  Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDCs)
are attractive materials for next generation nanoscale optoelectronic
applications. Understanding nanoscale optical behavior of the edges and grain
boundaries of synthetically grown TMDCs is vital for optimizing their
optoelectronic properties. Elucidating the nanoscale optical properties of 2D
materials through far-field optical microscopy requires a diffraction-limited
optical beam diameter sub-micron in size. Here we present our experimental work
on spatial photoluminescence (PL) scanning of large size ( $\geq 50$ microns)
monolayer MoS$_2$ grown by chemical vapor deposition (CVD) using a diffraction
limited blue laser beam spot (wavelength 405 nm) with a beam diameter as small
as 200 nm allowing us to probe nanoscale excitonic phenomena which was not
observed before. We have found several important features: (i) there exists a
sub-micron width strip ($\sim 500$ nm) along the edges that fluoresces $\sim
1000 \%$ brighter than the region far inside; (ii) there is another brighter
wide region consisting of parallel fluorescing lines ending at the corners of
the zig-zag peripheral edges; (iii) there is a giant blue shifted A-excitonic
peak, as large as $\sim 120$ meV, in the PL spectra from the edges. Using
density functional theory calculations, we attribute this giant blue shift to
the adsorption of oxygen dimers at the edges, which reduces the excitonic
binding energy. Our results not only shed light on defect-induced excitonic
properties, but also offer an attractive route to tailor optical properties at
the TMDC edges through defect engineering.
