Two-dimensional molybdenum disulfide (MoS 2 ) is an emergent semiconductor with great potential in next-generation scaled-up electronics, but the production of high-quality monolayer MoS 2 wafers still remains a challenge. Here, we report an epitaxy route toward 4 in. monolayer MoS 2 wafers with highly oriented and large domains on sapphire. Benefiting from a multisource design for our chemical vapor deposition setup and the optimization of the growth process, we successfully realized material uniformity across the entire 4 in. wafer and greater than 100 μm domain size on average. These monolayers exhibit the best electronic quality ever reported, as evidenced from our spectroscopic and transport characterizations. Our work moves a step closer to practical applications of monolayer MoS 2 .

modulate their electrical, optical, and structural properties by introducing impurities for doping. [5-10] So far, the chemisorption and charge transfer through surface functionalization are effective approaches to achieve doping, but it has a relatively weak influences on band structures or improvements of electronic properties. [11,12] By contrast, substitutional doping is more stable and capable of tailoring the bandgap of 2D-TMDs without introducing in-gap states. [13-16] Such substitutional doping could be realized through replacing the host transition metal or chalcogen atoms with other elements during synthesis to form alloys such as Mo 1-x W x S 2 , MoS 2x Se 2(1-x) , and V x W y Mo 1-x-y S 2z Se 2(1-z) , etc. [17-21] Recently, oxygen doping of 2D-MoS 2 has attracted considerable research interests. Previous studies show that post treatments of intrinsic MoS 2 in air, ozone, or oxygen plasma could induce oxygen doping, evidenced from the enhanced catalytic reactivity, quenched photoluminescence, and improved electrical conductivity. [9,18,22-24] Such doping processes usually lead to oxygen substitution and oxidation as well and consequently yield highly disordered or fragmented lattice structures. In situ oxygen substitutional doping in 2D-MoS 2 with a controlled and nondestructive manner is thus highly desired to preserve its original lattice structure, but still remains challenging so far. In 2D semiconductors, doping offers an effective approach to modulate their optical and electronic properties. Here, an in situ doping of oxygen atoms in monolayer molybdenum disulfide (MoS 2) is reported during the chemical vapor deposition process. Oxygen concentrations up to 20-25% can be reliable achieved in these doped monolayers, MoS 2-x O x. These oxygen dopants are in a form of substitution of sulfur atoms in the MoS 2 lattice and can reduce the bandgap of intrinsic MoS 2 without introducing in-gap states as confirmed by photoluminescence spectroscopy and scanning tunneling spectroscopy. Field effect transistors made of monolayer MoS 2-x O x show enhanced electrical performances, such as high field-effect mobility (≈100 cm 2 V-1 s-1) and inverter gain, ultrahigh devices' on/off ratio (>10 9) and small subthreshold swing value (≈80 mV dec-1). This in situ oxygen doping technique holds great promise on developing advanced electronics based on 2D semiconductors.