A tomically thin transition metal di-chalcogenides (TMDs) are 2D materials that consist of two hexagonal planes of chalcogen atoms X (S, Se) bonded to transition metal atoms M (Mo, W) arranged hexagonally. Bulk TMDs play an important role as dry lubricants and have properties different than those of 2D monolayer and few-layer thin films. 1 Two-dimensional TMDs have unique electrical and optical properties, evolving from an indirect to a direct band gap when the number of layers decreases down to a monolayer. 2À5 The very large exciton binding energy, due to the 2D nature, 6 leads to strong photo-luminescence (PL) at room temperature. 7À10