Spontaneous Crystal Fluctuation in Hydrocarbon Polymer–Coated Monolayer MoS2, MoSe2, WS2, and WSe2 with Strong Photoluminescence Enhancement

A large number of experimental results have been published on the improvement of optical properties in monolayer transition metal dichalcogenides (TMDCs) by probing with photoluminescence (PL) intensity. A remarkable improvement in PL intensity in sulfur-based compounds, such as MoS2 and WS2, has almost been established by chemical treatment methods, such as p-type dopants and superacids; in contrast, selenium-based compounds of MoSe2, and especially WSe2 have a very limited strategy. One potential strategy for improving PL intensity in all compounds would be to remove the substrate effect by mechanical floating of the monolayers from the underneath substrate material. However, a freestanding monolayer structure requires fabrication efforts, and a post-preparation method has not been established well. In this paper, we developed a universal post-preparation method for eliminating the substrate effect by a spontaneous fluctuation of monolayer TMDCs via coating with a simple hydrocarbon, paraffin. Because of the large thermal coefficient of paraffin, the strain is effectively applied to the TMDCs in the coating process. The paraffin-coated monolayer TMDCs show spontaneous fluctuation; and in an extreme case, the fluctuation forms the shape of a bundle. During the fluctuation and deformation, the substrate effect was removed; as a result, the PL intensity improved remarkably in both sulfur- and selenium-based compounds of MoS2, WS2, MoSe2, and WSe2. This work suggests a universal strategy for improving the optical property of monolayer TMDCs by removing the substrate effect. It also extends the design of applying TMDCs as optoelectronics materials due to the transparency, flexibility, and biocompatibility of the paraffin.