Spatial nonuniformity of excitonic properties in exfoliated WS2 monolayers

Summary form only given. Monolayers of transition metal dichalcogenides (TMDs) are promising new materials for future 2D nanoelectronic systems.
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With their tunable direct gap in the visible range of the optical spectrum and high surface-to-volume ratio, these 2D semiconducting systems are ideal for field-effect transistors, photovoltaics, light-emitting diodes (LEDs), single-atom storage, molecule sensing, quantum-state metamaterials and electrocatalytic water splitting applications.
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Furthermore, the outstanding stretchability of 2D crystals is promising for strain engineering and related applications.Here, we report on the extraordinary photoluminescence (PL) and Raman properties, not only of the physical but also of intentionally created via femtosecond laser ablation4, boundaries of mechanically exfoliated WS2 monolayers. In particular, it is shown that the edges of such monolayers exhibit significant Raman shifts as well as remarkably increased PL efficiency compared to their respective central area. The observed strain varies among the different edges of an individual monolayer and can be inhomogeneous, depending on the mechanical history of the edges during exfoliation (Fig.1). Moreover, there is a 3-fold enhancement of the PL intensity at the edges compared to the monolayer center (Fig.2), with the emission channels being of different origin. The relation of the observed properties with the chemical nature of the edges is analyzed via Scanning Auger Microscopy (SAM) with high spatial and analysis-depth resolution and discussed accordingly. Finally, we report on the existence of biexcitons from 78 K up to room temperature. We envisage that these novel findings could find diverse applications in the development of TMDs-based optoelectronic devices.