even electronic structures. This study provides valuable insight of synthesizing, processing, and characterizing 2D crystals in engineered approaches and paves a way for introducing them into electronic and photonic application.