Langmuir films and uniform, large area, transparent coatings of chemically exfoliated MoS2 single layers

Abstract

By manipulating colloidal dispersions of chemically exfoliated molybdenum disulfide (MoS₂) into an appropriate spreading solvent, we demonstrate, for the first time, the ability to form stable, floating MoS₂ Langmuir films without the use of surfactants or significant material loss into the aqueous sub-phase. While the floating sheets can be compressed into a densely tiled film by the barriers of a traditional Langmuir–Blodgett trough, we also report an edge-to-edge aggregation and spreading driven densification phenomena that allows the film to be built up from the outside of the trough inwards during the deposition process. Continued deposition allows us to fill the entire trough with a dense (85–95% coverage) film of discretely tiled 1T MoS₂ nanosheets and to coat substrates as large as 130 cm². The transfer efficiency is found to be as high as 120 m² of coated area per gram of deposited MoS₂. Comparing the transfer efficiency to the theoretical specific surface area of MoS₂ provides a method to estimate film thickness and exfoliation efficiency. Atomic force microscopy and optical absorption measurements are used to corroborate this estimate of 2.7 layers for the traditional n-butyllithium exfoliation method used. We demonstrate that the films can be built up layer-by-layer and investigate the optical and electrical properties of the films before and after conversion from the 1T to 2H polymorph.