Synthesis of Transition Metal Dichalcogenides

Novel properties and new phenomena that arise from two-dimensional (2D) materials have stimulated huge research interest recently. Among the 2D material family, semiconducting transition metal dichalcogenides (TMDCs) with an intrinsic band gap are complementary to the gapless graphene [1, 2]. Hence, both single-layer and multi-layer TMDCs are promising candidates for high-performance electronic [3-7] and optoelectronic [8-14] applications. The thickness uniformity and the lateral size of a single domain are important concerns for the exploration of the properties, function, and fabrication of electronic devices. Considerable efforts have been made in the synthesis of 2D semiconducting TMDCs, and the main theme focuses on large-scale growth of single crystalline monolayers and their heterostructures. Various methods have been developed for the large-scale production of monolayer and few-layer TMDC nanosheets, including the top-down layer exfoliation from bulk crystals [15-19] and the bottom-up chemical synthesis. Among all the approaches, the liquid phase exfoliation of TMDC layers from their bulk crystals generates a range of well-dispersed TMDC monolayers with a high yield [20]. This method has immense potential for solution-based printable and flexible electronics. However, the wet chemical method may unavoidably alter the lattice structure of thin TMDC layers and introduce extrinsic defects during the exfoliation process. It is essential to develop a synthetic technique that can produce wafer-scale TMDC materials that are also compatible with the current nanoelectronic fabrication processes. Considerable efforts have been made in developing controllable synthesis of 2D semiconducting TMDCs. Several synthetic routes, including thermolysis of thiosalts [21, 22], sulfurization (or selenization) of metal (or metal oxide) thin films [23], physical vapor phase transport [24], and chemical vapour deposition (CVD) [25], have been developed. Among all these methods, vapor phase deposition shows great promise towards high-quality TMDC production [1]. With this technique, monolayer TMDCs that have single domains with scalable size, controllable thickness, and excellent electronic properties can be obtained. In addition, the electro-dispersive properties and the semiconducting nature of TMDCs can be tailored by tuning the synthesis conditions. Remarkably, vapor phase deposition can also be employed to synthesize TMDC ternary alloys, in-plane heterostructures, and multilayer van der Waals stackings, which pave the way for exploring new devices and physics based on 2D materials.