  Individual monolayers of metal dichalcogenides are atomically thin
two-dimensional crystals with attractive physical properties different from
their bulk layered counterpart. Here we describe the direct synthesis of WS2
monolayers with triangular morphologies and strong room-temperature
photoluminescence (PL). Bulk WS2 does not present PL due to its indirect band
gap nature. The edges of these monolayers exhibit PL signals with extraordinary
intensity, around 25 times stronger than the platelets center. The structure
and composition of the platelet edges appear to be critical for the PL
enhancement effect. Electron diffraction revealed that platelets present zigzag
edges, while first-principles calculations indicate that sulfur-rich zigzag WS2
edges possess metallic edge states, which might tailor the optical response
reported here. These novel 2D nanoscale light sources could find diverse
applications including the fabrication of flexible/transparent/low-energy
optoelectronic devices.
