Synthesis, Properties, and Stacking of Two-Dimensional Transition Metal Dichalcogenides

In 2004, the study about monolayer carbon film, namely graphene, opens a new era of materials science research: Two-dimensional (2D) materials. It attracts the tremendous interests in the unique properties when materials’ dimension is reduced (Novoselov et al., 2004). In the recent 10 years, there has been rapidly increasing study focusing on 2D materials beyond graphene (Bhimanapati et al., 2015). A large family of 2D materials have gradually been revealed including conductors (graphene), semiconductors (transition metal dichalcogenides, TMDs), and insulators (hexagonal boron nitride (hBN)). In this family, TMDs fit the application on transistors and photosensors best due to its appropriate band gap (visible light range). Additionally, the band gap of TMDs can be tuned by alloying or stacking different TMDs (Lin et al., 2014a), defect engineering (Chow et al., 2015), and chemical doping. To realize the application, the preparation of TMDs is the first question to the scientific society. In the first part of this chapter, we review widely used synthesis methods of TMDs (mainly focusing on MoS₂) and compare the advantages and disadvantages of different synthesis methods. The properties of TMDs synthesized by different methods are correlated to the synthesis techniques. Stacking 2D TMDs layers with other 2D crystals to create “van der Waals” (vdW) heterostructures has been a common technique to explore new properties out of 2D material-based systems. In the second part of this chapter, the very first examples using graphene and hBN that initiated this field of research would be introduced to help readers build a general knowledge on vdW heterostructures. Subsequently, other vdW heterostructures that utilizes TMDs as a component to create novel optoelectronics would also be briefed. In order to place vdW heterostructures on a practical platform that can synthesize and produce their electronics, many synthetic techniques have been developed to grow different 2D materials together. Some cases of synthetic vdW heterostructures that have been developed are presented at the end of Section 7.