Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDCs)are attractive materials for next generation nanoscale optoelectronicapplications. Understanding nanoscale optical behavior of the edges and grainboundaries of synthetically grown TMDCs is vital for optimizing theiroptoelectronic properties. Elucidating the nanoscale optical properties of 2Dmaterials through far-field optical microscopy requires a diffraction-limitedoptical beam diameter sub-micron in size. Here we present our experimental workon spatial photoluminescence (PL) scanning of large size ( $\geq 50$ microns)monolayer MoS$_2$ grown by chemical vapor deposition (CVD) using a diffractionlimited blue laser beam spot (wavelength 405 nm) with a beam diameter as smallas 200 nm allowing us to probe nanoscale excitonic phenomena which was notobserved before. We have found several important features: (i) there exists asub-micron width strip ($\sim 500$ nm) along the edges that fluoresces $\sim1000 \%$ brighter than the region far inside; (ii) there is another brighterwide region consisting of parallel fluorescing lines ending at the corners ofthe zig-zag peripheral edges; (iii) there is a giant blue shifted A-excitonicpeak, as large as $\sim 120$ meV, in the PL spectra from the edges. Usingdensity functional theory calculations, we attribute this giant blue shift tothe adsorption of oxygen dimers at the edges, which reduces the excitonicbinding energy. Our results not only shed light on defect-induced excitonicproperties, but also offer an attractive route to tailor optical properties atthe TMDC edges through defect engineering.
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