Visualization of defect-induced excitonic properties of the edges and grain boundaries in synthesized monolayer molybdenum disulfide

TL;DR

This study uses high-resolution photoluminescence microscopy to reveal nanoscale excitonic phenomena at the edges and grain boundaries of CVD-grown monolayer MoS₂, highlighting defect-induced optical property modifications.

Contribution

It demonstrates the first sub-micron spatial resolution PL imaging of large monolayer MoS₂, revealing defect-related excitonic features and their theoretical explanation via oxygen dimer adsorption.

Findings

Edges fluoresce 1000% brighter than interior regions.

Parallel fluorescing lines observed at edges.

Giant blue shift (~120 meV) in excitonic peaks at edges.

Abstract

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.