Wafer-Scale Synthesis of MoS₂ Atomic Layers with Different Mo Precursor Solutions and Application to Thin-Film Transistors

Recently, transition-metal dichalcogenides (TMDs) have attracted much attention as new materials for electronics devices, electrocatalysts, photocatalysts, sensors, batteries, and bio-applications. Most TMDs are two-dimensional (2D) materials with a single layer. Bonds between each layer are made up of Van der Waals bonds, while intra-layer atoms bind together as covalent bonds.<br/>Chemical vapor deposition (CVD) with sulfur gas is the most popular method for synthesizing large- scale 2D materials with high quality. Various MoS₂ can be obtained from this method using various precursors with different properties, process temperatures, and substrate materials. Solution process methods show advantages for preparing films with large size, high throughput, low cost, thickness control, and an environmentally friendly process. Even though there is sulfur in the precursors of the solution-process synthesis methods, supplementing the sulfur that is lost in the high-temperature CVD process is unavoidable.<br/>In this study, two precursors of ammonium tetrathiomolybdate and ammonium molybdate were used in different solvent formulations to make MoS₂ crystalline by using simple thermal annealing and bottom-up thermolysis methods. These methods were relatively easy to handle, safe, and environmentally friendly processes. Importantly, these MoS₂ atomic layers were completed without additional sulfurization using CVD, but just with a single step of annealing because of sulfur-rich formulations.<br/>The 2D atomic layers were controlled to 2 to 7 layers with precursor concentrations with both formulations, which were confirmed by STEM-FIB. Thin-film transistors (TFTs) were prepared from the solution-processed MoS₂ on Al₂O₃ and SiO₂ dielectric with thermal evaporated Al source and drain electrodes. The results show improved mobilities of 9 to 48 cm² V^-1s^-1 and reasonable on-off ratios of around 1.0×10⁵ with solid output saturations. These new methodologies can be applied to multifarious devices and have the potential for scalability in 2D MoS₂ materials.