Talha Nisar

• Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising candidates for future electronics. Currently, the growth of large area TMDs thin films/flakes is one of the biggest challenges. A novel method for the growth of ultra-thin and large area tungsten disulfide (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. Ammonium tetrathiotungstate ((NH4)2WS4, ATTW) was used 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 and 90 oC. 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. The thickness ranged from a WS2 monolayer to five monolayers, as verified by AFM. Similarly, large area molybdenum disulfide (MoS2) ultra-thin film deposition is one of the big challenges in the recent years. Electrodeposition provides an opportunity to grow such ultra-thin films on large scale. However, the transfer of the electrochemically grown film to desired substrate is challenging. In this work, the polymer coated electrodeposited MoS2 films on Au are separated mechanically from the underlying substrate by using ultra-sonication. Collapse of micron-sized bubbles produced by ultra-sonication at the interface of Au and silicon substrate provides enough energy for separation due to their weak adhesion. The Au layer is then removed by standard Au-etchant (K/KI) and the polymer coated MoS2 film (two monolayers) is transferred to a desired substrate. The obtained MoS2 films by electrodeposition and spin coating were also used for hydrogen evolution reaction. The HER activity measurements for electrodeposited MoS2 showed an overpotential for 30 nm thick MoS2 of -0.33 V, whereas for smaller thicknesses below 10 nm a value of -0.22 V was found. This indicates that ultra-thin films of MoS2 show a better HER activity than thick films. The obtained Tafel slope of 44 mV per decade for 5 nm MoS2 is better than values reported for single crystal MoS2 and other 2D materials such as WS2.