Large-scale synthesis of high-quality two-dimensional (2D) semiconductors, such as molybdenum disulfide (MoS₂), is a prerequisite for their lab-to-fab transition. It is crucial to systematically explore and understand the influence of key synthetic conditions on the nucleation, uniformity, and quality of MoS₂ wafers. Here, we report the epitaxial growth of high-quality and uniform monolayer MoS₂ films on 2-in c-plane sapphire by chemical vapor deposition (CVD) method under optimized growth conditions (0–1 mg NaCl, adequate S/Mo ratio, and the addition of 0–1 sccm O₂). We systematically explore the influence of critical synthetic conditions on the nucleation, and stitching of MoS₂ domains over the wafer scale, including the dosage of the alkali metal salt NaCl additive, the evaporation temperature of MoO₃, the distance between MoO₃ and the substrate, and the flow rate of O₂. Among them, the dosage of NaCl and the S/Mo ratio have important influences on the quality and film coverage of MoS₂, while the flow rate of O₂ plays a key role in controlling the nucleation density and domain size. We further discovered that a-plane sapphire could easily guide the unidirectional growth of MoS₂ without the need for other specific synthetic conditions compared with c-plane and m-plane sapphire. The field-effect transistors (FETs) fabricated from the full-coverage films show an average and the highest mobilities of 28.5 and around 45 cm²·V⁻¹·s⁻¹, respectively.

High-performance field-effect transistors (FETs) based on atomically thin two-dimensional (2D) semiconductors have demonstrated great promise in post-Moore integrated circuits. However, unipolar p-type 2D semiconductor transistors yet remain challenging and suffer from low saturation current density (less than 10 μA·μm^–1) and high contact resistance (larger than 100 kΩ·μm), mainly limited by the Schottky barrier induced by the mismatch of the work-functions and the Fermi level pinning at the metal contact interfaces. Here, we overcome these two obstacles through van der Waals (vdW) integration of high work-function metal palladium (Pd) as the contacts onto monolayer WSe₂ grown by chemical vapor deposition (CVD) method. We demonstrate unipolar p-type monolayer WSe₂ FETs with superior device performance: room temperature on-state current density exceeding 100 μA·μm^–1, contact resistance of 12 kΩ·μm, on/off ratio over 10⁷, and field-effect hole mobility of ~ 103 cm²·V^–1·s^–1. Electrical transport measurements reveal that the Fermi level pinning effect is completely effectively eliminated in monolayer WSe₂ with vdW Pd contacts, leading to a Schottky barrier-free Ohmic contact at the metal-semiconductor junctions. Combining the advantages of large-scale vdW contact strategy and CVD growth, our results pave the way for wafer-scale fabrication of complementary-metal-oxide-semiconductor (CMOS) logic circuits based on atomically thin 2D semiconductors.