Transition-metal dichalcogenide monolayers (TMDs or TMDCs) are a class of two-dimensional materials that have the chemical formula MX 2, where M represents transition metals from group IV, V and VI, and X represents a chalcogen such as sulfur, selenium or tellurium. MoS 2, MoSe 2, MoTe 2, WS 2 and WSe 2 are TMDCs. TMDCs have layered structure with a plane of metal atoms in between two planes of chalcogen atoms as shown in Figure 1. Each layer is bonded strongly in plane, but weakly in interlayers. Therefore, TMDCs can be easily exfoliated into atomically thin layers through various methods. TMDCs show layer-dependent optical and electrical properties. When exfoliated into monolayers, the band gaps of several TMDCs change from indirect to direct, which lead to broad applications in nanoelectronics, optoelectronics, and quantum computing.