**4. Introduction of metal dichalcogenides**

In recent years, metal dichalcogenides (MX2) have attracted a lot of attention, like other 2D materials where M is transition metals and Xs are S, Se, and Te such as WSe2, molybdenum disulfide (MoS2), tellurium disulfide (TeS2), etc. MX2 can be a semiconductor or metal depending on the oxidation state of their metal atoms. As graphene-like compounds, they have similarities to graphene, resulting in new opportunities for detecting and building sensors, lithium batteries, optoelectronics, and energy storage. Many researchers have shown that the exfoliated layers of MX2 have large energy of bandgap and are semiconductors. In addition, they have the properties of fluorescence and photoluminescence [73].

There are currently only a limited number of reports on single-layer and fewlayer group IV–VI and III–VI layered compounds. Therefore, the 2D properties of these materials are largely unknown. However, MX2 shows a variety of electrical and optical properties that are suitable for catalysts, nanotribology, optoelectronics, and lithium-ion batteries. These materials are expected to exhibit extraordinary properties after reaching a thickness to single layer or multilayer [74].
