Phase Engineering of Two-Dimensional Transition Metal Dichalcogenides

Since the successful isolation of single-layer graphene with an atomic thickness, various van der Waals (vdW) materials have been intensively studied owing to their unique properties. Among the families of vdW materials, transition metal dichalcogenides (TMDs) have served as representatives because of their diverse band structures and intriguing quantum states, unlike those observed in their bulk counterparts. Particularly, unconventional polymorphic phases of TMDs increase the degrees of freedom in device fabrication and property modulation. As variations in structural phases significantly change the electrical, physical, and chemical properties of materials, phase engineering is essential for the new paradigm of TMD-based devices. In this review, diverse strategies that can induce and control structural phases in TMDs are explored. After introducing the polymorphic phase changes and the resulting electronic band structures, the various empirical approaches used for manipulating phases in vdW materials, including phase-selective synthesis and post-synthesis treatments, are summarized. The group-VI TMDs are considered as reference, and the analysis is extended to other TMDs across various groups in the periodic table. In addition to providing a comprehensive survey of the recent progress in TMD applications, the challenges for TMD applications and potential opportunities in emerging fields are discussed.