Phase engineering of two-dimensional transition metal dichalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have gained much attention in virtue of their various atomic configurations and band structures. Apart from those thermodynamically stable phases, plenty of metastable phases exhibit interesting properties. To obtain 2D TMDs with specific phases, it is important to develop phase engineering strategies including phase transition and phase-selective synthesis. Phase transition is a conventional method to transform one phase to another, while phase-selective synthesis means the direct fabrication of the target phases for 2D TMDs. In this review, we introduce the structures and stability of 2D TMDs with different phases. Then, we summarize the detailed processes and mechanism of the traditional phase transition strategies. Moreover, in view of the increasing demand of high-phase purity TMDs, we present the advanced phase-selective synthesis strategies. Finally, we underline the challenges and outlooks of phase engineering of 2D TMDs in two aspects-high phase purity and excellent controllability. This review may promote the development of controllable phase engineering for 2D TMDs and even other 2D materials toward both fundamental studies and practical applications.