Variation of photoluminescence spectral line shape of monolayer WS2

TL;DR

This study systematically investigates how the photoluminescence spectra of monolayer WS2 vary, revealing edge effects and the roles of excitons and trions influenced by structural defects and adsorbed molecules.

Contribution

It provides new insights into the spatial variation of PL spectral features and the mechanisms behind edge-related PL enhancement in monolayer WS2.

Findings

Trion signals are suppressed near edges, enhancing excitonic features.

Edge effects are more pronounced near armchair edges.

Depletion of free electrons near edges affects PL spectra.

Abstract

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.