  Monolayers of Transition Metal Dichalcogenides (TMDs) are atomically thin
two-dimensional crystals with attractive optoelectronic properties, which are
promising for emerging applications in nanophotonics. Here, we report on the
extraordinary spatial non-uniformity of the photoluminescence (PL) and strain
properties of WS2 monolayers exfoliated from the natural crystal. Specifically,
it is shown that the edges of such monolayers exhibit remarkably enhanced PL
intensity compared to their respective central area. A comprehensive analysis
of the recombination channels involved in the PL process demonstrates a spatial
non-uniformity across the monolayer's surface and reflects on the
non-uniformity of the intrinsic electron density across the monolayer. Auger
electron imaging and spectroscopy studies complemented with PL measurements in
different environments indicate that oxygen chemisorption and physisorption are
the two fundamental mechanisms responsible for the observed non-uniformity. At
the same time Raman spectroscopy analysis shows remarkable strain variations
among the different locations of an individual monolayer, however such
variations cannot be strictly correlated with the non-uniform PL emission. Our
results shed light on the role of the chemical bonding on the competition
between exciton complexes in monolayer WS2, providing a way of engineering new
nanophotonic functions using WS2 monolayers. It is therefore envisaged that our
findings could find diverse applications towards the development of TMDs-based
optoelectronic devices.
