  The origin of the variation of photoluminescence (PL) spectra of monolayer
tungsten disulfide (WS2) is investigated systematically. Dependence of the PL
spectrum on the excitation power show that the relatively sharp component
corresponds to excitons whereas the broader component at slightly lower energy
corresponds to negatively charged trions. PL imaging and second harmonic
generation measurements show that the trion signals are suppressed more than
the exciton signals near the edges, thereby relatively enhancing the excitonic
feature in the PL spectrum and that such relative enhancement of the exciton
signals is more pronounced near approximately armchair edges. This effect is
interpreted in terms of depletion of free electrons near the edges caused by
structural defects and adsorption of electron acceptors such as oxygen atoms.
