  By creating defects via oxygen plasma treatment, we demonstrate optical
properties variation of single-layer MoS2. We found that, with increasing
plasma exposure time, the photoluminescence (PL) evolves from very high
intensity to complete quenching, accompanied by gradual reduction and
broadening of MoS2 Raman modes, indicative of distortion of the MoS2 lattice
after oxygen bombardment. X-ray photoelectron spectroscopy study shows the
appearance of Mo6+ peak, suggesting the creation of MoO3 disordered regions in
the MoS2 flake. Finally, using band structure calculations, we demonstrate that
the creation of MoO3 disordered domains upon exposure to oxygen plasma leads to
a direct to indirect bandgap transition in single-layer MoS2, which explains
the observed PL quenching.
