Recent progress in large-area synthesis of monolayer molybdenum disulfide, a
new two-dimensional direct-bandgap semiconductor, is paving the way for
applications in atomically thin electronics. Little is known, however, about
the microstructure of this material. Here we have refined chemical vapor
deposition synthesis to grow highly crystalline islands of monolayer molybdenum
disulfide up to 120 um in size with optical and electrical properties
comparable or superior to exfoliated samples. Using transmission electron
microscopy, we correlate lattice orientation, edge morphology, and
crystallinity with island shape to demonstrate that triangular islands are
single crystals. The crystals merge to form faceted tilt and mirror boundaries
that are stitched together by lines of 8- and 4- membered rings. Density
functional theory reveals localized mid-gap states arising from these 8-4
defects. We find that 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
