This study investigates the microstructure and properties of grain boundaries in highly crystalline monolayer molybdenum disulfide (MoS₂). Researchers have refined chemical vapor deposition (CVD) to grow large-area, highly crystalline MoS₂ islands up to 120 µm in size, with optical and electrical properties comparable to exfoliated samples. Using transmission electron microscopy (TEM), they correlate lattice orientation, edge morphology, and crystallinity with island shape, demonstrating that triangular islands are single crystals. These crystals merge to form faceted tilt and mirror boundaries, stitched together by lines of 8- and 4-membered rings. Density functional theory reveals localized mid-gap states arising from these 8-4 defects. Mirror boundaries cause strong photoluminescence quenching, while tilt boundaries cause strong enhancement. In contrast, the boundaries only slightly increase the measured in-plane electrical conductivity. The study also characterizes the two most common types of grain boundaries in MoS₂: tilt grain boundaries, which occur where two nucleation islands intersect, and mirror twin boundaries, which occur in single islands. Using atomic-resolution imaging, they determine that the grain boundaries are stitched together predominantly through lines of 8- and 4-membered rings. They model the electronic structure of the boundary to show that the defects lead to the appearance of mid-gap states. By correlating grain imaging with photoluminescence and electrical transport measurements on individual grain boundaries, they show that these grain boundaries strongly affect the photoluminescence observed from the MoS₂ monolayers, but only slightly increase their in-plane electrical conductivity. The researchers also fabricated field effect transistors (FETs) to study the effects of grain boundaries on electrical transport. They found that the grain boundary has little effect on channel conductivity, but the parallel device exhibited similar behavior with 25% larger on-state and 60% larger off-state conductivity compared with pristine devices. This pattern was repeated for four parallel devices fabricated on two flakes, although with considerable variability in the degree of conductivity increase. The relatively larger parallel conductivity is consistent with a picture of conduction through mid-gap states at the grain boundary. The work presented here is an important step to incorporating molybdenum disulfide into two-dimensional electronics. The highly crystalline structures allowed systematic study of the crystal edges, grain structure, and grain boundaries by electron microscopy. By knowing the grain structure, they were able to identify crystal orientation and location of individual grain boundaries on the surface and systematically compare the optical and electronic properties to electronic structure calculations. The combinations of electron microscopy, optical spectroscopy, and electronic transport show that grain boundaries in molybdenum disulfide play an important role on the optical properties and slightly increase the in-plane electrical conductivity of this two-dimensional material.This study investigates the microstructure and properties of grain boundaries in highly crystalline monolayer molybdenum disulfide (MoS₂). Researchers have refined chemical vapor deposition (CVD) to grow large-area, highly crystalline MoS₂ islands up to 120 µm in size, with optical and electrical properties comparable to exfoliated samples. Using transmission electron microscopy (TEM), they correlate lattice orientation, edge morphology, and crystallinity with island shape, demonstrating that triangular islands are single crystals. These crystals merge to form faceted tilt and mirror boundaries, stitched together by lines of 8- and 4-membered rings. Density functional theory reveals localized mid-gap states arising from these 8-4 defects. Mirror boundaries cause strong photoluminescence quenching, while tilt boundaries cause strong enhancement. In contrast, the boundaries only slightly increase the measured in-plane electrical conductivity. The study also characterizes the two most common types of grain boundaries in MoS₂: tilt grain boundaries, which occur where two nucleation islands intersect, and mirror twin boundaries, which occur in single islands. Using atomic-resolution imaging, they determine that the grain boundaries are stitched together predominantly through lines of 8- and 4-membered rings. They model the electronic structure of the boundary to show that the defects lead to the appearance of mid-gap states. By correlating grain imaging with photoluminescence and electrical transport measurements on individual grain boundaries, they show that these grain boundaries strongly affect the photoluminescence observed from the MoS₂ monolayers, but only slightly increase their in-plane electrical conductivity. The researchers also fabricated field effect transistors (FETs) to study the effects of grain boundaries on electrical transport. They found that the grain boundary has little effect on channel conductivity, but the parallel device exhibited similar behavior with 25% larger on-state and 60% larger off-state conductivity compared with pristine devices. This pattern was repeated for four parallel devices fabricated on two flakes, although with considerable variability in the degree of conductivity increase. The relatively larger parallel conductivity is consistent with a picture of conduction through mid-gap states at the grain boundary. The work presented here is an important step to incorporating molybdenum disulfide into two-dimensional electronics. The highly crystalline structures allowed systematic study of the crystal edges, grain structure, and grain boundaries by electron microscopy. By knowing the grain structure, they were able to identify crystal orientation and location of individual grain boundaries on the surface and systematically compare the optical and electronic properties to electronic structure calculations. The combinations of electron microscopy, optical spectroscopy, and electronic transport show that grain boundaries in molybdenum disulfide play an important role on the optical properties and slightly increase the in-plane electrical conductivity of this two-dimensional material.