Recent progress in large-area synthesis of monolayer molybdenum disulfide, anew two-dimensional direct-bandgap semiconductor, is paving the way forapplications in atomically thin electronics. Little is known, however, aboutthe microstructure of this material. Here we have refined chemical vapordeposition synthesis to grow highly crystalline islands of monolayer molybdenumdisulfide up to 120 um in size with optical and electrical propertiescomparable or superior to exfoliated samples. Using transmission electronmicroscopy, we correlate lattice orientation, edge morphology, andcrystallinity with island shape to demonstrate that triangular islands aresingle crystals. The crystals merge to form faceted tilt and mirror boundariesthat are stitched together by lines of 8- and 4- membered rings. Densityfunctional theory reveals localized mid-gap states arising from these 8-4defects. We find that mirror boundaries cause strong photoluminescencequenching while tilt boundaries cause strong enhancement. In contrast, theboundaries only slightly increase the measured in-plane electricalconductivity.
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