**2.2 Tungsten disulfide (WS2)**

The synthesis of tungsten disulfide (WS2) can be done by three main methods, namely hydrothermal method, atomic layer deposition (ALD), and CVD. A simple hydrothermal method was used to form WS2/C composite using Na2WO4·2H2O and CH3CSNH2 as raw materials, polyethylene glycol as dispersant, and glucose as the carbon source under annealing at a low temperature in argon atmosphere [5]. ALD was employed to form mono-, bi-, and multilayer WS2 nanosheets by controlling the number of cycles of ALD WO3 with plasma enhancement using WH2 (iPrCp)2 and oxygen [6]. The synthesis process of large-area WS2 films based on CVD can be described as follows [7]: (I) the Na2WO4 precursor coated on SiO2/Si substrate was loaded into quartz tube of CVD process. (II) Argon was flowed into the quartz tube until temperature reached 850°C. (III) A liquid phase of dimethyl disulfide ((CH3)2S2, DMDS) was introduced with a bubbling system for 30 min to form the WS2 film.
