Metastable MoS₂ : Crystal Structure, Electronic Band Structure, Synthetic Approach and Intriguing Physical Properties

The 2H molybdenum disulfide (MoS₂ ), as a stable hexagonal phase, has been one of the most studied transition metal dichalcogenides over the past decades. In the last five years, the metastable phases of MoS₂ (1T, 1T', 1T'', and 1T''') have seen a revival of interests. Different from the edge-sharing [MoS₆ ] trigonal prisms in the 2H MoS₂ phase, these metastable phases are composed of the edge-sharing [MoS₆ ] octahedra, in which the neighboring Mo-Mo distances differ. Due to the various crystal structures and different electronic configurations of the building [MoS₆ ] motifs, these metastable polytypes are endowed with intriguing physical properties and potential applications in diverse fields. In this Review, the recent research progress on metastable MoS₂ is summarized, especially with an emphasis on the diverse synthetic approaches and the newly uncovered physical properties. The phase structures and electronic band structures are also outlined. In the end, a perspective of the future investigation on metastable MoS₂ is discussed.