Visualization of Defect-Induced Excitonic Properties of the Edges and Grain Boundaries in Synthesized Monolayer Molybdenum Disulfide

Atomically thin two-dimensional (2D) transition-metal dichalcogenides (TMDCs) are attractive materials for next-generation nanoscale optoelectronic applications. Understanding the 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 that is submicrometer in size. Herein, we present our experimental work on the spatial photoluminescence (PL) scanning of large-size (>= 50-mu m) monolayer MoS2 grown by chemical vapor deposition (CVD) using a diffraction-limited blue laser beam spot (wavelength = 405 nm) with a beam diameter as small as similar to 200 nm, allowing nanoscale excitonic phenomena that had not observed before to be probed. We found several important features: (i) There exists a submicrometer-width strip (,-500 nm) along the edges that fluoresces similar to 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 zigzag peripheral edges. (iii) There is a giant blue-shifted A excitonic peak, as large as similar to 120 meV, in the PL spectra from the edges. On the basis of density functional theory calculations, we attribute this giant blue shift to the adsorption of oxygen impurities 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 tailoring the optical properties at TMDC edges through defect engineering.