Growth of ultra-thin large sized 2D flakes at air–liquid interface to obtain 2D-WS₂ monolayers

Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising candidates for future electronics. Currently, the growth of TMD large area thin films/flakes is one of the biggest challenges. A novel method for the growth of ultra-thin and large area WS₂ monolayer flakes has been developed by introducing a solution-based temperature-dependent process. This two-dimensional WS₂ growth process is low cost and environmentally friendly. WO₃ flakes are grown at the air–liquid interface using ammonium tetrathiotungstate ((NH₄)₂WS₄, ATTW) as WS₂ precursor. The process requires a moderate activation temperature as no flakes are formed at room temperature. Successful growth of flakes was observed in an aqueous solution of the precursor at a temperature between 70 °C and 90 °C. These flakes could be transferred to any substrate by a controlled dip-coating process. Large 2D WS₂ flakes with a lateral size of up to 100 μm were obtained after sulfurization. The thickness ranged from a WS₂ monolayer to five monolayers, as verified by atomic force microscope. The chemical reaction mechanism behind the formation of the flakes was investigated by FTIR, Raman, UV–Vis and x-ray photoelectron spectroscopy. The initial flakes were found to be made of WO₃, which were successfully converted to WS₂ by a post annealing step at 500 °C–900 °C. This simple and environmentally friendly growth technique can be used to produce large WS₂ flakes for next generation electronics. (paper)