Detection and Modulation of Gap States at the Grain Boundaries of Monolayer WS₂

In this work, an in-depth correlated study of the impact of grain boundaries on the excitonic and electronic properties of monolayer WS₂ is reported. Signatures of defect- and strain-induced gap states are detected and studied in the vicinity of the grain boundaries using tip-enhanced photoluminescence, Kelvin probe force microscopy, and conductive atomic force microscopy. These gap states demonstrate a trap-like behavior for the free excitons, resulting in the radiative recombination of the localized excitonic states at room temperature. The trapping behavior is also detected for the free carriers, indicated by the abundance of fixed charges at the grain boundaries. The carrier trapping is corroborated through (tip-enhanced) photoluminescence spectroscopy at the grain boundaries, particularly after photoinjection of carriers. Comparison of the photoluminescence response acquired under ambient and high vacuum indicates the high reactivity of these defect sites and physisorption of ambient species. The ambient molecules seemingly passivate the defect sites and locally modulate the layer properties.