3R-MoS₂ in Review: History, Status, and Outlook

Transition metal dichalcogenides (TMDs) of molybdenum and tungsten are layered van der Waals materials that exhibit a rich array of polytypes. The different possible arrangements of the constituents of the “two-dimensional” MX2 sheets (where M = group 4–10 elements, X = chalcogen) give rise to a host of interesting and tunable phenomena. Molybdenite, or molybdenum disulfide (MoS2) in its most abundant and thermodynamically stable form, 2H-MoS2, is perhaps the most widely used TMD, though the potential applications of its metastable polytypes have been recognized only recently. From among the polytypes, the 3R-MoS2 (rhombohedral) phase has attracted the most interest because of its thermodynamic stability, ABC stacking (as opposed to the AA′ of the more common 2H-MoS2), and lack of inversion symmetry. These properties make it an excellent candidate for photonics, optoelectronics, and catalysis. Because the literature on this material is rapidly expanding, this review seeks to summarize the history, known and predicted characteristics, syntheses, and applications, as well as common misconceptions of, and surrounding, 3R-MoS2. Although the review is chemically focused, it includes suggested reading to cover a broader scope.