Photoluminescence of monolayer MoS modulated by water/O/laser irradiation.

The low photoluminescence (PL) quantum yields of transition metal dichalcogenide monolayers have been a limiting factor for their optoelectronic applications. Various and even inconsistent mechanisms have been proposed to modulate their PL efficiencies. Herein, we use PL/Raman microspectroscopy and the corresponding mapping, atomic force microscopy, and field-effect transistor (FET) characterization to investigate the changes in the structural and optical properties of monolayer MoS. Relatively low power density (<4.08 × 10 W cm) of laser irradiation in ambient air can cause a slight PL suppression effect on monolayer MoS, whereas relatively high power density (∼1.02 × 10 W cm) of laser irradiation brings significant PL enhancement. Experiments under different atmospheres reveal that the laser-irradiation-induced enhancement only occurs in the atmosphere containing O and is more remarkable in pure O. In addition, physically adsorbed water can also induce PL enhancement of monolayer MoS. FET devices suggest that the adsorbed water produces a p-doping effect on MoS, and the laser irradiation in ambient air generates an n-doping effect, and both types of doping can enhance the PL intensity. The island-shaped defects caused by laser irradiation can be stabilized by oxygen atoms and act as trapping centers for excited trions or electrons, thus reducing the non-radiative recombination ratio and enhancing the PL intensity. The physically adsorbed water works in a similar way. A low power density of laser irradiation can sweep away the originally adsorbed HO on the surface, thus reducing the PL.