This study investigates the microstructure and properties of monolayer molybdenum disulfide (MoS₂) grown by chemical vapor deposition (CVD). The researchers refined the CVD process to produce highly crystalline islands of MoS₂ up to 120 μm in size, with optical and electrical properties comparable to those of exfoliated samples. Using transmission electron microscopy (TEM), they correlated lattice orientation, edge morphology, and crystallinity with island shape, confirming that triangular islands are single crystals. The crystals merge to form faceted tilt and mirror boundaries, which are connected by lines of 8- and 4-membered rings. Density functional theory (DFT) calculations revealed localized mid-gap states at these defects. Mirror boundaries caused strong photoluminescence quenching, while tilt boundaries enhanced it. However, the boundaries only slightly increased the in-plane electrical conductivity. The study provides insights into the impact of grain boundaries on the optical and electronic properties of MoS₂, which is crucial for the development of atomically thin electronics.This study investigates the microstructure and properties of monolayer molybdenum disulfide (MoS₂) grown by chemical vapor deposition (CVD). The researchers refined the CVD process to produce highly crystalline islands of MoS₂ up to 120 μm in size, with optical and electrical properties comparable to those of exfoliated samples. Using transmission electron microscopy (TEM), they correlated lattice orientation, edge morphology, and crystallinity with island shape, confirming that triangular islands are single crystals. The crystals merge to form faceted tilt and mirror boundaries, which are connected by lines of 8- and 4-membered rings. Density functional theory (DFT) calculations revealed localized mid-gap states at these defects. Mirror boundaries caused strong photoluminescence quenching, while tilt boundaries enhanced it. However, the boundaries only slightly increased the in-plane electrical conductivity. The study provides insights into the impact of grain boundaries on the optical and electronic properties of MoS₂, which is crucial for the development of atomically thin electronics.