Recent advances of phase engineering in group VI transition metal dichalcogenides

As the crystal quality and phase structure of two-dimensional (2D) transition metal dichalcogenides (TMDs) have significant impacts on their properties such as electroconductivity, superconductivity and chemical stability, the precise synthesis, which plays an important role in fundamental researches and industrial applications, is highly required. Group VI TMDs, such as MoS2, usually exhibit diverse polymorphs including semiconducting 1H and metallic 1T phases. Even great efforts are devoted to revealing the structure-dependent physicochemical nature of TMDs by modulating their phases from the stable to the metastable at the atomic scale, there are still challenges on the phase-controlled synthesis of Group VI TMDs with metallic or semimetal properties. In this review, methods such as ion intercalation, chemical doping, strain engineering, defect triggering, and electric-field treatment are examined in detail. Finally, challenges and opportunities in this research field are proposed.