Growth of two-dimensional Janus MoSSe by a single in situ process without initial or follow-up treatments

Two-dimensional (2D) Janus transition metal dichalcogenides (TMDCs) are highly attractive as an emerging class of 2D materials, but only a few methods are available for fabricating them. These methods rely on the initial growth of 2D TMDCs in one process, followed by an additional plasma or high-temperature (T) process. To overcome these drawbacks, we employ the new approach of NaCl-assisted single-process chemical vapor deposition, which consists of three steps that proceed only by altering the temperature in situ. In the first step, MoS₂ is deposited onto a SiO₂/Si substrate with the Mo and S atoms activated in different temperature zones. In the second step, S vacancies are formed in the upper layer of the grown MoS₂ by annealing. In the third step, the vacancies are filled with activated Se atoms. Throughout the steps, NaCl lowers the melting point of the constituent atoms, while the T in each zone is properly controlled. The growth mechanism is clarified by a separate annealing experiment that does not involve a supply of activated atoms. These results highlight a simple and cost-effective approach for growing Janus MoSSe, which is more useful for fundamental studies and device applications.