Francis Oliver Vinay Gomes

• A novel wet-chemical synthesis of layered MoS2 thin films on silicon and sapphire substrate has been achieved. The gap in understanding solution processed MoS2 deposition needs to be closed to exploit all its excellent properties for low-cost applications. Both top-down and bottom-up approaches such as liquid phase exfoliation (LPE), dip coating, chemical bath deposition (CBD) and spin coating were explored for the synthesis of ultra-thin MoS2 films from solution phase. In this work, as deposited Mo-precursor thin films were prepared based on the solubility and coating properties of Molybdenum(V) chloride in 1-Methoxy-2-propanol. Subsequent annealing of the deposited amorphous Mo-precursor films in the presence of sulfur and H2 resulted in the formation of layered MoS2 films. Highly crystalline films are obtained on sapphire substrates, while inferior quality was obtained on Si/SiO2. Generally, improved crystallinity of the deposited films was achieved by increasing the process temperature and performing the post-annealing treatment. Post-annealing at temperatures above 900 °C increased the uniformity of multilayer films, together with the increase of MoS2 grain size. For charge transport analysis, top-gate top-contact thin film transistors (TFTs) based on these solution processed MoS2 films were fabricated. Ionic liquid gating of the TFT devices exhibited n-type semiconducting behaviour with field-effect mobility as high as 12.07 cm^2/Vs and Ion/Ioff ratio ~ 10^6. While with another precursor even mobility of 16.09 cm^2/Vs can be achieved. X-ray photoelectron spectroscopy measurements revealed that the films annealed between 900 °C and 980 °C had an average chemical composition of S/Mo ~ 1.84. This simple liquid phase synthesis method with centimeter-scale uniformity and closed films down to 2 ± 1 monolayer is suitable for low-cost preparation. This holds also for other transition metal dichalcogenides thin films in next-generation electronics.